What's Compression Ratio got to do with it? Options

[ThinAir]

When I built my 2.0L I had the understanding that if you wanted to run Regular fuel the highest compression ratio you could have was 8:1. I don't have any idea now where that came from.

In the thread about his air-cooled event, Jake just posted this: "The 356 engine is a 2.4, it made 224HP @ 9:1 CR on pump gas."

Now I know that there is more to one of Jake's engines than just the CR, but it got me to wondering:
1. What is the maximum CR you can run with Regular fuel?
2. What is the relationship of CR to performance? Do you always get more HP by running higher CR if everything else is the same?

Understanding this concept might be helpful the next time someone builds an engine, buys one from Jake, or has one built for them by someone else.

[Prospectfarms]

I don't think its permitted to cite links to other forums but I wish I could for this thread.

The past ten years saw epic debates concerning cr and camshaft combinations. One argument was that that very high CR and overlapping valve timing are complementary. Is this what people reference as "squish?" There is an active member here that has compellingly supported that idea for a long time. A related concept is that an engine with moderate CR using a conservative cam grind will actually have a very high "dynamic CR," and risk detonation. In other words, ThinAir, don't fly-cut your heads very much unless you plan on changing camshafts.

Those older posts, many by a smart fellow named John Connelly, "Massive Type 4" and others, were so interesting and contained lot of very technical engine theory that was communicated in a way that anyone could understand. It seems that their ideas have become generally accepted now. All going to prove that any analysis of proper CR for a particular application requires evaluating the entire system -- as I have heard others in this forum repeat many times.

[Dave_Darling]

I don't think its permitted to cite links to other forums but I wish I could for this thread.

Sure, you can put links to other forums here! The only thing to be careful of is setting up an "us vs. them" sort of pissing match. And the danger of that is greatly reduced any more.

Is this what people reference as "squish?"

I believe that "squish" refers to areas around the edge of the cylinder where the piston leaves very little room between it and the cylinder head when at TDC. That tends to push all of the air and fuel into the middle of the chamber, around the spark plug. That tends to let you run leaner mixtures, and also I believe helps combat detonation.

Aggressive cam grinds do indeed go hand in hand with high (static) compression ratios. And yes, everything does work together as a combination.

--DD