[PRAY]

Just wondering if anyone has done a cranking compression test on their motor. Trying to see what others are getting. I got 240psi on all but one cylinder. I got 230psi on #5. I got 1-2% leak down on that cylinder. It is getting by the rings. I guess that one just didn't seal up well. I scoped that cylinder and the piston and walls look good. The rest of the cylinders didn't leak anywhere. Good on GM. The motor is sealed up tight.

[02fbody35th]

Just out of curiosity. How many miles were on the motor when you performed the test? Also what was your break in procedure?

[PRAY]

9,200 or so.

[SS 1LE]

Originally Posted by 02fbody35th

Just out of curiosity. How many miles were on the motor when you performed the test? Also what was your break in procedure?

Pretty sure he didn't baby it...

[PRAY]

Originally Posted by 02fbody35th

Just out of curiosity. How many miles were on the motor when you performed the test? Also what was your break in procedure?

I missed the rest of your question.

I broke it in like the rest of the engines I build except I should have drained the mobile 1 and used Joe Gibbs. I assume that these motors get some sort of run in at the factory and this motor had 32 miles when I got my hands on it. 90% of the break in happens in the first 20 minutes of engine life anyway. My procedure on any new motor I build is to fire it, get it to op temp and let it sit till it gets to ambient unless it has brand new springs. If new springs then I let it sit for the night. Then as soon as I am back in it and it is to temp I do progressive pulls to WOT. It usually takes about six or seven pulls. I apply different throttle inputs and rpm limits for each pull. But I leave the car in gear and allow compression braking to slow the car down to almost idle. Cylinder pressure seats the rings on accel and the vacuum created by leaving it in gear in on decal pulls fresh oil up into the cylinder walls flushing out any burnt oil, ring material and cylinder wall filings. Oil is all that seals a cylinder. So you have to file off the peaks while leaving the grooves free from debris. If you don't do that part then you get burnishing and you are screwed. This is also why you don't use synthetic oil for break in. It is to slick. Seating the rings is simply knocking off the high spots of the cross hatching left over after machining. There is only 6 to 8 lbs of outward spring pressure on rings. Cylinder pressure is what seats them. I drain the oil at 100 miles and again at 500 miles. I drive the car in the manner described above for the first 250 miles or so. No steady state and all accel and decal is done in a some what aggressive manner and left in gear. I don't use synthetic oil until I hit 2,500 miles. If I had a dyno I would do the break in on that.

However, with this car, I didn't take into consideration that the trans and rear were also brand new. I would never do a trans or rear break in like I would do an engine. I didn't sit back and think. I was concerned about sealing the motor.

I will have to do some cranking compression tests on other cars and some leak down to see how mine stacks up. I have read that what ever GM does with the motors first there is nothing you can do to them to help after you get it. The recommended GM break in procedures are for the trans and rear, not the motor. If you read them from that perspective then they make sense.

[s346k]

can anyone add to this? i tested the #2 cyl yesterday and got 275 lb. motor was dry and stone cold, been sitting for a week. 23k miles, bolt on/ethanol since 16k.

[laynlo15]

Man 275 is way higher then anyone I ever had. I don't think 10lbs is anything to worry about from 230 to 240. None of mine have ever been that high either.

[s346k]

interesting. why is mine so high? i will admit i only checked it because i had an... issue in #2. i did not check any of the other cylinders.

[lt4camaro]

Originally Posted by PRAY

I missed the rest of your question.

I broke it in like the rest of the engines I build except I should have drained the mobile 1 and used Joe Gibbs. I assume that these motors get some sort of run in at the factory and this motor had 32 miles when I got my hands on it. 90% of the break in happens in the first 20 minutes of engine life anyway. My procedure on any new motor I build is to fire it, get it to op temp and let it sit till it gets to ambient unless it has brand new springs. If new springs then I let it sit for the night. Then as soon as I am back in it and it is to temp I do progressive pulls to WOT. It usually takes about six or seven pulls. I apply different throttle inputs and rpm limits for each pull. But I leave the car in gear and allow compression braking to slow the car down to almost idle. Cylinder pressure seats the rings on accel and the vacuum created by leaving it in gear in on decal pulls fresh oil up into the cylinder walls flushing out any burnt oil, ring material and cylinder wall filings. Oil is all that seals a cylinder. So you have to file off the peaks while leaving the grooves free from debris. If you don't do that part then you get burnishing and you are screwed. This is also why you don't use synthetic oil for break in. It is to slick. Seating the rings is simply knocking off the high spots of the cross hatching left over after machining. There is only 6 to 8 lbs of outward spring pressure on rings. Cylinder pressure is what seats them. I drain the oil at 100 miles and again at 500 miles. I drive the car in the manner described above for the first 250 miles or so. No steady state and all accel and decal is done in a some what aggressive manner and left in gear. I don't use synthetic oil until I hit 2,500 miles. If I had a dyno I would do the break in on that.

However, with this car, I didn't take into consideration that the trans and rear were also brand new. I would never do a trans or rear break in like I would do an engine. I didn't sit back and think. I was concerned about sealing the motor.

I will have to do some cranking compression tests on other cars and some leak down to see how mine stacks up. I have read that what ever GM does with the motors first there is nothing you can do to them to help after you get it. The recommended GM break in procedures are for the trans and rear, not the motor. If you read them from that perspective then they make sense.

Do you drain the factory fill at 100 miles which is synthetic and put in what? I also do not like synthetic oil for break in, unless the factory fill is not really synthetic.

[Tim M]

275 seems like an error. I recall typical SBC numbers of 240 requiring race fuel. A good running street engine is 200-220.

Nonetheless, I think the test is most useful to see if all cylinders are similar vice atmospheric conditions/engine temps/throttle openning, etc.

Testing one cylinder tells you very little.

[s346k]

Originally Posted by Tim M

275 seems like an error. I recall typical SBC numbers of 240 requiring race fuel. A good running street engine is 200-220.

Nonetheless, I think the test is most useful to see if all cylinders are similar vice atmospheric conditions/engine temps/throttle openning, etc.

Testing one cylinder tells you very little.

i was testing it to see if it even had compression. i was simply looking for a pass/fail answer to tell me if the car could be driven home or not. i plan on checking a few cylinders next time i get out in the garage. i will report back when i do.