RUNNING COMPRESSION TESTS
This is a summary of the responses to a question about a "Dynamic
Compression Test" sent out via the i-ATN e-mail list and posted on
Compuserve's "For Techs Only" forum. It seemed to ring a bell with the
most techs as a "running compression test," so I will use that name
here. Call it what you will, this test is an accurate if slightly
esoteric and time-consuming test of cylinder breathing. It is in fact
recommended by Detroit Diesel instead of a traditional static
compression test, it is included as part of Delmar's ATTP program, and
several instructors use it as part of their state emission training
programs.
HOW TO PERFORM A RUNNING COMPRESSION TEST
1. Start with a normal ("static") compression test. To eliminate rings,
valves, holes in pistons, that sort of things.
To do this: Unscrew all sparkplugs, disconnect fuel injector plugs,
disconnect coil primary plug, and have another battery nearby in case the
cranking of this engine runs its battery down. Prop open the thottle bores
being carefull not to drop anything in them. Carefully screw in your
compression tester (the press on type gauges are vastly inferior) to one
spark plug hole. Crank the engine over. Record the compression tester
reading on the first revolution of the engine and the fifth revolution.
Write these values down and go onto the next cylinder.
If you suspect you have bad rings, repeat the entire test above but
this time put an eyedropper full of SAE 70W gear oil in each cylinder
before screwing in the compression tester. 30 psi higher readings on a
cylinder on this 'oil-seal-assisted' second test indicates that cylinder
has worn out or broken rings.
2. The Dynamic Compression Test:
Remove prop from throttle bores. Put all spark plugs but one back in and
reconnect all electrical plugs taken off in step one above.Ground that plug
wire to prevent module damage. Disconnect that injector on a port fuel
system.
3. Carefully thread your compression tester into the empty hole. The test
can be done without a Shrader { press to release } valve, but most people
recommended leaving the valve in the gauge and "burping" the gauge every
5-6 "puffs".
4. Start the engine and take a reading. Write it down
5. Now goose the throttle for a "snap acceleration" reading. Reading
should rise. Write it down NOTE: Don't use the gas pedal for this snap
acceleration. The idea is to manually open then close throttle as fast
as possible while without speeding up the engine. This forces the engine
to take a "gulp" of air.
6. Now write down your readings for at least the bad cylinder (if there
is a single bad cylinder) and maybe 2-3 good ones. Make a chart like
this:
..........STATIC COMPR /IDLE -RUNNING COMPR / SNAP
Cyl1 ...........150 ..................75 ..........................125
Cyl 2 ..........175 ..................80 ..........................130
Cyl 3 ..........160 ..................75 ..........................120
7. ANALYSIS: Running compression at idle should be 50-75 psi (about half
cranking compression). Snap throttle compression should be about 80% of
cranking compression.
EXAMPLE 1 - RESTRICTED INTAKE
CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP
Cyl 1 150 ...................75 ...............................80
If Snap reading is low (much less than 80% cranking
compression), look for restricted intake air- severely carboned intake
valve, worn lobe on cam, rocker problem, "shutters" mispositioned in the
runners. (Toyota, Vortec etc. with variable runner length) Comparing
measurements between cylinders is important.
EXAMPLE 2 - RESTRICTED EXHAUST
CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP
Cyl 1 .......150 ...........75 .............................180
If snap measurements are significantly higher than
80% of cranking measurements, look for restricted exhaust on that
cylinder-such as worn exhaust cam lobe, or collapsed lifter. Or, if they
are all high, look for a clogged cat converter.
WHAT IS GOING ON?
When you do a normal compression test, you are checking cylinder
sealing, not cylinder breathing. When you check engine vacuum at the
manifold, you are looking at the breathing of the entire engine, by
checking vacuum at a common (plenum) source. You aren't testing a
specific cylinder. This test looks at the breathing of an individual
cylinder.
Say the engine is running at 18 inches vacuum. Atmospheric pressure is
about 30 inches, so the difference (30 inches - 18 inches = 12 inches)
is what the engine is sucking in. 12 inches mercury is equivalent to
about 6 psi absolute air pressure. Compressed at an 8 to 1 ratio, you
should get CONSIDERABLY MORE THAN 6 x 8 = 48 psi pressure if all the air
makes it into the cylinder and then gets pushed out. So your idle reading
on running compression is about 55 to 75 psi.
Contary to popular belief, an 8 to 1 compression ratio will give you
MORE than eight times the pressure. It would give you exactly eight times
the pressure - but only if you compressed the air and then gave the now-hot
air a few minutes to cool. Fast compression in an engine gives
considerably more pressure because the air doesn't have time to cool. This
kind of compression is called "adiabatic" which is a fancy name for "no
loss of heat."
When you snap the throttle, the manifold vacuum drops, so the absolute
air pressure going into the cylinder increases.
In fact, you can do running compression tests at various constant
manifold vacuum readings (by brake-torqueing the engine momentarily),
and the running compression should roughly correspond to the manifold
vacuum. For example, at 10 inches vacuum, engine should be breathing in
about 10 psi air pressure, so you should see a running compression
reading of about 80 psi (at 8 to 1 compression ratio).
If one cylinder reads low running compression compared to the rest it
means that the air didn't make it in. If one cylinder reads high, the
air didn't make it out (and the next pulse of air raised the pressure).
Many thanks to the people who responded through both Compuserve and
i-ATN.
