Inside the Patriot Missile

So, as with almost anything one might endeavor to write about, this one has a “back story” of sorts.  A number of years ago, I had an on-line discussion with the legendary drag racer Granddaddy Joe Smith.  In the course of that conversation, Joe told of one of the many engine configurations with which he competed in Top Fuel, that being a Shovel featuring one injector on each head.  The thing that caught my attention, was the way in which Joe achieved the individual intake tracts.  Ingeniously, he used two rear heads, turning one 180 degrees and mounting it in the front position.  What I found especially compelling of this set up was the how few special parts are required.  The cylinder base stud pattern on a Shovelhead is “square.”  That means rather than having custom built cylinders in order to turn the head around, one can simply turn the whole cylinder and head assembly 180 degrees.  Another neat little part about this method is that since the intake and exhaust ports remain in their original orientation with respect to the cam it does not require the use of a specially ground camshaft as some other conversions might.  One other note concerning this conversion would have to include a credit to Turk Dale, another legendary drag racer who also campaigned a Shovelhead in this same configuration.

Now, seeking to put into practice a principal presented to me by an early mentor in the field of Harley mechanics, the second thought I had (after, “I really want to build one of those!”) was, “who can I find to pay me to build one of those.”   That’s right, the principle is, “let someone else pay you to experiment.”  I even came up with cheaper way to build such a motor: start with an Iron Sportster.  However, after several years of the idea rattling around in my head, it became clear that I would find no one in the market for such an exotic piece, at least no one willing to spend more than nothing to make it happen.  But once my good friend Kevin “Teach” Baas let it slip that he had a Sportster motor that was destined to be a project for the students in Kennedy High School’s Chopper Class, the old light bulb started to glow.

If I couldn’t make any money building a cool conversion such as I envisioned, the next best scenario would be to donate my labor to a worthy cause, in what would eventually come to be the vintage style drag bike known as the “Patriot Missile.”

Kevin has been teaching a class designed to get his high school students interested in what we used to call “the industrial arts” at Kennedy High School in Bloomington, Minnesota.  To make the idea of learning how to weld and fabricate more attractive, Teach came up with the idea of building choppers as a focus of the class.  It has been very successful, to the point where the idea has caught on at various schools around the nation.  After all, imitation is the greatest form of flattery.  And I guess that makes this engine build a form of flattery aimed towards Granddaddy Joe and Turk.

In case any of you are wondering how well this conversion, pioneered by Turk and Granddaddy Joe on Shovelheads, applies to an Iron Head, the answer is, “just fine, thank you.”  There is only one caveat, and it actually makes the whole process even one step simpler.  Since the cylinder base pattern on a Sportster is NOT square, you can’t reverse a cylinder, but the beauty of it is that you don’t need to because on an Iron XL the headbolt pattern is square! 

 

Since this engine build was destined for the drag strip, I took the liberty of machining a goodly portion of the cooling fins off both the heads and the cylinders, however I did spend the extra time to mock everything up first with the fins intact to confirm that the conversion would also be viable on a flat tracker or even a street bike. If you chose to build your own version of this, you will also find that it will entail some trimming of fins for pushrod tube clearance and such, but that is a fairly easy hurdle to clear.  During the process, I even machined spark plug wrench clearance into the stock front rocker box, which because it now resides on a reversed rear head, has its spark plug on the pushrod side.  Using a front rocker box in its original orientation on a reversed rear head is not quite a bolt on.  Some of the rocker box mounting holes wind up being about “a 1/4 of a hole off” which is easily remedied with a long end mill of the correct diameter.  However, the two mounting holes closest to the center of the head do keep the rockers in proper relation to the valve tips.  Since this was to be a drag bike, before I was through with the rocker boxes, I split them, lightened and polished the rockers, and added grease zerks to the shafts to provide lubrication.

The iron heads were where most of the time was spent, and indeed the area that requires the most modification for the conversion.  Rather than simply welding an extension onto each intake port, I chose to build separate “manifolds.”  The extra length required to provide carb clearance would have made it very difficult to do a good job of porting, so fabricating a couple extra flanges to make the extensions removable was time well spent.  Since porting work is one of my specialties, this project was a good opportunity to push the envelope a bit for Iron heads.  This began with a 2.060” intake valve, a full 1/8” larger than the largest valve Harley ever put in a Sportster.  To compliment this larger valve, the port diameter was also increased from its stock 1.565” to 1.680”.  The large intake valve resulted in the need to cut down a 1-3/4” exhaust valve to a more “valve to valve clearance” friendly 1.650”.  The use of hotter cams would have necessitated either an even smaller exhaust valve or perhaps a 2 inch intake.  If this was to be something other than a drag bike, I would have needed to convert the heads (or at least the one in the front position) to external drain lines, but because of its intended purpose that step could be ignored here.  As previously mentioned I fabricated steel flanges to weld onto the cast iron heads, but then switched to aluminum for the actual intake port extensions.  During initial mock up, I found that a straight port extension would not provide carb clearance without making them considerably longer than a theoretically correct tuned length called for.  Since a curved port extension would be difficult to come up with, I chose to put a 5 degree angle on each end of the extension, knowing that was a small enough change of direction to cause little loss in air flow.  The old round slide Mikunis, being narrower, would have been a better choice here, but I was fresh out of them, and have been for a couple decades.

As for the flow figures, the heads came up to 267 CFM at .600” lift (at 28" test pressure) on the intakes, and though they were still gaining at that lift, I didn’t bother checking them past that point since we were stuck with the .400 lift “P” cams anyway.  Curiosity may have gotten the better of me, but checking them at higher lifts would have necessitated shorting the tops of the guides.  The 267 figure comes out to be about 45 CFM more than a stock head of similar vintage, and even the .400” lift figures showed nearly a 40 CFM gain.

A bit of further explanation may be in order here before continuing with the engine description.  The Patriot Missile project, despite its name, did not have the luxury of being funded by the U.S.  government.  Most of the parts were either donated, “scrounged” or purchased by Teach.  The motor had reportedly been recently rebuilt before it was donated, but to stock specs.  The pistons showed this to be the case, being new +.040 JCC reproductions sporting stock compression ratio.  Stock Sportster “P” cams inhabited the cam chest.  Since high compression pistons and hot cams were not in the non-existent budget, I decided to do what drag racers did in the days before specialty parts were readily available and success depended less on cubic money

.

The cams would have to stay, but there was one thing I could do about the pistons.  After all, electricity, argon and filler rod all add up to peanuts compared to custom made high compression slugs.  After a goodly amount of time welding, grinding, machining and measuring, the domes came in at 60.7cc, which put the final compression ratio at just over 11:1.  Not surprisingly, the extra material in the dome added a fair amount of weight.  To avoid the necessity of splitting the cases to re-balance the flywheels, a like amount had to come out of the bottom of the piston domes, along with some other strategic areas of each piston.  Leaving out the middle ring also saved a little weight, while decreasing frictional losses at the same time.

In that same vein, selection of carbs was based on two criteria; price and availability.  Several years ago, I purchased a box full of old OEM butterfly Keihin carbs that another shop had designated as parts carbs.  That fulfilled both points of the criteria.  The Keihins were in hand and for all intent and purposes they were free.  Once a matched pair of them were separated from the herd, all that remained was to clean them up and find a way to get some air through them.  Never having bored one of these before, I decided to creep up on the maximum size possible, and found the 38mm carbs could be taken out to 41mm, though that was certainly pushing the limits. 

 

At this point in time, I have to say that I am quite pleased with the results, especially considering the extremely low cash investment.  Given the dead stock lower end, I don’t expect to see record setting performance, but I do expect to see a smile on Teach’s face when he takes it down the track at the Meltdown Drags in July, ….and I hope that it puts a smile on the faces of a couple of legendary drag racers who provided the inspiration!

Vintage Dual Carbs, Part 4

Okay, its high time I finish up this series on vintage dual carbs, or more accurately stated: dual carbs on vintage Harleys. In a previous post, I promised to offer what I see as possibly the best compromise between authenticity, performance and feasibility.

For a Knuckle or a Pan, there are three basic ways to achieve dual carbs. First is the obvious: major modifications to the heads (welding and machining) for one carb with an individual runner on each head. This method is probably the most difficult and/or the most expensive. If you are a good welder, machinist, and have porting experience, then it may just the ticket. If you lack one or more of these talents, well, ...that's where the expensive part comes in. It also suffers from the drawback that, for street use, leg interference is normally a major issue, especially with Linkerts. If using Linkerts or other small bore carbs, this method will also limit maximum air flow.

The second method is to mount two carbs on opposite ends of a common manifold, 'a la the Seely manifold of yesteryear. This is perhaps the simplest method and easiest to build. However, it does suffer from the same leg clearance problems of the first method; again, especially with the relatively long Linkerts carbs. One advantage this method does have over the first is the ability to either keep each intake track separate for true independent runner (IR.) operation, or to use the manifold as a common plenum so that each cylinder can draw from both carbs at once for maximum power potential.

A third option, as I see it, is a custom manifold for two carbs mounted side by side on the pushrod side of the engine. On a Knuckle or a Pan, since the intake spigots are at the far left hand (primary drive) side of the engine, this method has the advantage of providing more room to accommodate carburetor length. This allows the dual carbs to be the least intrusive in respect to legroom. The drawback is that this last method leads to a compromise between the tuning advantage of individual runner and the increased flow capabilities or a plenum manifold which allows each cylinder to draw from both carbs. Keep in mind (as pointed out in part one of this series) that a pure individual runner manifold with carbs on the pushrod side of the engine will necessarily suffer from a "pinched" port cross section. Given the limited amount of real estate between the cylinders/port spigots, I just don't see any good way around this.

That leaves us with one option (in my humble opinion) as the best overall compromise for vintage style dual carbs. That would be a manifold with a runner from each carb, which merges into a common runner before splitting off into each port. A variation of this would be for the carbs to mount on a "plenum" which then feeds into two separate runners which attach to the ports on the heads. In practice, the section of either of these manifolds which attaches to the heads will essentially mimic (if not actually begin life as) a stock "T" manifold (or in the case of Shovelheads "Y" manifold. For the sake of easy identification, let's call the one a "plenum manifold, and the other an X manifold (for a Shovel it would be pretty much X shaped, though for a Pan or Knuck it would look like an X with the top legs folded down).

X Manifold for Knuck

 

 

 

 

 

Plenum Manifold for Shovel

The natural question then becomes, which is better, a plenum type or an "X" manifold. Outside of building one of each, followed by real world testing of each (including dyno testing), I really cannot give a firm answer. Now if any reader out there would care to provide me with a generous research grant along with dyno facilities, I would be glad to find an answer. As you may surmise from the increasingly length of time between blog posts, I have no spare time to donate. However, we can come get some idea of how such manifolds may work out by use of computer simulations.

As engine simulation programs go, there seem to be three levels readily available. There is the very basic type which uses a limited number of inputs. While these can be fun and useful, they definitely have their limitations. The mid-level programs use many more inputs, provide for a higher level of accuracy, and of course cost more. Finally, there is the professional level simulation program with even more inputs and even higher purchase prices. The basic simulators generally can be purchased for $100 or less, the mid level in the $200-$300 range, with the professional versions in the $500 range.

The simulator which I own, Engine Analyzer from Performance Trends, falls into the middle category. To give you an idea of the data that is utilized in this program, here is a partial list: Bore, stroke, rod length, windage, friction losses, compression ratio, combustion chamber shape, valve diameters, flow efficiency, port diameter, port volume, port centerline length, manifold diameter, manifold centerline length, manifold flow efficiency, runner length, carb cfm, ...and that only covers the first three of seven screens of data to be entered. Obviously the more accurate the data entered, the more accurate the results. The real strength in a program such as this is not in the hard numbers it produces, but rather is revealed in the company name itself:  Performance Trends. While the actual horsepower and torque numbers produced by the program may not match real world results exactly, by changing input data one step at a time, one will see a "trend" in the results.

So, ...to the simulator! The logical first step was to enter everything in as accurately as possible to match a stock 74 cubic inch Knucklehead. Well, almost stock. I used the flow figures for an unmodified FHP/S&S Knuckle head along with an Andrews "N" Knucklehead cam. The N cam is what Andrews calls its "stock replacement" and though its specs don't quite match my own measurements for an OEM cam, or the S&S stock replacement Knuck cam, I already had the specs loaded into the program and deemed it suitable for these tests.

As I stated earlier, the actual numbers that the program predicts are of much less consequence than the "trends" that it reveals, though I might mention that the numbers generated looked reasonable, remembering that rear wheel HP recorded on a chassis dyno will be 10-15% lower Here was the first thing of note: changing from the stock Knuckle M35 carb to the slightly larger M74 resulted in a gain in average torque (across the tested band of 1500 to 6000 RPM) of 4-1/2 foot pounds, along with along with gain in average horsepower of 4. Peak torque went up by nearly 7 ft pounds and the horsepower peak went up by 5, but also peaked at 500 RPM higher than with the M35 (5000 RPM vs 4500).

M35 vs M74

Next up was to input the added runner length of the dual carb "X" manifold without the additional flow capability of a second carb. This model shows an increase in both torque and horsepower across the entire RPM range, which suggests that added overall intake tract length for the dual carb manifold is a performance enhancement all by itself.

But we really aren't going to the trouble of building a dual carb "X" manifold just for the length, are we? Inserting the flow from an M74 combined with an M35 results in even more impressive gains. Peak torque is up nearly 9 foot pounds compared to the M74 on a stock manifold, while peak horsepower increases by almost 13. Average torque across the tested RPM range is up by 8 with average horsepower up by 7. Perhaps just as important is that the dual carb model show the torque as higher than the stock carb and manifold at all RPMs, even compared to the single M35. That would suggest that we are not going to give up any lower RPM performance in the search for top end power with the dual carb manifold.

M74 on stock manifold vs dual carbs on X manifold

Perusing all of the data, it appears that the M35 on a stock manifold is limiting the peak torque to 3000 RPM and the peak HP to 4500. The simple change to an M74 raises the torque peak to 3500 and the HP peak to 5000, indicating the restriction of the M35. The addition of the dual carb manifold with two carbs leaves the RPM for peak torque at 3500, but raises peak HP to 5500.

At no point in the torque curve did the M74 produce less than the M35. In fact, subsequent "runs" with even much larger carbs still showed a horsepower peak at 5500 RPM (the peak number increased, but not the RPM at which it was reached). That indicates to me that the M35 was limiting the RPM of peak HP, whereas with an M74 there is some other factor that is limiting it (in fact increasing the cylinder head air flow in the test did result in the RPM for peak HP increasing).

Plenum vs X manifold shows which is clearly the better street choice

 

Of course these tests don't tell us much about throttle response and overall driveability, but that's where a little personal experience weighs in. While I have not had the opportunity to field test an "X" manifold, I do have some personal experience with a "Seely" style manifold with no divider which allows each cylinder to draw from both carbs. When I built such a manifold about 25 years ago, it seemed logical to set it up in much the same way as an automotive 4 barrel carb. If you consider a typical 4 barrel, you will notice a couple things. One is that the "primary" throttle plates are normally smaller than the secondary's. The second is that the linkage provides for the primary throttle plates to open part way before the secondary plates begin to open: this is commonly called progressive linkage.

Now think about what that means for a moment. The smaller primary plates will naturally result in better fuel mileage and throttle response when you don't "have your foot in it." The larger secondary plates will allow maximum air flow for WOT (wide open throttle) situations. That should make for a pretty good all around compromise between driveability, mileage, and power. And such proved to be the case. I fabricated throttle linkage (shown below) which allowed the M35 that I used as the primary carb to reach 1/2 throttle before the M74 secondary carb began to open, but which also caused both carbs to reach wide open at the same time. As an added airflow enhancement, I only ran a choke plate on the primary carb (though the choke shaft remained in place on the M74 to keep the low speed needle adjustment mechanism).

Throttle closed

Right carb at 1/2 throttle, Left about to start opening

Both carbs at full throttle

Starting was as easy as stock, and as I recall required abut the same procedure as with one carb. Throttle response was as good as stock. WOT performance was very good, and in fact when I would "roll on" the throttle, I could feel when the second carb started to open, much like you can feel the secondary's of a 4 barrel carb on your V-8 kick in. (Come on, ...at least some of you had to have ridden in an antiquated car with a 4 barrel carburetor.)

Of course this setup was not on a completely stock Knucklehead, which has the potential for skewing some of the results. The engine remained 74 inches, but it used two right side flywheels allowing quicker revving. It also had ported heads with larger valves, along with a 110 Sifton cam; you know... typical Knuck hop up stuff of days gone by.

Full disclosure; the one glitch that I never did quite work out entirely to my satisfaction (at least from a curiosity standpoint) was the idle speed. With the circuit breaker at full advance the idle speed would remain too high despite my attempts to disable the idle circuit of the secondary carb. I think it was due to my use of typically worn out Linkert carbs (with excess wear to the bodies at the throttle plates) allowing too much air to sneak through. Since I did not have any better carbs to try, I "solved" the problem by installing a late '60s auto advance circuit breaker which I did happen to have in my parts stash.  Problem  not solved, but symptoms masked.  Whether I was correct that the source of the high idle was worn out carbs, or if I simply did not do a good job of disabling the idle circuit remains to be seen, and any input from someone with similar experience is welcomed.

So, there you have it, an "X" shaped dual carb manifold; ...my best compromise solution for dual carbs on a vintage Big Twin. No need to do extensive and expensive modifications to your heads. By avoiding the short runner length of a plenum type manifold, RPMs for toque and horsepower peaks stay at a street friendly level. And perhaps one of the biggest advantages is that it sticks out less than 3 inches further than the stock set up, or about an inch more than an S&S E.

Ignore the linkage shown here - pic was taken early in the mock up process

Rumors of my Demise...

Well, I am not aware that there are actually any such rumors circulating yet, though it has been so long since I have posted you may have wondered.  The truth is I have just been so busy trying to keep my head above water in the shop that I have, to my shame, not been able to finish the final installment of Vintage Dual Carbs. 

One small part of the hold up is that I would like to make part 4 a little more than just a commentary on my past experience, however given the difficulty finding time to write, its unlikely I will find time to do actual real world testing any time soon.  That's where computer simulations come into play.  I would like to take the time to run some simulations on my Engine Analyzer software to re-enforce my thoughts for a good compromise for dual carbs on a vintage Harley.  Nothing takes the place of actual testing, but I fully expect that with some careful measurements and time with the program some valuable information will be revealed.

So stay tuned...

Vintage Dual Carbs, Part 3

In part three of this treatise on dual carb Harleys, I would like to put the venerable Linkert under the proverbial microscope. First stop will be an unlikely source for an avowed Knucklehead fanatic such as myself, that being the ’48 to ’57 Panhead Service Manual. The last page of the carburetor section has a handy-dandy chart listing most (all?) of the carbs used from 1936 to 1957. Along with applications, throttle disc angles and even transfer port dimensions, we find venturi sizes listed. Hats off to those technical writers of yesteryear; if only their modern day counterpart would follow their lead.

For our purposes here though, the venturi sizes are indeed what is most useful. The first one we might note is the 3 bolt M-5/ M-55 of early 61" Knucklehead fame. They featured a venturi bore of 1-1/16". OK, so what, you might say. Well, let me put that in terms of modern carbs: 27mm (rounded off). That friends, is not a lot of area to get much air through. But as we go down the list, the venturi sizes do increase, but not much. While we do find the M-25/ M-75 with its whopping 1-5/16 venturi on 1940 Knuckles and as an option on ‘41-’48 models, most Knuckles left the factory with the 1-1/8" M-35. Finally, with the 74" Panheads a 1-5/16" venturi became the standard bearer for brass bodied Linkerts on Harleys, being the largest Linkert offered. Again let’s put those sizes into terminology which will make it easier to compare with modern carbs. The 1-1/8" venturi M-35 series carbs were only 28.6mm (again, rounded off). The big kid on the block M-74 comes in at a whopping 33.3mm (do I really need to point out that this figure too is rounded off?). As a point of reference, when Harley switched to Bendix carbs in the mid 1970s, they put a 36mm version on the XL models, which just happen to come in at very near the same cubic inch displacement as the 61" Knuckle, and a 38mm on 74" Shovelheads.

 

Super E on left, M35 Linkert on right

As a further point of reference, let’s mention a couple modern performance carbs. S and S (why does it bother me so much that blogger’s html will not allow the use of the ampersand?) designates their carbs by the diameter at the manifold surface rather than the venturi size. An S and S Super "E" carb which is called 1-7/8" is actually 40mm at the venturi, while the "G" model’s 2-1/8" bore is 44.7mm at the venturi. Right in between these two carbs is the flat slide Mikuni HSR42. Keep in mind though, none of these three performance carbs relies on a choke plate as a starting aid. All of the Linkert carbs employ such a plate, which further limits their air flow beyond what the already smaller venturi does. And the measured airflow through these carbs bears this out. An S and S "E" carb flows a whopping 73% more than an M74 despite the venturi bore being only 20% larger.

 

Knuckledragger Carbs

 

Right about now you may be thinking, "What about all those old pictures I’ve seen of vintage drag bikes with a pair of Linkerts fitted?" In fact, those paying close attention may even wonder why my own vintage drag bike, "The Knuckledragger" breathes through a pair of them. Speaking for my own situation, I had two reasons to use Linkerts. Nostalgia and pragmatism. The nostalgia portion is self evident. The pragmatic stems from the simple reason that the heads I had in my possession for the project were already set up with Linkert 4 bolt flanges. Had it been a more serious performance effort, I would have reworked the heads.  As it is, I still cut the choke assemblies off, added radius inlets and swapped to the somewhat larger 1-5/16" venturi's on the M35 carbs to maximize air flow.

 

Granddaddy Joe Smith still ran Linkerts on his Knuckle just before switching to a Shovelhead

 

The reason you see Linkert in old pictures of early drag bikes is also pretty straight forward, though maybe not so self evident. Many of the drag bikes you see outfitted with a pair of Linkert carbs were run on nitro methane fuel . "Nitro" as it is commonly called among racers, is quite different than gasoline in that it has oxygen in its chemical makeup. That means that rather than relying on airflow to get enough oxygen into the combustion chamber, with nitro a goodly portion of that oxygen is supplied by way of fuel flow. Remember back in part 3 when I wrote this? "The problem is very seldom getting enough gas into the engine for high performance, the challenge is in getting enough air in." Perhaps I should have more specifically used the word oxygen rather than air and fuel rather than gas, but I think you get the picture.

 That is not to say that the addition of nitro turns the Linkert into killer performance carb with no other changes. Nitro requires a much, much, much richer fuel to air ratio. For a given amount of airflow, nitro will need about 7-1/2 times as much fuel as the same engine running gasoline. This requires some substantial modifications to the fuel delivery system all the way from the petcock to the float system to the jets. The potential results however, are nothing short of awe inspiring. A switch from gasoline to nitro methane comes with a potential of just over double the horsepower. Do I need to mention that the bike and the rest of the motor needs to be strong enough to survive double the power?

Now it looks like this series will drag out into yet another post. God willing, I plan to wrap it up in a post encompassing my thoughts on what makes a viable street application for vintage style dual carbs.

...a brief intermission...

This will be short.  No sooner did I mention that I did not recall ever seeing a pair of SU carbs used on a dual carb Knuckle than I received the pictures below from Glenn in Australia.  It seems that his friend Billy campaigns a Knuck with just such a setup.

The 79" Knuckle motor is reported to put 96 HP to the rear wheel on methanol.  Everything else about it is a secret.

 

 

What is there to say but WOW!

Vintage Dual Carbs, Part 2

Airflow.  That's what its all about, at least for the performance enthusiast.  More air (carrying the proper mix of fuel of course) = more power.  Even what is often the first modification, open exhaust, falls into this category as evidenced by the fact that a less restrictive exhaust will require richer jetting to go along with the increase in air intake.  Obviously the same goes for the air cleaner.  So what's next after that? 

Bigger carbs, bigger valves, and porting are all the radar screen for those in search of horsepower, and that's not a new development. If I am not mistaken both Chet Herbert in the late '40s and George Smith Sr. in the '50s ran Riley carbs on their famous dual carb Knuckle drag bikes.  The Riley carbs were originally a racing part for Model A Ford engines.  I can only speculate at this late date that the reason for their existence was for increased air flow. 

 So perhaps this is the appropriate point to mention a few fallacies.  One is that when running one carb one each head (aka individual runner), you need to use smaller carbs than when using one carb to supply both heads.  Think about this for a moment.  On a V twin engine, both intake valves are NOT open at the same time.  Ignoring for a moment any tuned length/ram effect, that means a carb that is too small to be a performance carb with a conventional intake manifold will also be too small to be a performance carb when used on a one carb per cylinder set up.  As a matter of fact it seems that the experts, i.e. anyone I regard as smarter than me (a long list to be sure), suggest that on a individual runner set up, the carb actually needs to be larger than for conventional set up due to the lack of a "plenum effect".  If I follow the reasoning correctly, it is because a plenum will actually help to dampen the large fluctuations in pressure in the intake tract from the valve opening and closing.  This effect would be smaller on a two cylinder than 4, 6 and 8 cylinder engines, but present none the less.

On the other extreme I have had someone with a dual carb set of heads, upon hearing my opinion that the carbs were too small, state that, "Yeah, but hopefully I can jet them up enough to work."  Well, sure you can jet them "up" to work.  All that takes is the right jet to maintain the correct fuel/air ratio.  The problem is very seldom getting enough gas into the engine for  high performance, the challenge is in getting enough air in.  That's what makes a carb too small for a performance application; lack of air flow, not lack of fuel flow.  But more on that later.

So, what was the hot set up for dual carbs "back in the day?"  Well, I suppose that would depend a lot on your exact definition of "back in the day."  Forty years ago (back in the mid '70s) a large round slide Mikuni was a common performance upgrade to replace the aging Linkert, as were SU carbs adapted from British automobile applications.  The Mikuni was a natural for dual carbs, though the popularity of said dual carb builds was on the wane primarily due to the more flow friendly intake tract of Shovelheads and Sportsters. I can't say I remember ever seeing a dual carb Harley with SU carbs, though the constant velocity design would seem to be nearly ideal for the job.

Going back a little further in time, it seems that Italian made Dellorto carbs may have been one of the most popular for use on dual carb heads.  Their SSI series slide type carbs were available in sizes up to 42mm making them a good choice.

 

highly modified Knuckle from the mid '60s sporting Dellorto carbs

 

 

Ron's vintage Knuck also features early Dellortos - more on this bike here

 

 

"Famous" Doug Gall used a pair of later Dellorto carbs

Now, cool as the Riley and Dellorto carbs may be, due to their scarcity one will probably not find many of them being used on modern "period correct" builds.  What you will generally find is Linkerts.  Yes, they seem to be constantly rising in cost, but compared to a Riley carb they are still dirt cheap. 

 

pair of modified Linkerts on The Knuckledragger

So, how does the Linkert stack up as a performance carb, you ask?  We'll look into that in part 3.

Vintage Dual Carbs, Part 1

Undoubtedly one of the most popular topics on this blog is that of dual carbs.  As I have mentioned before, I tend to be a sucker for exotic carburation myself.  Judging by feedback, both via email and in the comments section, I am not alone. 

Most of us are at least somewhat familiar with the fact that the Factory (as in Harley Davidson Factory) dipped their foot in the waters of land speed racing, placing legendary racer Joe Petrali on board a specially modified dual carb Knucklehead, during the spring of 1937 at Daytona Beach.  The result was a new one mile speed record of 136.183 mph; a record which would stand for 11 years.  The heads on that bike were modified in the conventional manor of one carb on each head.

This is a pic I snapped at the 2008 Cincinnati Dealer Expo.  I assume that it is a replica of Petrali's mount since the original color is normally portrayed as blue.

 

 

Less common are Panheads modified in like manor.  Though it seems that George Smith modified a number of them in days long gone by.

This is a shot of the George Smith dual carb Pan heads on a bike Teach did a few years back.

Though modifying the heads for dual carbs was a popular performance trick of the day, it was not the only way to achieve dual carbs.  One of the alternatives that I suspect to be a very early example of a dual carb manifold is shown below.  It was a mass produced aluminum casting made to fit the early "small port" Knuckle heads and featured a 3-bolt Linkert pattern.  IMHO it suffers from a couple of design flaws, but certainly was nicely done and the (unknown to me) originator was on the right track.

 

Vintage dual carb manifold for early, small port Knuckle heads.

 

 

My critique of this manifold stems from a couple points.  One is that the aluminum material of the manifold would be unlikely to hold up well to the sealing system of the day, that is "plumber nuts" with brass seals.  If you have perused a well worn OEM steel manifold, you have no doubt noticed how badly deformed the surface that the brass seal rides on becomes from use.  I suspect the relatively softer cast aluminum would fare much worse. 

 

My other issue has to do with airflow (not surprising since I deal with airflow for a good bit of my livelihood).  The mere fact that this manifold is an aluminum casting limits the airflow in this particular case.  Since the "spigots" of the manifold need to be the same O.D. as an OEM steel manifold in order for the  plumber nuts and seals to fit, it follows that the I.D. must be smaller that stock in order to provide some strength.  My educated guess is that an aluminum manifold nipple that shares the steel manifold's .075" wall thickness would probably not hold up to even the first tightening of the plumber nut.  On a similar note, and of even more concern on this particular manifold, is the runner diameter.  Ideally, the cross sectional area of the manifold runners would remain constant.  In the case of this particular manifold, the entrance of the port is nearly round at the carb mounting surface, as it is at the exit where it feeds into the head.  In between these two points, the runner takes an elliptical shape, maintaining a constant height, but  suffering from a severe narrowing in width.  The third picture above shows this, and yes...  it is as bad as it looks. The major reason for this was to keep individual runners, in other words keeping each cylinder's intake tract isolated from the other.  More on this later.

 

Now, given the tortuous path that a Knuckle or Pan present to the air/fuel mixture as it finds its way from the carburetor to the combustion chamber, it would not surprise me to learn that this manifold provided a performance increase despite it's inherent flaws.  Recently it appears that someone has undertaken the task of reproducing these vintage performance manifolds, since I often see un-finished versions for sale on eBay.  One notable change on these new ones is a flange the shape of the 4 bolt Linkert.  Few details are given and one cannot tell from the pictures whether any other improvements have been made.  If anyone has real world experience with this style manifold, either the original or new, I would be happy to hear about it.

Another early example of the quest for dual carbs is the "Seeley" manifold, something which I have written about previously here.

 The Seeley manifold was basically a re-imagining of the stock Linkert manifold, putting one carb on each side of the engine.  An obvious minor drawback to this type of dual carb manifold is leg clearance on the spark plug side, particularly if the relatively "long" Linkert is used.  In fairness though, the customary modification to Knuck heads for dual carbs suffers from the same issue.  As a side note, I am certainly no historian of the Seeley manifolds, but I have seen pictures of an aluminum version very similar to the steel one pictured above.  And that brings up a question; was the steel one an early prototype or was it possibly a home made copy?  The aluminum version is stamped with the Seeley name, the steel is seemingly unmarked.  One thing I would note is that the aluminum version suffers from the same drawback as the dual 3 bolt manifold mentioned earlier with its thicker walls on the spigots.

Over the years there have surely been many variations on these three basic dual carb designs, and probably a few that I missed as well.  The fact is, I have built a number of variations myself.  During the 1980's and '90s I modified a number of Knuckle heads for dual carbs as part of my quest for drag strip performance.   The fact that even then I was three generations removed from the latest technology in Harley head design would seem to reveal something about "where my head was at" (as they used to say). I wanted performance, but was not willing to entirely give up the "cool factor" (as they also used to say) to get it.  I would venture a guess that the same could be said for many today who  are drawn to the concept of dual carbs on vintage motors.

Of course motivations will vary from one man to the next, and where you fall in the spectrum of desire for "the look" verses "performance" will have a lot to do with how you approach a dual carb conversion.  If the look is more important to you than the performance, then any of the designs listed above will fit the bill nicely and I say go for it.  If, on the other hand, you lean more toward the performance end, then there are a few other things to consider.  One of the biggest of these is air flow.  Face it, even in the vintage world, today's motors tend to be larger than yesterday's, and a larger motor wants more air.  In the early days an 80" Knuckle was a big motor, and possibly the most common size used for drag racing.  In the years since, 84 and 86 inch engines have become common for street builds.  What I am suggesting is that what may have been a performance upgrade on a 61 inch motor could very well be a restriction on a 96 incher.

Airflow and how it relates to these vintage style dual carb modifications will be the subject of a soon (hopefully) upcoming post.