Compression Test

[bemiller]

Hey guys, hope everyone enjoyed the long weekend! I did a compression test on my racecar today. The 383 has 12.5 TRW pistons and 906 heads it checked from 155 to 175 only one cylinder checked 155 and only one checked 175 most of them were 165 to 170, is this motor good another season? Or should I consider new rings. Also does 170 sound right for 12.5 to one? The service book for another car I have says put 3 squirts of oil in cylinders and check again. If goes up need new rings. Any help appreciated!!

[J-BODY]

Another good test to run would be a leak down test, especially on the lower compression cyl. Compression #'s are going to vary from engine to engine, especially when using larger lift and duration cams in race oriented engines. It sound like to me that your engine is in OK shape for another season. The oil in the cylinders will help to seal the rings better during the compression test, but I think the leak down test is the way to go.

[bemiller]

Where can I get the adapter to do a leakdown test, I have an air compressor. But none of the books I have give any info on how exactly to do it, just say hook up compressed air and listen for leaks. Sorry for all the questions but I'm learning.

[J-BODY]

The leak down tester can be purchased from any tool dealer or I even think Sears has one. I've also seen them in the Jegs catalog, Tavia I believe, and they were priced around 70 to 80$. I bought mine from the Matco distributer for around 70$. The tester has two gauges with a regulator so you are always using the same amount of air pressure for the test, and one of the gauges will show you the amount of leak down in a percentage value, and has a general good, medium, bad value on it. I did a compression test, and leak down on my 498 after running two events four years ago, and check it twice a year to see if there is a obvious pattern developing. I also have a cylinder that is starting to lag a little (#7) and through testing have found that the leakage is at the rings, so I know what I get to do next winter. (it better make it!!)

[bemiller]

This may a stupid question but do you have to make sure both valves are closed to do a leakdown test? You do right? Otherwise how would it pump up. So it's kinda like adjusting valves only not. Thanks for your help!!!

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[bemiller]

oops signature wrong in last reply, trying to make sure I got it right.

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Check out http://www.donet.com/bemiller/ and let me know what you think!!

[J-BODY]

The only stupid question is the one not asked. You are correct on the valves being closed. I start at #1 tdc firing and follow the firing order. I have also always done this with the engine warm so the tests will be somewhat consistent.

[dusterrcr]

Bemiller,My 440 also has 12.5:1 with 906 heads.this motor is 9 yrs old,my compression ck recently showed 165 to 170 lbs also,so your letter was good news to me! (motor was cold)my first ck showed # 5 at 130lbs ! then I remembered I had done an emergency adj at the last race.(they were calling us to the staging lanes & this rocker was really hammering)there was almost zero lash on #5 intake.I opened er up to .028 & it went right to 165 lbs. I plan to ck/replace bearings on bottom end patch a couple oil leaks (pan gasket) and ask it for a couple more seasons. try this with a shivvy motor!--A question for the class: can these static compression numbers determine octane requirements? is 165-170 that high? I currently use racing gas,can I use 93 octane super unl with a lead additive without risking damage?---always looking for a cheaper way to race.--thks---later

[bemiller]

My motor is also over 5 years old, even had NOS pushed through it a couple times!!! Pulled the pan last year to check bearings and other than a couple specs of dirt they looked brand new. Car ran 10.80's on the bottle, but I figured it was a waste in a bracket car so I removed it. I put over 100 runs on the thing last season. Mopar rules!!

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Check out http://www.donet.com/bemiller/ and let me know what you think!!

[J-BODY]

Dustercr, I would say that it would not. The topic is getting out of my league but basically there is a difference in cylinder pressure when the engine is running compared to when you are only cranking, like during a compression test. I believe this is where measuring your compression comes in. I also believe it's time for someone to step in and give out the technical answers to this question. HELLLPP!!!

[340king]

Dustercr you raise a good question, one that I wish I had the answer to. As I have stated in past posts, cranking compression is just that. What happens at race rpm can be quite different from one combination to the next. See the posts for the subject posted by Speedy about Dwell.

There are many things that influence knock(detonation) in an engine. Trying to predict it with just one parameter sounds great, but is not really practical. Things like heat retention or heat removal efficiancy of the cylinder head assembly have a lot of influence on knock. Combustion chamber shape and clearance create an entire new set of variables.

What we want out of our race fuel is knock resistance, performance and consistancy. Any concessions in these areas is intollerable to me. One day helping J-body's dad race, we ran short of fuel. The car slowed down 0.3 of a second just in time for the final round. Guess what place we got? The substitute fuel, while sufficient in knock resistance, did not perform as good as the VP C12. I hope I have the correct J-body here, white Mirada?

VP recommends selecting a fuel based on two main requirements. The first is octane requirement. The second is rpm range. They recommend matching the vapor point/boiling point with the rpm range. The reason for this is that raw fuel droplets don't burn, atomized vapor burns. Higher rpm engines have less time to vaporize the fuel and need lower vapor points/boiling points.

I hope this helps. Now on the original subject, your compression looks good for your compression ratio. The best way to check this is to keep track from the beginning like J-body does. This way any change is immediately noticed and a problem is identified.

[Mopar400hp360]

Simon and Garfunkel - "I am just a poor boy though my story seldom told" You can make a leak down tester out of an old spark plug and an air tank with a register. Break the porcalin off the spark plug then weld - solder it if you have to - I find certain pipe fittings to work as well - adapt the right fitting for you air hose. Charge the tank, screw in your modified plug and connect your air line then turn your air on if your tank has a valve, the just sit back and watch the gauge, it should not move at all.