Compression test readings - ATTN Rebuilders

[natenkiki2004]

I've been working on another car, doing tests to see how healthy the engine is overall and just finished compression testing it. This got me thinking about the compression test I did on my Explorer so I went back and looked at the readings. Some cylinders are low (20-25 PSI difference) but not horribly. Not bad for the mileage really.

My question though is that the readings seem high to me, as they did at the time. They also seemed high to my well-seasoned mechanic friend. Some cylinders topped out at 195 PSI. I'm wondering if anyone else has seen readings this high? Particularly on a rebuild with fresh parts? I'm wondering if my heads are pretty carboned up. It's a long shot but maybe it's what is causing my incessant pinging with regular gas despite my best efforts to fix it.

 

[the_don]

When you did the rebuild did you reuse the pistons or new? how about the rods? did you have the heads machined any?
the high compression itself would explain the pinging when using 87 octane have you ever tried running 92?

 

[natenkiki2004]

When you did the rebuild did you reuse the pistons or new? how about the rods? did you have the heads machined any?
the high compression itself would explain the pinging when using 87 octane have you ever tried running 92?

You misunderstood I've not torn into the block nor pulled a head. I've torn into the lower intake, rocker assemblies, timing assembly and had the pan off but I haven't done anything to the block itself. The previous owner replaced a head with a used one from an Aerostar (budget & time constraints) but the other side is virgin.

I'm wondering if my compression test results are abnormally high for this engine which would indicate heavy carbon buildup and a potential source for pinging. It's more curiosity than anything else.

 

[malohnes]

I did a compression test after new 95tm heads, headers, etc 2 1/2 years ago. I hope this helps.

In order, 197, 185, 185, 190, 193, 180. The last one was suspect but my friend said w/I 10-20% was sufficient given a 145k motor.

 

[natenkiki2004]

Thanks for the readings malohnes. Aren't the 95tm heads higher compression?

 

[the_don]

My 93tm heads run about 145-155, since you have two different heads i would expect to see one bank much higher then the other, if this is the case then you have found your cause of the pinging, if your numbers are "erratic" between cylinders then a carbon build up would be likely.

 

[malohnes]

Thanks for the readings malohnes. Aren't the 95tm heads higher compression?

Yes, they are higher, about 1 point higher. I didn't get readings pre repair since I knew it was bad heads. According to my info it's now about 10:1.

 

[2stroke]

That seems very high, I believe 145-165 to be normal. Was one bank 190 or so, and the other 160?

 

[malohnes]

That seems very high, I believe 145-165 to be normal. Was one bank 190 or so, and the other 160?

Nope. Fresh after top end rebuild with 95tm heads. The only questionable cylinder was #6 which could have been from not sealing plug fitting tightly when taking readings. It was still w/i 10% of spread between hi and low and its not considered an issue from my research, so I left it.

 

[natenkiki2004]

My 93tm heads run about 145-155, since you have two different heads i would expect to see one bank much higher then the other, if this is the case then you have found your cause of the pinging, if your numbers are "erratic" between cylinders then a carbon build up would be likely.

The virgin bank is actually higher, the side with the 93TM head from an Aerostar is a bit lower. Most are 190-195 but there's one or two around 175-180.

Isn't compression the same between 90TM and 93TM, they just built the castings better?

 

[natenkiki2004]

That seems very high, I believe 145-165 to be normal. Was one bank 190 or so, and the other 160?

Not sure if you're addressing me or malohnes.

Here's my exact readings in September 2014:
Cylinder 1 - Test 1: 190-195, Test 2: 195
Cylinder 2 - Test 1: 190-195, Test 2: 190-195
Cylinder 3 - Test 1: 190-195, Test 2: 195
Cylinder 4 - Test 1: 175, Test 2: 165-170
Cylinder 5 - Test 1: 185-190, Test 2: 185
Cylinder 6 – Test 1: 180-185, Test 2: 185

It was the driver's side head that got replaced, cyl 4-6, the one with lower compression. This test was dry on a cold engine but I did not hold the throttle open, I just cranked longer.

Looking through my books, I can't find a hard PSI rating but rather that the lowest cylinder needs to be at least 75% of the highest cylinder.

 

[malohnes]

The virgin bank is actually higher, the side with the 93TM head from an Aerostar is a bit lower. Most are 190-195 but there's one or two around 175-180.

Isn't compression the same between 90TM and 93TM, they just built the castings better?

I believe you are correct.

 

[malohnes]

Not sure if you're addressing me or malohnes.

Here's my exact readings in September 2014:
Cylinder 1 - Test 1: 190-195, Test 2: 195
Cylinder 2 - Test 1: 190-195, Test 2: 190-195
Cylinder 3 - Test 1: 190-195, Test 2: 195
Cylinder 4 - Test 1: 175, Test 2: 165-170
Cylinder 5 - Test 1: 185-190, Test 2: 185
Cylinder 6 – Test 1: 180-185, Test 2: 185

It was the driver's side head that got replaced, cyl 4-6, the one with lower compression. This test was dry on a cold engine but I did not hold the throttle open, I just cranked longer.

Looking through my books, I can't find a hard PSI rating but rather that the lowest cylinder needs to be at least 75% of the highest cylinder.

Cylinder 4 seems a bit low. Rings or issue in head-valve? The rest are nice and strong.

I can't find info but I haven't looked hard to find psi-comp comparisons yet. Further, I was told by two engine builders that 15% is considered the most one should allow if your considerate to performance and efficiency. Other opinions vary but given your tightness in all other cylinders, it seems logical to check that particular one for issues.

 

[natenkiki2004]

Cylinder 4 seems a bit low. Rings or issue in head-valve? The rest are nice and strong.

I can't find info but I haven't looked hard to find psi-comp comparisons yet. Further, I was told by two engine builders that 15% is considered the most one should allow if your considerate to performance and efficiency. Other opinions vary but given your tightness in all other cylinders, it seems logical to check that particular one for issues.

I agree however, I likely won't be going down that road. I feel I've done too much to the engine already and that if it pukes, I'll swap over the one from my 1994 that's in better shape. Until that happens, it'll run as-is. I do have to run premium gas though and I was hoping that if the compression numbers seem high, maybe the heads are carboned up and I could have that taken care of.

 

[malohnes]

http://www.gregsengine.com/cylinder-compression-testing.html

Im sure you know this or have researched it but this may be helpful. I might try to retest my engine again as I'm pretty certain I didn't have my engine fully warm or with TB open.