<Unless you are planning to use the car for racing only, using racing fuel,
do not exceed 9 to 1 compression. Your engine is virtually guaranteed to
ping and run on, causing expensive damage to pistons and bearings, if you
exceed this figure. 92 Octane just cannot handle that high a compression
ratio.>
Lawrie, I more or less agree, but I think you are being just a touch
conservative. I have run engines at 10:1 on pump premium without problem, but
you do have to watch the timing. I agree that 9 would be safer for most
people.
The other thing to be considered is that the increase in ratio doesn't really
get you much unless the head has been prepared. The stock MG cylinder head
is, being as polite as I can, a piece of lamentable crap from a design and
flow point of view. It has hot spots, it pings and bangs and it doesn't make
nearly as much power as it should in stock form. It was a step back from the
MG TD head, and I could never understand why the factory didn't do a quick
redesign back in the old ZA Maggot days.
I spoke with a factory rep in the old, old days, and asked about the 'design'
process and the decisions taken. I was told that there was an interesting
story behind it but that they couldn't talk about it while John Thornley was
around. Well, John has passed on, and so likely has my informant, and I'd
sure like to know what went on. I imagine it had something to do with a
decision like this:
"Let's see - the ZA cylinder head, for which we have the patterns and
machining tools, will do an adequate if somewhat less than stunning job, and
the result will still be quicker than the TF (mostly due to body mods). We
could cast up a new head, but with design, testing and whatnot, it would add
about 1 pound to the cost of the car. Even though we could get another 12 bhp
or so, the car wouldn't sell any better for it, so let's not bother"
Now I agree that many racers have managed to get rather stunning results
using the MGB head, but it is really somewhat similar to carving and
polishing a turd - we are all surprised at the result, while muttering (sotto
voce, for those more polite than I am) "Why on earth couldn't they have
started with something decent?"
If anyone knows what (if anything) was going through the minds of the factory
on this project, please step forward and tell us. That 'secret story' has
been bugging me for years.
Bill S.
Regular drivers: Pontiac, 8.5:1 with 13.5 psi boost, Jensen (383) with 10:1,
both running with no problems or pings on 92 octane.
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