IRONMAN

Has anyone done any flow tests of holley 800 or others with the flame arrestor on? How many CFM do you loose with the flame arrestor installed. My guess is about 40CFM. Should we be sizing carbs to compensate for this loss? I am not sure if I need more CFM for a 510 BBC at 5600 - 5800? Thanks, The Ironman.

excaleagle42

a few things to take into cosideration are : atmospheric pressure is 14.7 lbs. at sea level, the higher you go the less it is, the amount of vacuum your engine makes, (more radical cam = less vacuum) the hotter the air = less density, what the intake runner size is, oval port or rectangular for air flow velocity. and engine efficiency, these are a few things off the top of my head that would change the differential pressure across an aircleaner, wheather you have the best or the most restrictive.
i melted a 454 by going to small on carb size. but if you really want to scientifically calculate carb size, remember this, "bigger is better but less is more"

RumRunner

excaleagle42

You make some very good points, but you contradicted yourself. Since carburetors work based on air velocity, or pressure differential putting a carburetor on your 454 that is too small would effectively run rich providing that it was working properly. Same amount of air as with a larger carburetor (since an engine is just an air pump) but higher velocity = richer not lean. You may have had a problem with the carburetor itself or your fuel delivery system.

IRONMAN

I guess what the real question is is how do carb manufacturers test the carbs for flow rating and how does the flame arrestor effect the actual rating in a real world application.

excaleagle42

i don't follow you on where i contradicted myself. any way, if what you said is true, that smaller is richer because velocity increases, we can all run 400cfm two barrels and take off those big old double pumpers to get more fuel flow. what happened in my case was, the carb was simply jetted to lean, i'm sure you know the leaner, the more power but if it's too lean it's too hot and thats why i blew molten piston chunks out my exhaust until i backed off the throttle. i still made it home though. the higher the velocity, the more fuel you get until your engine overcomes the carb capacity.

excaleagle42

carb flow rating is calculated by 1.5 inches of mercury pressure drop through it. since an engine cfm rating requirement is calculated using horsepower, volumetric efficiency and max rpm's, it won't match up with a carb exactly because at wide open throttle there is more than 1.5 inches of mercury in your manifold. i hope this helps clear things up. if not, feel free to e-mail me directly. excaleagle42

IRONMAN

Thanks, I may just try a few larger carbs and different arrestor combos to see what is best.

RumRunner

Ironman
Run a vacuum gauge on your engine if you're running more than 1.5"hg at W.O.T. you may have a restriction, next try it without your flame arrestor. If the flame arrestor is restricting your flow what ever your vacuum is at W.O.T. without it should be much lower (at least a .25" hg)
Doug

mcollinstn

higher velocity simply gives better fuel atomization. Jetting is determined per application. Any modification to an engine that reduces pumping losses and makes air flow more easily through it will require a size-up in the jetting. If you have an intake-side restriction, then huge exhausts will not reduce pumping losses and will not require jetting changes. A motor with an exhaust-side restriction that is fitted with huge headers will net a significant reduction in pumping losses and will require a jet change. Same for vice versa on intake side.

Take a small carb that is jetted correctly on a small motor and bolt it unchanged on a big honker of a motor and you will usually be lean (but not always). No real rule of thumb here, except to always listen for a lean condition and do a plug reading whenever you make a change.

RumRunner

All things being equal change carburetor size on the same engine the smaller carburetor will flow more fuel through the same size jets