I just bought a motor and wanted to do a compression test to it. The motor is not in a car so I was wondering if I just turned the crank with a ratchet will this harm anything? Thanks!

 

lol...no way you will have enough cranking power to get an accurate reading.

If you wanted to, do this

Make sure oil is in crank case
Remove spark plugs
connect tester
Hold throttle body wide open
Jump starter to a charged 12 v battery and let her turn over
Make sure tranny is in neutral also.

Just ground the engine instead of trying to ground the starter itself

 

Thanks for the help. I think I'll try it soon.

 

Oh yeah. I also don't have a intake manifold on there right now, so I obviuosly can't hold the throttle wide open. But this should still be fine right?

 

I don't see why not, just make sure you don't suck or drop anything into the intake ports.

 

Good point.

 

I think the engine has to be warm to get a good reading....but not sure.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Lsos &raquo;</TD></TR><TR><TD CLASS="quote">I think the engine has to be warm to get a good reading....but not sure.</TD></TR></TABLE>

Yes it does. You can still do it anyway just to see if you get consistent numbers. But yeah, it will be more accurate and possibly different numbers when at running temp.

 

yes u will get a better reading if the motor is warm, if u wanna do it cold atleast drop a little oil into the cyl first it will make the cold reading a little more accurate.

 

Cap full of oil?

 

yes a cap full is perfect sorry i didnt specify..

 

Oh no problem. Thanks everyone for all the help. Really aprreciated.
I don't have to worry about the engine bouncing around everywhere right if I have it sitting on a tire?

 

haha a ratchet. I dont think that will work. If the car doesnt start your better off doing a leakdown test at that point.

 

I sorta dropped that stupid idea. lol.
The motor is out of a car and just sitting in my garage. I'm not sure on how to do the leak down test. Gotta research that some more. Can it be done with the motor out of the car?

 

Absolutely.

You have a couple of options. Summit Racing sells a nice inexpensive leakdown kit - SUM-900010 for $65. You can hook it up to an air compressor or if you dont have an air compressor, you can use an air tank and just fill it up at a friends house. Thats what I do.

But honestly if you dont care about leakage % and you just want to troubleshoot where the compression is leaking, you can just compress air into the cylinder without the leakdown gauges. Just buy an adapter to screw your air line into the spark plug threads and save yourself some money.