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.
Friday, February 14, 2014
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7 comments:
Hi Lee,
Thanks so much for posting a response to my question on shovelhead porting and polishing. I must admit I was tempted to weld n grind as per Figure 5 and 6 but will consider this in light of the information you have provided.
Thanks again for parting with your wealth of experience and knowledge, I'll let you know how I go...
Regards...Dave
The concept of a "choke" above a valve that increases flow seemed counter intuitive to me. But as i thought about it I remembered something I had learned about called Coandă effect described by [http://en.wikipedia.org/wiki/Coand%C4%83_effect] as:
"Another demonstration is to direct the air flow from, e.g., a vacuum cleaner operating in reverse, tangentially past a round cylinder. A waste basket works well. The air flow seems to "wrap around" the cylinder and can be detected at more than 180° from the incoming flow. Under the right conditions, flow rate, weight of the cylinder, smoothness of the surface it sits on, the cylinder will actually move. Note that the cylinder will not move directly into the flow as a misapplication of the Bernoulli effect would predict, but at a diagonal.
The effect can also be seen by placing a can in front of a lit candle. If one blows directly at the can, the air will bend around it and extinguish the candle."
Making horsepower in an engine is about burning gasoline. The more gasoline you burn, the more horsepower you make. But, you need air to burn gasoline and it's way harder to get air into a cylinder than it is gas. The object being to encourage air around the curved spaces of the intake plenum and into the cylinder. This is probably why a tuner spends his time with shape and not cross sectional area. This is probably why I've done more harm to the intakes I've "modified" than I've helped. The problems with the curved spaces of intakes is probably what led Buell and Rotax and a host of others to opt for a straight down flow. But even then, you run into the top of the valve where the air has to be encouraged to flow around a bend and into the space between the open valve and the head.
A visual demo of the Coandă effect can be seen at:
http://www.youtube.com/watch?v=aF92B6Gon3M
There are other videos as well but unfortunately we live in a world that would rather believe in "ET" than God so a lot of the videos are wasted trying to make a case for flying saucers that TOTALLY could have flown across space using the Coanda effect. Whatever.
Something I had never thought about before:
http://victorylibrary.com/tech/crod-c.htm
Ahh, we ignorant must be a trial to those of you that are wizards but the ignorant never tire of an ever-changing and surprising world.
That's just great Dave (Ervin)- I still had not wrapped my mind around the Coanda effect and its implications, when you add something new.:-)
Fortunately I am already familiar with the rod/stroke ratio controversy (the controversy not in whether there is an effect, but how important it is). From what I have read, many of the top performance engine builders consider it to be near the bottom of the list of the many things to take into consideration when "designing" a motor.
If I had half the knowledge you have I'd know twice what I do know. And, in fact, half of what I think I know is under suspicion of being "internet knowledge."
Rod ratio isn't something easily modified. It's just something I had never considered. I've read that stroking a motor lowers the top RPM as piston speed increases to the limit of metallurgy but big motors (V8's) have normal strokes equal to bodaciously stroked motorcycle motors and they rev just fine. Or not.
Many internet forums are poor places to find information (search youtube for over unity concepts, for example) and it's hard to sift the worthwhile from the slag. Here's an example. I have no experience to comment on the process they refer to as "powre Lynz." Call me skeptical.
http://www.allpar.com/fix/holler/performance-and-economy.html
(I used to have an 87 turbo Sundance that was more fun than a car full of girls; till I overheated it)
Skeptical is also a good description of my take on much of these "do it yourself miracle mods." Not to say that every aspect of what they claim is inaccurate, but for the most part the results are highly exaggerated if not outright fabrications. This kind of stuff has been around almost forever.
I remember a guy coming up to my booth at a swap meet once (years ago) singing the praises and selling those turbulator things (without being honest enough to pay for a booth himself, I might add) When I told him that if he gave me one to test I would put it on my flow bench and then on an engine and give him my professional opinion on it, he turned and walked away without comment.
That doesn't mean someone did not get rich selling them though!
Someone once said that lures were to catch fishermen, not fish. And cat food is sold to cat owners not cats.
A bunch of us were once stationed overseas. We were cloistered together in apartments and driving communal cars. One gentleman, now, sadly gone, got some of those magnets that increase fuel mileage which he installed in one of the cars. Now, I'm not saying that tomfoolery was the sole order of the day but I'd be surprised if some of our co-workers didn't stay up late dreaming of pranks as the results were spectacular in some cases.
When Dick installed the magnets on the car some of the others took it upon themselves to add a half gallon of gas into that car about every morning. Dick could not have bitten harder on that hook if it had been made of original sin. Every day was an exhortation of the magic qualities of those magnets. Then the boys began siphoning out a half gallon a day. Needless to say the bragging stopped and a very sullen and quiet Dick faced each day. Eventually the boys spilled some gas and the game was up.
The magnets were worth every penny, but not to Dick, who paid the bill for his own fool's cap.
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