Testing And Pressure Theories

//Testing And Pressure Theories

Testing And Pressure Theories

Street car racing is a serious game. Lots of money is spent in pursuit of what has amounted to incredible almost unbelievable performances by muffled street tire equipped cars. I remember back in the very early 80’s when Bob D’Andrea had a camaro that would run 9’s with a big block. Now his little mustang is in the 8’s. Times they do change and you have to change with them.

Awhile back there was a theory going around about reducing the size of carburetors in a nitrous application. The idea behind that was to force the engine to ingest the nitrous mix rather than the carburetor mix. The discussed theory was that this would offer us ultimately more pounds of oxygen in the cycle.

One thing I have learned and have constantly proven is that engine horsepower shows up on the racetrack every time.

Every time you build the carbs to accelerate the engine the vehicle travels faster. The concept behind the theory is good. When I heard this being discussed, I had already been down that road. Two years ago we tested this theory on two Pro-Mod cars and qualified 2 and 3rd, two very similar cars with the same power plants and tune-ups. One with small carbs, the other with large carbs. Both cars ended up having the same plug color, with the same fuel requirements. However, the larger carbs scored more MPH.

Lets look further. When smaller carbs are introduced the theory possibly failed because one thing emerged immediately. Pumping restriction. There is alot of air that that thing needs at high rpms. Nozzles attempting to supply that air displayed in this test showed that they couldn’t afford to supply it at the exchange of the low pressure spots created by the smaller venturis. Reducing the carb size (reduction in cross section) decreases cylinder fill and any chance at positive pressure spots in the intake created by the sealed hood scoop (similar to how a main jet works…smaller the jet the less fuel…smaller the venturi the less pressure in the manifold). This increase in pumping restriction created by the smaller venturis at the finish-line shows you a decrease in top end charge and the MPH has dropped off on all of our tests.

I do see some validity to the theory and believe me we have concepts working on it. But not in the direction of smaller carbs.

Yes smaller carbs can run faster. You can put too big a carb on something.

But in relating to the theory, at this time the best thing to do would be to have the carburetors go the opposite way. Larger carburetors at the finish-line would be the key. Meanwhile adding as much high percentage oxygen at the same time. The larger carbs would keep you from losing out on the positive pressure created by the scoop and decrease the losses created by errant unfilled pulses in the intake. The larger venturis allowing the lower pressure areas to migrate faster to the open air vent (venturi area). Now we’re back to our “decrease in pumping restriction” theory that always seems to win “on the track”.

When you decrease area/restriction to get rid of one venue, you have to similarly increase area or reduce restriction with something else. But why give up any potential by creating a restriction if the engine shows it will ingest from a closer location anyway. In our test that is what it did. Whereby giving up the theory of increasing restrictions at the venturi to force it to ingest from a more localized spot disproved the concept. Now that we are led to believe it won’t work, it makes sense why it didn’t. But anytime your cutting edge or R and D’ing, you have to try it.

I have carbs that make less power on the dyno than other carbs and yet they are faster on the track. Go figure.

Anyway, if that theory proved true an increase in nozzle size would’ve been required in the Pro-Mod car with the smaller carbs. But that didn’t happen. I have done tests on the dyno that showed support for the theory, but when the subject got to the track, the larger carb won.

I’ve heard of trying to prove this theory by closing throttle blades down as the run progressed and sometimes a slight increase in performance was exhibited. But what that may have shown was an alteration in fuel curve or fuel distribution caused by the repositioning the throttle blades. This would alter the A/F ratios and cyl to cyl distribution that may be more to the engines liking. Confusing? No its just fuel and air and getting it there.

In a nitrous Pro-Mod engine, the fuel that makes the 1300 hp and the other 900 hp worth of fuel has to work in harmony (including the extra fuel required to keep it cool). Simply restricting the engine to force the plant to ingest the nitrous mixture might provoke thought but in the real world .. its the path of least resistance that wins. The engine will take whatever it can ingest. If you can get it to take in the more volatile nitrous mixture without burning it up then that’s the key.

But there is one thing that is missing as of today’s date. At this point in time the problem really lies in the high combustion temperatures that gasoline creates when mixed with high oxygen content in a confined area. The real key to burning more nitrous is the ability to get rid of heat or create less of a peak temperature reading. Pipes and pistons will only take so much heat for so long and then they melt. You’re creating a blast furnace in there. Hence the use of larger carburetors and a fuel with more latent heat absorption would be the key to more power and speed in a nitrous application at this time.

Thanks for reading. See you at the track!

By |2002-01-26T00:55:04+00:00January 26th, 2002|Uncategorized|Comments Off on Testing And Pressure Theories