DIY: Compression Test For engine out of car

[328ijunkie]

See Vid: http://www.youtube.com/watch?v=oy6vmqLFy5I

Did this for a customers S52 i just picked up. Very easy to do. Either do what i did and use a junk auto trans bellhousing or bolt up a trans, bolt up a starter, Positive to battery Then just ground the starter or motor to the battery. Top terminal (the one that has a 10mm nut on it) to the + on the battery will fire the starter. I used a fused link with a 7.5amp fuse in it just for safety.

(Make sure theres some oil in the pan lol)

[Jalves619]

Can I perform this test on a long block. I don't see any reason why not having an intake or anything past the valves would affect it, but I've never done a compression test before, so it is possible I'm overlooking something blatant.

I understand the numbers will be lower due to the engine not being at operating temp, but I should be able to get a general idea of the engine's health by comparing the cylinders.

[328ijunkie]

Yes you can easily. Im still figuring out a way to do a proper leakdown test on a motor out of a car. A new thread will be formed when i figure that out lol

[dsobering47]

leakdown? why cant you just use a leakdown tester? I did that on my engine when it was on the stand and found my first 3 cylinders were crap.

[328ijunkie]

You realize a cold leakdown test will tell you nearly nothing...

[dsobering47]

I'm not a pro on engine rebuilding, but why would a cold leakdown tell you nothing? I would figure the rings should be expanded and sealing regardless of temperature?

[dsobering47]

I would say it's the same as doing a compression test on a cold motor...yea, the numbers might be lower than if it were up to temp, but the numbers would still be fairly important. Correct me if I'm wrong though.

[328ijunkie]

Things haven't expanded and so it will be way off, ive done them on cold motors and had 20+ % when cold and <5 hot

[dsobering47]

So the only worthwhile test for an engine that's out of a car (dead cold) would be a compression test? I did a leakdown on mine before I pulled the head and had numbers like:
1) 30%
2) 25%
3) 16%
4) 7%
5) 5%
6) 6%

Head was rebuilt and is straight/true. Found 6 intake valves were no dice.

[328ijunkie]

Well a cold compression test isnt ideal either but it will show issues before a cold leakdown will from my experience. And yeah you def had some issues there.

[george graves]

I'm almost shocked that the motor doesn't move around more than it does. Do you have it lashed down or something?

[328ijunkie]

nope. Keep in mind all spark plugs are out so its not really building compression and this is my shop battery which gets moved around alot and charged intermittently so the starter isnt cranking with full power.

[george graves]

Yea, I noticed that it sounded very weak.

Just be careful with a full battery - It'd suck to have a block roll over on your leg or something.

[328ijunkie]

Been there, done that :(

[MattM]

cold compression wont tell you anything about the rings or cylinder walls or HG... which is usually why you do a compression test

i guess if your looking for catastrophic stuff go for it..(broken valves/rods/etc)

basically if you get 0 it fails, anything higher then 0 and it passes...