sharrowspeed924s

What are some good compression numbers for a 87 924S the numbers I have from my motor are
1. 120

2. 130

3. 120

4. 120

John_AZ

A lot depends on the test. Did you remove the DME/Fuel pump relay to prevent "washing" the cylinders of gas (per the WSM)?

Altitude? Miles on the engine?

Other views:
http://forums.pelicanparts.com/porsc...est-944-a.html

J_AZ

V2Rocket

those are pretty low, try again..

if they stay low, try putting a tiny bit of oil in the cylinder (to seal the rings) and see what the numbers come back with

John_AZ

+1^
When you pulled the head, did you do any machine work on the valves or check the valve seating/sealing?

https://rennlist.com/forums/924-931-...l#post10494113

J_AZ

sharrowspeed924s

Well I had a problem with it not starting earlier this year but that was partly because of it been cold the other problem came up as so as it was warm enough to start it oil got in to the intake ( which is why I check the compression) but oil problem I believe has gone away. The head has been work on but that was done a guy who rebuilds heads for a lot of shops in Alabama so I have a strong feeling the value seating/sealing this good

VirginiaF1

Howdy.. I believe we both have the M44.09 engine.

Last fall, I pulled 181, 179, 180, 182 on a cold engine @sea level & 70F,
WOT w/ no oil in CB for a 95.5% of factory spec after 25 years.

2.5L Factory Compression PSI at sea level should be CR x 18.5..?
So @ 10.2:1 CR, the original/rebuilt PSI should be:
10.2 x 18.5 = 189 for our n/a 8-valve, yeah?

John_AZ

Clarks Garage "minimum" compression numbers:
http://forums.pelicanparts.com/444455-post4.html

J_AZ

VirginiaF1

Thanks John... Too bad Mr. Fletcher doesn't post any longer.

Not to mimic the alchemy of an oil thread but:
How/why do some of us see numbers so much higher than his minimums?
Carbon buildup?

sharrowspeed924s

Could my numbers be low because of my location

VirginiaF1

Hi again..

Your 120-130's are way different from mine? Clearly..
Clark's Numbers:

944 Turbo - 120#
83-86 944 - 140#
944S, S2 - 160#
87 - 88 944/924S - 150#
Your 130 is somewhat closer to this 150; but significantly under..

Atmospheric pressure diffs between ND and Richmond would not explain our giant disparity...


Here are some other test readings from last summer:
http://forums.pelicanparts.com/7523223-post25.html
http://forums.pelicanparts.com/7523308-post26.html
http://forums.pelicanparts.com/3387736-post1.html

I've yet to see a definitive explanation for all these variances..
(Other than User Error)

nynor

your compression numbers should be the compression ratio x atmospheric pressure, which is ~14.7 at sea level (not 18.5). however, at altitude, atmospheric pressure is less, so, i'd expect to see only 80% of the normal compression reading at my altitude (~4500ft).

it is my understanding that a bigger issue would be large differences in the readings between cylinders, which the OP does not have, IMO. also, if the reading is being done in minot, nd, the elevation there is 1620ft. i think the numbers are spot on: 87 924S has a compression ration of 9.7:1 (raised to 10.2:1 in 88), giving a compression of 140PSI at sea level (9.7x14.5). so, at the altitude of minot, nd, i'd expect to see in the neighborhood of 125PSI.

my two cents.

darell

wingnut

These dynamic compressions are subject to several variables like the air density and the condition of the battery and starter and only provides a good/bad result. The best info from the dynamic test is any imbalance between cylinders.

I do not doubt others here are familiar, but I mention it for those that are not. The more informative test is the differential compression test, also called a 'leakdown' test, which is not subject to these conditional variables, and the pressure applied can point to where a the leakage is happening. This is the standard method used when inspecting and diagnosing aircraft engines. If going this route, be sure to use the differential tester with the smaller orifice sized for cylinders less than 5.125 in. diameter. Suppliers include Aircraft Tool Supply ( http://aircraft-tool.com/ ).