3.9 V8i intake manifold

**BigJon** · 13th July 2010, 09:20 PM

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).

**BigJon** · 13th July 2010, 09:58 PM

Or perhaps putting a spacer under the plenum chamber where it bolts to the manifold? I have seen plastic (thermal) spacers for Ford EFI V8s in the past.

d@rk51d3 · 13th July 2010, 10:05 PM

I know the AU Falcon 6 cyl has a 2 stage intake. Long for low down torque, which then switches to a short for HP.

I'll have a look in the Rover tuning guide to see if anything is recommended there.

**LOVEMYRANGIE** · 13th July 2010, 11:08 PM

Twin plenum intake and mandrel bent stacks opening 4 a side into the inlets.

There's no height to play with under the bonnet. Have seen a few custom joined manifolds around and look quite good.

Guess it comes down to cutting a hole in the bonnet or not.....

Good to be different and try something outside the square tho!

Cheers

Andrew

**BigJon** · 14th July 2010, 07:24 AM

Originally Posted by LOVEMYRANGIE

There's no height to play with under the bonnet.

As noted, I have 30 mm more than standard due to a body lift.

**PhilipA** · 14th July 2010, 08:13 AM

The best way is to fit a Thor manifold.
It has very long runners and Hemholz resonance at low revs.
Google it and do a search here.
Regards Philip A

**Traco** · 14th July 2010, 08:17 AM

Quite a few TVR owners have done this. They use the same 14CUX hotwire efi. There are TVR Rover V8 specialist kits around in the UK that contain a manifold spacer and different length trumpets.

Have a look on some of the TVR forums for contact details - Google TVR Gassing Station and scroll down.

**BigJon** · 14th July 2010, 08:25 AM

Originally Posted by PhilipA

The best way is to fit a Thor manifold.
It has very long runners and Hemholz resonance at low revs.
Google it and do a search here.
Regards Philip A

I have seen a few threads. Best - yes? Expensive and complex and non standard looking - yes?

I have a couple of standard manifolds to play with.

steve_35 · 14th July 2010, 08:26 AM

Check this page out has a lot of info on intake manifolds If you haven't already

Rover V8 inlet manifold photo archive

It shows the differences in plenums and how to mod yours

**BigJon** · 14th July 2010, 08:35 AM

Steve,

I have looked at that page. Most of the mods seem to be increasing maximum power at the cost of low rev torque, quite the opposite of what I would like to achieve.

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