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]

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.

I've been told that power = CR. We all know that higher CR's can mean dangerous premature combustion, knocking-pinging-detonation, blah blah.

Lots of general discussion of CR/octane/premature combustion (AKA "knocking, Pinging and detonation" related but not exactly the same) on some of the AC VW forums (I'm thinking of the name of a Latino dance...) Many drag racers there who really "get into" HP.

Too easy to say that CR/Octane/Pre-combustion "depends on a lot of factors." Obviously. The details are an interesting subject for anyone with an air-cooled or boosted engine. Higher temps and more air pressure increased chance for premature combustion. FYI, The demise of mass production of air cooled automotive engines (South America 2004) came about because of the increase in of bio-fuel mix that require higher compression. (octane of Alcohol is high) And this is the short answer to your query: compression = heat.

I will add that many people think of CR in terms of dimensions (Stroke x Comb. chamber volume x piston diameter) while the various charts CR/Octane/combustion are scaled on actual CR. Actual or "dynamic" CR is simply a formula that takes into account the degrees of the camshaft lobes, i.e., the valve timing, i.e. rocker angle of a particular engine, as well as its dimensions.

Here's a link to help formulate actual CR: http://www.kb-silvolite.com/calc.php?action=comp

From what I can tell, the rule that 87 gas is limited to 8:1 or less CR comes from a certain interpretation of a 40 year old chart on CR/Octane/detonation by early VW air cooled performance engineer, Gene Berg. This chart is in wide distribution and indicates much lower detonation thresholds at certain CR than generally discussed.

There are some who say, for example, that they have a 10:1 CR that "does fine" on 87 octane gasoline. They may have calculated the CR based on stroke and volume alone and that number is usually higher than the actual CR.

Reaching the end of any knowledge on the subject, I'll note that efficiencies in chamber design, porting, ignition and fuel atomization are means to manipulate the octane limitations of CR.