quick compression Q

[MrK]

My compression reading is around 215psi. Is there a way to work out what CR(compression ratio) that is?

[RobertK]

For a 325i that's kinda high.. of course you have a euro 325i and they had higher compression ratios then the NA 325is IIRC. Unfortunately I'm not sure how much higher.

[MrK]

Yeah and on top of that, I can't just look up the CR numbers on a euro-spec 325i, since mine has different pistons on it than stock...

[SpecM]

-Did you build the engine? Should've measured while you were in there...

-Give us some details about your engine.

[MrK]

All I can tell you is that the only thing changed on the bottom end are the pistons themselves. I hardly have time to do any work on my own car, so I have a friend of mine who's a bmw mech that does most of these things for me. He put in these pistons, and I can't reach him to ask him any questions, cuz he's on vacation. I thought there was a direct relationship between PSI and CR (given identical cylinder bore and rod length)? For example, if engine A read 160 PSI, then it's a 9.0/1 CR, and if engine B (same model engine) read 200 PSI, then it's a 10.3/1 CR etc etc... isn't there something like that?

[Brew]

I thought there was a direct relationship between PSI and CR (given identical cylinder bore and rod length)? For example, if engine A read 160 PSI, then it's a 9.0/1 CR, and if engine B (same model engine) read 200 PSI, then it's a 10.3/1 CR etc etc... isn't there something like that?

Nope. Only way to get the true compression ration is to measure the volume of the combustion chamber at BDC and at TDC.

[madjurgen]

Nope. Only way to get the true compression ration is to measure the volume of the combustion chamber at BDC and at TDC.

How do they do that btw?

[Brew]

How do they do that btw?

I'm not sure if its possible with a built engine.
One would need to very accurately measure the volume of the combustion chamber, the cylinder volume at BDC, and then take into account the thickness of the headgasket, any dish, dome, reliefs etc in the piston and the deck height of the piston. Obviously some very precision measuring.

[madjurgen]

Now that I think about it you could get a pretty good measurement using simple methods at home.

[MrK]

Update: According to the Bosch Automotive Handbook you CAN get an approximate relationship between crank compression and compression ratio.

Link: http://www.type2.com/library/engineg/comrat.htm

Basically it says that "Compression = ((CR-1) to the power of about 1.15) x 14.7"

(14.7 is atmospheric pressure at sea level)

So this answers my question. Funny thing is, I'm running a stroker right now, and I shaved my block, so my compression ratio is different, but this means there IS a way to figure out one from the other.

For example, right now I've dialed my cam a few degrees back to reduce crank pressure until I can tune my engine properly to prevent pinging. My current compression reads around 160psi...

((X-1)^1.15) * 14.7 = 160, therefor X= 9.0 CR (approximately)

[Brew]

Straight from that link:

For the most part though, knowing the compression will tell you very little about CR (as the table above shows), especially if you measure a compression of 110psi (could be a CR of anywhere from 6.5-7.3:1)

The fact of the matter is, you can take a bone stock m20, with perfect compression to factory specs, and put in a schrick 306 race cam and totally change the compression numbers. That formula has no way to account for cam timing and duration.

Honestly, I wouldn't get too worked up about compression ratio. As long as your compression is even across all 6 cylinders, and you have good leakdown, and you have the ability to tune out detonation, you're all set.

[MrK]

Brew,

I'm not saying that you were wrong when you said that it's difficult to measure CR if the engine has already been built. I'm just giving those ppl that don't really feel like taking their engines apart a way to calculate an approximate CR by simply using crank compression and this equation.

[Brew]

I realize that, I just wanted to make it clear that this formula is very approximate and that compression ratio really doesn't matter all that much as long as you make power without any detonation.
Its definitely an interesting formula, but it doesn't take into account many of the variables that effect cranking compression.