[mechanic17]

"Funny how the N/A internal combustion V-8 engine shows so many similarities following a few basic guidelines for setting them up, no matter what brand they are........Cliff"

In the article BruceWilkie linked to earlier (post #45), the author uses two examples of good vs. poor (for CR purposes) combustion chamber design, the Cadillac Northstar and the Hemi. I recall him saying the 2000 Caddy redesign allowed them to get up to 10-1 SCR on 87 octane (the second example I know of in this thread after LT1s.)

So it appears those more modern chambers that everyone begged Edelbrock for (in Pontiac heads) are also a big part of that ideal combo. Now we even have them coming into the market for the masses.

--so, I'll try a brief recap, as the thread grows, of responses to the original question:

I. Major factors/variables beyond SCR include:

-Dynamic compression ratio /cranking compression (as determined by cam events like LSA and IC--NOT overlap as is often assumed)

-RPM range the engine operates in (determined by gearing and driving purpose)

-Quench distance (pistons down the cylinder)

-head material (iron vs aluminum)

-cooling system efficiency

-fuel quality, in addition to octane

-combustion chamber design

I'm sure I missed some--did it from memory.


II. The major points of contention seem to be:

-Should a "safe" SCR (8-1? 9-1?) be observed by non-tuners, even those who read these forums and are trying to learn more about high performance? Is SCR even useful as a guideline, given the many other variables?

-Should camshaft LSA be wide or tight for a Pontiac, and for what purpose (part-throttle cruising? drag racing? dual-purpose? remembering that this is the "Street" section)?

-What is the useful limit of dynomometer data for street-perf vehicles?

I'll be happy to edit omissions/inaccuracies if they're pointed out.

[BruceWilkie]

Scratch LSA please! Its the intake closed point that determines how many degrees of crankshaft rotation you are in compression mode. Stroke and to some amount rod length determine distance in compression mode. Yes hot exhaust contaminating the intake charge can affect cylinder pressure too. However cylinder pressure is mostly affected by the intake closed point. Cylinder pressure affects compression temperature ultimately affecting octane requirement.
Real kicker is exact seat closing points arent advertised. In fact advertised seat timing varies from manufacturers often clouding things trying to compare one brand to another. Area under the curve and valve seat angles play a role here as well as valve timings and lift curves overall on both intake and exhaust.

LSA and ICL are coincidental centerlines of chosen valve timing events. Cams are NOT designed on predetermined LSA or ICL. (though looking at different brands shelf grinds you'd think so)

Originally Posted by mechanic17

Should camshaft LSA be wide or tight for a Pontiac, and for what purpose (part-throttle cruising? drag racing? dual-purpose? remembering that this is the "Street" section)?