Thread: 3.9 V8i intake manifold

Originally Posted by BigJon

My question is if a 30mm difference in trumpet length would be noticeable. I would guess that increasing each individual trumpet by 30mm would have a far greater effect than putting a 30mm spacer at the throttle body (8 times the difference?).

I have enough room under the bonnet to increase the height of the manifold by 30mm easily.

Well, shortening makes a measurable difference so there is no reason why 30mm long wouldn't, but it's just how much of a difference.
Have a look at www.g33.co.uk I think it is or search google for Ginetta. Somewhere in there is a formula for working out runner lengths.
It's biased towards TVR's but there are also a lot of landrover references also.

Cheers

Andrew

Think your a little confused big John , Adding to the Trumpet length may have a very (read tiny) small increase of torque at lower Revs .
Adding something in front of of the Throttle body will only increase your Plenum Volume , and probably Slow down your MGV . Waste of time and effort both ways . Get the heads properly ported to up the Volumetric efficiecy first !

Well, shortening makes a measurable difference so there is no reason why 30mm long wouldn't

Just remember that the shortening is usually accompanied by increasing the size of the trumpets and porting the runners .

BTW 400HPongas there is a thriving industry in the USA in selling TB spacers . Whether they achieve much is a different matter.
Any J**p owner worth their salt seems to have a spacer added to their 4.0
Regards Philip A

How much clearance is there on the inside of the plenum between the trumpets and the plenum
I had mine apart a coupe of weeks back and i got the impression there was only a very small gap not enough to lengthen the trumpets by 3cm

Originally Posted by steve_35

How much clearance is there on the inside of the plenum between the trumpets and the plenum
I had mine apart a coupe of weeks back and i got the impression there was only a very small gap not enough to lengthen the trumpets by 3cm

Quite right, the theory discussed here is to add 3cm spacer to the plenum and then put in 3cm longer trumpets and yes there's room under the bonnet because of a body lift.

Righty oh

I thought the spacer went under the trumpet base

400HPONGAS, I don't know why you think I am confused. I am talking about increasing the intake manifold runner length by making the trumpets longer. This should, in theory, move the torque curve lower in the rev range.
I didn't bring the throttle body spacer into the conversation. What is MGV? I don't want to pull the heads off this engine to port them. I have a 3.5 in the shed which will be rebuilt with a stroker crank to increase capacity, those heads will get work.

Steve35, if you read all the posts in the first one I mention putting a spacer inbetween the two halves of the plenum chamber to make it taller. That will give the room for the longer trumpets.

Didn't read that line propa

So what would the difference be would a spacer under the trumpet base do exactly the same thing as making the trumpets longer