The V8 Owners Forum -

I've noticed there are a few companies selling EFI manifolds bored to 45mm and ported, just wondering if there are any gains to be had from these over a standard 38mm manifold that's had a touch of porting at the head end.

My spec is 3.9, 10:1 CR, crower 50233, stg3 heads, 71mm plenum - around about the 265-270 bhp mark at just over 6000 revs.

To my mind 45mm runners seem massive, and perhaps too big, when the inlet valves are only 41mm and the valve throats are 36.5mm ish.

Cheers
B

Hi
there is no gain on a road engine with factory head castings (even if ported to kingdom come) with using 45mm inlet trumpets, unless of course it is on TVR they are when they are worth at least 65bhp
you have answered the question yourself, 41mm inlet valves means the hole in the middle of the seat inserts will be no more than 37mm less the area of the valve stem. . . . there is a reason Rover made the trumpets 38mm hole down the middle.
best regards
Mike

My ex 4.6Efi made 280hp with stock 65mm plenum and 38mm trumpets.. Head has got 41.5mm inlet valve and 35.5mm throat.. Also it will run STD 3.9 ecu chip. Then i replace bigger trumpets, 45mm and it will run very lean, so have to fit programmable ecu..

In my mind you will need slightly bigger trumpets to make venturi effect.
And if i was you, i will also add more compression..