2006 Compression Test

[Mad_Trbo]

Hello all, I'd like to introduce myself. I am an avid auto enthusiast who enjoys extracting power out of unique platforms. I am in the midst of purchasing a 2006 XLR - V and I had the dealership perform a compression test for me.

I located compression stat results on this site. I have the LC3 documented as between 131 - 158 psi. I assume that is on a warm engine, I am curious to know if anyone can verify results they have seen on an 80k plus engine.

The results I was given are dismal if peformed correctly and I probably need to reconsider the purchase, cylinders 4,6,7 came in at 125 and all others came in @ 120. I am guessing this may have been on a cold engine, but I assume a cadillac tech knows the proper steps for a compression test.

 

[xlrlist01]

Hello all, I'd like to introduce myself. I am an avid auto enthusiast who enjoys extracting power out of unique platforms. I am in the midst of purchasing a 2006 XLR - V and I had the dealership perform a compression test for me.

I located compression stat results on this site. I have the LC3 documented as between 131 - 158 psi. I assume that is on a warm engine, I am curious to know if anyone can verify results they have seen on an 80k plus engine.

The results I was given are dismal if peformed correctly and I probably need to reconsider the purchase, cylinders 4,6,7 came in at 125 and all others came in @ 120. I am guessing this may have been on a cold engine, but I assume a cadillac tech knows the proper steps for a compression test.

What did the tech say about the numbers? Often times the difference (range) is as important (or more so) as the number and in your example the numbers are reasonably close, 120 to 125. Given the need to keep the throttle fully open (I assume this was done correctly) and assuming crank using the starter, s/c disconnected and an effective compression ratio of around 8 (ie having taken inevitable induction losses into account from the 9 of a V), atmospheric pressure is a little under 15, that gives ~120. Supercharged motors use slightly lower compression ratios than non-supercharged as a rule. The days of 10+ ratios are pretty much history for standard road going use (unless using direct injection or you have a standard XLR). However, when the motor is running various methods are used to increase the apparent effects, they come down to tuned port designs and variable valve timing mostly. When at crank speeds I wouldn't be sure what exactly is going on especially in a s/c motor, but 120-125 suggests normal. If tests were run with the s/c connected then any number might result. It would depend on how worn the s/c vanes are, how hot it is and a whole host of imponderable effects. Heck, the lifters are hydraulic and as oil pressure drops, they leak down slightly, so you will gradually reduce the effectiveness of the valves. Can the starter spin the motor fast enough to keep the valves fully supplied? All this combines to make totally consistent readings unlikely. Hence my comment that a big difference is more of a concern than the actual numbers, provided the numbers look reasonable, which, I suggest they do.

A rough measure of engine condition is fuel economy, but if you really want to know, get a rolling road power check. Worn motors usually burn oil too. Watch from behind while it is accelerated hard (on a fully warmed up motor).

 

[Johnxlrv]

What did the tech say about the numbers? Often times the difference (range) is as important (or more so) as the number and in your example the numbers are reasonably close, 120 to 125. Given the need to keep the throttle fully open (I assume this was done correctly) and assuming crank using the starter, s/c disconnected and an effective compression ratio of around 8 (ie having taken inevitable induction losses into account from the 9 of a V), atmospheric pressure is a little under 15, that gives ~120. Supercharged motors use slightly lower compression ratios than non-supercharged as a rule. The days of 10+ ratios are pretty much history for standard road going use (unless using direct injection or you have a standard XLR). However, when the motor is running various methods are used to increase the apparent effects, they come down to tuned port designs and variable valve timing mostly. When at crank speeds I wouldn't be sure what exactly is going on especially in a s/c motor, but 120-125 suggests normal. If tests were run with the s/c connected then any number might result. It would depend on how worn the s/c vanes are, how hot it is and a whole host of imponderable effects. Heck, the lifters are hydraulic and as oil pressure drops, they leak down slightly, so you will gradually reduce the effectiveness of the valves. Can the starter spin the motor fast enough to keep the valves fully supplied? All this combines to make totally consistent readings unlikely. Hence my comment that a big difference is more of a concern than the actual numbers, provided the numbers look reasonable, which, I suggest they do.

A rough measure of engine condition is fuel economy, but if you really want to know, get a rolling road power check. Worn motors usually burn oil too. Watch from behind while it is accelerated hard (on a fully warmed up motor).

Looks like fun at least. If you can get a dyno run in, that can help validate things. With 40k mikes stock, I was at 394 hp and 377 torque at 5800 rpm in 4th gear...where the 155 mph speed limiter shut me down. With no tune and a mustang dyno, wondering what you would get. It's also a great time to be outside of the car and watching and listening for things.

Wish you luck

 

[PTpro]

Come on guys, you are talking about a nine year old car that still kicks a$$.

My opinion is; if your 2006 V doesn't burn oil and runs close to what it used to, be happy, don't worry! Mine, which I bought new, has 70K miles on it and doesn't burn a drop of oil and still runs with the big dogs. I never back down from a drag race and still can run 0-60 with the best of the new V's.