David (listers),
I think the question you need to ask is: what is the optimal Cranking
cylinder pressure? As I recall back in the early seventies there was a
lot of talk about how to extract the most energy from a specific amount
of fuel. Part of it was the dynamics of actually getting it into the
chamber and the rest was about how much compression you needed. The Big
three in the eighties explored the limits of how little compression you
could get by with to limit pollutants and along comes competition cams
with a cool idea lets close the intake valve sooner on the 8.5:1 motors
and see what happens. Instantly the cranking compression went from about
110 - 120lbs to 150lbs and it REALLY woke up those old low compression
engines. You may remember the cams series called Comp Cams Hi-Energy
cams. This little bit of trivia helps explain some of the effects that
you would see if you put a mild cam into a high compression motor. If a
cam is designed to generate 150-160lbs of pressure in the cylinder
cranking at a specific valve closing timing event then if you had a high
compression piston with a small chamber head such that a 12.0:1 ratio
was used the net effect would be to create cranking cylinder pressures
of 225 lbs or higher.
Old super stock racers used to use the magic number of 195lbs cylinder
pressure as the cranking pressure target for gasoline. If you were
creating more pressure than that you were doing two things that were not
in your best interest: 1 using unnecessary horsepower to continually
increase the pressures in the cylinder and 2 that increased load was
showing up in extra heat that you now need to get rid of. Of course
this is discussing static (cranking) cylinder pressures and running an
engine at RPM (dynamic) creates a completely different model
environment.
The real reason for extra compression is for higher RPM engines as the
operating RPM increases the valve events start to run into physical
limits and a cam needs to start the events sooner and have them occur
for a longer period of time (increased Duration). Due to this
overlapping of events some cylinder pressure is leaked off of these high
RPM cam profiles and the extra compression ratio over 10.0:1 is a way of
compensating for it.
So the short answer is to optimize a particular combination shoot for
185-195 cranking cylinder pressure with the components installed. At
least in my opinion.
Best Regards,
Tim Ronak
Business Development Manager
Akzo Nobel Coatings
23961 Via El Rocio
Mission Viejo, CA 92691
Off: (949) 305-5393
Fx: (425) 955-6268
Cell: (949) 289-3357
Business email: timothy.ronak@crna.akzonobel.com
Personal email: timronak@cox.net
VM: (800) 234-6747 ext. 2257#
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