Power potential is limited by air flow.  It’s easy to get more fuel into the engine, but a lot more challenging to get more air in.  On a naturally aspirated engine, there is only about 14.7 psi available to force air through the intake tract and into the cylinder.

The air must work its way through the air-box and filter assembly, the carburetor, the intake port and the intake valves.  Each one of these components takes its toll on the flow.  I wanted to test the entire intake tract using three different carburetors to get a feel for the power potential of each carburetor.  The contenders are the stock Mikuni constant velocity carburetor, a 40mm Kehin constant velocity carburetor (ala Big Twin Harley), and an S&S Super E carburetor (same pedigree).

The Mikuni is billed as a 40mm carb, but if you examine the carb closely you will note that the bore in way of the throttle plate is 40mm, but the venturi is actually oval (40mm x 27.4mm).  That works out to a cross-sectional area of 984.6mm squared, which is only equivalent to the cross-sectional area of a 35.4mm circle.  So, the stock carburetor is really a 35mm carb, not a 40mm.  It would be nice if one of you guys/gals could double check my calculation and confirm its accuracy.  Is a 40mm x 27.4mm oval cross section equivalent to a 35.4mm circular cross section?

In the interest of full disclosure:  I intend to install the S&S on my bike.  First, I like the idea of having a bike that’s different, and it will be cool to have the only Savage with a big S&S mixer.  Second, it flows well.  Third, it was never meant to be installed on a 40 cubic inch single so it will be fun and challenging to get it to work correctly.  I will do my best to remain impartial on this comparison.

I did tests on the three carbs with no airbox attached, and also tested them with an airbox configured for maximum flow.  Let’s face it, not much point in installing a high-performance carb and then smothering it with a low-performance airbox.  The high-performance airbox configuration was the same as that outlined on my previous airbox flow tests (K&N filter with the airbox plastic cover removed, decorative tin in place).   So, a maximum flow run looked like this.

As I mentioned in my earlier post, there is a formula floating around that predicts horsepower based on intake flow measured at 10” H20, so I tested each configuration at 10” (actual, no conversion).  I wanted to try and determine if that formula is anywhere close to accurate.  The formula is: .43 (CFM)=HP.  The CFM value is measured @ 10” H2O.  I have now found this formula in two high performance publications and “Fast650” has also come up with the same formula.  The formula assumes that all other aspects of the engine are optimized.  The compression ratio, exhaust system, quench, ignition timing and advance curve, cam lift and timing, combustion chamber geometry, etc.  The formula must also assume fuel octane rating is appropriate for the BMEP, so 100+ octane gasoline would become part of the mix.  If any of you are familiar with this formula, it would be great if you could share your experience regarding its accuracy.

If we look at the performance potential using the formula, the stock carburetor on a stock cylinder head and modified airbox has the potential to support about 49 HP.  That sounds optimistic.  I can’t accept that number.  Who knows, maybe with 14:1 compression, .035” quench, dual plugs, optimized ignition timing, a finely tuned straight pipe, an aggressive cam and VP C16 fuel (or maybe some of their oxygenated fuel), 49 ponies off the stock head and carb are achievable.  Seems unlikely to me.  What do you folks think?
 
The formula predicts that the S&S carb has the potential to support about 58 HP on an engine with a high-flow head.  If you consider that the engine in question would be built to the hilt, I guess it’s feasible, but still optimistic.  There are a lot of high-power single-cylinder machines on the market these days.  Many approach 70 HP, but they have huge valves and sophisticated engine management systems.  Our cylinder heads have valves that are appropriate for a 400cc engine, not a 650cc, and our engine management system consists of a fixed-timing ignition system (with no rev limiter) and a carburetor design circa the 1940s.

So, what’s the bottom line?  The S&S flows better than the Mikuni or the Kehin.  Whether or not it will make more horsepower remains to be seen.  It will be difficult to set up and get tuned correctly.  The manifold was a bit hard to fabricate, but it’s done and fits perfectly.  It’s clearly not as sophisticated as the metric mixer, but hope rings eternal.

The Kehin is more sophisticated than the S&S, and it fits nicely (except the throttle cables are on the right side of the carb).  Some preliminary investigating into parts for a Kehin 40mm used on the KX650 and KLR650 shows that the emulsion tube is different, and the main jet is way smaller and uses a different thread series.  It also uses a different needle jet and jet needle.  The emulsion tube and needle jet for the Kawasaki are expensive, so in addition to acquiring a Harley Kehin, I’m willing to bet one would have to purchase the emulsion tube, needle jet, jet needle, etc. for a Kawasaki (or similar Kehin) to get that carb to work on a 40-inch single.  Do any of you have anything to share about the 40mm Kehin CV carb used on Harley Sportsters and Big Twins?  Has anyone out there mounted one of these carbs on an S40, and if so, how did it work?

I would really like to test a Mikuni 36 & 38mm VM, and a 40mm Mikuni HSR, and any other carb that has potential for installation on the LS650.  If any of you have an old jalopy carb you would be willing to loan for testing, I will be more than happy to suck its guts out.  I’m sure the info would be beneficial to all the forum members.  Drop me a PM if you are interested.

Flow testing and modification on the HammerHead are going well.  I have been able to achieve very good improvements without increasing intake port volume.  “Fast650” has been instrumental in that progress and has done a great job of mentoring and keeping me on track (and of course he donated the HammerHead).  He’s my sounding board.  I’m finished with the intake and am preparing to move to the exhaust (an obvious challenge).  Now that I can suck a full 15” on both ports, things should move along better.  Once I am finished with the cylinder head flow tests and mods, I will share all the data with you along with a detailed map of the modified ports (intake currently about 195 CFM @ 15” H2O).

Looks like you having way too much fun.  Kudos for the work involved on that.

I just wanted to throw in that my brother had the S&S carb on his bike for about 2 years and finally gave up on it.  His motor was hopped up a bit with a cam and what not, I dont think he put the effort into making it work that you are, but after some amount of time he just didnt want to deal with the S&S.  It was a back firing and gas dumping machine.

In addition that he could sell the carb for a lot of money and just use a Keihin that ran great and didnt need constant attention.

I think your chances of success appear to be much better.

DragBikeMike wrote on 03/17/19 at 15:11:23:


I think the bore to stroke ratio will be the main factor that limits power. More RPM=more power, and that long stroke places the limit at around 7000 RPM for acceptable engine life. You can spin it faster but you can start measuring time between top end rebuilds with a stopwatch then. Rings don't last long at those speeds, the heat anneals them and the ring seal goes away in no time. For the Savage, it will take boost or nitro to make the really big numbers unless you destroke it and increase the bore to keep the displacement similar.