ITB size for small port SC engine

[phelix]

I'm comtemplating an ITB conversion on a small port SC engine. Common lore seems to suggest a 40mm throttle body for a 500cc cylinder that will likely produce no more than 40 hp. One vendor is recommending a 45mm throttle body (perhaps that's all they have?) and says that because their manifold tapers to match the inlet port it'll be fine. Is this the case?

[Dpmulvan]

I’m curious as well, I’ve seen many builds using both 40 and 45’s

[Dpmulvan]

Guess it depends on a few factors, might help to have a flow bench.

[phelix]

I seem to recall the stock small port head flows 225 cfm.

[al lkosmal]

If the 3.0, small port is relatively stock...I typically install 40mm ITBs with 34mm base manifolds. This is also a tapered design....but the throttle body is 40mm and works well with the small port motors

I would expect that the 45s will work, but not ideal and likely you'll have issues with tuning the "tip-in", due to the larger size of the throttle.......and you'll sacrifice low end performance.

[phelix]

Thanks Al. The engine has uprated cams and pistons but unfortunately I have no info on specifics.

[al lkosmal]

Still, for a mildly hot-rodded street 3.0...I'd go with the 40s.....

regards,
al

[Raceboy]

40's would be maximum to use, otherwise you would lose quite a bit of mid-range torque. We have tested many variations and tens of hours on the dyno, it is somewhat possible to compensate for too big bore throttles with intake tract length (measured from the end of velocity stacks) but there is limit on 911 unless going with whale tale spoiler.