Carburetor Question

[W2racing]

One of the series we run - SLMR - unites different cars/engine packages together to form one class of racing. This is done with different car weights and different restrictor plate sizes based on engine size and type. We run a Wissota spec engine package. My question is this. If you have to run a restrictor plate under your carb, does it make any difference what size carb you use? We normally run a 750 Holley carb on gas. Has there been any dyno or track testing done to suggest you would have better track performance with a bigger carb (850) or a smaller carb (650) ? Or does it make no difference because of the restrictor plate? Any and all opinions appreciated.

[Egoracing]

A carb with a smaller bore would keep the amount of air hitting the plate to a minimum which could create a better flow. Think of a 2 inch stream of water hitting a 1 inch hole, a bunch is bouncing back which in a 3 inch pipe would fill the area with water flowing back away from the hole. If you had a 1 1/2 inch stream the amount of water bouncing away and coming back would be reduced.
I have never tested this but in theory it should allow it to flow better with less reverbiration back into the venturi.

[LM14]

We were stuck in this situation when IMCA went from open motors to spec heads. We had to run a restrictor plate with our combination because we didn't have spec heads.

We found a carb that the venturi size closley matched the restrictor hole worked best. A big carb puddled fuel and wasn't as sharp or consistant on the track. We tried our really good 390 carb and it actually worked better because the bores matched well.

The carb spacer on top of the restrictor plate can make a huge difference. If they spec a straight bore spacer like IMCA did, the little carb worked better. When we put a tapered spacer on that matched the carb on top and the restrictor on the bottom it let us run a bigger carb successfully.

Hope it helps, still have my nice 390 if you are interested ;)

SPark