what is the right way to do a compression test

[Viablo]

Finally broke 2,000 miles on the GTS and am going to change the spark plugs. So i bought a compression tester so i can get baseline numbers for the future. I decided to give the Zr1 a tune up and compression test also before it goes on its merry way.
I look up in the viper manual and it just says to remove all plugs, etc, etc,- No big deal.

Look in the ZR1 manual and says that before doing compression test 'Engine should be at operating temperature, remove spark plugs, etc etc'"

Then as we all know for aluminum heads we should allow the engine to cool before removing spark plugs or the threads in the cylinder head may be damaged. At least that's what the corvette manual syas.

Looked at the instructions that came with the compression tester and it also says the engine should be at operating temperature.

So i ask you, what is the right way to do a compression test?

Thank you .

 

[garolittle]

Interesting. Sounds like a good question for Mr. Jon B.

 

[GTS Dean]

One of the reasons they went to cast pistons in the Viper was so that they could run tighter piston to wall clearances and reduce cold-start emissions. Cast aluminum pistons expand less with temperature, so would probably give somewhat higher pressure readings with cooler block temperatures.

I checked my '96 some months back. I drove it awhile, then let it cool for about an hour with the hood open before removing the plugs and testing compression. I spun the engine over with the throttle open and watched the gauge until it stopped climbing. As I recall, the numbers were in the 160's to 170's.

 

[Keller]

Last time I did a compression check it was on my 97 Supra which also has an aluminum head. I did it while the engine was hot. No damage done to the threads. Just an idea if your worried. Let your engine cool and remove your plugs. Put a small amount of oil on the threads and reinstall. Run the car, remove the plugs and test. The oil on the threads should keep any damage from occuring.

 

[Bad_Byte]

I've done 3 compression tests in recent times, 2 cold and 1 warm.
The cold produced low 160's and the warm produced high 170's. I would assume forged pistons have something to do with the variation? yes/no?

 

[Eddie N]

you can do a compression check either cold or hot.. just find someone who has done a cold compression check and compare with their numbers...

- eddie -

 

[Tom and Vipers]

The thing about removing spark plugs from cold aluminum heads is for the special case where the plugs have not been removed in recent history - in which case, they will be "stuck" in the head.

Even cold, you might strip a head.

For plugs that have been recently installed and not overtorqued, a hot head is not a problem. (If you really are worried, retorque the plugs cold so that they are "loose." Does anyone remember having to run a plug "fingertight" because of header clearance in Chevys?)

Remove all plugs.

Hold throttle wide open when cranking. (I think this was a left over from carbs, however, I think it triggers a computer "choke open" lean condition. You may want to disconnect your injectors .)

You also may want to disconnect your coil packs. I get Viper and LS1 comfused but running a coil with an open secondary circuit causes very high voltage to build up in the coil and in one shop manual (which one I don't know) they say you can damage the coil w/o a plug connected.

 

[Chuck 98 RT/10]

Consistant presure across all the cylinders is what has always been the most important. I wouldn't think engine temp would be much of an issue.