Compression test results....

[boostjunkie]

Well, I decided to do a compression test on my car. Now after doing all of the correct steps (disabling ignition/fuel, WOT) I got dry readings of:

cyl 1 - 245psi
cyl 2 - 243psi
cly 3 - 241psi
cyl 4 - 245psi

I ended up with numbers ALOT higher than what people normally find. The service manual states anything above 203psi with less than 16psi diffrerence between cylinders.

Now people tend to find somewhere in the neighborhood of 210-220psi which makes me wonder...

What does an excessively high compression mean? I do know it would mean sooting, however, for all of the cylinders to be within 4psi of each other I find that hard to believe. I also run SeaFoam thru the car every 10,000 miles and use premium all of the time (it's required). Spark plugs never showed any fouling.

I know people talk about certain cars being factory freaks, but could a higher compression across the board be one of the causes?

[scottzg]

miscalibrated compression tester?

[evolutionmovement]

Could you be using a slightly longer headed spark plug?

Steve

[scottzg]

Could you be using a slightly longer headed spark plug?

Why would that affect compression? Wouldn't that be compression ratio?

[boostjunkie]

miscalibrated compression tester?

I would expect that if I didn't try two other compression testers in the shop with the same result.

[vrg3]

Scott - The compression ratio is what causes compression.

Jason - What was the barometric pressure when you did the test? If it was higher than standard pressure (1 atmosphere) then you'd read higher compression numbers than standard, right?

[boostjunkie]

That is true. But would it cause an increase of around 30psi?

Very puzzling.

Another thing. My car was sitting outside (in the frigid weather) just before I drove it into the bay. Once inside, I remember the water temp gauge read two hash marks short of warmed up... about half-way warm. I know, I know, it's supposed to be done when the engine's warmed up...

But wouldn't that have given me a lower reading since the rings wouldn't have been seated as well?

[vrg3]

No, it wouldn't account for the entire 30 psi... The way I figure it, barometric pressure would have had to be 14.7/215*245=16.75 psi to account for all of it. But maybe it was some of it. And maybe Steve's suggestion about the plugs is some of it.

I wonder if intake and exhaust modifications would make noticeable differences in compression. It seem plausible.

[entirelyturbo]

Didn't you pull the spark plugs to do the test? If so then the type of plugs won't matter

[vrg3]

Oh. Right.

[boostjunkie]

Intake is only an extra pipe run thru the stock air box.

Exhaust is stock.

My particular car aside, does a higher compression across the board (for the same engine) account for anything in terms of performance? Power? Torque? Throttle response?

Also, is the base number dependent on bore and stroke of each cylinder? Volumetric efficiency? Compression ratio?

I tried to ask my automotive professor but he didn't know how compression relates to variances in performance. Only how it affects performance in a negative way.

[evolutionmovement]

Didn't you pull the spark plugs to do the test? If so then the type of plugs won't matter

D'oh!

Higher compression will yield more power, better gas mileage, increased engine braking, but will likely not affect throttle response. Higher thermal efficiency. It's simply the ratio of cylinder volume at bottom center compared to cylinder volume at top center.

How many engine turns are you using to test compression? It could be building up.

Steve

[scottzg]

Scott - The compression ratio is what causes compression.

Right, but i thought that a compression test measured how much pressure the cyl could hold before leaking. Is that wrong??

[evolutionmovement]

You're thinking a leak down test where you fill the cylinder with air pressure from an external source to see where and if it's leaking. It's a more accurate test.

Steve

[LaureltheQueen]

all 4 of my cylinders were exactly the same with 1 being 1psi higher than the rest. Turbo power

[vrg3]

In a compression test you're supposed to reach a steady state after several turns. The tester has a check valve that only allows the reading to increase. Doing extra turns shouldn't materially change the number.

What the test measures is how much air pressure is developed in the cylinders. This is a function of many, many parameters, which is why it's considered to be just a basic sanity test. It reflects the health of the piston rings, the sealing of the valves, the compression ratio, and even the condition of your battery and starter motor.

Abnormally high compression probably won't be beneficial. Steve's list of all the advantages of an increased compression ratio is correct; but, if an engine exhibits higher compression than it's supposed to, there's a good chance that whatever is causing it is also kind of screwing with the shape of the combustion chamber, right? So that could cause trouble with detonation control and whatnot. Higher compression is generally associated with a slower burn and a higher likelihood of detonation anyway.

But I don't think you have anything to worry about, Jason. While it's kind of odd that most other owners are reading relatively consistently lower numbers than you are, it's not a big difference and can partly be explained by other factors.

[professor]

I've never worried much about the absolute number on the compression tester...many factors come into play...unless someone has modded the engine and may have shaved the heads too much, or changed pistons, rods, etc.

The fact that they all came out the same sounds good and you are likely in good shape.

Taking in cold ambient air, into a warm engine could account for those high numbers, the ambient atm. pressure being higher could only account for maybe 5% of it, but very cold intake air could add 20%, if it warms rapidly in the cylinder after being drawn in.

[boostjunkie]

In a compression test you're supposed to reach a steady state after several turns. The tester has a check valve that only allows the reading to increase. Doing extra turns shouldn't materially change the number.

Abnormally high compression probably won't be beneficial. Steve's list of all the advantages of an increased compression ratio is correct; but, if an engine exhibits higher compression than it's supposed to, there's a good chance that whatever is causing it is also kind of screwing with the shape of the combustion chamber, right? So that could cause trouble with detonation control and whatnot. Higher compression is generally associated with a slower burn and a higher likelihood of detonation anyway.

Yeah, I did 5 cranks. Just kept cranking until the reading settled and stopped.

The reason why I was asking about increased performance was because of the nature of ring seating. By convention, no set of rings makes a complete seal around the piston. There is always some space that allows the ring to expand. The resulting space between the ring ends might be lessened, promoting better sealing of the cylinder. I was thinking this phenomenon as an explanation. Maybe, maybe not.

[vrg3]

It's possible, but there is a spec for the ring gap and I would expect manufacturing tolerances to be fairly tight.

[evolutionmovement]

The gaps are very small and you have a couple rings. There are gapless piston rings, but I wonder how much of a difference they really make.

Steve

[professor]

if you squirt some oil in the spark plug hole, you'll be gapless for a few strokes. if your reading goes up more than a few psi, suspect worn/cracked rings and likely blowby. I don't consider any compression check complete without doing this.

usually the reading goes up between zero and 2-3 psi which is normal. I've seen it go up 20 psi on one cylinder, not coincidentally the plug on that cylinder looking fouled. it usually goes up temporarily to the reading on the other cylinders, not above.

[boostjunkie]

The gaps are very small and you have a couple rings. There are gapless piston rings, but I wonder how much of a difference they really make.

Steve

Gapless rings are quite a misnomer. There is no ring that has no gap as there needs to be some slack for the expansion of the metal.

I'll do a wet test, along with another dry test this week and see what it turns up.

[vrg3]

Gapless rings don't have a gap that allows stuff through though. They do contact the cylinder wall for the full 360 degrees. It's kind of a ring-within-a-ring, and the overlap can vary in order to account for expansion. The smaller inner ring is still in contact with the cylinder wall at the "gap."