[Cloud9...68]

I'd like to do a compression test on my engine, which has about 105,000 miles on it, so I want to make sure I'm following the best know standard procedure for performing the test. Here's what I think the procedure should be - please let me know if there's anything I need to change or add. I only want to do this once, as I'd like to remove and replace the plugs as few times as possible:



1) Make sure battery is fully charged (I plan to run mine on a trickle charger until it tells me it's fully charged).

2) Warm up the car fully

3) Remove all four spark plugs

4) Remove the DME relay (is there a more convenient way to disconnect the fuel delivery?)

5) Thread the compression tester into spark plug thread #1

6) Crank the engine 8 times (Is this too many? It's just a number I've randomly selected in the past)

7) Repeat for cylinders 2 through 4

8) Celebrate, or cry, depending on the readings



Thanks.

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[xrad]

That will work. Don't forget to release pressure in the gauge before next cylinder. Leakdown test is of course more sensitive, but not always more helpful. You should check the engine while hot, but I would also check while cold and compare the findings.

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[Cloud9...68]

[quote name='xrad' post='64740' date='Dec 27 2008, 07:05 PM']You should check the engine while hot, but I would also check while cold and compare the findings.[/quote]

Interesting - what will this tell me? I have to tell you, I cringe at the thought of removing and torqueing the spark plugs into this car's head any more times than absolutely necessary, which is part of why I haven't replaced the plugs in five years. Thanks.

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[flash]

it could give you an indication of the condition of the rings, as they expand when hot

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[Cloud9...68]

[quote name='flash' post='64751' date='Dec 27 2008, 11:23 PM']it could give you an indication of the condition of the rings, as they expand when hot[/quote]

Is there a spec for an acceptable difference between cold vs. hot compression readings?



The car runs well, but with 105K miles on the clock, many of which are track miles, I'm wondering if a head rebuild (which would be the first step on a slippery slope toward the quest for more power) would be in order.

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[flash]

not really a "spec" - it should go up just a bit warm over cold - maybe 5 pounds

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[xrad]

Cloud9: time for new plugs. I use Bosch platinum single electrode with standard gap. 6$ each and work fine. As for torqueing the plugs, you should always make sure your plug socket is on the plug fully and squarely when removing, AND always hand thread when installing. As for tightening, snug them all down until firmly seated, then just give and extra 1/8-1/4 turn at most. I use a touch of aluminum paste on the plug threads (put the paste on the plug threads about 1/2 way up from the tip so it does not go in cylinder, and only use a LITTLE)

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[Cloud9...68]

Yes, I'm sure new plugs are in order, as it's been about 25K miles since they were replaced. And yes, I always use anti-sieze paste on the plug threads, am ultra-careful about making sure the socket is properly seated on the plug before removing, and super-ultra careful to make sure they're not cross-threaded when installing. I always torque the plugs to the recommended setting. But I still figure the threads only have so many "torqueings" in them, so I do this as infrequently as I can, especially after stripping one of the water pump threads in the block when doing my belt/water pump replacement this past spring. I guess we're all heavily influenced by our experiences...

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[xrad]

yeah, you have to be careful with plug threads. I find that the 1/8th + turn is fine. After doing this hundreds of times, you can get a feel for when the plug washers lock up.

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[flash]

i am amazed at how much further than "snug" it actually takes to hit the torque spec - makes me nervous too - i definitely use a needle torque wrench and not a click type



just a note on plugs, and you can go to sparkplugs.com for more info:



like all single electrode platinum plugs, the bosch plugs last a long time, but don't give as much performance in tests - it's the trade off for the longevity - they are a good plug though - however, even the NGKs are specced for some insane lifetime, and everybody i know changes them long before their published life expectancy, so i'm not sure how effective the advantage of the platinum is - i'm sure i change them too early, but i do that with a lot of things, just to be tip top



over the years, i have done my own testing between a number of plugs, and have seen benefits of one plug over another - the copper have always been the worst in all areas - least life - least power - i'd upgrade one way or the other - i know a lot of people swear by the original plug, and say they have no "problem" with them - i am not suggesting that the copper plug doesn't work, but there is a difference between "no problem" and "better"



there are a lot of choices - each one has a different advantage - much of this goes to the condition of the engine though, so it may not make any difference at all if your compression is down



for now, i'm sticking with the iridium, as all i am after is maximum performance

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[xrad]

Might as well get an 'inch/ounces' digital torque wrench...

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[flash]

lol - actually....................

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[Cloud9...68]

OK, here are my compression test results, with the engine both cold and fully warmed up, following the procedure I proposed at the start of this thread. The cold readings were all over the map - as follows:



Cylinder 1 = 200 (!)

Cylinder 2 = 174

Cylinder 3 = 189

Cylinder 4 = 140 (!)



After fully warming the engine up, here's what I got:



Cylinder 1 = 178

Cylinder 2 = 175

Cylinder 3 = 176

Cylinder 4 = 174



To be honest, the warm readings were so close that I would argue that most of the difference can be accounted for by my interpreatation of the gauge, not to mention the precision of the gauge itself. So, within the degree that's discernable, the readings in all four cylinders are identical. I couldn't find a spec for the compression numbers on a healthy engine, but doing a quick search revealed other people seeing numbers in the 180's, so maybe my numbers are a little low, but of course this could be accounted for by gauge-to-gauge differences. I guess I'd need to do a leakdown test to know for sure. But I've very pleased with the tight distribution in my numbers, so even if the engine is starting to get a little tired, there doesn't seem to be an urgent need to do anything about it. Comments?



I don't know what to make of the cold readings. I've never done a compression test cold before, so I don't have any frame of reference. It seems like an inherently inaccurate technique, because the oil is nowhere near its specified operating temperature, so I wouldn't expect it to seal around the rings properly. And the 200 psi reading in cylinder 1 is just goofy. I'm using Redline 10W40, by the way, and it's unseasonably warm in Austin right now, a stunningly gorgeous 70 degrees, low humidity, with tons of sunshine.



The engine was dyno'd at 213 rwhp, and 204 lb-ft, when it had 83,000 miles on it five years ago. It now has about 105,000, but has seen a total of just over 18 hours of hard track time in the last two years, and that number is climbing fast. It uses very little oil, and the oil pressure gauge reads as follows (fully warmed up engine):



Idle = 2.0

1000 rpm = 2.1

1400 rpm = 3.0

1500 rpm = 3.5

2400 rpm = 4

3600 rpm = 4.2 (max)



I am embarassed to say my spark plugs look pretty worn. Their color is perfect - a nice light tan, and no sign of oil anywhere, but they look like they should have been replaced at least 10,000 miles ago. A new set of NGK Iridium IX's is on the way, and should arrive today. I'm going to go ahead and replace the cap and rotor as well, as this is also long overdue.



Oh, and one last comment: The heat shield helped make the job of measuring the compression much more comfortable. It really is noticeably cooler in the engine bay now.

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[flash]

regarding the warm numbers, if that was 8 cranks, the numbers, while even, are low - they are typical of a 100k engine though - they run well like that, but you are down on power by about 12%



as a reference, i am up well over 190 (closer to 200) at 47k - gauges can vary a little though, so that may be part of the reason for your low readings



regarding the cold numbers, that bothers me a bit - they shouldn't be that far off - i'd do a leakdown

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[Cloud9...68]

Flash,



Your comments make sense. I just got off the phone with Pete (for a different reason), and went through the numbers with him, and he said he has seen gauges vary a lot, which may at least partly explain my relatively low readings. But given that they are so even, I agree that while I'm probably down on power some, which is definitely impacting my lap times, there doesn't seem to be a dire need to pull the head just yet. Depending on how finances look later in 2009, I've love to send the head to Pete to do a rebuild (at least a Stage 2, and maybe a Stage 3), add his exhaust, and either his Stage 2 or Stage 3 chip. But for now, looking at the glass half full, my somewhat horsepower-challenged car will continue to force me to focus on maximizing my speed through the corners.

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[PorscheG96]

Good info in this topic, thanks.

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[flash]

lol - i totally understand - i've just been waiting for my numbers to drop a bit so i can "justify" yanking the head and doing some fun work

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[Cloud9...68]

[quote name='PorscheG96' post='64856' date='Dec 30 2008, 02:52 PM']Good info in this topic, thanks.[/quote]

Glad you've found it helpful - that's what this forum is all about. Pete told me to keep an eye on my oil consumption (it's quite minimal at this point), because the exhaust valve guides tend to wear significantly at about 100K miles on these engines. Once significant wear sets in, things go downhill pretty fast, because the exhaust valve starts to move around inside the guide, accelerating the wear. The symptoms are excessive oil consumption, and in the extreme case, low compression, because an enlarged guide can allow the valve to not seat properly.

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[xrad]

Cloud9: I usually average three reads per cylinder. Your warm numbers look good.

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[Cloud9...68]

[quote name='xrad' post='64864' date='Dec 30 2008, 06:55 PM']Cloud9: I usually average three reads per cylinder. Your warm numbers look good.[/quote]

How many times do you crank it over per test? I did exactly eight cranks per test.

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