> I have the old cylinder heads with the big combustion chambers, and
> a newer 18V block with the larger piston domes, so my compression
> ratio is abysmally low. How can I figure out how much to deck the
> head? I have a steady supply of 93 octane pump gas, so I'd like to make
> the most of it.
>
> If it helps, the block is a 18V, low-compression model (18v585Z L1202),
> and I believe it's been overbored .030. The pistons that are in there now
> have a dish in them, but I don't know if they're the same dish size as
> what was originally in the engine.
>
> I think I'm looking somewhere in the range of .040 to .060 off the head.
> Can anyone help me narrow it down more?
Well ... determining the piston dish and the combustion chamber size
(head could have already been machined) is the key.
Assuming a +.030 overbore ...
Bore 3.19"
Stroke 3.5"
Radius 1.595"
Chamber Volume 43 cc <----- pre 18V head
Gasket Volume 3.3 cc
Unswept bore 4.58 cc
Dish 6.25 cc <---- you probably have a greater dish
Swept Vol 458.076 cc
UnSwept Vol 57.13 cc
Capacity 1832 cc
CR 9.01:1
You can remove material from the head to decrease the capacity of
the combustion chamber ...from "How to Power Tune MGB 4-Cylinder
Engines for Road and Track", by Peter Burgess, published in 1996:
"A general rule of thumb for skimming the B heads is that
10 thou (0.01 inch or 0.025mm) of metal removal equals
an approximate 1cc reduction of chamber volume. I must
repeat that this is not a hard and fast rule, due to the
previously mentioned variances between heads, but can
act as a guide to how much material to remove to achieve
the desired chamber volume."
... but there is a max that can be removed from the head so, IMHO,
you would be better off fitting pistons with will less dish ... 'flat tops'
would give ya approx 10:1 without modifing the head ... ;^)
> What's a good compression ratio to take full advantage of 93 octane
> gas?
Good question ... one that I can't really answer ... my guess is that its
between 9.5:1 and 10:1. Your mechanical/vacuum advance will play a
big part in this ... so if you are running the 18V dizzy you will wan to fit
an early model 'B' dizzy ...it has a more appropriate advance curve. Cam
timing (IOW, when the exhaust valve closes) also has an impact.
> Lastly, if I do it and it turns out that I took off too much and the engine
> pings on 93 gas, what can I do? Retard the timing?
>
... yep
> Double-gasket the cylinder head?
... yep
Safety Fast! ... larry.g.unger@lmco.com
'61 MGA 1600 MkII with 'flat tops'
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