[flash]

if they are too high, you can get detonation

also, carbon build up does not allow full and even combustion of the fuel

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[Cloud9...68]

Plus it's really a false indicator of high compression numbers. In other words, a high compression reading should be a result of excellent sealing of the rings against the cylinders, and of the valves against their seats. Carbon build-up in the combustion chambers could be serving as an "extra factor" that's driving up your numbers, and it could be masking the true sealing performance of the rings and valves. But the consistency between cylinders is a good sign, so hopefully your numbers will still be good after a Run-Rite (or similar) treatment.

Another option would be to do a leak-down test. This test charges your cylinders with gas (just air, I think), and measures the rate at which the pressure drops. So, in a case like yours, it could help separate ring & valve sealing performance from possible carbon build-up. But in any case, if you've never done a good manifold/combustion chamber clean-up, it would be a good idea to go ahead and do one. Good luck, and let us know what you find.

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[Nikoror]

just a note about compression ratio (actual static compression ratio) and efficiency: the relationship is very far from linear, so a 10% drop is nowhere near a 10% loss of power (and a 10% increase is nowhere near a 10% gain)

The formula for Otto cycle thermal efficiency is eta = 1 - 1/(r^(gamma-1)) where eta = thermal efficiency, r = static compression ratio, gamma = ratio of specific heats (for air ~1.4).

So a 10:1 engine has 1-1/10^0.4 = 0.6019% theoretical max efficiency while a 11:1 engine has a 61.678% theoretical max efficiency, so a 10% increase gives only a 2.47% gain in efficiency (respectively a 10% decrease gives a 2.41% drop)



this is also the reason why boosted cars don't care about compression ratio as much as they care about boost pressure (where the relationship is much closer to linear)

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[Cloud9...68]

That's correct; thanks for pointing it out. I suspect 968s with high mileage, unless there's something really wrong with the engine, aren't actually down on power a whole lot vs. when they were new. As evidence, most people who have been forced to have at least top end rebuilds (for things like broken belts) haven't noticed much, if any, by way of improved performance. Also, when car magazines do long terms tests on cars, and check acceleration at the end of the test period, they're usually as fast as, if not faster than, when they were new. I recognize that this is typically after around 60K miles, not the 100K+ that so many of our cars have, but I can tell from my own experience in rebuilding my engine at 116K miles that the valve seats were almost as good as new, and the cylinder walls showed essentially zero wear (not sure about the rings, though), but I suspect they were fine as well, as rings aren't known to wear very fast on these engines If you take good care of them, these engines can go a long way.



Unfortunately, they do have a weak link in the head gasket, but if you're really pinched for cash, and are handy with a wrench, yanking the head at around 80-100K, and slapping in a new gasket, and nothing else, should carry you to upwards of 200K if you take good care of it, imho.

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[flash]

everyone knows i love the math, but all the thermal efficiency theory in the world doesn't add up when you actually dyno the car - a tight motor will have about 200 at the wheels - a soggier one at 100k miles will have about 187 - do the math there - if your cylinders are at 185, you'll only be putting out about 190hp at the wheels - that's a 5% drop in power for a 5% drop in compression - don't ask me why it does that, but it does, math theory or not - it may be coincidental other wear, but they do seem to go hand in hand - also, torque is affected more than horsepower, but i don't have those numbers handy



as for boosted cars and not caring about compression ratio, that's not right either - just a half point in compression drop can buy you 2-3 lbs of boost if you're near the edge already - that's a lot

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[Cloud9...68]

So you actually have comparative dyno results on cars with fresh, tight engines, vs. more tired ones? I guess that's pretty definitive, other than the usual uber-painful discussions about dyno comparisons. My car was dyno'd by the previous owner at about 80K, with a claimed 213 rwhp and 204 lb ft-lb, but who knows what kind of dyno, ambient conditions, phase of the moon, yada yada yada. My most recent compression at 116K miles had three of the cylinders at 185, and one at 190, which sounds pretty average. There are an awful lot of variables to consider when talking about engine longevity, for sure.

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[flash]

yes - somewhere around here in the pile of data i've accumulated



213 is actually pretty good for a 100k engine with a chip and airbox - so are your compression numbers - i've seen lower for sure - on a fresh motor, you should see over 220 rwhp with the racer x and airbox (discounting of course all the dyno difference hubbub) - fresh motor compression numbers should be 200 or better

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