Race Engine Forum UK

What effect would we get if we fitted a larger throttle body when the flow bench is telling us the original throttle body is sufficient ?

Is there a rule of thumb by an ammount the body should out flow the head ?

Is it one throttle body per head Chris, one feeding both heads?

GC

GC it is one per cylinder.

OK, Chris,

all you need is same flow as head - more is no advantage at all, but not overly detrimental (within limits). I would be concerned about velocity thru the throttle plate if it was much more than 10% higher.

GC

BTW - going back to an earlier post by you about disparity in flow from one Ducati barrel to another, I did some follow-up tests lately using a Vauxhall XE 16v head to look closer at this, just BPF tests mind you. The upshot of it is that if a part of the splitter vane is biased slightly off centre ie: away from a barrel, that barrel captures more air than other one and the other barrel flows commensurately less. I always figured that might be the case but I was surprised how sensitive it can be.

This may sound a stupid question, but how much does the spindle interfere with the flow on Throttle Bodies compared to spindless Throttle Bodies??

Andy.

The 998 heads even when big valved and at 14mm lift still flow some 20cfm less than the 54mm throttle body does individually. There is a tuner out there offering 58mm throttle body conversions for this engine saying that it makes better power from 8000rpm onwards.

If what were are all assuming is correct there is no reason at all why it should make more power with the 58mm bodys as the 54mm all ready out flow the head right ?

Well, all my years of exp have taught me to be circumspect about power claims.

I won't give them any credence unless I know/trust the person and his methods, or see well-constituted dyno curves. And you should not either. In this case I would want to see overlaid and back-to-back dyno results (from the same engine).

With that in mind I can't think of a reason why, at present, that bigger throttle would make any difference.

GC

I would be more worried about drivability when going to big TBs, especially on a bike driven on track where I suspect crisp throttle response is very important out of the corners.