Thread: 3.9 V8i intake manifold

Originally Posted by cockie55

Hey Big Jon did you ever get around to putting this spacer on top of your manifold??

I haven't been home yet! I am three weeks into a four week work stint.
I was in the NT, now I am in WA.

Just to add my two bob's worth. Do any of you remember the big block Chev's used in the McLaren Canam sports-racing cars? They used unequal length ram tubes to smooth out the peaks and valleys in the torque curve of a ram tuned induction system. From memory they used two each of four different lengths. I also remember they used Crower fuel injection with big monster throttle butterflies of almost 3" diameter. They were supposed to have a near smooth torque curve from 3500 rpm up to 7500-8000.

Originally Posted by BigJon

I have read that the length of the intake runners in the intake manifold has a bearing on where in the rev range torque is made.
The later design intake manifolds have longer runners than the early type and the torque curve is correspondingly lower. PhilipA has made comments that fitting the later style manifold transformed his 3.9 engines characteristics.

I have a spare intake manifold for my 3.9 and I have been having a thought (dangerous, I know ).

Would there be a worthwhile improvement to bottom end torque if I got 8 longer trumpets made up (say 30mm longer) and had a spacer made to lift the top part of the manifold to give clearance for the longer trumpets?

I have room under the bonnet for it all to fit (30 mm body lift). Would the correspondingly larger volume of the plenum chamber have a significant effect?

How would I go about making the spacer? I would imagine the trumpets would be the easy part (just weld a section of correct size pipe on the bottom, all the trumpets start the same length, they just sit in different depth holes).

I always think adding cubic inches is usually the cheapest and best modification for a daily driver/street car. You are pushing a two ton brick with a small engine. Bung in something with six or more litres. Think of the beast on your avatar.

Thanks Jakeslouw for the tip. I bought that bottom halve of the Thor intake manifold (as I have the Plenum & Throttle body but nothing else)

PhilipA you can now see the direction I am going with the Thor manifold (so I will be in touch)

If you have a look at the book "Tuning Rover V8 engines" by David Hardcastle you will note that JE Motors did some experimenting with plenum trumpets.
Cheers