Compression test numbers

maachan · 05-04-2002, 08:28 PM

There is question about compression test figures.
There is a outfit in California that say the test result is 18/19 x compression ratio. What is this number 18/19 comes from?
There is other post that says the test figure is atomospheric pressure x compression ratio. If you do the math, there is substantial defference between them.

The more I think of this the more I get comfused. If I could know what those numbers, 18/19, comes from, then I may be able to make my educated decision.

Maachan

maachan · 05-05-2002, 07:37 PM

Hi,

Can anyone shed the light on this?

Maachan

maachan · 05-06-2002, 08:55 PM

Hi,

What I am asking, really, is what is the acceptable compression test result? What kind of number is called for an engine overhaul? When I tested my engine, the numbers were 145, 145, 145, 153, from #1 to #4, in April, 2000. I tested again in April 2001. The numbers were 150, 150, 160, 156. I am going to test it again this year, soon.

I thought these numbers are good. But then........

Maachan

fletch944t · 05-08-2002, 03:56 AM

Hi Maachan,

Your compression test numbers depend on which model 944 you have. Minimum readings on the compression gauge should be as follows:

944 Turbo - 120#
83-86 944 - 140#
87 - 88 944 - 150#
944S, S2 - 160#

After a rebuild, your initial compression number should be less than your second compression test (just as you indicated). It takes a while for the rings to fully seat so, the compression numbers should improve at least the first few times you check it.

Hope this helps.

maachan · 05-09-2002, 10:17 PM

Hi,

Thank you, fletch, for your input.

Sorry I failed to mention, my car is 87 924S. Does the number for 83-86 944===140# apply to my car? Well, the engine is not rebuilt engine as you might thought. It has at least 130,000. The trip counter is broke for a long time, even before my ownership. It reads 93,000. I think I had added at least 30,000.

I had time to check the compression this lunch break. The numbers are 150, 152, 150 and 158. If the minimum for the car is 140, I have some miles left in it. But if it was 150, ,,,,,,,,,,,,

Anyways, where do you find those minimum numbers?

Maachan

1.2gees · 1.2gees's Garage

So should I be happy or sad that my turbo reads ~138?
Ahmet

fletch944t · 05-10-2002, 02:40 AM

Hi,

For you Maachan, the 1987 924S had slightly higher compression than the early 944s (9.7:1 as opposed to 9.5:1). So, use 142# as your minimum compression numbers.

For you Ahmet, 138# is perfectly acceptable. The numbers I quoted are minimum compression numbers where you need to take action to fix a problem. My turbo numbers were 130-135# after a fresh rebuild. Those will increase slightly as I get a few more miles on the engine and the rings seat better (only 4000 miles to this point).

97XRAY · 97XRAY's Garage

So here's the numbers from MY compression test yesterday:
182
205
180
185

Knowing that the car is need of, and now having front end seals, and that's it's a euro spec early 944, what do you guys make of these numbers?

fletch944t · 05-10-2002, 07:34 AM

Hi Jake,

Quite honestly, I'm not sure what to make of those compression numbers.

Take a look at the plugs to see if there are major differences in the color of the plugs. That will probably give you the best indication. You might also take a long cotton swab and run it down into the plug hole to take a sample. You might possibly have a leaking injector, excessive oil leakage (valve seals or piston rings), or a leaking head gasket on that cylinder.

maachan · 05-10-2002, 07:21 PM

Hi,

Thank you guys for posting your comments on this thread.

fletch, where do you find the minimum numbers for the cars? Is there a formula to get that number?

There seems some ways to go down to 142s from 150s. I think that's a good news for me. I cannot live without my car running. If the car needs extensive repair like rebuilding engine would kill me, finacially and mentally.

From my experience working on aircraft engines, compression test numbers differ from test to test on one engine. Like, #1 cylinder was lowest last time, but this time is #4, and such. The same may apply to auto engine.

Maachan

fletch944t · 05-11-2002, 05:09 AM

You can get the compression ratio for your model car from Kevin Gross's 944 FAQ (under Model Information). A search for "944 FAQ" should come up with the address. Compression ratio numbers are given in terms of the ratio of pressure in the cylinder to atmoshperic pressure. So, taking the compression ratio for your car (9.7:1), multiply 9.7 times atmospheric pressure (14.69#) and you get 142.49#.

Bear in mind that 14.69 is atmospheric pressure for sea level. As you increase altitude from sea level atmospheric pressure will gradually decrease. So, if you want to be completely accurate, you can find out what standard atmospheric pressure is in your area and multiply that by the compression ratio. However, most standard atmospheric pressures are given in inches of mercury absolute, so you'll have to convert to psia.

Since sea level is the highest atmospheric pressure you should ever see (barring weather), multiplying by atmospheric pressure at sea level gives you the highest minimum compression spec - which is conservative. In other words, as long as you're above the minimum compression number at sea level, you know your compression is good regardless of the altitude where you live.

Sorry about the length of the post. I geeked out for a moment.

maachan · 05-11-2002, 07:37 AM

Hi, fletch,

Thank you.

I thought the compression ratio has to do with cylinder volume and combustion chamber volume or something like that. Anyways that may translate into pressure ratio.

The explanation you gave makes sense to me. I wish I could get some input from the shop where the formula for compression numbers should be 18/19 x compression ratio. I would like to know how they got that number.

If you do the search on this boad with wards "compression check", you will see what I am talking about.

I am interested in other people's opinion on this.

Maachan