Hi Brian,
I emailed Mike Dowley on this subject a few weeks ago, regarding my N
crank, -.030 on the big ends, -.020 on the mains. I told him I'd crack
tested it.
His reply:
"Cyl.Head Reface to 3.19/32" to give 6.7:1 C.R (remove 3/64")
3.35/64" to give 7.2:1 C.R. (remove 3/32")
Personally I would go for option 1 bearing in mind you have an almost
70 year old crankshaft - albeit uncracked."
Colin Campbell's analysis (my summary):
The big end bearing load at tdc on the WORKING stroke increases with
compression ratio much more rapidly than does the C.R. However the
inertia load at tdc for the NON-WORKING stroke at higher rpm
(4000+?) is much higher than the WORKING stroke bearing load at tdc for
most engines with stroke/bore ratios of 1.25 and more. See Colin
Campbell's "The Sports Car - Its Design & Performance", editions 2 or 3,
about pp 84 & 98 respectively, not in edns 1 or 4).
An MMM engine's S/B ratio is between 1.25 or 1.46.
Therefore, the increased stress on our engines due to increased CR may
not be as critical as might be supposed at first thought.
As long as you have good exhaust valve inserts, Bill Putnam's suggestion
of a blower seems the way to go.
(Bob Rich, I'm sure you agree).
Cheers ... Pat
Bill Putnam wrote:
>
> At Monday, 10 March 2003, you wrote:
>
> >MMMers,
> >
> >What are the restrictions on increasing the compression ratio on
> MMM engines.
> ...
> >All comments and advise appreciated.
> >
> >Brian Kelly.
>
> If you're looking for more power, I would suggest a blower as a more
> effective route. Less stress on the engine per added bhp vs. CR
> increase. Also, you'd need quite a bit of an increase in CR to give
> the powerband that even 5 psi boost will do.
>
> Bill Putnam, who, once he drove a MMM with a blower, became entirely
> addicted to them!
>
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