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The Scarlet one returns...
1,040 Posts
Discussion Starter · #1 · (Edited)
Recieved my GM throttle body during the week and have set about the prep work. Jobs done so far:

Throttle body
Cleaned up with a wire brush and some petrol
Painted (red of course!) :)
Unnecessary brass pipes (3 no) removed
Unnecessary holes (2 no) plugged with self tapping screws

Throttle cable adapter
Y shaped part shortened to 23mm
Slot to take throttle cable cut in Y shaped part
M4 bolt found and head filed
Ball and socket shortened to 23mm
Bolt removed from ball and socket
Both parts bolted together and mounted on throttle body

OK, with this lot done the throttle body looks like this...


The Scarlet one returns...
1,040 Posts
Discussion Starter · #2 · (Edited)
Latest developments...

Throttle sensor plate fabricated and fitted.

Throttle cable bracket fabricated and fitted.

Still to do...

File mounting holes.

Measure electrical resistance in throttle sensor.

Fabricate gasket.

The story so far:

Well, today's little projects turned out to be more dfficult than i thought. The throttle sensor plate was a case of 'third time lucky' - i managed to systematically drill the three holes in the wrong place two times in a row, completely buggering up the two aluminium plates Dieter sent through to me. So i had to start from scratch. :mad: Still, got it done in the end. :)

The throttle cable bracket also caused me grief. The piece of aluminium plate i had put aside for the job mysteriously dissapeared leaving me with no alternative but to bodge a bracket from a length of aluminium strip. :mad: Not only is this bracket missing one of the fixings, it is also 2mm thick, which could cause problems with the bolts that secure the throttle body to the plenum. :eek: I have tried the throttle cable clip in the 'keyhole' and this seems to fit (if a bit tight). :)

What's Dieters prediction regarding bhp/torque figures for this TB in comparison to the Trophy TB?

The work involved on this project is clearly more involved than I would expect than the Trophy unit. So. clearly isn't a task the average enthusaist could undertake, or am I over reacting?

The Scarlet one returns...
1,040 Posts
Discussion Starter · #5 ·
OK, i have hit a couple of problems!

The first one is that my resistance meter is analogue rather than digital meaning that readings of less than 1 Ohm are nigh on impossable to fathom!

What i have done to get around this is 'guestimated' the position using the original position on the old throttle body as a guide. Not sure if this will work, i have no idea how critical this is, but am working on the theory that any accurate measurements taken from the sensor will be buggered up by my ham fisted drilling of the hole for the retaining screw anyway!!!!

Secondly, as i mentioned earlier i have had to bodge the throttle cable bracket a bit and this is now causing difficulties, not least of which is the thickness of the metal which is making securing the throttle cable tension adjuster tricky.

Lastly, because the GM thottle body flap opens the other way to the MG one, the axle that the throttle position sensor attaches to rotates the other way. The way around this is to swap the outer two wires in the sensor, thus reversing the polarity of the sensor. Can i get them out of the plastic clip? Am i a bit miffed at the moment? You betcha! :mad:

OK, so this is how i intend to play it from now on. I have sent a 'help me' Email to Dieter, but, it being a bank holiday weekend i have no idea when he will open it. In the meantime i have stopped work for the day as i can't proceed any further without his advice. If i have not recieved a reply by mid day tomorrow i am going to put the old one back as i need the car on Tuesday :(

:( shame:(

57 Posts
Hi all,

If SF had said before that he's reporting here, then things would have run a bit easier... LOL

>Unnecessary holes (2 no) plugged with self tapping screws
Mate, they must be sealed closed. Did you add any adhesive ?

I think you are on swapping the wires now. *g*
Spade connectors in retainer housing. The connector can be opened at the rear and the spade connectors need to be pushed out to the back.
A rough mechanic would just cut both wires and use barrel clamps to re-connect. *shame*:mad:
Spade connectors always get pushed in from rear and secured with small tabs, bent upwards.
This tabs can be bend back with thin wire pushed in from the receiver side.
At the MGF throttle sensor connector and at several more the spade connectors are sealed also against the retainer housing with coloured sealings, direct crimped to the spade connector and wire.

If you got the outer swapped then and try to find out whether the sensor is mounted in the same now converted angle?

I think, what you did may work *approximate*. The right idle angle and resulting resistance can be verified under running engine, though.
If to low, then it may not start (IMO)
If to high then the engine will run at about 2000 revs ++.

I haven't seen your sensor plate yet, but hopefully you got some play in the sensor mounting holes to turn the sensor some degrees.
If not... LOL, then prepare for the third try.
Remove the screws and turn the sensor careful until you get about 900 revs.
Then mark the location and drill again two holes... if material to drill availiable. Looks like you get a swiss cheese !

Err, don't worry, I did two sensor plates until I got satisfied ;)
My friend Matthias, who is a professional mechanic made plates ... ROFL, you should have seen !! A shame for each mechanic professional ;) He said at last. It works, so what ?

Due to the power and torque question,
I have no idea. The game was cheap, I had my engineers fun and the car feels better.
That's it.
Roger Parker just reported about improvements with the Trophy 52mm. I think it was something about 5bhp at the VVC, together with improved air filter system (cold air).

>IF Haynes ever need a tech author scarlet is the man..
Cheers ! But who would pay for a booklet which is already much better in the web :)

If not known. This poor bearing at the original TB is IMO the main trouble for sticking flaps.

The Scarlet one returns...
1,040 Posts
Discussion Starter · #8 ·
Final progress report…

Mounting holes filed
Sealing gasket made
Wires swapped over (cut wires in the end, tried a paper clip but it didn't work so resorted to mechanic's method!)
TB fitted - it fits woohoo!!! :)
Throttle cable fitted
Small breather hose fitted
Large breather hose…

Hmmm, it appears that the hose that fits the large port on the TB is a bigger diameter than the connection on the stepper motor. This is rather annoying as the hose I have bought is quite a substantial item and is unlikely to fit even with a jubilee clip (which I do not have, and with the car in pieces, no way of obtaining) :(

But there’s worse to come!

With the GM TB fitted it became obvious that the ITG air filter is too big. The filter is probably the largest one available and comes in two forms (I have both). The old style one is on an L shaped pipe that narrows in bore from the TB to the filter and sits in the far corner by the petrol tank. This pipe is too big to fit in the engine bay with the GM TB and the filter fitted, furthermore, because it narrows in bore, cutting it down would mean that not only would it not fit on the TB, but it would also stop it fitting the old one; more of this later. The type 2 ITG filter is the one from the Maxogen kit. This has a straight pipe of much shorter length and is a constant diameter. The Maxogen filter is the same diameter as the old TB (ie bigger externally) and therefore the Maxogen pipe is too big to fit on the GM TB.

So, after 4 days of work this was the situation…

Everything is fitted and works except the large breather pipe and the air filter. The breather pipe needs replacing with a more flexible alternative (garden hose?), one that can be ‘shrunk’ to fit the stepper motor connection and attached with a jubilee clip (which also needs obtaining). Also a new air filter is required, one which is either smaller (K+N or maybe PiperX) and/or a new connection pipe is needed, one which is a constant bore to suit the GM TB and the ‘old style’ ITG filter. Neither of these things will happen by Tuesday (which is when I need the car again) so I have admitted defeat and re-fitted the old one.

To be perfectly honest this was a REALLY difficult job, caused mainly by it’s position in the engine bay, and is not something I am anxious to try again. I am sure it works and works well; the difference in internal bore between the two TBs is incredible, just why MG designed their own one with an internal narrowing beats the hell out of me, crazy. However, 4 days of constant messing around trying to overcome one problem after another, filing this, making that, drilling holes and fiddling with hoses, electronics, gaskets, plates, brackets and cables is IMO too much work for a performance gain, no matter how great. On top of this, I have doubts about my handiwork and this would be a constant worry for me – especially given the amount of touring I have planned this year.

So, there we have it. It’s beaten me :(

Overall, yes I am a bit disappointed, but if I hadn’t tried then I wouldn’t know would I? Also, I had a good look in the engine bay and cleaned up a few things (like my air filter, cleaned and re-oiled). So it wasn’t a waste of time and I am glad I did it, although i'm a bit down about the result.

Thank you for all your help Dieter, I certainly wouldn’t have got anywhere near as far as I did without you.

Andrew, very tired and very dirty!

57 Posts
Dooh, bad news :eek:

the filter fitment trouble may be that the GM TB is 38mm longer (just measured) then the existing plastic thing.

GM TB big breather hose is 19mm diameter versus 16mm of the existing. I think that is the minor problem.

*g* I'd not give in after so much works. :D

Tired mechanics should not decide anything :rolleyes:
... but sleep over the problem... think about it, go one step backwards and then again two steps foreward.

CU later :cool:

MGF and ZTT pilot :)
863 Posts
Keep at it Andy, don't give up! Dieter's installation looks fantastic! I saw it last year when I was last in Germany... :)

Very very impressed with the work that you've done so far: the red painted TB looks well cool. :)

I wonder how much of you current problems are down to the combination of GM TB's extra length plus the VVC plenum you have fitted? The VVC plenum moves the TB a further (I'm guessing here) 25 mm to the left over the standard plastic plenum, so it is perhaps little surprise that you may be running short of room. As a temporary stop gap, it may be a good idea to fit a smaller cone filter to the GM TB - perhaps straight on until a silicone tube of suitable dimentions can be purchased (Demon Tweeks sell them, in a variety of colours and shapes- including 90 degree elbows).

If you need a smaller filter, I have a Pipercross Vector which you can borrow (probably the best bet, as the internal venturi evens out the airflow more before the throttle butterfly than a 'normal' cone filter).

We have the technology Andy... let me know how I can help! How are you set this weekend? If we're not moving I'll come round and lend practical/moral support! :eek:)

The Scarlet one returns...
1,040 Posts
Discussion Starter · #12 ·
Before i begin, all diameter dimensions are 'guess-timated'

This is the air filter problem

Old style ITG

Large cylindrical filter with an opening for the pipe of, say 60mm diameter. This then connects to a long L shaped pipe (a short leg and a long leg) which is 60mm diameter at the filter end (the short leg) and 70mm diameter at the TB end (the long leg). Now, the GM TB is around 60mm external diameter, therefore the only way to fit the L shaped pipe is to reverse it. Note, because the GM TB is longer than the MG one even if the pipe was the right diameter it would be too long.

Like this:

Original fitment:

MG TB (70mm diameter) to long leg of pipe (70mm diameter) narrows to short leg of pipe (60mm diameter) to ITG Filter (60mm diameter)

New fitment:

GM TB (60mm diameter) to short leg of pipe (60mm diameter) widens to long leg of pipe (70mm diameter) to ITG filter. Old style ITG will not fit, new style (Maxogen) does BUT, because the pipe is reversed the filter and pipe is now too long for the engine bay (the long leg of the pipe runs front to rear of the engine bay as opposed to side to side) :(

ITG Maxogen

Large cylindrical filter with an opening of say 70mm diameter connects to straight length of pipe also 70mm diameter. GM TB external diameter is smaller, say 60mm. Pipe is rigid and will not compress this much. :(

Incedentally, the picture i sent to Dieter of the GM TB and ITG filter (on his web site), the filter is on the short pipe and is just balancing on the TB, it is actually very loose. I will try to take some more pictures to show you what i mean - it's difficult to explain in words.


I ran out of time. It was fitted and then i needed to get a jubilee clip for the hose or replace the hose and when the filter wouldn't fit as well i had to make a decision. I had one day left to either carry on or re-fit the old one. In the end i decided to re-fit the old one as i knew this would work, carrying on with the GM TB would probably involve making another sensor plate and cable bracket and i just didn't have the time or the materials to do so.

Having thought about it for a day or two, i will probably give it another go in the near future. I've done so much work already its silly to throw it all away.

Also, i am having a problem now with a sticking throttle, could be the plastic TB - they are prone to do this, but is more likely either the throttle sensor (although i cant see how) or the throttle cable (it looked a bit kinked under the parcel shelf lining). The car is booked into my dealership on Thursday to get this sorted out, hopefully it is not too serious. Symptoms are

1, Start car, idle is normal (ie just above 1000rpm from cold)
2, Drive a little, then coast, rpm stays at 2500!
3, Drive some more, then coast, rpm climbs to 3000!!!

Sticking throttle body or cable i imagine will produce symptom number 2, symptom number 3 is baffling me at the moment, maybe the throttle sensor...

Anyway, this weekend is out Rob :( i just want to get the car running properly again. Then i think i need to get the throttle bracket and sensor plate made up properly. The sensor plate should now be easier as my last one (3rd attempt!) is pretty much there, it just needs milling to produce 2 slots for the sensor position fixings - 2 small nuts and bolts is probably the best way to do this. As i mentioned earlier, my throttle cable bracket was a bit of a bodge job - the plate i had set aside for this job mysteriously dissapeared and all i was left with was a length of aluminium. I was not happy with this and didn't have the time or the materials to remanufacture.


1,439 Posts
Bummer! I have a spare standard TB if you need one.

Tenacity by boy, through sheer blo0dy-mindedness one can achieve anything!!

You're a braver man than me though ... :D

The Scarlet one returns...
1,040 Posts
Discussion Starter · #14 · (Edited)
Thanx for the offer Andy, but things have moved on a little since then! ;)

The story continues...

OK, this is what has happened since my last posting. The car was booked into my dealership (Jones of Rayne) last Thursday to sort out the intermittant idle problem, this turned out to be a warped plastic throttle body (i told you they were prone to it!) Anyway, i recieved a phone call in work with the news and was then told that it was probably caused by overtightening of the jubilee clip that secures the ITG air filter pipe. It occured to me then that the best way to overcome this is with an alloy throttle body and if i was going to pay for a new alloy one i might as well get the Trophy spec item. This was ordered in (V.O.R.'ed) and fitted Friday, so i have not had the chance to take the car on a run yet to feel the differences, but already it seems to have a bit more mid range grunt - this of course could also have been caused by my cleaning and re-oiling of the air filter. :D

So, i am now in the possession of a standard plastic throttle body, an alloy Trophy one (fitted) and 2 GM ones, so am in a good position to give comparisons. Rob Bell popped over today (Sunday) for a good look at the items and to see what work is involved in getting the GM to fit (we also talked cold air induction boxes as well - some VERY interesting dimensions were taken from his Rover 820 air box and the ITG item, but this is for another thread...)

The important thing to come out of today for me at least anyway, is that i have a renewed enthusiasm for the project. We are talking about getting the GM TB finished and maybe doing some back to back rolling road tests with the Trophy spec item and the standard one, as well as the various air filter options. :D

Incedentally, i have now had a good opportunity to examine the old plastic one and it appears that the real reason the throttle was sticking was because i had overtightened the throttle sensor retaining screws causing them to project out the back of the sensor and foul the rear rotor arm on the back of the butterfly mechanism (difficult to describe this). So, i now have 4 throttle bodies! :cool:

MGF and ZTT pilot :)
863 Posts
Andy, I retrieved all the pix from my camera and started to work on them last night. Hopefully on a web site near you soon! :)

Having seen Andy's handy work so far, I can confirm that he really was 90% finished before a couple of unexpected problems stopped play - and looking at the work that Andy's done so far, I can confirm it is all really first rate, as would be expected from looking at the pix of the red TB above :D

Having spent a lovely Sunday afternoon measuring everything up, I think that the dimentional problems that scuppered the first attempt to fit the GM TB can readily overcome :D

Yeah, I reckon some back to back RR testing of these TB's would be really useful. As we now have a 'spare' potentometer to work with (the Trophy item that Andy had fitted apparently being supplied with a new one), work on the GM TB should be quite easy (if challenging!) to complete...
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