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.

[Dave_Darling]

By default, when people talk about compression ratio they are talking static compression. That is, the volume available in one cylinder when it is at BDC versus the volume at TDC. Usually, when people talk "dynamic compression", they refer to it as just that.

A higher compression ratio means you're squeezing the intake charge more, which means it pushes harder when you light it on fire and it expands. That is where the higher torque and the higher power come from.

If you squeeze the charge too hard, it will tend to light itself on fire in part from the heat caused by squeezing. This happens unevenly, in an uncontrolled fashion, and generally when you don't want it to happen. That will be audible as "pinging", and it is bad for your motor. It creates even more heat, and pressure when and where you don't really want it. I've seen pistons with holes blown through them from pinging...

Lots of other things influence if and when the mixture self-ignites. These vary from the presence of hot spots in the combustion chamber, to the shape of the chamber, to the exact mixture of air and fuel, to the atomization of the fuel that is in there. And on and on and on. And fuels with a higher octane rating will tend to resist pre-ignition better than those with a lower rating.

Modern cars tend to have better designed combustion chambers, heads and pistons that cool more evenly, excellent atomization, and so on, and so they are generally more resistant to pre-ignition than old outdated cars like.... Well, like ours. Modern engine management systems also can detect pinging, and they can tweak the ignition timing and the fuel mixture to prevent it. All of which means that modern cars can run safely and happily with a much higher (static) compression ratio than our old aircooled cars can.

--DD