Racing Fuel Systems

I have an old Holley 4778 700cfm DP. Would there be any flow gain if I went to down leg boosters over the original straight leg boosters. I ask this question because it seems to that the hood across the minor dia . of the booster is reducing airflow slightly. I have tried to use this carb to slow my car down to run a slower index class and it runs .08 off the index. If I could pick up that .08 by changing boosters I'd try it. Dan

You would want a .450 step downleg that BLP manufactures. It will flow the most air. For a 700 probably a .145 passage thru on gas.

.450 is the hole thru the center. A stock booster is around .380 and has no step, you can get .435 and .450 with steps cut underneath. The steps help with signal and atomization.

My understanding is that there is little to no flow difference [ ie, air flow past the booster ] with either type, but the DL booster is more sensitive & flows fuel at a lower pressure.

There is a gain thru the booster, and a little outside as the booster is machined on the outside as well. I did a 650 body with .450 DL's for someone in Australia, they picked up 15HP on the dyno.

Thank you for your input JMark .

I can tell you I had a Holley 4777 650 double pumper carburetor that I took out the straight leg boosters and installed down leg stepped boosters and it made quite a difference in throttle response and a stronger signal when hitting a quick half to wide open throttle and was a lot crisper and smooth when going from the transition circuit the mains flowing through the boosters. That was my experience doing that. I used a .140 sized inside dimension of the booster tube going through it.