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MG MGB GT V8 Factory Originals Technical - Lowering EFi plenium

Hi chaps,

I'm converting my flap-valve injection to hot-wire as my winter project, and it occurs to me that the removal of the water heater plate on the plenium, combined with a shortening of the inlet manifold trumpets would give over 1cm reduction in height.

Has anyone had practical experience of this, I'm not sure it serves a useful purpose, and am tempted to by-pass it to see what happens.My under-bonnet temperatures are quite adequate to vapourise fuel, and lowering the plenium would give more heat transfer from the rocker cover.

Any ideas , I would prefer practical rather than hypothetical discussion

Michael barnfather

This has all you will need to get more than an inch off the height.
any reasonable engineering works can handle.

The loss of the heat pad under the throttle is not an issue in our climate (might be in Murmansk) but there is a rather important drain into 4 of the inlets (6mm from the foot of the trumpet stacks) and these need some thought - do you reproduce or do you accept the consequences of topping up the foot of the plenum chamber with oily waste.

Others will comment on whether this mod will get the plenum under the conventional bonnet and whether the arrangement changes the characteristics of the engine appreciably.
Roger Walker

I have 5 B hotwire V-8s that I have put together & there is NO hot water at the intake & I cut .800 off the bottom of the trumpet stack & I make up a set of motor mts. to lower the motor & I cut the slot in the frame motor mt. 3/8" futher down & with this I have NO hood mods.
Glenn Towery

Re: oily waste-don't forget that there is a vaccum take off on the front portside that should remove any build up.
Re: engine charactoristics/changes, I rather suspect that shortening the trumpets may have been the cause of a problem I was having with idleing after bursts of accelleration. Not long after I got this thing going I found that after longish runs on the free way the engine would not always drop back to idleing speed but would sometimes stall. I had to reset the idle speed (with stepper motor blocked off) by adjusting the covered screw on the plenum. I've seen another car with the same problem/solution.
Easy enough to do, if you know it needs doing.

Mind you I've shortened my plenum assembly 33 to 34mm, which is as much as you can get even with the heater removed because of the rocker cover getting in the way.
If you weld up the vacuum take offs on the LHS you can take up to 12mm of the top of the tray. I moved the brake survo take off to the back of the plenum by substituting one of the stepper motor bolts for a hollow brass hose fitting. You can get 16mm off the bottom of the tray and up to 5mm off the cover( 8 if you weld up the bottom stepper motor hole).
This adds up to more than 33/34mm you'll be noticing and you almost certainly won't need the full 33mm so I suspect that the best thing to do is set up the manifold then do some carefull measuring and you can chose how much comes off what/where. Some people take it all off the trumpet tray, some don't weld the vacuume take offs but get enough off the other components. The important thing about the trumpets is that they compensate for the different lengths of the manfold air channels so you need to ensure that the two at each end are shorter than the four middle ones , by the same amount. ie don't shorten all the trumpets by the same amount as the middle ones sit on a shoulder, a shoulder which is further down at the front and back of the tray and which the machineing process will remove. In other words you will need to shorten the end trumpets more than the middle. You can work out how much to take off the four middle trumpets by simply adding up the amounts you chose to take off the cover and the top of the tray. For the four end trumpets, ensure that the orrigional "middle trumpet/end trumpet" height difference is maintained.

If I were doing it again I would take 5mm off the cover, 14 off bottom of the tray and 8 off the top of the tray. (I'd still shift the vacuum take off).


Peter - your speed sensor working ?

No, don't have one. However when stationary I would imagine that the sensor would be sending zero anyway, and the car was stationary (of course!)when I checked and reset the base idle. What was happening, I think, was that the stepper motor/computer was compensating, but not all of the time. Anyway it was an easy fix and it runs fine now.
I've got a second trumpet tray in a cupboard somewhere and at some stage I will re-machine and get rid of the extra under bonnet clearance. If the base idle alters then I'll know for sure.

I've read that you may gain an advantage by shortening the trumpets (in my case leaving them the same) and having greater clearance between the top of the trumpets and the cover, better air flow. Does that make any sense?


I will let you know eventually , I've acquired some very short, overbored trumpets (allegedly V8 Developments), and some standard Range Rover one for comparison.

In the dim and distant past people used to put long(or short) trumpets on SU's and Webbers, one gave more torque, and one more HP but I can't remember which was which.

Michael barnfather

I have had no problems with my 3.9 Hotwire after removing the intake cooler. I found that I needed 1 3/4" to clear the hood, so I cut 2" out of the plenum, and ram pipe manifold. The pictures show what I did pretty clearly on my website under engines. I have not seen this done in such a way before, but It works very well. I cut a new lid out of a 1/4" piece of aluminum and had it welded to the plenum. I also had to shorten the ram pipes to clear the new lowered plenum. I milled 11/16" out of the bottom of the ram pipe housing. I also had to machine the flange where the vacuum lines attach buy 1/4" so the fuel rail would not be obstructed. I relocated the vacuums using new nipples. I milled 1 1/8" out of the top of the plenum and 1 3/8 off of the ram pipes. With all this modifying I was able to fit the fuel injection under the Original hood with a quarter inch to spare. I do have access to a well equipped machine shop so I may have modified a little more than most people, and it took me about a day to complete this project.
Evan Amaya

Peter - yes I have heard airflow is sufficiently important to warrant a second throttle in some installations - I added FI primarily for fuel economy so I haven't altered the trumpets or the plenum.

Good to know,
I'll go get that tray re-done and transfer the trumpets across. Left as they are should give an addittional 5-7mm clearance (and I want to give the cover bolts a little more alloy to grab onto).

This thread was discussed between 03/12/2003 and 11/12/2003

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