do they make a difference to performance? they appear to be 3 different lengths

6"

3.4" (which i have now)

and 1.15"

i'm tempted to get the 1.15" as that would leave me with plenty of space between ram pipe and chassis compared to the .5mm i've got at the moment!

just interested to know if it makes any difference other than room. also would i get sufficient induction having the carbs enclosed under the bonnet?

Theoretically the shorter the induction the further up the rev range the power would be , but I dont believe there is a massive difference to how it feels in practise , my bike carbs are on quite short stubs , but the engine still pulls nicely from low down.

Also if you mean that you have something very near your open bell mouths , that would not be good, i believe something like half the diameter would be the smallest gap you should have.

The length of the tract is overall from valve to end of bell mouth so you can add length on the outside of the carb.

[Edited on 24/3/06 by Surrey Dave]

There is an ideal length, but it varies from engine to engine. Just aim for a nice straight shot into the head and you've won half the battle. Then allow plenty of spave for the filter and you'll be fine. As said above it should make a difference, but you probably won't notice it on the road.

Makes a huge difference, but also depends on exhaust lengths and inlet/exhaust volumes.


I'll write more tomorrow, need sleep

Yes , if we are talking high revving racing engines on hot cams, it would make a lot of difference, but with low revving car engines on soft camshafts , the effect is probably masked.

When I used to tune high revving 2 strokes , it seemed more detrimental to go long rather than short with the inlet length.

yep, agree on the cam characteristics effect, but then there is still a considerable effect even on cams with low overlap.

Peak power will be broadly similar, so a typical "peak figure" from a rolling road or dyno will not show any great change, maybe a slight reduction in power with long inlets but not much. What will change is the power/torque curve, and this effect can be very significant, even with mild cams.

By way of an example, a mildly tuned, with mild cams, Vauxhall XE 16v engine with peak power of around 220hp @7200 rpm (as a typical sort of figure) can have the power at 5000rpm varied by 10-12hp by adjustment of inlet tract lengths.

At 7200 rpm this would maybe only yield a 1-2hp change.

PS on any "road" car engine spinning at "road" revs then the best inlet length is normally the longest that you can fit, on a 7 nearly always entailing a hole in the side of the bonnet with IR systems:



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XE throttle bodies