How do i do a compression test?

[Rock]

I wanna see if i screwed the engine up. How do i do a compressions test? What is this thing all about?

[shocker1322]

On a related subject, how do you find compression ratio? I know it is Vinitial/Vfinal, where V is the total volume the cylinder contains. Can it be found using just a pressure gauge and dividing by atm (14.7 psi)?

To do a compression test, there is a pressure meter that you screw into the sparkplug hole. Then you crank the engine over a few times. The meter will read the highest pressure until you clear it. This will only give you how high the pressure is going on the compression stroke. Another good test to do is a leak down test. This is sort of a backwards compression test. Inject the cylinder with 100 PSI of pressure, then see where the pressure stabilizes. This shows how well the piston holds pressure. Anywhere around 90 PSI is good. 70 is pretty bad, you might have a scratch on your cylinder wall or bad rings.

[Taz's Master]

Shocker, the only way I have read of to find the CR is to fill the cylinder at bdc and tdc with liquid from a container that reads the volume it contains to see how much it takes to fill the cylinder. Then do the same with the combustion chamber, add, then divide. I'd love to hear of another way, and use it to find the CR of my buddy's engine.

By the way, do you know of any local recyclers with 944's in their yard?

[shocker1322]

yeah, was hoping to avoid tearing an engine apart for a compression ratio. Just was thinking that if your ratio is 2:1, then at TDC you should have 2x atm. Just going on that assumption, and it might be wrong.

[toolboy62]

shocker, your assumption would be correct if you had a cam with no overlap, but that isn't generally the case with an automotive engine. Therefore at cranking speeds there will be less pressure in the cylinder than the mechanical CR would indicate.

nate

[shocker1322]

Ah, gotcha. Thanks for clearing that up.

[AFJuvat]

For a good rule of thumb,

Multiply the compression ratio by 18 and 19, that will give you a window.

944 NA: 9.5:1 CR

9.5 x 18= 171
9.5 x 19= 180.5

Crank the car 3 - 4 times per cylinder to get an accurate reading.

AFJuvat

[s928s]

THERE IS A COUPLE WAYS...ooops !!

There is a couple ways you should check compression. comparing the results would give an strong indication what part of the componts went bad.

I was always told to check compression with all of the plugs removed, if a cylinder is leaking across into another cylinder this would fool the reading. check thecompression at all cylinders and note the reading off the gague. all cylinder should be aprox 10-20 psi of each other. it one varies greatly from the others there is an internal problem.
add a small amount of oil to each cyl. and recheck, if the weak cylinder regains its pressure, it indicates a ring or cylinder lining issue. ( wet test)
If the bad reading does not change it relates to a coolant or valve train issue. Most compression testers come with an adapter to add compressed air to the cylinder. Add air to each cylinder and listen for an air leak. air sound through the exaust manifold is an exaust valve issue. Air leak through the intake is a intake valve issue. In extream issues with an internal coolant leak the same testing will make bubbles in the overflow.( easiest to pull the bleed screw on the upper coolant inlet to see, and leave the cap on the resivoir...
the only other note is if 2 ajacent cylinders have the same low compression, the headhasket is leaking across the cylinder gasket.

[idontknow]

Need compression ratio? It's not really an involved process but it doesn't change unless you have bigger pistons, a longer stroke, or your cylinder heads had some work done.

1. Determine the displacement of your engine. Displacement formula is:

BORE X BORE X STROKE X .0031416 = DISPLACEMENT

Example: 100 x 100 x 78.9 x .0031416 = 2479cc

2. Determine the swept volume of one cylinder:

Example: 2479cc divided by 4 = 620cc

3. Find TDC. Remove the #1 Spark plug. Use a large syringe filled with light oil and fill the cylinder to the bottom of the spark plug hole. Make sure you count the CCs.

4. You now have all the measurements to determine your compression ratio.

(one cylinder swept volume + head cc) / head cc

Example: 620 + 73 = 668 / 73 = 9.5:1

[ilikemy944]

Uh...
to do a compression test, get a tester.
Remove coil wire
remove a spark plug, attach tester, crank engine four times.
Check number, repeat for all four cylinders.

[VaSteve]

Originally Posted by ilikemy944

Uh...
to do a compression test, get a tester.
Remove coil wire
remove a spark plug, attach tester, crank engine four times.
Check number, repeat for all four cylinders.

Which coil wire do you remove?

[ilikemy944]

From coil to distributor - just in case
Also remove DME relay.

[Jrboulder]

You need to find the atmospheric pressure at the time of the test in PSI (easy to find online)

You sure won't get the same compression numbers at different altitudes.

Jackson

[Stephen03]

A Diff. test is all that maters, but requires a bunch of ****ing around to get the piston on compression stroke TDC, Thats all that is required for airplanes so I feel its a more informative test to do anyways. Just too bad we can't change out the whole cylinder like can be done on a radial.



Enjoying a beer after changing the belts and some other jobs while I was there.

[Jrboulder]

[QUOTE=Stephen03;6275287]A Diff. test is all that maters, but requires a bunch of ****ing around to get the piston on compression stroke TDC, Thats all that is required for airplanes so I feel its a more informative test to do anyways. Just too bad we can't change out the whole cylinder like can be done on a radial.
QUOTE]


I sometimes fantasize about putting an IO-320 in my 944.

Jackson

[Stephen03]

[QUOTE=Jrboulder;6276017]

Originally Posted by Stephen03

A Diff. test is all that maters, but requires a bunch of ****ing around to get the piston on compression stroke TDC, Thats all that is required for airplanes so I feel its a more informative test to do anyways. Just too bad we can't change out the whole cylinder like can be done on a radial.
QUOTE]


I sometimes fantasize about putting an IO-320 in my 944.

Jackson

IO-320? Haha what about a TSIO-540?

[Jrboulder]

[QUOTE=Stephen03;6276331]

Originally Posted by Jrboulder

IO-320? Haha what about a TSIO-540?

But what would you save for the 928? an IO-720 ?

[whitesmile]

[QUOTE=Jrboulder;6277199]

Originally Posted by Stephen03

But what would you save for the 928? an IO-720 ?

Yup, what do u save? I'm just curious about that.

[egil]

Originally Posted by ilikemy944

Uh...
to do a compression test, get a tester.
Remove coil wire
remove a spark plug, attach tester, crank engine four times.
Check number, repeat for all four cylinders.

Remove DME relay in addition, so that you have no injection.

Also, I prefer removing all plugs first, it is no more work. My feeling is that the cranking RPM is more consistent this way, and anyway, the load on the starter is less.

[Stephen03]

[QUOTE=whitesmile;6277277]

Originally Posted by Jrboulder

Yup, what do u save? I'm just curious about that.

R985, Minor mods required.