The V8 Owners Forum -

hello all,

Anyone know what the height from the top of the trumpets to the inside roof of the plenum should be on the efi engine...

i am running 45mm trumpets and modified base, but a standard plenum....

But the new trumpets are really low, and now the distance to the roof is alot larger than it was.

Just wondering if i should be going for a carbon plenum to match with my trumpet base.

Or what problems the low trumpets will cause to the airflow/fuelling inside the plenum.

any thoughts welcome.

cheers

scott

These are basically thoughts, based on a little knowladge but I think you are into very specialist teritory!
The distance from the inside of roof to top to the end of each trumpet will effect the pulse tuning of the inlet tract and bring it close enough and it will resonate at 1.5 X the frequency in one mode when compared to moving it far enough away. I suspect it is placed at the moment so that at low engine speeds it resonates in the high frequency mode and at higher revs it resonates in the other mode giving you pulse tuning in two parts of the rev range. This is more intuitive belief and from reading some articles on how other EFI manifolds operate.
As far as flow is concerned, bringing the top close enough for this higher frequency mode to opperate you reduce the flow at the high rev (high demand) range (I am much more certain of this).
The up-shot of this Is if you want to favour low and mid range power and have a nice flat torque curve you should lower the roof of the plenum so it is the same distance relative to the new length of the inlet tract as in the original set up. If you want top end power and are not so worried about the bottom end then mover the roof away from the ends of the trumpets, as far away as the underside of the engine cover will let you.
Best regards
Mike