An analysis of cranking compression numbers!

Post by **QuinGarcia** » Mon Mar 17, 2003 9:14 pm

Hello, I have a stock sr20det on the dyno right now with about 45,000 miles on it. I just bought the motor. I ran a compression test and got these numbers: Cylinder #1: 155 psi , Cylinder #2: 131psi, Cylinder #3: 127psi, Cylinder #4: 145psi.Heres how I ran the test, for those of you that are skeptical.1) warm up engine2) Hold throttle all the way open3) pull plugs4) Install compression gauge5) Crank engine for 5-7 "puffs" on each cylinder6) read gauge

I also ran a cold compression test and got similar numbers. And after i ran this cold test i poured a capful of oil into each cylinder, then tested it again, Cylinders 2 and 3 improved about 5 psi, while 1 and 4 improved by about 15 psi. I did the oil thing cuz it's a test to see if the rings are bad. If the numbers improve a lot after the oil is added, then it's rings, right? I wonder if my improvement is large enough to be sure that the rings are bad.

The service manual says that 155 is good, and 128 is minimum, and I have heard that you want no more than 10% variance between cylinders. Well, my middle cylinders are on that borderline, and I definitely have more than 10% variance. I wonder if I overheated the engine on the dyno without knowing and blew the headgasket? But I have not found any coolant in the oil, or oil in the coolant, and the engine is not exhausting lots of steam or anything. However the engine ocassinoally burnsa little bit of oil under certain operating circumstances (not yet determined exactly which situation, but i think after we rev it, right when it starts decelerating again)

Another thought: In the process of troubleshooting my new fuel injection setup (Electromotive TEC-3) I have run excessively rich during certain rpms for a couple days now(only about two hours of total running time on the dyno). I was thinking that this excessively rich condition could cause oil washdown on my cylinder walls and ruin my rings (I've heard of this happening on some engines before). But I question if I ran ran it long enough to really trash my rings. So, SR20DET experts, tell me what ya think. Is the compression too low? If so, what might be the problem and how do I pinpoint it? Could the headgasket be blown with regards to compression, but still isolate the oil and coolant from eachother? Is the washdown theory viable? Could my engine wear out the rings after 45,000 miles and a couple hours of crappy running on the dyno? Gimme everything situation you think is possible, and your advice!! I really dont want to pull this thing apart if it doesnt need it, and I dont want to rebuild if it doesnt need it.

Website · Post by **the_invisible** » Mon Mar 17, 2003 9:14 pm

It is not unusual to see the middle two cylinders with lower compressions. But with the 10% variances, something could be wrong. Also, I was told that the numbers don't really matter. What matters is that the variances across the cylinders are low.

If the motor consumes oil, then it could possibly be the rings.

BTW, what do you mean by overheating the motor on the dyno? Wasn't it connected to the radiator during the dyno?

Post by **GodzillaFan** » Mon Mar 17, 2003 9:14 pm

-Is the compression too low? I think so.-If so, what might be the problem and how do I pinpoint it? you just did. (get a leakdown test...) -Could the headgasket be blown with regards to compression, but still isolate the oil and coolant from eachother? Possible - but there can still be coolant getting in the oil - just not in a visible quantity, oil analysis can help with that.- Is the washdown theory viable? doubt it -Could my engine wear out the rings after 45,000 miles and a couple hours of crappy running on the dyno? miles are a poor indicator of engine condition, when you dont know the motors history.you should have done the compression test before you started to tune the motor...

Post by **QuinGarcia** » Mon Mar 17, 2003 9:14 pm

Invisible, yes the engine was connected to a radiator while on the dyno, but unbeknownst to us, this dyno was plumbed incorrectly, and our autometer gauge was reading incorrectly, so we didnt know that the engine was too hot. We've now rectified that problem. Why is it normal for the middle cylinders to have lower compression??Godzilla, so a leakdown test is just a more accurate version of a compression test, right?? It still is not going to tell me what is causing the cylinder to leak down, right? I could have 50% leakdown, but I wouldnt know if it was the valves, rings, or headgasket? How would I tell which one it is? Listen near the headgasket for rushing air? Open the throttle body and listen by the intake for air rushing back into the intake plenum? Listen for rushing air by the exhaust manifold? During my leakdown test, at what percentage do I say, "Ok, this motor's screwed, lets buy a new one".

Post by **lagvoid** » Mon Mar 17, 2003 9:14 pm

Sorry to hear your troubles, but do you mind warning others that are in the market for sr engines who to avoid?

Post by **QuinGarcia** » Mon Mar 17, 2003 9:14 pm

I don't think it's right to say who I bought the engine from because I am not sure that the engine is fully screwed and the company I bought it from only guaranteed the fact that it would run, not run well.

Post by **I H8 UR DSM** » Mon Mar 17, 2003 9:14 pm

Post by **curbsurfer** » Mon Mar 17, 2003 9:14 pm

sounds like you have a very small headgasket leak between cyl. 2&3. it happens on DSM's all the time. overheating will cause this. if you can, go to napa and buy the kit that checks for exhaust fumes in the radiator. if it fails, then you headgasket is bad.

Post by **QuinGarcia** » Mon Mar 17, 2003 9:14 pm

So, we ran a leakdown test and saw that the exhaust valves on cylinders #2 and #3 were leaking (we heard rushing air out of the exhaust manifold for both cylinders. So that means we must have burnt the valves on cyls #2, and 3. So, we apparently need a valve job, and to replace the valves on those cylinders. But we are going to buy another engine while this one is being repaired. We called Heavy throttle and Venus Auto parts and they have a waiting list. Who has an SR20det in great shape that we could buy right now? As far as repairing the old engine goes, I'm just going to pull the head and have a valve job done and replace the burnt exhaust valves (perhaps even all the exaust valves). Where can I find the exhaust valves for my engine? Are they sodium-filled? Where can I buy valve guide seals for our current engine?? thanx

Website · Post by **the_invisible** » Mon Mar 17, 2003 9:14 pm

Hey, it looks like the overheating didn't damage your engine. It's just that the engine was already like that when you bought it.

Sounds like your engine wasn't expensive at all, maybe this can be your excuse to rebuild the valvetrain and change the headgasket as well. You can get new valve, springs, and rocker arms from phase2 or HT, both of them are selling them for reasonable price. Maybe upgrade the cams and cam gears as well?

Post by **QuinGarcia** » Mon Mar 17, 2003 9:14 pm

No, the engine was actually pretty expensive $2000+, wejust need to have a running engine asap. I will probably replace the valves, headgasket, valve guide seals, and get a valve job done on the sealing surfaces, and check the deck surfaces to be true. If the cam, sporckets, and valvesprings arent too expensive, I might get them too. But I need this engine to run as smooth and clean as stock, so I dont want to make any upgrades that might jeopardize that.

Website · Post by **the_invisible** » Mon Mar 17, 2003 9:14 pm

If you want your engine to run as smooth and quiet as stock, do not get the performance cams. You can also skip the performance cams also as they are a pain to play with and tune. They mess up the timing if not tuned properly. A thicker headgasket, I heard, may also make the idle a little rough. 1.1mm is good,

If you want to fix your problem and smooth idle, get new valves, valve guide seals, and rocker arms and springs. A 1.1mm head gasket is also good. This can fix your problem and will make your top end much more reliable at higher boost and higher RPM.

Post by **QuinGarcia** » Mon Mar 17, 2003 9:14 pm

What is the thickness of the stock headgasket? Why do you suggest running a 1.1mm gasket?

Post by **lagvoid** » Mon Mar 17, 2003 9:14 pm

Someone can correct me on this, but I believe the stock headgasket is 1.2mm thick.

Website · Post by **the_invisible** » Mon Mar 17, 2003 9:14 pm

The stock gasket cannot be 1.2mm. It is a little over 1mm. Thicker gaskets lower the compression ratio and allow for higher boost.

I suggest 1.1mm because it should allow more boost than stock gasket and shouldn't make you loose too much power with stock boost. Also, the thicker gasket tends to create rougher idle, or so I heard.

Post by **lagvoid** » Mon Mar 17, 2003 9:14 pm

I was seaching through old posts and came across this onehttp://nicoclub.com/zerothread?id=16220&highlight=head+gasket

Thats where I came up with the 1.2mm being stock, dauntt always seems to know what hes talking about.

Website · Post by **the_invisible** » Mon Mar 17, 2003 9:14 pm

Apex'i makes 1.1mm headgasket for the SR20DET. If the stock headgasket is 1.2mm, then Apexi's increasing the compression ratio on the SR20DET.

Post by **lagvoid** » Mon Mar 17, 2003 9:14 pm

Well, Apex'i makes a few different head gaskets for the sr20det. Different sizes to lower or higher compression, your choice. Phase2 sell the 1.1mm head gasket while heavy throttle sells a .8mm and 1.5mm head gasket.

Website · Post by **the_invisible** » Mon Mar 17, 2003 9:14 pm

Oh that's good to know!!! More choices is always better!

An analysis of cranking compression numbers!

An analysis of cranking compression numbers!

Re: An analysis of cranking compression numbers!

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