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MG MGB GT V8 Factory Originals Technical - Rover SD1 V8 Conversion

I've recently bought a Rover SD1 9.35 to 1 compression ratio V8 engine, which will be fitted to my 1978 MGB GT once i've gathered the rest of the bits i need.

I've done quite a bit of research but i have a few questions,

Is there any performance mods worth doing to the engine before it is installed?

What rear axle/diff shall i use? i'm currently toying with using a Rover SD1 axle and getting it shortened and modified to fit straight on as i've told of a company that can do this.

Although i've read that you can simply change the ratio of the standard axle to suit - but is this going to be strong enough?

Gavin J

Sounds like you've got a good motor.
Have a look on the RPI site wrt engine mod's. Skys the limit. Someone else can tell you best value for money.
My cars a '75 with a 3.9 low compression motor. I'm using the standard, unaltered in any way , back axle.
This is coping without any problems I can detect. Apparently this (and your) axle was originally designed for a light truck/Van then adapted for the MGB.and I really like the standard 3.9 ratio and don't want to change it. It's right for the car. I need the low first gear for car parks ect and peak hour traffic. I find I can accelerate powerfully from standstill in second gear to well over 40MPH with out changing gear. I could not do this as well (I believe) if the ratio were 0.3. and first gear would over rev. There have also been complaints from people who've changed ratios that their fuel economy had become worse as a result, motor working harder.

However if I were to change the axle (in pursuit of a limited slip diff') I would keep a similar ratio and go for an axle with an off set differential. i.e. 4WD of some sort. If you choose carefully you will find one where the short side/axle is close to MG width. Then all you need do is shorten the longer axle tube to match and buy an 'off the shelf' short side axle to match. That way you don't have to shorten and respline two axles (and the other tube of course) and spare parts arenít a problem. I have read that the Nisssan Skyline Silhouette is suitable and uses a 28 spline diff' with a number of ratios to choose


The SD1 halfshafts aren't up to the job I replaced them at great expense with uprated ones.

You can get a LSD for the SD1 axle, and various ratios... mines a 3.07 (vitesse) with 15" wheels...I tried a 2.9 from an automatic, bit it's a tad too high to do 30mph in's only about 900RPM !

M Barnfather

I'm running an SD1 3.5 in my roadster with stock MGB rear axle but the gears have been swapped for 3.07 out of an MGC. So far (2500 miles later) no problems.

On the highway, it's turning at about 2800 rpm at 70 mph on 15" wheels.

I believe Dan LaGrou at D&D Fabrications makes up Chevy S-10 pickup rear axles for the MGB. Don't know about shipping to the UK but might be worth a look.
Simon Austin

Cheers for the replies so far.

Seems like choosing what route to go with the rear axle is going to be a hard one.

I use the mg for trackdays so the axle will need to be nice and strong. The most straight forward option would be to change the ratio of the current axle but my only big concern is whether it can handle the power and the stress put onto it on trackdays. So it would be an expensive mistake if i paid for the ratio to be changed for it to eventually break.

Heres a pic of it in action from Friday at Donington,

Gavin J


Which Company to modify SD1 axle?

The SD1 drag shafts will cost appox £400

FL are running a 5.0V8 with as far as I am aware a B axle.

The Cossie IRS would be worth considering although not cheap.



I've been told that a company called A1 Fabrications near Salisbury will convert an SD1 axle - tel: 01722743063 - although i've not phoned up and got a price yet.
Gavin J


The supplies listed in V8 book no longer trade. The SD1s were batch produced to keep costs down. A one off may be expensive but Andy (see link) should be able to help, certainly for shafts



Andy did my shafts for me , so should have details, they are a fabulous piece of kit, and so far no signs of the splines twisting... I was assured that they were good for 500hp.

Michael barnfather


"I have read that the Nisssan Skyline Silhouette is suitable and uses a 28 spline diff' with a number of ratios to choose "

Whilst searching for info on Skyline diffs I came across this rather derogatory commentary...

Food for thought. Perhaps Ford 8" or 9" is the way to go.



Hmm, Glad you found it, was seriously looking at using it based on a south Aus' guys recommendation, wouldn't touch it with a barge pole now!. However I still think the Off-set diff' approach is a very good idea, and there must be plenty of candidates. Might do to take a tape measure to 4WD wreckers.
On another subject,
You once said you knew of someone in the Melb' metro' who did a gearbox/speedo 'box of cogs' adjustor. Do you recollect who that was? The only other one I know of is in Bendigo and I seem to recollect was more expensive.

Ringwood Speedos - they used to be located on the NW corner of Maroondah Highway and Heatherdale Road. I have no idea how they charge. Some friends in rallying (from the group G - anything goes days) recommended them.

Hope they can assist.



Thanks Dom,
I'll give them a call. The ratio of the MG speedo to Supra gearbox/15inch wheels is 0.7 and although I've got very good at the 7 times table, it would be nice to have it right. One of those jobs that I kept meaning to get around to 'someday'. Mostly, I suspect, because it's not actualy a legal requirement to have a speedo on pre KPH cars, legal loophole.

When you get around to plenum machining you might want to give the guys at MOTOR IMPROVEMENTS a call 9534 8007. They are opposite the St.Kilda Town Hall. For $100 they machined a pully and trumpet tray (which makes the second now) and made a very neat job of it. Welded up the vacuum take offs and then ground off bumps etc.

This thread was discussed between 11/10/2005 and 16/10/2005

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