I've been reading a bit on the DIY EFI stuff recently. Not really in a position to do it but one thing stuck in my mind. If it is possible to produce 200 ish BHP from 4 x 34mm throttle bodies (i.e. 500/600cc motorbike), with much better throttle response than larger ones. Then why do people bother fitting 45mm and larger ones ? Just seems daft.

If you can point me in the direction of where to purchase a 600cc motor bike with 200bhp i would be very happy


2 15mm inlet valves can only flow so much air most cars only have one throttle body so you may have 8 15mm valves to feed or perhaps i dont know what im on about ill wait for a more technical answer

ah, I didn't mean a 500/600cc bike could produce 200bhp, but that it has been implied on some of the forums that 4 x 34mm throttle bodies often fitted to these sized bikes could flow enough air to produce 200bhp. Maybe I'm wrong but if dunnell can recommend that Weber 40's with 34mm chokes produce the best power and torque on a 2.0L zetec and 34mm throttle bodies are likely to flow more air than these then. It follows that with the right injectors small throttle bodies should be able to give high power outputs ?

just a thought ?

dosnt the venturi size also tune where the power is produced a bit like a large or small turbo

With carbs more air velocity is required to improve mixing, this is less vital with injection where the injector spray produces much better mixing.
34 mm venturis in DCOE webers start to run out of steam circa 130 bhp on 4 pot.

I'm not at all worried about running out of airflow with my 600cc GSXr TBs. I can see the install advantage of 750s though, the bigger diameter may be closer to the size of the port area, meaning less transition in the inlet manifold, but for airflow, I'm right with you.

More likely you would be wrapped round a tree somewhere.
Bigger venturi move the airspeed further up the rev range giving top end power instead of throttle response at lower speeds as the air speed will increase quicker through a smaller venturi but be limited by the size at higher revs, that's my take anyway. Fuel atomisation will be better with the faster moving air through carbs but as said injectors will provide better atomisation and more precise fuelling throughout the rev range.

I fitted 35mm bodys to my car and as the inlet tracts are only 32mm could see no advantage in going bigger.Oversize throttle bodys will cause the throttle response to be non linear, as they may be capable of actually suppling to much air at small openings so act more as a switch than a progressive throttle.
Paul.

All depends on lots of things!

One of my Vauxhall XE engines runs 48mm throttle bodies, and on the dolling road these consistently show a higher output than 45mm bodies, throughout the rev range.

What they do have though is fluffy throttle response at low revs, clears up over 3000 though.

my 7 will be running 45mm jenvey sf bodies.

If you look at the people running much smaller bodies on "200hp" engines, most of these have not had a back to back comparison done with larger ones, you don't miss what you don't know you don't have!!!

According to the Megasquirt site 4X34mm throttle bodies will give up to 266bhp http://www.megasquirt.info/v22manual/minj.htm#throttle
scroll to near bottom of page to find calculator.