Tom,
I agree with your statement that the combustion chamber shape affects
how the fuel combusts and the resulting effect on octane requirements.
However, we were not discussing changes to the combustion chamber but
rather shaving the head. Pop-up pistons should be designed (note I say
"Should") to provide the best combustion for the head that they were
designed to work with. But regardless of which method is used to create
12:1 CR, I think you will have to agree that there is no way that one
could run that ratio on normal pump fuel.
On my thoughts about shaving versus pistons, since parts for our cars
are getting more scarce over time, I try to do as little removal of
metal as I can get away with. Since new pistons are still being
produced, and you can't "add" height to heads, I would rather not remove
metal from either heads of the top of the block except as necessary to
provide a level surface.
Regards,
Joe
Tomislav Marincic wrote:
> Sorry, Joe, for once I disagree with you. Combustion chamber shape
> plays
> a tremendous role in how well a given compression ratio stands up to a
> given octane
> fuel. Flame propagation varies quite a bit with shape and "squish area"
> which is part
> of the reason you can buy a modern high-performance motorcycle with 11:1 CR
> or more
> and run it on pump gas with no problems.
>
> In the case of the Spitfire, I think (but cannot prove) that it
> would be far
> better to get dished pistons (from the 7.5:1 cars) and shave the head to
> achieve a given
> CR than to install the flat-top pistons and shave the head less. I'm
> familiar with your 250
> psi compression horror story, but I ran my old MG Midget 1500 (Spit engine)
> with a
> radically shaved head and no problems on premium unleaded except dieseling
> after shutoff.
>
> Best Regards,
>
> Tom Marincic
--
"If you can't excel with talent, triumph with effort."
-- Dave Weinbaum in National Enquirer
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