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.

It just occurred to me that perhaps the highest CR is dependent upon the fuel delivery mechanism. In my case, I'm running stock L-Jet FI on that 2.0L.

too many variables to give a short answer.

combustion chamber design,digital f/i and ignition, and modern cam profiles have up'd the c/r per octane on fuel.

that's the best i can give you as far as short answers

[jake will say "it's all in the combo"!]

CR is the key to HP (& TQ too), cuz it's thicker air! ... 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.

Pump gas means only that it came from a commercial pump. It could have been 93 octane (or whatever Premium is in GA).

The alternative is race gas or AVgas, both very high octane fuels.

My 914 engine has 9.2:1 CR and will ping if I put regular in it. Runs like A champ with Premium. My Honda Fit has a 10.4:1 CR and runs like a champ on 87 and kinda doggy with Premium. It's all tuning.

Zach

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.

The bolts in the cases can only take so much..

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.

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

Everything.

[EDIT: I just realized that I'm once again doing a bit of a OT thread hijack, given the original question. My bad ] My experience has been (with my 11:1 CR 968) that altitude (and thus lower mass of oxygen per given volume) is a prime factor as well. I've run very successfully on mid-octane and occasionally lower while motoring around from 5200 ft - 10,000 ft.

Since you live at about the same average elevation than I live and work in Ernie, I would bet that (since both of our motors are NA) that your motor would run well enough on lower octane gasoline ... but then again, since you're carb'd I dunno. I know the computer in my 968 probably dials back the fuel mixture given what the MAF and O2 sensors are reading at high altitude, but your carbs...

CARB'D??? Never! Factory 2.0L D-Jet FI

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.

Click to view attachment..and then add in some un-natural aspiration (boost) and compression can go to nirvana
Marty

Of course, it says so right in your signature ... damn! I'm going batty. My apologies.

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.




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

Great information. Thanks.

"Aggressive cam grinds do indeed go hand in hand with high (static) compression ratios. "

Yes, I think that's a better way of describing it.

I know this is somewhat sophomoric but I am still amazed by the idea that when valves stay open longer the dangers of high static cr are mitigated by a reduced dynamic cr, while the corollary effect is that many advantages of high compression are thereby retained; and, that it even decreases certain engine stresses like CHT. If so, does the concept fit the needs of Ernie, or anyone else who plans to open their engine case and is willing to pay to upgrade the moving parts and a little machine work? In other words, with today's technology, why would anyone ever build an engine with stock (static) cr?

Not planning to change engines in the near term, these are rhetorical questions to me, but clearly relevant to ThinAir's OP.

Compression ratio is directly related to environment and intended use. set it at 10:1 and talk to me after you've driven I-5 south from Sac to LA in mid-day August. Here in So Cal with beach, mountain and desert conditions, using my driving habits as base and allowing for dynamic correction I would never advise a client to cam and compress for more than 9:1
As well for amature air cooled road racing purpose the higher the (actual) compression the more restricted the use, fact is in this venue while staying within mean racing environmental conditions expect severely diminished returns when exceeding 12:1 compression.

I'm not planning on an engine build anytime soon, but I'm always curious to understand the principles involved for when that time comes.

In may case, I "designed" an engine that would be pretty much a stock 2.0L that runs factory FI. What I changed was the use of flat-top Euro pistons and an Elgin cam that is more "performance" than stock, but made for factory FI. I calculated the deck height to give me 8:1 CR. I run regular unleaded fuel and it seems to do well.

Although I'm pretty satisfied with what I've got, I always wonder about some of the decisions I made and I would likely buy something next time from a builder like Jake or McMark. My questions about CR are to help me better understand how to evaluate options when the next time comes around.