Compression testing an engine on a stand: Grassroots Motorsports forum: Grassroots Motorsports Magazine

Subscribe get a full year for only $23.95 Sample Free Test Drive—no strings attached

April 8, 2010 5:04 p.m. skullsroad New Reader

So I bought a motor for my SVT Focus and thought I would test the compression. I don't know if I'm doing it right.

I pulled all the spark plugs out. Valve cover is off (wanted to check condition of camshaft lobes), throttle body is zip-tied wide open.

When I turn the crank pulley it shows I'm only getting 30-35 psi in all 4 cylinders. After a few cranks I hear a release of pressure on cylinder 1 (very faint) even though I'm testing number 4.

I bought this engine to avoid having to rebuild the head on my shot engine. Am I doing something wrong or does it look like I'm diving into my first engine build?
April 8, 2010 5:18 p.m. Toyman01 Dork

Not turning it fast enough if I had to guess. Not to mention the cylinder walls are probably dry from not running in a while. Also a cold engine may not have as much compression as a hot one.
April 8, 2010 5:21 p.m. Run_Away New Reader

Compression tests have to be done at a certain rpm, there are different specs for cranking speeds, compression on one cyl while the engine is running etc etc. If you cannot mount the starter with the engine out you're pretty much limited to a leakdown test.
April 8, 2010 6:17 p.m. fifty Reader

A leak down test might be better if it's out of the car
April 8, 2010 11:57 p.m. skullsroad New Reader

Thanks for all the info. I can't wait to get this car running. I got the car for free so all the extra money goes towards upgrades.

So far it was 1200 for the 48k motor, 230 for the timing belt kit (tensioner, idlers and belt), valve cover gasket and serpentine belt (didn't know Continental made belts). Just ordered Massive underdrive pulleys and unbreakable motor-mount (200 for both). Next on the list is a Competition Clutch lightweight flywheel (320), Redline fluids ($?) and a Torsen differential (600). Hopefully by summer I'll have a suh-weet daily driver with just a 2000-2500 investment. I've never owned a car newer than 1992.
April 9, 2010 6:43 a.m. Toyman01 Dork

Keep it under $2010 and take it to the Challenge.

That's a win even if you come in last.
April 9, 2010 7:20 a.m. Soma007 New Reader

There's no way you're going to turn the motor over by hand fast enough to get a good reading.

You can do it while its on the stand if the starter is attached. Just hot wire it to a spare car battery.
April 9, 2010 7:58 a.m. Paul_VR6 Reader

I've done it with an impact on the crank pulley before. It's good for getting relative numbers, but not absolute.
April 9, 2010 1:19 p.m. Armitage Reader

You can always throw on the trans and hook up the starter to a battery and jumper it manually. Make sure you've taken the breaker bar out of the crank pulley before you engage the starter :)
April 9, 2010 5:12 p.m. skullsroad New Reader

I don't have the garage space to take off the tranny from the old motor and put it on the new one unfortunately. Plus my engine stand would tip over. I'll try the leak down test.

When I cranked the pulley when I first got the motor in the garage I think I turned it the wrong way and it felt like something slipped/jumped. What did I do?
April 9, 2010 6:04 p.m. m4ff3w SuperDork

Slipped the timing belt?
April 9, 2010 7:21 p.m. a401cj New Reader

fifty wrote:

A leak down test might be better if it's out of the car

that will only tell him if one cylinder is good compared to another...unless he uses an FAA standardized tester