Crank Compression Test - Test Pressure

A Cranking Compression Test Can Tell You A Lot

Mike Petralia Oct 1, 2004 0 Comment(s)
Sucp_0410_02_z Crank_compression_test Gauge 1/8

You might talk about it often, but do you really know what compression is all about? Every engine has to make it and those that don't are hurtin' for sure. But, here's the confusing part, for this story, we're not interested in some killer 14.5:1 compression ratio that could make an engine detonate to death as easily as it can make it sing. No, we're talking about cranking compression this time. Cranking compression is the pressure, measured in pounds per square inch (PSI) that builds inside your cylinders when the valves are closed and the piston moves up the bores to compress the mixture.

Your compression ratio, on the other hand, is simply a mathematical calculation comparing the volume of the cylinder with the piston at bottom dead center vs. the volume of the cylinder with the piston at top dead center. In that case, if you have 10 times more volume with the piston at BDC, you have a 10:1 compression ratio. Ironically, the compression ratio has little to do with cranking compression.

Once you've built an engine, the compression ratio cannot be changed without replacing the heads, head gaskets, or pistons, or changing the stroke, which are all factors that affect it. You can, however, change the cranking compression at any time with a few minor adjustments. Cam timing plays a critical role in the amount of cranking compression your engine makes, and generally, the sooner the intake valve closes, the higher your cranking compression will be so advancing the cam will increase cranking compression and retarding it will lower cranking compression.

Sucp_0410_04_z Crank_compression_test Kit 2/8

First step in a cranking compression test is removing all the spark plugs. We use this Summit Racing carb tool tray to keep the plugs, and our tools, organized.

A higher cranking compression typically translates into more power at the flywheel, but it can also mean death and destruction if it's not controlled with higher-octane gas, just like a high compression ratio would. Also, an older engine or one that's been abused heavily in its lifetime may not generate that much cranking compression, which could mean something's worn inside the motor. The cool thing about cranking compression is that it can be easily checked at any time using very few tools.

All you'll need to do to test it is remove all the spark plugs and install a compression tester. Then disconnect the coil, hold the throttle wide open, and crank the engine over several times with the starter. You'll see pressure build, and finally peak, on the compression tester's gauge. Make sure to count the number of engine revolutions it takes to reach max pressure and repeat the test for each cylinder, cranking the engine the same number of revs every time.

What You're Looking For
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.

Another interesting thing about cranking compression is that it can actually improve over time, especially when your engine is brand new. A freshly built engine may have inconsistent cranking compression figures because it's rings and valves have not fully seated yet. Run the engine for a few thousand miles and your numbers will probably improve. A cranking compression test is also an excellent way of determining the health of the engine in a car you're thinking about buying. Its non-intrusive nature means you won't be taking any risks. The whole test is no more difficult than changing spark plugs and the things it can tell you are worth the effort.