Throttle Body Sizing

[joshkemp]

I'm working on fuel injection on 3.5 hp briggs and stratton engine for an SAE competition. I matched the diameter of the throttle plate to the diameter to the throttle plate diameter of the carb (0.75 in=19mm). It idles very high, even after fixng intial vacuum leaks. After looking over the website here, it seems that I have made the passage too big, that I should match the venturi diamter or use the equation given which gives a diameter of 8 mm.

Would a restricter plate accomplish the same thing as a smaller bore? I plan on casting a new throttle body to address these issues, but does a 8mm bore seem resonable? It says you should adjust that value if you are running 1 throttle body per cylinder, how much larger than 8mm should it be?

[Big Creek]

Generally throttle plate size should not effect idle but throttle plate fit to the bore will. You are probably "leaking" too much air around the plate.

Throttle size on a single cylinder is much larger than what is needed for multible cyl on a plenum. Most of the high performance crowd seem to go for ITB"s a little larger than the area of the intake valve(s). I would think a litlle smaller would suffice for your application. I am sure others with more experience than will chime in here.

Todd

[joshkemp]

I just cut the briggs carb in half to get access to the venturi and its bore is 0.5 inches. I think I will just match this size for now and can always enlarge it if necessary.

The clearance I have between my throttle plate and the wall is already smaller than it was for the briggs throttle plate. With a 0.5 in bore there should be less area for leakage

[PSIG]

The size of a throttle is primarily based on user-friendly controllability. If you don't need smooth linear throttling then a large TB with minimal restriction will give the best efficiency and engine VE when coupled with a properly tuned intake manifold. Else, a balance between smooth throttling and minimal WOT restriction is the target.

The venturi in a carburetor is a necessary evil to provide the vacuum to draw fuel into the throttle bore. This restriction is not necessary and it is common practice to remove the venturi section or machine it out of EFI conversions, making the larger throttle plate the primary restriction in the system.

Hope that helps,
David

[the bubbler]

Hello,

I'm injecting a small single too, although bigger than your engine- 300cc 13.5hp.

I managed to snag some parts via ebay, off of a Peugeot Jetforce 125cc.

That bike uses a small 26mm TB, with a built in TPS. This is manufactured by Bing.

Bing also produce a smaller TB, which will probably suit you down to the ground-

http://www.bing.de/shared/images/datenb ... ype71e.pdf

Something else worth a look may be throttle bodies off of a Aprilia SR50. These are 50cc efi two stroke.

Nice little fuel pumps on these too.

Andy