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MG Midget and Sprite Technical - Maximum power for rear axle

Assuming the axle had an LSD, uprated drive shafts and better location, what do we think is the maximum power it could reliably take before considering alternatives?

What would those alternatives include? Ford Anglia, narrowed RX7? Others?

I'm looking at coping with up to 250bhp for road & track day use.

Your feedback please.
P R Clark

I broke mine with around 200 bhp and 150 ft lbf. I now run a modified ford English axle with atlas competition half shafts and side gears. It is used on the road but also on the drag strip and power is now 270bhp and 260 ft lbf using nitrous and a turbo. So far ok. A cheaper option would be a Volvo 240 axle, narrowed, don't think a standard English axle would take it. A ford 8'' would do it but is a monster to fit in these little cars. Hope this helps. BTW how are you planning to get 270 bhp?
steve cowling

270 hp, wow, i got to see that !!

at your power out put, your wasteing your time with a mg /lucas rear end, probably want to consider a pick up truck rear end or have a custom made DANA 60

Im assuming to get 270hp your doing an engine transplant... maybe take the axle from the donor car

As to the MAZDA RX7 Rear end, thats a great choice... ive got one ready to be modded but no cash or ambision to do it

The GOOD ... once cut down its very close to the specs of the midget rear end as far as size, its rated to 250hp, it has LSD, 13 inch wheels and disk brakes, and various pumpkin gear sizes ratios and a popular rear end for racers

The BAD.... you want the 1st generation, and it was only available for the 1st 2 years before rx7 went to independant rear suspension... i want to say 1987 to 1989, it was known as a GSL mazda rx7

Curious to what you have... id think a ford ranger pick up or a small chevy rear end would be a good choice

And a side note... because the tires sit so close to the hub id also look at having what known as DOVE TAIL rear springs made for rhe new axle... that should give you a better clearance of wheel to hub ona modified rear end

Also... upgrade to 7/16 inch wheel studs from the 3/8 inch

Also for that kind of power... im thinking a WATTS 5 Link rear suspension im not sure that ladder bars, with a panhard rod on telescope shocks will be enough for a rear suspension

But please do give up some extra details on the 270hp engine.... you have to remember even an early chrome bumper 1275 came from the factory with 60hp.... so this will be a massive very cool build project

I hope you can help us with front suspension, brakesx master slave peddles safety hardware and safety roll cage desighns and what soild wheels is it a normal asperation, torbo or blower induction..... im looking forward to this set up

Very cool

Prop

1 Paper

Opps... i forgot to mention just an uprated rear end made reliable...depending on what you build id say around 90 to 120 hp

Ive got the best of all worlds rear end in stock form eith the correct year of axles and pumpkins and ive never had an issue with it and i do moderatly crazy crap with my car all the time in the back roads of farm country and my engine is coming in around 95 hp to 110 hp ..not tested but comparing to other like engines similar to my set up

Prop
1 Paper

Ford English Axle. Anglia 105e - fits OK. Much stronger diff and shafts, and also better hub sealing arrangement.

250 should be OK providing no mad standing starts, but you'll wear stuff out very quickly.

A 'best case' midget axle should handle about 200bhp or so with a bit of mechanical sympathy. I have had 120 bhp through my standard shaft, standard hubs standard diff for about 5 years, only recently have I gone LSD, double hubs and beefy shafts. My car gets absolutely hammered.

How are you getting to your 250bhp?

If you're on Facebook there's a good group for the major modifiers - K Series and Zetec Midgets and MGBs.

Rob Armstrong

We have ~215BHP in standard axle with uprated fine spline halfshafts and LSD. Used extensively for autotests with sticky tyres, which places far more stress on the unit in a one event that a whole season of track when the only time yuou are in first is away from the line.
No reliability issues. Only significant things in 15 years of aggressive use:
1 the collapsible spacer between the pinion bearings collapsed further, replaced with solid spacer and shims.
2 the axle housing cracked at the spring mounting recently, addressed by suitable reinforcing.
Paul Walbran

Thank you for your feedback so far.

To answer some of your questions, I run a 150bhp K-series in my current Sprite but would like to do something more extreme for road & track. I'm considering forced induction either turbo or supercharger for another K or maybe a Zetec.

My target would be 225 - 240bhp but used the 250bhp figure to include some contingency.

There are a few big hp cars out there including Eddie Brown's supercharged K & Pete Adamson's (?) turbo.

I understood the A-series axle to be marginal when higher power is applied (although my current stock set up has proved reliable so far) so along with a change to 6 link location I was wondering what axle to use.

Cheers
P R Clark

Ours is supercharged K series, top power not quite what it could be in that the injectors run out at 6500 RPM and it goes lean above that, so with larger injectors we could get usefully more. But the torque is there, and it is torque which kills drive trains. Add in the multiplier of first gear and you'll see why autoresting is so tough on them.
Paul Walbran

Are you talking to Will Corry at all? If not it might be worth it :) Pete's car runs a Ford axle I think.

While we're on I've had many issues with hubs leaking. That seems to be the achillies heel of the axle imo. Like Paul says, they can take some grunt if being careful. (and sometimes not!)
Rob Armstrong

Rob, that's novel, saying we are careful! First time for everything, usually people are much blunter and observe that we thrash the living daylights out of it - to which we happily reply, guilty as charged :-)

Paul Walbran

Good to hear from you K-boys again!

My axle is the same as Eddy Browns: a Frontline build/setup using LSD, fine spline halfshafts, double bearings.

Sofar its been industructable (knock on wood)


Paul, how diffecult/easy was it to put on a supercharger?
Rewiring? inside-enginework?

Any photo's of the installation or when the work was in progress?
A de Best

Paul, had you considered an independent rear end with Sierra diff?
David Smith

David, although we had changed the engine to a K, we were keen to retain as much MG componentry as we could. (In fact the only non-MG item is the (Supra) gearbox, the engine being from an MGF.) So with the rear axle we thought that in the first place we'd give it a try and go to something else if we had to. In the end, that hasn't been necessary at all. No doubt properly set-up IRS would improve handling, but the original suspension has a nice feel to it so inertia prevails! And other projects get priority, like the K-turbo Midget (waiting for Matt to come home from his OE) Andrew's MGF rally car, the WA restoration I started in 1968, and Jennie's Aston Martin V12-into-MGB ... That will keep us out of mischief for a while.

Arie: Quite a bit of work but worth it. IIRC you have more room than us, as our engine is upright.
Here's an overall view from the front, then a couple of the manifold which was a modified VVC inlet one.






Paul Walbran

Top view. There is no functional connection between blower and manifold (air goes to an intercooler before returning to the manifold) but the two are bolted together for mounting reasons. At this point of construction there were only two of the mounting bolts in place.


Paul Walbran

View from underneath.
A is the plennum
B is the supercharger outlet
C is a lug for a stay at the rear
The red arrows and yellow marks indicate other fastening points


Paul Walbran

And a vie from the front top


Paul Walbran

Nice one Paul
With the rear axle choices--
My opinion, which probably doesn't count for much, is that the rear axle shouldn't be too stiffly located with track rods panhard bars and all that sort of stuff
Best result for me in my hillclimber was--
An extra half leaf in the front half of the spring to reduce windup and a large double acting gas shocky mounted from the lh bottom seatbelt mounting,through the rear bulkhead to the top of the pumpkin--This setup lets the axle move around to find grip but takes all the shock off the drive shafts
Works for me
willy

105e Anglia housing and axles--3.7Escort ls pumpkin
William Revit

Volvo 240 series diff had 3 gear ratios depending on the transmission options:

3.9:1 automatic

4.11:1 4-speed

4.56:1 4-speed w/OD

Cheers

Gary
79 MGB
gary hansen

Thank you to all of you who contributed to this thread.

It seems that an Atlas axle could be made from new components which would comfortably take the output and could easily be incorporated in a six link setup. However it is a very pricy option.

The Ford English axles are not as robust as the Atlas so I question whether they offer much benefit over the stock BMC axle.

I liked the idea of the RX7 FB rear axle as some incorporate a LSD & disc brakes in one package but they are proving few and far between.

Likewise Volvo 240's.

I've not reach a conclusion yet as there is no immediate time pressure but I'm leaning towards a stock axle with uprated shafts, disc brakes and LSD then seeing what breaks first.

It seems some are running them at greater than 200bhp reliably with suitable upgrades.

Paul W, any updated pictures of your K turbo with 6 link rear end? I saw some early images on your website.

P R Clark

This thread was discussed between 08/06/2017 and 15/06/2017

MG Midget and Sprite Technical index

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