Carb testing & flowbenchin'

[98-1074649673]

Anyone out there testing carbs on their flowbench? Not just CFM's I'm looking to talk about the relationship of fuel signals in the different circuits ie idle, transition and main. I can not find alot of info on the net anywhere where it talks about the relationship of numbers and such?

I've played with my carbs on the flowbench and see changes in numbers but am not really sure what it represents. Anyone care to take on a discussion on this subject? Brainstorm, put out your theories be it right or wrong, spark a thought process..... I know everyone is now using fuel injection and carbs are "old technology" but we still use them on our motors.

I did find some info not to long ago that made me think, it talked about testing carbs at lower flow depressions ie 8-10" since that is what an engine is doing at wide open throttle. This would give u a better idea of what the carb is seeing and how it needs to work in that environment. Probably would be a good idea to test a running engine at various rpm's to see what vaccum signals are present in the intake tract. This will be one more gage to install on my dyno project.

So lets hear from some of you who are doing actual testing . . . lets get this forum rolling!!

[84-1074663779]

Not so sure that steady state flow is really relevant. As air pulses and flows in both directions through the carb, the venturi signal can pull more fuel than it would if the same net airflow was steady state.

I really think carb tuning needs to be done on a real engine, with the cams and manifolds you intend to use. There are also nasties like exhaust reversion that can upset things quite a fair bit as well.

A dyno and an air fuel ratio meter might be the best way to go about it initially, but even then on road behaviour might not be absolutely perfect. It never is.

Ah, it all brings back memories..................

[98-1074649673]

ok let me clarify some more of my question then, what you would be or are looking at on the flowbench is how throttle plate dia venturi dia and lengths along with anything else u can think of effects airflow quality and quantity, Not the actual setup of the carb for on a working engine ie tweaking of jets and such, this would be done on the dyno. Now compare any changes u made on the flowbench to what now happens on the dyno so u are looking at the whole package.

I think of testing on the flowbench as a "slice in time" (but of longer duration for flowbench study) of what happens when reversion and pulses are not a concern (I do know they effect things alot). U are seeing that moment of time when these things are not happening and working from there . . . does that make sense?

[84-1074663779]

I agree that mixture quality is just as important as mixture quantity, and this really comes down to combustion. Things like swirl, tumble, compression ratio, chamber shape, and squish, come into it as well.

A real running engine and suitable instrumentation is the only way that I can see of trying to figure what is actually happening in there.

[DaveMcLain]

My carburetor guy (Bobby Oliver at Competition Carubretion) uses his flow bench for at least one aspect of a carburetor build. He uses it for booster postitioning. He says it's always amusing to look at carburetors and carburetor building tools where the boosters are lined up perfectly or they are selling a fixture to line them up perfectly. That's great and all, but what if that's not where the booster really wants to be in the venturi(most probably) due to casting variations which slightly alter the shape of the venturi entrance on every carburetor.

What he does is install the boosters he's going to use and then he bends them around into the position that flows the most air for a given amount of signal produced by the booster. When he finds that sweet spot he then leaves it there, he puts some epoxy at the base of the booster to make sure it does not move or come loose.

A flow bench can also be used to get your jetting in the ballpark. It will be close enough to get you going if you have no idea at all what a particular carburetor needs.

[Jesse Lackman]

You can also direct wet flow to hot cylinders by moving the boosters. I've done this during dyno testing and verified it by watching the egts before and after.

[DaveMcLain]

Most of the time they cut a notch in the bottom of the booster to change the mixture distribution or in some factory applications they have a special booster with a little lump sticking out to one side or the other. I can see where tweaking the booster might be made to help some on a bad intake or with a restrictor plate....