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.

[Tom_T]

CR is the key to HP (& TQ too), cuz it's thicker air! (IMG:style_emoticons/default/biggrin.gif) ... pun on thinair!
... that's why modern cars with turbos are all running premium fuel (excl. turbo diesel of course).

I have a good article on it somewhere that gives the CR vs. Octane reqts. that I'll try to find, but you can get a good idea by looking at new car specs for which CRs require premium vs. those for regular or mid-grade..

IIRC 8.0:1 was about the max you could run the 1970's unleaded regular on - & remember that unleaded initially ONLY came in regular, until they fully switched over to unleaded only at all grades in the 80's. The problem could come with knocking/pinging in those areas where regular can be as low as 85 (vs. 87), then maybe bump to their mid-grade 88-89 in those areas if it pings, or premium 90-91 if it still pings.

Pinging/knocking is extra bad for our aircooled/oilcooled Type IV engines, cuz it makes them run hotter.

BTW - the 2.0's & 1.7s/1.8s were made 7.6:1 for CA smog regs of the day, which now you can tweak in other ways & still keep within smog limits & higher CR (although 75 & earlier are smog exempt in CA). Porsche/VW only wanted one 2.0 for all 50 states, so all USA bound GAs had the 7.6:1 CR heads, vs. the 8.0 "Euro" heads for the rest of the world GB motors, but probably 60% of the 2.0s went to the US market.

[Bleyseng]

CR is the key to HP (& TQ too), cuz it's thicker air! (IMG:style_emoticons/default/biggrin.gif) ... pun on thinair!
... that's why modern cars with turbos are all running premium fuel (excl. turbo diesel of course).

I have a good article on it somewhere that gives the CR vs. Octane reqts. that I'll try to find, but you can get a good idea by looking at new car specs for which CRs require premium vs. those for regular or mid-grade..

IIRC 8.0:1 was about the max you could run the 1970's unleaded regular on - & remember that unleaded initially ONLY came in regular, until they fully switched over to unleaded only at all grades in the 80's. The problem could come with knocking/pinging in those areas where regular can be as low as 85 (vs. 87), then maybe bump to their mid-grade 88-89 in those areas if it pings, or premium 90-91 if it still pings.

Pinging/knocking is extra bad for our aircooled/oilcooled Type IV engines, cuz it makes them run hotter.

BTW - the 2.0's & 1.7s/1.8s were made 7.6:1 for CA smog regs of the day, which now you can tweak in other ways & still keep within smog limits & higher CR (although 75 & earlier are smog exempt in CA). Porsche/VW only wanted one 2.0 for all 50 states, so all USA bound GAs had the 7.6:1 CR heads, vs. the 8.0 "Euro" heads for the rest of the world GB motors, but probably 60% of the 2.0s went to the US market.

Nah, the 2.0L heads are all the same! To get the 7.6 CR Porsche supplied the GA and GC motors with dished pistons vs the GB flat topped pistons.
"W" and "EA" 1.7L motors had a CR of 8.2 to 1 to get that 80hp.

I am running 9 to 1CR and use Premium or Plus when I feel cheap and it pings on regular.