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MG MGB Technical - History of HP reductions

Is there a brief history of the HP reductions that took place over the life of the "B" and possibly simple ways to get some back that don't include major internal engine modifications? I'm sure there is lots on tihs in the archives but I wasn't able to find anything with the searches I did. I have 74.5 and 79 Bs.
Tom Gillett

Hi Tom

I am not sure the Bs ever made different power if they had been power tested to the same standards. We used to get about 65 at the wheels on all standard Bs on the old Clayton rollers and around 62 with the new Dynocom rollers (3rd gear Clayton and 4th gear Dynocom). Some late rubber bumper Bs seem to have three angle valve seats, they also seem to have been skimmed/milled for some reason after leaving the factory, these make same top end bhp as port/throat restricted even with three angle seats but make very good mid range torque.

Bs work well with raising CR to 9.75:1 and three angle seats with some throat fettling.

Peter Burgess Tuning

Tom is in the USA Peter, so he will have the "federal" versions and his '79 should have the strangled single carb engine. There were significant power losses in US cars as the rubber bumper years went on weren't there?
Mike Howlett

I believe the exhaust gas recirculation system also affected power output for US engines - the basic engine was the same as the UK models so removing any of the "emissions" gear and reverting to twin SU carbs and the right distributor "should" put the US engines back on a level playing field with the standard UK engine. Then as Peter says, up the CR and port the head, perhaps overbore the block and you will have a good driveable car.
Chris at Octarine Services

In addition to the single ZS carb, the integrated intake/exhaust manifold with cat attached is not as well designed as the original separate intake and exhaust manifold system. Especially in the area of the exhaust system where the early manifold and down pipes formed a tuned exhaust system which functions quite well. On my current RB car, I have switched over to a factory exhaust manifold and down pipes with a free flowing cat behind the down pipes. Meets emissions requirements and seems to perform quite similar to my CB car.

As to the "loss of power", my memory is that the method of testing the horse power was changed during the life of the MGB, with the early cars having an HP rating taken from the flywheel and later cars listing an HP rating taken at the rear wheels. As most understand, there are some significant losses of HP between the fly wheel and the rear wheels. So, the true HP ratings could only be determined by taking original, stock cars and testing several examples on the same rolling road to see what actual differences exist.

Les Bengtson

OE US spec single carb made 45 at the wheels on the old Clayton if that helps? Apologies for only thinking of non US models, thats the problem with us Brits, well, one of the many problems :)

Peter Burgess Tuning

in addition to previously mentioned the best way to keep power is to regularly fully service, maintain and repair the whole car and to regularly drive the car over reasonable distances with the occasional long and/or blow out run

after that if all the systems, components and parts on the car are in good condition and working well then a rolling road set up (with someone who actually knows what they're doing rather than just having the equipment) should have the engine running as well as it can in its present condition and keeping

don't just think of the engine as it could be other systems and items on the car holding it back - regular full servicing, maintenance and repairs will help to reduce this and without these basics you can't successfully progress
Nigel Atkins

In the early '70s, with the advent of the single row timing chain, the camshaft timing was advanced for increased power at lower rpms and reduced emissions. In addition to a reduced compression ratio, from 8.8 to 1 to 8 to 1, power also dropped a bit at the higher rpms. RAY
rjm RAY

I don't know how accurate it is but this table shows the performance changes on the American spec MGB, C and Midget over the years, from 94/98 BHP initially down to 62 in 1976 and recovering a bit to 67 by 1980.
Paul Hunt

Paul's HP numbers reflect about what I always remember hearing.

RAY, I thought it was actually a different cam. Was it only a timing change to the same cam?

C R Huff

Charley, been away from my computer for a few days visiting various doctor's offices for a bum shoulder. The early engines, with a mechanical tach drive, used one cam, but from '65 until the end of production, the same cam was fitted. Only the cam timing was changed. RAY
rjm RAY

Good to know, RAY. That would make it easier to return a late model engine to the earlier configuration.

C R Huff

Not so. I had this same argument with a fellow club member. I once had documentation to go with it. The late model USA smog engine had a cam with less duration to help with emissions. The difference can be heard in the idle.
Carl Floyd

Maybe the specs are in the Haynes?

C R Huff

Never forget that the figures before 1972 were gross BHP and after were SAE net, which can be 15% lower to start.

Otherwise, the figures Paul cited are correct, so early high compression cars in the low 80s net, dropping quite a bit to 62 in single carb low compression form.

If you want a late engine (N. America) to be early spec you have to do carbs, exhaust and compression. The cam, IIRC, doesn't make that much of a difference so is optional.

If anyone is going to do this, I strongly suggest that you go to about 9.5 compression rather than 8.9. The engines love compression.
Bill Spohn

This thread was discussed between 17/02/2014 and 18/03/2014

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