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MG MGB Technical - which compression ratio

Help,
Because of a recent head gasket problem I have had to remove the engine from my 67 MGB roadster. It has a Piper BP255 cam and a CR of 8.8 to 1 with 1.1/2 SU's, 123 dissy, Cast iron manifold and Falcon SS exh.
I would like to go for a BP270 cam and a higher CR but am worried about detonation. What CR can I go to without causing problems, I normally run on 97 or 98 Ron fuel with no additives.
G Cherry

9.75:1 should be ok with a BP270 cam.

Peter
Peter Burgess Tuning

This chart may help you decide on bore and cyl head capacities -green lines are standard available piston sizes.

Chris at Octarine Services

Only trouble is you aren't showing deck heights, piston annulus vol, dish or head gasket vol.

Peter
Peter Burgess Tuning

Hi peter - those are all built in using the standard figures as suggested in your book!
Chris at Octarine Services

How can you have standard deck heights? Depends on skimming. Also depends on crown height and dish. Unmilled/skimmed blocks with standard-type pistons tend to be 25-35 thou down the bore. This has to be measured before milling as it cannot be guessed at in case the block has been skimmed before. The book content you mention is a guide for how to calculate CRs

Peter
Peter Burgess Tuning

Hi Peter & Chris thanks for your comments.
The only other inf I have is Bore size 3.220 pistons +60 thou giving 1868cc. inlet 1.62, exh 1.34, 3 angle seats. I'm hoping the bores are OK because I don't want to go any bigger. Will I be OK at 9.75 to 1 with detonation, as I have several friends with 1950 and fast road engines that experience lots of trouble.
Graham
G Cherry

Graham

It depends on being properly setup, there is no way anyone can guess the fuelling and timing requirements, it needs confirming on a rolling road. Quite often problems cease to exist when properly setup. That being said, you should not encounter problems at 9.75:1 CR with a 270 cam.

Peter
Peter Burgess Tuning

Peter

Thanks, I am far more confident of which way to go and will probably come for a rolling road session after a 1000 miles or so when the engine has settled in.

Graham
G Cherry

What difference in HP will the CR increase from 8.8 to 9.75 make?

And when I am at it: For my 948 MG Midget engine I have not managed to find any flat top pistons, so the CR is 8.3, not 9.0 How many HP will I lose?

On the other hand it has a superb Burgess Econotune cylinder head, so I guess it will be a little more powerful than standard anyway!

Tore

Tore

Tore,

In short I don't know what the exact difference will be, but I'm hoping there will be a modest gain from the BP270 cam without loosing to much flexibility at lower revs. Perhaps Peter will comment on that one.

Graham
G Cherry

Hi Graham

The 270 we use is very sweet, you will find the specs of a BP270 may vary if Piper evolve it, you have to time it in and set the tappets to what it says on the paper accompanying the cam. I assume you are timing the cam in?. I cannot tell you how yours will work as we do not know the head or engine build.

Tore, I trust you are well? maybe 2-4 hp less but smoother at low rpms.

Peter
Peter Burgess Tuning

Hi Peter,

Yes thank you, life is excellent at the moment! And this has been a fantastic MG summer in Norway :-)

Tore
Tore

Hi Peter,

The engine was originally built by Don Loughlin 16yrs ago and he now has it back to rebuild after the head gasket problem, so he will time the cam in.

Graham
G Cherry

Hi Graham

That is good to know Don is a good'un.

Peter
Peter Burgess Tuning

This thread was discussed between 25/08/2013 and 29/08/2013

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