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Compression Ratios


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Okay, the compression ratio for an EA82 engine is 1 to 9.5 This means that at TDC, the compression should be at 9.5 atmospheres of pressure.

 

My compression gague reads pressure in PSI, therefore I have to make a conversion. One atmosphere is equivalent to 14.7 PSI. Therefore if my engine's compression is perfect, I should be reading 9.5 x 14.7 = about 140 PSI?

 

Is this correct, or are there factors I am not considering?

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Compression checks don't really have much to do with compression ratio. It does a little, but compression ratio is a static measurement. Cylinder compression is a dynamic measurement. When your checking with a guage, your looking for consistency. The cylinders should all be within 10% of one another. Valve wear, ring wear, cam specs, cam timing, all have an impact on the cylinder compression but the compression ratio is still the same. As an engine gets worn the compression will decrease, but the ratio is the same. Look for the numbers to be similar, any dramatic difference is a sign of a problem. But 140 sounds good. Good luck.

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  DirtyMech said:
Okay, the compression ratio for an EA82 engine is 1 to 9.5 This means that at TDC, the compression should be at 9.5 atmospheres of pressure.

 

 

No, the compression ratio is a ratio of volumes, not pressures. Depending on the speed at which the compression occurs (this influences the amount of heat lost etc.), the pressure ratio will change. Also, the low pressure is not atmospheric, it is lower by the amount of intake vacuum.

 

You need to compare your compression readings with the values specefied in the fsm.

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Compression tests dont work like that, Valve timing determine how much of the piston volume is traped. Also you are not sampling one stroke but several strokes of the piston. you are loking at the maximum compression. the 9.5 to 1 is purely a mathmatical number.

 

Also compression testing, you are looking for a variation between cylinders. You can 4 cylinders test at 130 psi and be ok. you can have three at 160 and one at 120 and have a problem.

 

nipper

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Ahhh sweet, now it makes a little more sense. Once I put the engine back in my car I'm going to crank the engine via the starter while the carb and intake is still off and make sure the readings are consistent. Thank you all for the clarification.

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Just out of curiosty, i think a sooby engine can be done while on the bench, since the starter bolts to the engine and not the transmission itself, or am i worng. Hate to have you bolt it all together just to find a issue..

 

 

 

nipper

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Another compression ratio question -- I had an EA82 (SPFI, non-turbo) engine rebuilt. It came back with turbo heads (so the mechanic says, he says there's one fewer bolt holes for A/C struts on the turbo heads, and that it's not a problem).

 

Make sense? I think (hope) these are new heads on the otherwise rebuilt engine. But I thought the turbo engine has a different compression ratio and that the space in the heads was what made that difference.

 

What, me worry?

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There are a few ways to make differnt compression ratios. One way is the changing the swept volume using the piston stroke (shorter vs clonger connecting rods). Another is by milling the deck height of the cylinders, the third is by milling the the cylynder head decks. I dont know how which one subaru uses. TUrbo heads do breath a bit better then normal heads.

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  Hank Roberts said:
Another compression ratio question -- I had an EA82 (SPFI, non-turbo) engine rebuilt. It came back with turbo heads (so the mechanic says, he says there's one fewer bolt holes for A/C struts on the turbo heads, and that it's not a problem).

 

Make sense? I think (hope) these are new heads on the otherwise rebuilt engine. But I thought the turbo engine has a different compression ratio and that the space in the heads was what made that difference.

 

What, me worry?

 

You seriously need to consider a new mechanic. After all the stuff he fed you about your tranny.. Now this.

 

BTW, in an EA82, the static compression is influenced directly by the pistons. the volume of the heads is the same across the board.;)

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Now this is scary (I know, I've been scared for months).

 

You tell me the turbo heads won't bolt up to the SPFI.

 

The mechanic told me he's been out test-driving the car yesterday and the engine was fine but for that one bolt hole/strut difference.

 

It's SPFI. Maybe the heads are, oh, lordy lord lord lord, my head hurts.

 

I've got to get on the phone to the engine rebuilder tomorrow and ask what kind of heads they sent out and if this makes any sense at all.

 

 

(Today the car should've had front rotors replaced, which I believe it did need, based on the car's history -- they were as thin as allowable when I bought it, I knew they'd need replacing -- and I didn't know not to put the parking brake on hot rotors (!) til I read it here today).

 

Now it's the local mechanic who says it's a turbo head based on only missing one bolt hole for the A/C.

 

Could be it's not a turbo head -- but lacks a bolt hole for the A/C for some other reason? Is that possible?

 

Yeah, as soon as I get this away from them, I'll be back to looking for a Berkeley area Subaru mechanic.

 

"Older, wiser, and wallet lighter" as the saying goes.

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Yeah.. umm.. no..

 

If your car has turbo heads and an SPFI intake, I absolutely guarantee you it is not out romping around town on test drives ;)

 

The difference in the AC bracket? could be the heads are from a different year, from a car without AC, could be carb heads (pretty much interchangeable with SPFI heads), could be that the mechanic can't find his rump roast with two hands and a flashlight.

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