In a message dated 8/01/00 12:25:47 AM Pacific Daylight Time,
owner-mgs-digest@autox.team.net writes:
> Yes, we are talking about a 'B'. I have not yet calculated the CR of
> my car. Does anyone know the chamber volume of a stock head? The shop
> manual claims that the High Compression motor is 8.8:1 and the low is
8.0:1.
>
OK, the standard high compression pistons that give you 8.9 (on the 18 V
engines, 9.0) are not flat top. They are shallow dish. A real flat top gives
you something toward 10:1 (I haven't bothered digging out my books to check
exactly) and I thought this was what you meant when you said flat. The
combustion chamber on the early heads is around 42-43 cc, BTW.
The original HRG heads came in 4 different versions. The Mark 3 was suited to
MGB engines and did have a slightly smaller chamber, but being alloy they
were more tolerant of heat and compression. A street ratio of 9.6 resulted,
without change in pistons, so you would only use flat top pistons if you
wanted to raise it further.
I have no idea what chamber volume the reproduction x-flow heads are using,
and you should check with the manufacturer before choosing pistons.
If you _do_ want to raise the compression for a bit more snap, some head work
(unless you use a x-flow) and a different cam are mandatory, or you are
wasting most of your effort.
Bill
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