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MG MGB GT V8 Factory Originals Technical - Shorter trumpets plenum

Here's somthing that actually works.
I origionaly shortened the plenum to the greatest degree possible. Welded the vacuum take offs; 16mm off the bottom of the trumpet tray, 12mm off the top and 5mm off the top cover. Of course I shortend the trumpets 12 plus 5mm.
After refitting the motor, I was able to move it down 10mm or so.
I then observed that I had 14mm bonnet/hood clearance, and I also had a second trumpet tray laying around. This tray I had welded as before but took 10mm less off the top than last time. In an experimental frame of mind I refitted the same set of trumpets.
This effectivly gives me 10mm greater clearance between the cover and the trumpets and in theory should allow easier air flow. It actually does work. The motor revs noticibly more freely and has just a little more go than before.
So any one about to shorten a plenum, take note and get more zing for your efforts.


By the way, the maximum I would recommend taking of the trumpet tray is 15mm bottom and 10mm top of tray. 16 and 12 is just a tiny bit too much. It works ,but it looks horribly thin and fragile, with just bearly enough thread for the top cover bolts. Also this time I took only 14mm off the bottom, could have gone a smidge further.
Peter

Peter,

I've got a standard plenum , but the base and trumpets came from a TVR of some kind, via V8 Developments, they are very short, and a much larger diameter than standard.

I always thought that there was not enough room between the trumpet top and plenum to give a good airflow at full throttle, I was told that shorter, wider trumpets should produce more top end power at the expense of mid-range torque, but can't say that I have found this to be the case.


Mike
M Barnfather

Hi Mike
It's quite noticably, not imagination. Motor just runs better. Definity more accelleration, better reving.
Could be I had a plenum leak before, and now not. I really doubt it though, given general lack of leaking inlet symptoms and the difficulty I had unsticking it (I'd used lots of silicon, ultra grey).
I can see why flowing the heads gets so much more power given the small but noticable improvement made by simply increasing the space above the trumpets a tiny bit. That will be next on the list of things to do (head flowing).
I would guess that Range rovers are after torque rather than power. With our much lighter flywheels (and cars!) TRV's (and MG's) want power. Because I've not shortened the trumpets any more than they were before, no mid range torque has dissappeared but then again I've always felt that around 2800 revs the car has heaps of go, thats where it really kicks in. No shortage I can detect!
I've often wondered why they bothered with a bonnet bulge (and such an ugly Range rover style square one!) with the RV8 when they were casting a special MG plenum cover anywhay. How do they decide how long a trumpet is anyway?. If there is a compression effect from a long inlet then why not turn them 90 degrees and have them as long as possible?
Peter

This thread was discussed between 10/10/2005 and 11/10/2005

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