Valve Potential Flow

We all fall prey to it occasionally; working on a project in which we have a preconceived idea of the results, but when you come near the end, they appear disappointing at best.  What did I do wrong? Can I fix it? When is “it will have to do” good enough?  Or, the ever-present option: do I need to start over?

But then sometimes when you find yourself in such a position, you need to take a moment an re-evaluate those expectations you had when you started. 

To make a long story short, I recently ported a set of head castings that I don’t often deal with, using smaller valve sizes than what I would normally select (for reasons I will not go into).  Once ported and flow tested, I was left wondering what I might have done differently.Then I remembered the actual valve sizes involved and it occurred to me that perhaps I expected more flow than I should have given the “smallish” valves.

For years I have had a chart that I produced for my own use, utilizing information from SuperFlow’s instruction manual for common valve sizes which provides “maximum potential flow” for the given valve size.  When I consulted the chart, I realized that I was dealing with a valve that was not on my chart.  The good news, however, was that once I did the math, I found that my work was completely up to snuff. 

But of course, never one to take small victories and move on, I decided that it might be a good time to update my chart.  And when I say update, I mean completely re-vamp it, put it into Excel and make it “interactive” so that with absolutely minimal input, all the data will be calculated for any valve size. So, if you have Microsoft Excel on your computer, you should be able to follow the link and download the “chart” and add valve sizes as needed.  

Valve Flow Potential @ 28" 

  

If you do not have the Excel program on your computer, you should still be able to download the chart though you will not be able to add sizes and will be limited to the valve sizes that I have listed.

This chart enables you to input the valve head diameter along with the stem diameter and everything else will be calculated for you.  Column A is a short description of where the valve is commonly used.  Column B is the valve head diameter and column C is the stem diameter.  Once the figures are entered into columns B & C, the rest of the fields will fill in automatically.  Column D gives the result of subtracting the square of the stem diameter from the square of the head diameter.  For the most part this figure can be ignored (though it is used by the program for further calculations).  Column F is the net valve area (which takes the area displaced by the stem into account).  From there the columns alternate between the potential flow at the given “valve lift to diameter ratio” and then what that valve lift is in inches. 

 

A quick word about lift to diameter ratio.  Many of those who do porting work find it most convenient to test flow at every .050” or .100” of valve lift, while others do all their testing at lift to diameter ratios. In the Harley industry, the lift to diameter ratio is not used often, though the concept is simple. The ratio is the derived by multiplying the valve diameter by a percentage; commonly 10%, 15%, 20%, etc.  This is expressed as: .10d, .15d, .20d, (you get the picture).  In other words, a 2.0” diameter valve would have a .25d lift of .500”.   The lift to diameter ratio has a couple of advantages, in that it allows one to compare the flow efficiency of two ports using different size valves rather than just the raw CFM of flow (where the larger valve will nearly always show up as the winner). This would be handy in a shop that ports a wide variety of head types. Also important, though often overlooked is the .25d flow.  At .25d (where lift = ¼ of the valve diameter) the “curtain area” of the valve is equal to the valve head area.  At that point, the amount of valve lift is no longer the primary deterrent to flow.  In porting, this has important implications when deciding what portion of the head needs improvement.

Hope this may be of use to some of you …

By the Numbers, Part 2

Well, as proposed, in light of the first post in this series in which we looked at minimum Cross Sectional Areas of your intake port and how they can affect flow potential. One thing that we saw was that preferably we would like to see the minimum CSA to be the throat (Also Known As venturi, AKA neck, AKA choke) just under the valve seat. I might add two points in regards to that assertion. One is that this may only be applicable to Harleys and not to other heads with significantly longer intake tracts. The other point is that I am not advocating larger than needed port CSA's, only that one should take into consideration those parts of the port that are actually a limiting factor to flow. We saw that the minimum CSA is in at the throat under the seat with the Evo/ Twin Cam intake port, but only at the points where the port is round and easy to measure. But what about where that port is no longer round (which is most of it). Just a quick visual confirms that the real minimum CSA is probably not at either the round port opening or the throat under the seat.

Evo

 

 

 Twin Cam

The first obvious thing you see looking into the port from the manifold end, is a "hump" in the floor. And right here let's take a moment to thank the Lord that we are dealing with an overhead valve motor right now so that there is no confusion as to which part of the port is the floor (hint - if you were short enough you could walk on it). Just imagine the confusion if we were forced to look at a Flathead port like we did to explain the concept of curtain area in our last post. Even I'm not sure which would properly be termed the floor and roof on a Flathead. Would one have to turn the Flathead cylinder upside down in order to keep the "short side" radius as the floor and the " long side" as the roof? Or do you leave the Flathead cylinder in the orientation that God created it in, and rename the short side the roof and the long side the floor? The potential for confusion is immense. Whew. Good thing were talking OHV right now! By the way, the short side radius is just what the words mean. The length from the port opening to the valve will be considerably shorter on the floor than it is on the roof (again from an OHV point of view).

As I said, the hump in the floor of the port is pretty obvious and changes the port shape from round to a sort of "D" shape (tipped 90 degrees counter clockwise). Obvious as it is in an Evo, the hump is even more pronounced in a Twin Cam. Though it's going to be very difficult to measure the Cross Sectional Area once we get into the port where it is no longer round, we can certainly get some sense of it from a few measurements.

I keep a suitably modified inside caliper on my porting bench. You may notice from the picture that there I have two sets of notches (filled with a bit of red paint for clarity) as reference points. These arbitrary reference points are a handy way to locate a fixed point for measuring since the port walls are tapered. By putting the caliper into the port so that the matching reference points align with the face of the port opening, one can easily take multiple measurements at a set depth. My calipers are marked at .750" and 1.500".

On this particular Evo head, the port opening measures a nominal 1.610" (it not quite round). But at a depth of .750" in from the port opening it measures 1.745" side to side and 1.375" top to bottom. So the port has gotten .135" wider, but at the same time it has become .235" shorter. That makes it pretty self evident that is a smaller Cross Sectional Area than at the port opening. When we make the same measurements on the Twin Cam head, the results are no better. The port opening of this particular head measures 1.650", but 3/4 of an inch in, the measurements are 1.845" side to side and 1.235" top to bottom. The question you must ask is this: did the increase in the width of the port make up for the decrease in height?

 

Remember what I mentioned about the preferable place for the minimum Cross Sectional Area being the throat under the valve seat? Since it is quite obvious that is not the case with these heads in their stock configuration, we can be pretty sure that to get the full flow potential from these heads, something needs to be done to enlarge this "pinch point" so that the minimum Cross Sectional Area is in fact under the seat where it belongs. 

Big Hint: don't cut down the "hump" to get the port closer to round. The hump is there for a good reason - and that reason is twofold, though related and codependent. The primary reason is form over function. Fatbob tanks have been a signature feature of Big Twin Harleys since the 1930s, and in order to keep the air cleaner from interfering with those tanks, the ports (both Evo and Twin Cam) had to be kept as low on the heads as possible. A port entry with a higher angle of approach to the valve would have made far more sense from a performance standpoint, but the factory knows the audience they are playing to. But in an admirable attempt to have their cake and eat it too, the Motor Company added the "hump" after the low entry point in order to help gently turn the air around that short side radius.

Don't get me wrong though. Just because the minimum Cross Sectional Area is in the wrong place, it doesn't mean the significant flow gains cannot be realized by porting without addressing that fact. In other words, if you (or the person porting your heads) never noticed this point of restriction, it doesn't mean the heads can't have considerably better than stock flow, it just means the flow will not reach the optimum. Now I should mention that there are probably some practitioners of the porting arts out there who will plea that some "pinch' of the port CSA is a good thing to have at the 'hump" and that it helps turn the air. I am not sure that I am smart enough to agree or disagree with that theory, but to attempt to prove it, I think one would have to be able to accurately measure the CSA of that pinch point and then by varying its size, empirically show how much constriction yields the best results. Whatever method you use to get there, you will know that you arrived at a near optimum port flow when it matches the CSA of the throat under the seat multiplied by 133.

Now keep in mind that this "secondary" choke point in the vicinity of the hump is pretty difficult to measure (as previously mentioned) due to its irregular shape. But adding to that irregularity is the valve guide boss. That right, by the time you are 1/2" into the port, the cross sectional area is not only being reduced on the bottom by the hump, but on the top by the valve guide boss. By now that cross section looks more like the drawing below:

Anyone want to take a stab at giving an accurate Cross Sectional Area for that shape? 

(it is possible, but quite time consuming) 

Okay, once you have the port flowing close to that magic 133 x the minimum CSA of the throat, you will know that you have the rest of the port to the point where it is no longer the limiting factor. And since this post is intended to informational in nature rather than a full blown "how to" we will leave the "hump" section of the port.

Keeping in mind the ideal of the minimum CSA being at the throat just beneath the valve seat, what then happens when you add a larger intake valve? Well, if you don't open up the diameter of the throat, then the minimum CSA stays the same and thus the theoretical maximum air flow remains the same. The larger valve has not helped at all except that it gives you more room to turn the air more gradually if you shape the seat correctly, but that only gets you closer to the maximum possible flow through that same minimum CSA.

But here we probably need to add another number to the mix. That is the relationship between the valve diameter and that of the throat below the seat. This is expressed as a percentage of the throat diameter verses the valve diameter. In other words if you have a stock Evo/Twin Cam intake valve that measures 1.843" with a throat of 1.625", the percentage would be 88.2% (rounded off). Today it is commonly accepted in the porting world that the range for this percentage is from 88 to 91% for best performance, though early Superflow literature suggested 85%. The exact number within that range may be tailored by application, with the lower RPM engines with low lift cams benefiting from the lower percentages. 90% is generally a good safe figure to use; 91% can be too big; while 92% is definitely too large and will likely hurt performance (though not necessarily flow). With that in mind, what happens when you put in a 1.900" intake valve (as is most common when porting Evo and Twin Cam heads)? If you open up the throat to 90% of that 1.900" valve, it will be 1.710". That has obviously moved our minimum CSA from the throat to the port opening (or to the "hump" as it may be). In other words, the 90% valve throat has a flow potential of 305 cfm, but your port opening will only handle 275.

Again, I would like to stress that just because your port opening is only of stock size, that does not mean that a 1.900" intake valve will not help your flow. We are dealing with theoretical maximum potential here, not hard and fast rules on how your heads must be configured to improve performance.

One thing that I might mention here relates back to part one of this series, having to do with curtain area. If you were to put 1.900" intake valves in your heads, keeping the stock port opening diameter the lift required for the curtain area to equal your minimum CSA will only go up an insignificant amount (.002") because that minimum CSA has not changed much, it is merely located in a different place in the intake tract.

since we are looking for the lift at which CSA equals the curtain area, insert the

CSA figure in the formula in place of Curtain Area

Remember the minimum CSA with the stock valve was 1.999 (throat CSA minus stem CSA). With the larger valve and matching larger throat under the seat, the minimum CSA is now at the port opening which is 2.074 square inches. If however, the port opening was not the place of minimum CSA, and the throat was, then the lift required for the curtain area to equal that point would be .372" (that is arrived by calculating the CSA of the throat [2.297], subtracting the CSA of the stem [.075], and then dividing that by the product of 3.1416 multiplied by the 1.900 valve diameter [5.969] ).

To get some perspective of how this lift = curtain area point changes when taken to an even more high performance application, let's consider the Screamin' Eagle 2.175" intake valve used in their "Hurricane" heads. IF they used a 90% throat, it would work out to 1.958" I.D. or a throat CSA of 2.935 square inches when corrected for the valve stem diameter. Plugging that into our formula we find that the valve curtain area will not equal the throat area until a lift of .430". That might change your perspective a bit on what might be considered low lift flow.

Just one more warning at this point. This calculated curtain area does not give you the lift needed for a given valve size or port size. It only gives you the lift at which the curtain area is no longer a primary limiting factor to flow. It is not the lift at which the your port will reach maximum flow (unfortunately not even close) but it does give you some insight as to how important the valve size and seat shape is at lifts below the point where the curtain area equals the minimum CSA.

At the end of part 1 I said that I would eventually get to how much air flow your engine really wants. Looks as though that means there will be a part 3.

By the Numbers, Part One

I am not a "numbers guy" the way some are. I don't find joy in numbers just for the sake of numbers. One reason might be that I generally don't remember them especially well, though there are a few exceptions; mainly those numbers burned into my brain from repetition, repetition, repetition. For instance I have not yet, and may never forget my social security number since that dark night on the bus taking a load of new recruits from the airport in San Antonio to Lackland Air Force Base for Basic Training. A very intimidating uniformed fellow informed us in no uncertain terms that "YOU WILL HAVE YOUR SOCIAL SECURITY NUMBER MEMORIZED BY THE TIME YOU GET TO THE BASE!" Possible consequences for failure were left to our imaginations, but for an 18 year old fresh off the farm ..., well let's just say I didn't spend much of that ride gazing out the window.

Some of the other numbers that seem to be forever stuck in my brain are strictly due to much use. Its been many years since I have had to look up the center to center rod length for a pre Twin Cam big twin. 7.43875" for the late and 7.46875" for the early. I'm pretty sure that comes from balancing well over 100 sets of flywheels (last time I counted) over the last 35 years.

But other than that, numbers don't hold any special favor in my consciousness. What numbers can do for me, on the other hand ..., that I can get excited about. That's why I just had to share a couple of formulas that I recently ran across while searching some of my reference material. The particular reference book I was perusing was "Engine Airflow" by Harold Bettes. He's a smart guy, so I've read through the book more than once. By the way, I titled this post before noticing Mr. Bettes has a like named chapter in his book, but I didn't get it from him - I would assume that I inadvertently stole it from somewhere else.

One of the first formulas that caught my attention on this particular day concerns determining curtain area of a valve. If you're not familiar with the term, picture a valve lifted off its seat, with a "curtain" hanging around the perimeter of the valve  to the seat. Okay, maybe it will help if you begin by picturing this on a Flathead motor where the valve head faces up so that you don't have to concern yourself with how a curtain could defeat gravity and "hang" from a valve "up" to its seat. Are you picturing it now? Good. The area of that "curtain" has a good bit to do with flow potential. The simplified formula for computing this is the following:

Before we get into applying this formula, let's consider something else. Cross Sectional Area (or CSA). When dealing with heads, the CSA we often are concerned with is the Minimum Cross Sectional Area. Now, it is self evident that there is limit to the volume of air that can pass through a given size hole at a given pressure. There is one place in every intake tract that is smaller that the rest of the system, and the point with the smallest cross sectional area can be the limiting factor in air flow (in the real world it will take a well ported head for this to be true). That smallest point in the intake tract, or minimum cross sectional area is only easy to measure where the intake tract is round, but for my purposes right now, that is okay because the Harley ports are round (or at least they were meant to be) at the port opening. Another point where the intake tract is round and thus easy to measure, is the "choke" or "venturi" just under the valve seat.

So, if we were to take, say for example an 80" Evo... Why an 80" Evo, you ask? Well, it is clearly the direct forerunner of the Twin Cam. The ports are quite similar, and in fact they share the same port opening diameter and even the same intake valve (at least to 2004). That port opening diameter is 1.625".

Happily the choke I.D. (under the valve) also is 1.625". I say happily because that is the one place in the intake tract that you absolutely must have a "choke" point so it is the obvious place for the minimum CSA. If you are wondering why Harley left a second minimum CSA at the port opening, they didn't, but we'll get back to that in a moment. First lets convert that diameter to Cross Sectional Area. The formula is this:

That gives us a CSA of 2.074 square inches for a 1.625 diameter opening. In other words the CSA for the port opening is 2.074". But not quite for the "choke" under the seat, because that also happens to have a valve stem protruding through the center of it. Once we calculate the CSA of that stem (which generally measures .310" diameter) and subtract it, we see that our choke just under the seat of 1.999 square inches is indeed the minimum CSA.

Now back to that curtain area formula. One of the cool things about algebra is that it allows you to turn formulas around to suit your needs. In this case, computing the curtain area for your intake valve at full lift might be handy, but wouldn't it be even more interesting to see what lift it would take so we can be sure the valve curtain area is not the limiting factor. In other words, what valve lift would it require to equal the minimum CSA of the port. Turning that curtain area formula around would look like this:

If we take our previously calculated minimum CSA of 1.999 (which is our desired curtain area) and divide it by the product of pi (3.1416) multiplied by the valve diameter (1.843), we come up with .345" valve lift. So, you can see that the Harley engineers did their homework in providing a stock cam with more than enough lift (.472") to provide a curtain area theoretically large enough that it does not become a limiting factor. If one were to take things a step further, Bettes' book also includes a formula for computing the minimum CSA required for a given Cubic Feet per Minute (CFM) of air flow measured at a 28" test pressure. That formula is:

Or, once again to turn that formula around for my own purposes:

If we were to plug our minimum CSA into that formula we would find that theoretically, our stock Evo (or Twin Cam) heads with stock valves and valve seats, together with a stock cam, should have the potential to flow nearly 292 CFM (1.999 x 146) @ 28" test pressure. Wow.

But back in the real world... That theoretical flow potential of 146 CFM per square inch of Cross Sectional Area is a figure that has been calculated and confirmed by a number of people way smarter than me over the years, and while correct, it admittedly does not take into account any friction losses, and perhaps most importantly it does not account for loss of flow due to expansion which must happen when the air flows out of the port and into the cylinder. According to Patrick Hale's "Engine Pro- The Book" this figure when adjusted to take these other factors into account, along with "benchmark" results (as of 2004) reduces the target to a more reasonable 133 CFM per square inch. Still, that leaves your OEM head with a potential of 265 CFM (1.999 x 133).

But if we were taking a trip back to the real world when we reduced the flow potential to 133 CFM per square inch, then its time to get our heads out of the clouds, because even if the clouds are in the real world, most of us don't ride our Harleys there. So keep in mind that it will take a very good porting job to take full advantage of the potential flow through that minimum Cross Sectional Area.

On the next installment of this multi-post series, I'll try to take a look at a few things such as why the minimum cross section we just spent our time calculating may not be where the actual minimum CSA is located, what happens when we add porting and bigger valves, and eventually on to how much air flow your engine really wants.

Valve Stem Protrusion; Knucks, Pans, and Shovels

Harleys are very rebuildable, and I would go so far as to venture that they may be among the most commonly rebuilt (using the term "rebuilt"somewhat loosely) of any engine family in existence. Such a supposition is somewhat bold, given the minuscule number of Harleys compared to the vast oceans of, say, small block Chevys. But face it, which engine is more likely to wind up in a scrap yard when it is in need of major repair?

Given that, along with the often less than spectacular life span of a top end rebuild on Knuckles, Pans, Shovels and Sportsters, many if not most have seen multiple valve jobs over the decades. Naturally with each valve job performed, the valves will seat a little deeper in the head. The method of gauging how much deeper is via the valve stem protrusion specification. Valve stem protrusion is one of those specs that is sometimes overlooked and to some extent misunderstood when dealing with Harley heads.

At issue are a several things. In no particular order; valve spring installed height, shrouding of the valve in the chamber, compression ratio, and finally rocker arm geometry. Having less than the minimum can lead to the devastating result of your valve springs reaching coil bind while your cam is still trying to lift the valves higher. Not a good situation and can usually be summed up as 'broken parts."  On the opposite end of the spectrum is the Panhead that looks as though the pan covers have been bashed out with a ball peen hammer (because they have indeed been bashed out with a ball peen hammer) so that the valve spring collars would not hit them.

Shrouding of the valve in the chamber from the valve being too deep is fairly easily remedied by a judicious modification of the chamber during the process of a valve job, though this too can overdone resulting in issues down the road when new seats are installed.  Along with deep valve seats comes a reduction in compression ratio (aggravated via de-shrouding) by making the chamber larger.  That may or may not be an issue depending on a number of factors.

Valve train geometry is also at issue, but I will attempt to address that later in the post.

To examine this subject I would like to start in the middle and work our way forward in time before going back to the beginning - that beginning being the Knucklehead.

On page 75 of the Harley Davidson Panhead Service Manual - 1948-1957 Rigid, we find what seems to be first official mention of the specification (at least that I can find).

The spec, which the drawing refers to as "Valve Seat Tolerance" is pretty self explanatory. It is the distance from the tip of the valve stem to top surface of the collar of the valve guide. The illustration also shows a gauge which was available for those lacking precise measuring tools or for quick checks. The gauge is simply a cylinder that straddles the guide. The "step" at the top of the gauge indicates minimum and maximum height; if the tip of the stem falls between the top and bottom of the notch, the stem protrusion is within spec.

The 1978-1/2 to 1984 FL/FX 1200/1340 4 Speed Service Manual (note the title may not be growing in length but it certainly is in use of numbers) shows the same illustration (page 3-18) for 1979 and earlier, but it might be worth noting that it offers a different illustration and spec for 1980 and later.

The difference, at least in part, is due to the changeover to valve guide seals. Earlier heads, both Pan and Shovel, only required a machined pad that was at least the diameter of the valve guide collar to locate the guide since the lower spring collar rested on the collar of the guide. The addition of seals made it necessary to rest the lower spring collar directly on the head to provide room for the seal, so the machined portion of the spring pocket was increased to the diameter of the lower spring collar.

Late vs Early

At first glance one might assume that the different spec is due to taking the measurement to a different surface, since it is now from the tip of the valve to the surface that the bottom of the guide collar seats against. And maybe that's the case, however, things don't seem to quite add up. If the collar on the guide is nominally .100" thick, then all is well. Add .100" to the early 1.500" to 1.545" spec and you come up with the '80 and later spec of 1.600 to 1.645". Ignoring the '80-'81 guides that used a .075" snap ring instead of having a guide with an integral collar, there is still the question of the gaskets that were under the guide collar on earlier motors. I had to look pretty close to even find the part number (18196-51) for this gasket in a Harley parts book since it does not appear in any later copies, though I have a small collection of them left over from top end kits. Measuring a random sample of these showed that they ranged in thickness from about .030" to .040". The James Gaskets catalog lists them as .031" thick with the application being 1951 to 1978.

Hmmm,... so with a window of only .045" in minimum and maximum stem protrusion, we find a variance of at least .030" just in whether or not a gasket was installed under the guide when rebuilding. And what about '48 to '50 Pans and '79 Shovels? Won't they show up as nearly at maximum protrusion right from the factory? And what does that mean when considering '80 and up which certainly never used the gasket? Now the .100" difference in stem protrusion spec doesn't add up so neatly because you have an "effective" guide collar thickness of .130" (collar + gasket) for many years.

Add all of this together and I think its safe to conclude that stem protrusion specification is probably not something will "make or break" your valve job unless you wander too far afield. My guess is that the spec was added after the fact as a guideline for mechanics rather than a part of the original design parameters of the Motor Company.

And if all doesn't throw enough margin of error into the equation, then consider this. If the Motor Company's stem protrusion specs theoretically provide correct valve train geometry (and that is a gigantic stretch given shops such as Baisley High Performance have presumably made a fair chunk of money over the years from their service of correcting Harley rocker arm geometry), then that still means that when you increase valve lift via a performance cam, you have also changed the stem protrusion numbers which should theoretically retain correct geometry.

Here is basically how it works. If you were to draw one imaginary line through your pushrod and another through the rocker arm's ball socket to the center of the rocker shaft, when your cam is at one half of its lift, the line should form a 90 degree angle. Likewise, an imaginary line from the center of the rocker shaft to the pad of the arm should also form a 90 degree angle with the centerline of the valve stem at that same half lift point. That way at zero lift the line through your rocker arm should be the same amount below 90 degrees as it is above 90 degrees at full lift. But that means that if you increase the lift of the valve with no other changes, then the angle with the valve closed will remain the same , but the 90 degree relationship between pushrod and rocker will no longer be at 1/2 lift. To get back to the theoretically correct valve train geometry you would need to lengthen the valve by an amount equal to 1/2 the increase in lift. Or, you could get the same effect by sinking the valve that amount. And guess which is easier and more cost effective, sinking the valve or having a custom valve manufactured?

All of that is to say that with a performance cam, the theoretically correct stem protrusion increases at a rate of half the increase in valve lift. In practice this also has the added benefit on a Harley of providing the increased valve to valve clearance during overlap (commonly referred to as Top Dead Center lift) which is needed for those performance cams.

Now, with all that to digest, I'll pause briefly before continuing with the question of valve stem protrusion on a Knucklehead.  Stay tuned.

The Multiple Valve Job Low Down Performance Blues

The term "tired" is one often used to describe the engine condition of our antique (or on the verge of antique) bikes.  Often times that is an apt description.  Worn cylinders/pistons/rings/valves/guides and pitted valve seats all take away from the performance we originally enjoyed.  Most of the time a fresh top end rebuild would be all it would take to return the old girl to her former [performance]glory.  But with the passing of the years and the oft repeated "freshening" we may wake up one morning (after an evening of thrashing gears with buddies on their Twin Cams) thinking that maybe the old girl never was as quick as we thought we remembered.  Perhaps you've heard of the syndrome: "the older I get, the faster I used to be..."

But wait, ...maybe it's not all due to a faulty memory.  What if your trusty old steed really has lost a step or two over the years despite a fresh top end rebuild?  This could creep up on you slowly (like most everything else that creeps up on you).  Repeated valve jobs can actually decrease performance despite sealing the combustion chambers better than the tired condition it was meant to improve upon.  The reason is compression ratio.  Compression makes power!  That's not a secret, but sometimes we forget that each time we grind our valve seats we are also increasing the combustion chamber size and lowering the compression ratio. 

As a matter of fact, I have previously written on this subject here.

One valve job; no problem.  Another couple valve jobs -still maybe OK, ...depending on how much it takes to clean up the seats each time.  And that is often where things can get quickly out of hand.  Take a cheap, generic valve guide of unknown origin.  Is the bore of the guide concentric with the outer diameter?  If not, then when you install it and start to grind or cut the seat, you will be off center from the last valve job which will force you to cut deeper to get a good seat.

 

This Shovel shows evidence of multiple valve jobs, but it is not uncommon to see seats considerably deeper than these

Just to reiterate, and to give you some examples as to how this works out, I have made a few calculations.  One thing that these calculations take into account  is that the deeper a valve is sunk into the head, the more likely that there will be additional material removed from the head so that the valve is not shrouded by the chamber, which would otherwise disrupt air flow.  The figures given assume that at a depth of .050" deeper than stock, no un-shrouding was done.  Further, they assume that when sunk .100" deeper than stock, the valves will have been un-shrouded.  The additional amount of material removed for un-shrouding purposes was determined from actual measurements using 1-3/4" diameter valves.  Of course your results would vary somewhat unless you used the same tooling and dimensions for the un-shrouding process.

So on a 61" EL model Knuckle which had a 7:1 compression ratio in stock configuration, sinking both valves .050" would lower the ratio to 6.78:1.  Not so bad.  But sinking the valves a total of .100" past stock would result in 6.33:1.   At this point you have a lower compression ratio than the low compression E model's 6.5:1.

What about on a 74" FL Knuck?  .050" lowers the ratio from 7:1 to 6.78:1, but sinking them .100" makes your motor 6.45:1.  (disclaimer: your 74" Knuck is probably not 7:1 anyway, since the pistons available today are unlikely to have the same dome volume as the original Knuckle piston)

Now Panheads are going to be especially prone to this scenario since seat replacement is, shall we say, ...problematic.  The FLH's 8:1 compression ratio drops to 7.7:1 with the valves sunk .050" and to almost exactly the same as the low compression FL model when sunk .100".  If you start out with the low compression FL you will end up with Knucklehead type compression ratios: 7:1 when the valves are .050" deeper and 6.65:1 at .100".

With their larger intake valve, Shovelheads suffer from the effect even more.  A 74" with 8:1 ratio will be reduced to 7.66:1 if both valves are sunk .050".  Make that .100" deeper than stock and your motor is all the way down to 7.18:1.  An 80" Shovel also rated at 8:1 will similarly be reduced to 7.69:1 with a .050" deeper valve job, and down to 7.24:1 at a depth of +.100".

One thing you might take away from all this is that there is very little reason to buy the low compression version of the replacement pistons on the market today.  In fact, it may cause you to select higher than stock compression pistons for many applications, especially if hotter cams are being employed.  Remember, compression makes power, and if you want old reliable to be as fast as she was in her younger days, you need some of that.

A Note in Passing

As it so happens, right now I have two sets of Evolution cylinder heads in my shop that I have seen before.  Both of them were previously ported by me.  What make this worth mentioning is the "when" that I last saw them.  One set has a porting job which I performed in 1994; 20 years ago.  It was the 131st set of heads that I had ported.  You may wonder how I can be so sure about work that I completed so long ago, but its simple.  I stamp an ID number into the heads I port and keep records.

Now, I don't know much about the life this particular set of head lived in the intervening years, but it was ready for new valves and guides.  Not totally shot, unusable junk; just out of spec on the valve to guide clearance and stem taper.  The present owner is not the one who I did the work for originally, and since another local shop brought them to me to freshen up, I don't know how many miles or owners they have gone through since '94. 

16 year old porting

 

The second set of heads received my porting work a scant 16 years ago.  They were the 273rd set of heads that I ported.  I have a little more info on this set due to the fact that I count the owner as an old friend, though I suppose that he would point out that it would be more accurate to say that I am his old friend, since I am about 10 years his senior.  This set has gone in excess of 100,000 miles, many with sidecar attached,  since I last saw them.  The rest of the motor is quite worn out, to the point of needing a new crankcase pinion race, oil pump, and even an oversize breather gear due to the amount of debris that has passed through it.  Somewhat surprisingly the guides did not need replacement on this set of heads, which is a testament to the design of the Evolution valve train.

 

guides still good - seats after valve job

 

 

Shovelheads Again

Unfortunately I was unable to get pictures to load for this post -  sorry - that would have made it much easier to follow and understand the material presented here.


It is not uncommon for me to receive a question in the comments section of my blog posts.  Sometimes it is an easy answer, but other times it requires a little more... and that may lead to a whole new post.  Such is the case here.  I recently  received the following in the comments section  of an older post:


 I'm interested in building a big bore shovel and have toyed with some do it yourself porting...there's some interesting views from the nightrider web site: How to Build a High Performance Shovelhead Engine.  I don't have a flow bench so was contemplating just smoothing out intake harsh edges and general polishing in lieu of redesign? Appreciate your thoughts if you care to comment...




First of all, a thank you to Dave for asking a very good question.  Probably most wrenches who worked in a dealership in the 1980's or before will recognize the sheets copied on the nightrider site.  I don't remember if they handed them out at the factory service school when I attended, or if my set was passed down to me from a previous attendee.  Either way, this info has been out there for a long, long time.  For reader's convenience I scanned my copies and attempted to place them her in the text, but to no avail.  Apparently the man behind the curtain at "Blogger" is too busy conquering the world to keep the picture uploading feature working at the moment. 







The material on shaping the intake ports presented therein (figures 5 and 6 in the link) has probably been the basis for a number of porting jobs.  However, before you drag out your TIG welder, consider this: if executed properly, this modification will indeed increase performance by way of greater flow.  Executed poorly, however the port modifications described can result in a net loss of performance. 






Unfortunately the difference between well executed and poorly executed can be very difficult to ascertain without the aid of a flow bench.  The reason is that the modification to the floor of the port leading to the valve seat (commonly called the short side radius) is one of the areas of a port that has the most potential for gain in airflow, but is also the most sensitive to shape.  The fact is, this area is one of the worst features of a stock Shovel head casting and also the most difficult to "fix."  It cannot be optimized by grinding; the problem is there is already not enough material there.  What is really needed is more material - just like this old performance paper suggests.








Stop!  I already warned you to hold up on dragging out the welder!  If you are going to start welding, you also may want to consider this; you will also need the ability to machine your heads for new valve seats.  Here is the reason.  If you look at Figure 6 in the link, you  will notice a dimension labeled 1.64 DIA. This smaller dimension just under the valve seat is commonly called the "choke" or venturi.  Let's stick with calling it the choke since there is also a similar situation in your carburetor also called a venturi.  And just to keep things on the up and up, I should mention that in porting discussions another choke is often mentioned, that being the place in the port that has the smallest cross sectional area other than the one just below the valve seat.  Of course that leads me to feel the need to point out that there is a 3rd item in the intake tract called a choke, which of course is in the carb and used for starting.  You can completely ignore that one!  So to sum up, there are 3 chokes and 2 venturi, but the only ones we are concerned with for this discussion are in the head.




This choke dimension (the one just under the valve seat - remember?), or more precisely the relationship of this dimension to the valve head diameter,  has an important relationship to airflow past the valve.  Now the nominal head diameter of a Shovel intake valve is 1.937 (1-15/16) often referred to as 1.94.  The 1.64 dimension means that this modification is calling for the choke to be just less than 85% of the valve size.  While it is easy enough to see where this 85% figure came from, putting my stamp of approval on it is a little harder.  The early SuperFlow  flow bench instruction books show a diagram of the ideal intake port area and shape and show the 85% relationship.  The key word there is ideal.  The only less ideal port shape than the Shovelhead which comes to mind are the tortuous switchbacks in the Knuckle/Pan intake tract.




Be that as it may, if you were to take your ordinary everyday Shovel head and measure the inner diameter of the valve seat insert, you may be surprised (or not) to find a number like 1.820".  Now if you do a little reverse engineering you will find that gives a choke percentage of almost 94% (1.820 divided by 1.937).   Now 94% is a far cry from 85%, but it gets worse (at least form the 85% perspective).  When  you were measuring intake seat insert, did you notice that it  that it really didn't line up with the aluminum of the port very well?  You aren't that observant?  Go back and look.  I'll wait....




Okay, visualizing the direction the mixture must travel, what do you think all of those 90 degree corners will do for your air flow?  What do you mean you left the head out in the shop?




So what you could do is remove the valve seat inserts, weld in all the areas shown in the drawing (and you may as well do something with the equally offensive exhaust port while you are at it) machine for new seats that have both a larger outer diameter and a smaller inner diameter, grind the port to match the diagram and do a valve job.  Piece of cake, right?  Oh... and before you decide to take a shortcut using some sort of porting epoxy, don't even consider leaving it hanging out over the seat insert as shown in the diagram.  It's life expectancy in an air cooled engine on the street will be far less than you like.





But, that is not to say there is no hope for you do-it-yourselfers.  First of all let's go back to that 85% choke figure.  It's probably a decent ratio for an exhaust valve, and maybe even for the situation in the diagram, but are you really going to spend the time and money trying to duplicate it?  Many, if not most cylinder head porters will tell you (if they are willing to tell you anything) that a good rule of thumb is to make the choke 90% of the intake valve diameter.  I have heard some of the very best say that you may sometimes need to go as high as 91% but absolutely no higher.  At least part of the reason is easy enough to visualized.  Air likes to turn in maximum increments of 15 degrees (which explains the angles used on a valve job) but it needs a little length for each of those angles - about .060" is enough.  But on a stock shovel seat insert with its I.D. at 94% of the valve, how much room is left on the inside for a 60 degree once the 45 degree angle is cut (or ground)?  Little to none, that's how much; and forget about adding a 75 degree angle.  Not much help in turning that air!




It is often said that one of the biggest factors in performance is the valve job.  But for the reasons stated above, I say: "Not on a stock Shovel!" On a stock Shovel about all the valve job can do is make a seal.  Sadly there is no material present to put good valve job on to help get that air turned.  Want to change that?  Put in a 2 inch intake valve.




Simple as it is, it improves several things.  First of all, suddenly there is enough meat left in the seat insert to add a couple more angles under the 45 degree seat.  Now your valve job can be a little more conducive to flow that the simple on/off spigot it was before.  Plus, now your choke percentage is a far more reasonable 91%.
 


As to the actual valve job, if you remember the 15 degree airflow rule of thumb, it becomes fairly obvious.  Just make sure that the outer edge of your 45 degree seat coincides with the outer edge of your valve.  That will leave the maximum room below that 45 for your 60 degree, 75 degree, and in the unlikely event that the I.D. of your seat insert is too small, a 90 degree.



Once you have an actual performance valve job in place, Dave's instinct to just smooth out the harsh edges is about right.  The turn that the floor of the port makes just before the valve (called the short side or short turn radius) is always a major offender on Shovel heads.  If the seat insert does not line up with the aluminum of the port in this area, don't be afraid to do a little grinding on said insert as part of putting a radius on this turn.  If you are more ambitious, get yourself a set of inside calipers and work at keeping the cross sectional area constant from the port opening to the short side radius.  And don't forget, if you want both heads to flow the same amount, you will want that cross section constant from front head to rear head also.




Finally, one disclaimer:  You don't get something for nothing. A 2 inch intake valve is heavier than a 1.94.  If your valve springs were marginal before, they are even less likely to provide good valve control with a heavier valve.  That's not too hard or expensive to take care of.  A bigger concern may be valve to valve clearance.  All things being equal, a .060" larger intake valve will be .030" closer to the exhaust valve when they pass each other during overlap.  The hotter the cam you have, the more likely that you will have issues.  If you decide to go with 2 inch intakes, you should check this whether it be via a full blown mock up on the engine or a bench check using the TDC lifts listed for your cam.


Of course that brings up at least one more question.  Is it possible to get a good flowing Shovel intake port without going to a 2 inch intake?  The answer is that you most definitely can.  With the aid of a flow bench many (myself included) have been doing it for many years.  But to do so one needs to make up for that poor seat shape somewhere, and that "somewhere" is most easily found via a flow bench.