Trevor is right.
Preignition can be a very serious problem with higher
compression
ratios and less than adequate octane levels. I've rebuilt engines that
needed pistons
because of this. Several Ford inline sixes ( 300 cid ) for example. These
had broken
ring lands and scored the walls of the cylinders. In each case the owners
used regular
and heard pinging for a while before I got the engine ( these engines
were designed for regular )
What can we do ? ( I want to build a higher compression motor
too )
First, make sure that any porting work includes matching the combustion
chambers
for size. Open them all up to the largest...after it's been smoothed out.
That will slightly
lower the comp. ratio and the smoothing off any edges helps quite a
bit--hot spots
are big problems ! Same with the piston tops, if there's any edges.
Compression ratio is a measurement of volume.
Cranking compression is different and it's what we work with.
Cranking compression
can be affected by altitude ( Atmospheric pressure at sea level = 14.7
and at 5000 ft. = 11.8 )
It's also affected by cam overlap. A narrow overlap bleeds off some
compression and
adds H.P. on top end. R.V. type cams have a wider lobe angle and pump
up compression slightly
as well as low end torque.
Ignition timing can play a big factor too Too much initial
timing builds heat fast.
I've retarded the timing on a number of pinging engines and the problem
stopped.
( temporary solution at best )
Ambient temperature plays a factor too. A motor that's fine on a crisp
fall day can run into
trouble on a hot summer day !
Octane booster can help, but you may not always have it in
the car when you need it.
( No booster is available in Resume Speed , Iowa for example...so your
engine will
self-destruct when you reach St. Louis. That's Ok because we all know the
Missouri loves company )
Obviously, this is just the tip of the ice burg on this topic.
Do your research before building your motor, as trying to fix it later
may not help enough.
Another thought....where will octane rating be next year. Will High Test
be available ?
Oh for those thrilling days of yesteryear when Sonoco Blue was available
on every other corner
at 102 octane and under $0.50 a gallon ( I never had a problem with my
12.5 comp. 302 Chevy with
the first design Trans Am road race cam and dual quads, even on the
hottest day :-)
Happy Motoring, Greg
On Wed, 12 May 2004 06:46:54 -0400 <trevor_easton@dofasco.ca> writes:
> David wrote
>
> James et all,
>
> A 10.4 to 1 C R is all fine to talk about, but how does one
> stop
> the dieseling after shutoff when using carburetors????
>
>
> My worry would not be so much dieseling as the preignition and
> subsequent damage during normal running. You can't always hear the
> pinking/pinging. Water injection maybe?
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