Compression Test Results. Variance is too high?

robwclark

robwclark

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OK, my truck is an 06 model reg cab stick with a stock longblock, american racing long tube headers, VEC3, & a paxton w/meth injection. The truck used to put down 620-640 rwhp with a perfect 12:1 a/f ratio. The truck is down about 50 rwhp, runs super rich, and the #1 cyl has considerable oil in it. I finally got around to doing a compression test and was wondering if anyone has any idea why I am seeing such high numbers. Now I know the variance is too much and I will probably need a rebuild, but why the high numbers? Here are the results:

Driver side
#1 207 - very oily plug
#3 202
#5 205
#7 218
#9 215

Passenger side
#2 188
#4 185
#6 188
#8 190
#10 192

So 185 is the lowest & 218 is the highest. And, yes I performed the test correctly. I wonder why the oily #1 cylinder is one of the higher results? The passenger side has considerably less compression than the driver....thats just weird. Also, a side note. Did you guys know it takes about 15-20 revolutions to get peak compression. I ran the battery down just doing this stupid test. My past experience has been like 7 or 8 revolutions before it stopped rising on an LT1 Trans Am. I expected like 160 to 180 max. Anyone else ever do a compression test and have any idea why these numbers are so high? Also, do you guys share my opinion that the variance is too much from 185 to 218? Thanks,

Robert Clark
 
robwclark said:
OK, my truck is an 06 model reg cab stick with a stock longblock, american racing long tube headers, VEC3, & a paxton w/meth injection. The truck used to put down 620-640 rwhp with a perfect 12:1 a/f ratio. The truck is down about 50 rwhp, runs super rich, and the #1 cyl has considerable oil in it. I finally got around to doing a compression test and was wondering if anyone has any idea why I am seeing such high numbers. Now I know the variance is too much and I will probably need a rebuild, but why the high numbers? Here are the results:

Driver side
#1 207 - very oily plug
#3 202
#5 205
#7 218
#9 215

Passenger side
#2 188
#4 185
#6 188
#8 190
#10 192

So 185 is the lowest & 218 is the highest. And, yes I performed the test correctly. I wonder why the oily #1 cylinder is one of the higher results? The passenger side has considerably less compression than the driver....thats just weird. Also, a side note. Did you guys know it takes about 15-20 revolutions to get peak compression. I ran the battery down just doing this stupid test. My past experience has been like 7 or 8 revolutions before it stopped rising on an LT1 Trans Am. I expected like 160 to 180 max. Anyone else ever do a compression test and have any idea why these numbers are so high? Also, do you guys share my opinion that the variance is too much from 185 to 218? Thanks,

Robert Clark
Click to expand...

You are cranking the engine too long to record accurate compression readings. All compression testers that I have seen have a check valve below the guage so the reading will be held for you to view once you've completed the crank cycle. Continuing to crank the engine is allowing cylinder compression pulses to move the guage reading up above what the true cylinder pressure is and the check valve is holding that stacked pressure reading.

Try running the test with only 3 compression pulses on each cylinder and see if you are (a) closer in your variance and (b) much closer to 160 psi. Then run a cylinder leakdown test. Remove the oil cap, coolant cap, and on cylinders that have excessive leakdown, look and listen carefully for escaping compression from crankcase, coolant system, intake and exhaust.

As to the oil, could be rings, valve guide or seal. The oil can mask a leaking cylinder somewhat as it slows compression loss by slightly filling voids in the guide or rings.

Good luck and I hope it's nothing too severe.
 
As mentioned, 4 or 5 bumps during the compression test should be plenty. Your numbers will only go so high anyway.
But the cylinder-to-cylinder variance is what to look for and your high-to-low isn't too far out of range. Right now you are just under 18% (comparing the 2 extremes).

If you have a very oily plug, you will have a very oily cylinder and that will drive your compression test numbers high in that hole (less swept volume in there now); plus the oil can create a pretty effective seal for your test.

As for your power loss, a rich condition can kill power numbers as much as you've indicated.

I wouldn't drive the truck much more under w.o.t. conditions as your oily cylinder could be prone to detonation (oil vapors are LOW "octane"); plus the fact- that rich condition you have isn't doing your oil any favors and that is being pumped through the whole engine.

DO find out why you are getting oil in the cylinder. And as mentioned, do a leakdown on the oily cylinder and see if you can pinpoint either upper cylinder or ring seal...A bad seal and/or valve guide is a pretty easy fix.

During your compression test, watch the first couple of readings (it is kind of a mini leakdown test). The "better" cylinders will read higher right away...

As for an engine side to side variance- All I can say is I have seen it before and it seems to "go away" after a proper rebuild.

Ronnie
 
Last edited:
Pops@JMB said:
You are cranking the engine too long to record accurate compression readings. All compression testers that I have seen have a check valve below the guage so the reading will be held for you to view once you've completed the crank cycle. Continuing to crank the engine is allowing cylinder compression pulses to move the guage reading up above what the true cylinder pressure is and the check valve is holding that stacked pressure reading.

Try running the test with only 3 compression pulses on each cylinder and see if you are (a) closer in your variance and (b) much closer to 160 psi. Then run a cylinder leakdown test. Remove the oil cap, coolant cap, and on cylinders that have excessive leakdown, look and listen carefully for escaping compression from crankcase, coolant system, intake and exhaust.

As to the oil, could be rings, valve guide or seal. The oil can mask a leaking cylinder somewhat as it slows compression loss by slightly filling voids in the guide or rings.

Good luck and I hope it's nothing too severe.
Click to expand...



Well, something I forgot to mention is the truck has 15k miles with the blower on it, still stock internals, only one of the paxton pumps is firing, and its had the piss ran out of it by that chris sanders guy before I got it. I am pretty sure the motor is getting weak. The rich condition is most likely because the compression issue is costing power, therefore same fuel going into the motor that isnt as tight as once before. I ran the same test but only turning it over 8 cranks on each cylider and the compression was about 160 to 175, except on #4 cylinder which after 8 revolutions was at 195. Thats why I redid the whole test and cranked it about 15 rev before the gauge quit climing.

That first test with 8 rev's is as follows:

driver:
#1 158
#3 162
#5 171
#7 168
#9 178

passenger:
#2 174
#4 193
#6 166
#8 162
#10 171

I have a nice snap-on compression tester that has the valve that lets you unscrew it and still hold the pressure on the gauge(maybe they're all that way). Now I just followed the instructions on the snap-on tester and it says in there to turn the engine over until the needle stops climing. I dunno, it seems like not a very accurate way to test by turning the motor over only 3 revs while the compression is still rising quickly. Any other advice is greatly appreciated. Thanks guys.
 
robwclark said:
I have a nice snap-on compression tester that has the valve that lets you unscrew it and still hold the pressure on the gauge(maybe they're all that way). Now I just followed the instructions on the snap-on tester and it says in there to turn the engine over until the needle stops climing. I dunno, it seems like not a very accurate way to test by turning the motor over only 3 revs while the compression is still rising quickly. Any other advice is greatly appreciated. Thanks guys.
Click to expand...
i can't speak for JMB'spops, but ronnie knows his stuff, and i'm sure JMB'pops
is very knownledgeable with viper engines as well, i'd listen to what they had to say. my question is how would one cylinder burning so much oil affect and AFR meter in the truck, maybe show it runnning rich when it's really not?
 
Did you pull every plug out at the same time and do the test on each cylinder ?Or did you remove one plug at a time and do the test? Pulling all the plugs won't kill your battery off ,it will be easier for the starter to turn over , you may get more consistent readings.
 
do a leak test, that will tell you what u want to know
 
Hi Boots, long time no see. hows things going for ya??
 
AZBLACKMONSOON said:
Did you pull every plug out at the same time and do the test on each cylinder ?Or did you remove one plug at a time and do the test? Pulling all the plugs won't kill your battery off ,it will be easier for the starter to turn over , you may get more consistent readings.
Click to expand...

I pulled all the plugs, then did the test.
 
Harbor freight has the leak down kit. Cheap, price and quality.

just like a compression test but you end on the compression stroke and watch the gauges, one has a ck vlv the other doesn't this checks how much your cylinder leaks over a givin time....

As Voodoo, Prof, Ronnie will tell you it's the only way to really see the engine. You could buy a scope and take a peak at the pistons and valves.
 
Jay makes a good point, see if someone will lend you their scope...a look at the top of your pistons would be good info at this point...if it looks like a diamond mine, you have problems...
 
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