Sunday, September 21, 2014

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.


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