MGB Roadster conversion

18519 Views 18 Replies 6 Participants Last post by Rf 4.3, Mar 4, 2022

67BGTEV

Discussion Starter · Jan 29, 2015

All,
I'm now planning for my next Conversion, which is going to be a 1979 MGB Roadster. This is a rear wheel drive.

I wanted this to be a fast and high performance drive. I just bought a Remy HVH250. I'm close to getting a rinehart pm100dz. I'm planning to run it close to 500V.

I need your guidance on the reduction ratio for this setup. MG diff is 3.9:1. I read up few threads about 2 speed Powerglide trannies and I don't think it is a good idea to go for Powerglide tranny, as its final ratio is 1:1. I don't want to spend too much money on tranny/reduction

Miz, RobA, Duncan, and all other experts, please share your thoughts

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Duncan

· #2 · Jan 29, 2015

Hi
I don't think I'm an expert but here is my tuppence worth

Top speed - what do you want?
What is the motors max rpm?

Divide A by B - and you get your maximum reduction ratio

any greater reduction than that will reduce your top speed

Sit that to one side

Weight of vehicle (MGB) - weight on rear axle
Size of tires

My tires are almost exactly 2Ft in diameter - so each Lbft of axle torque = 1Lb of force at the road surface (and at the axle)

General rule of thumb road tires have a max coefficient of friction of 80%

So if you have 1000lbs on the back wheels the most force you can apply is 800lbs
So the most torque is 800ftlbs (2ft tires)
If your diff is 4:1
You need 200ftlbs at the motor - to MAX out less is OK

Just plug your numbers in - see what you want and then have a look at what you can get

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Duncan

· #10 · Feb 11, 2015

Hi
Can you find a higher ratio diff?
3.9 would work
But 7.8:1 would be better!
Up to 36mph - 1400ftlbs - tire spin city
By 55 mph torque has dropped to 862ftlbs - still spinning
By 73 mph torque has dropped to 574ftlbs - now the same as the 3.9:1

Anything between those would be good - higher motor revs = better cooling

What is available for your rear axle?

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Duncan

· #12 · Feb 11, 2015

Hi
The MGB used a lot of Austin/Morris parts as MG was part of the Austin/Morris/Leyland.... conglomerate

I think you will be able to go to at least 4.4:1 with standard bits
And with the long history in motorsport there may be higher ratios available

It may be worth "Going Custom" as you will end up with impressive performance

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