Compression test results - Alfa Romeo Bulletin Board & Forums
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post #1 of 35 (permalink) Old 09-23-2018, 02:06 PM Thread Starter
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Compression test results

So I've been using a TON of oil (2 quarts in 500 miles!) on my newly acquired '81 S2. I'm running Mobil 1 20W-50 V-Twin oil, I switched from Mobil 1 10W-30 which the PO had been running. I live in Texas, and felt the heavier oil would do better in the heat.

I did a compression test today, long rod was disconnected from the bellcrank, wire from coil to the dizzy disconnected, engine was hot ~180 degrees, I pulled each plug when I tested each cylinder, but the others were in place - here are the results:


Cylinder 1 - 92 lbs DRY - 160 lbs WET
Cylinder 2 - 90 lbs DRY - 150 lbs WET
Cylinder 3 - 90 lbs DRY - 150 lbs WET
Cylinder 4 - 92 lbs DRY - 140 lbs WET

Looks like the rings are the culprit - can I really be losing that much oil with cracked rings? The car does not smoke, it is running a bit rich (plugs electrodes are black after about 1500 miles) What's the next step in zeroing in on the problem? Any other ideas?

Many thanks, as always,

GC

'81 Spider (Weber's, Euro Bumpers, Custom Interior) Ivorio over Red | '54 Austin Healey 100-4, Bone Stock save for M cam, Carmine over Black.

Last edited by Crowbar44; 09-23-2018 at 02:27 PM.
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post #2 of 35 (permalink) Old 09-23-2018, 04:18 PM
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Do a leakdown test to identify the sources of the compression leaks.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #3 of 35 (permalink) Old 09-24-2018, 12:03 AM
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I did a compression test today, long rod was disconnected from the bellcrank....
so you did not do the test WOT? (wide open throttle)
I'd do it again.

Dom - Alfa Spider 1990 S4 - formerly: Alfa 101 Sprint, 2600 Sprint, Montreal - family classics: Jensen Interceptor II, '58 Hooper RR Silver Cloud I, Shadow II, '60 Corvette.
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post #4 of 35 (permalink) Old 09-24-2018, 08:18 PM Thread Starter
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I didn't do it WOT...assumed the fuel rushing into the cylinder would corrupt the results...
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post #5 of 35 (permalink) Old 09-25-2018, 05:27 AM
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so you did not do the test WOT? (wide open throttle)
I'd do it again.
+1

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post #6 of 35 (permalink) Old 09-25-2018, 05:50 AM
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It does not need to be at WOT but the throttle should be at least partly open.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #7 of 35 (permalink) Old 09-25-2018, 07:22 AM
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Compression tests are of little value in cases of oil consumption. Compression ring have little to do with oil control and oil rings have little to do with compression. But if you are going to do a compression test, do it right. All spark plugs out during the test and WOT. WOT may not be absolutely necessary, but it is consistent.
What is it you hope to know from the compression test? You already know the engine uses oil and a compression test tells you nothing about the condition of the valve guides and stem seals or of the oil control rings.
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post #8 of 35 (permalink) Old 09-25-2018, 09:34 AM
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A throttle opening of 10% will supply sufficient air for the engine to rev at 6000 rpm. That is about 200 times higher than the engine is spinning during a compression test. The WOT requirement is an old wives tale.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #9 of 35 (permalink) Old 09-25-2018, 10:07 AM
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A throttle opening of 10% will supply sufficient air for the engine to rev at 6000 rpm. That is about 200 times higher than the engine is spinning during a compression test. The WOT requirement is an old wives tale.
Just how does one be certain the throttle is sufficiently open? There's no need to guess if the throttle is wide open. It's not a matter of old wives, just of convention, convenience and common sense.
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post #10 of 35 (permalink) Old 09-25-2018, 10:26 AM
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to me it is just better to stick to a simple procedure
WOT, hot engine, all plugs out.

for instance, injection will go into 'clear flood mode' with WOT whilst cranking (say <600 rpm)....meaning the injectors will not spray.

Dom - Alfa Spider 1990 S4 - formerly: Alfa 101 Sprint, 2600 Sprint, Montreal - family classics: Jensen Interceptor II, '58 Hooper RR Silver Cloud I, Shadow II, '60 Corvette.
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post #11 of 35 (permalink) Old 09-25-2018, 10:33 AM
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to me it is just better to stick to a simple procedure
WOT, hot engine, all plugs out.

for instance, injection will go into 'clear flood mode' with WOT whilst cranking (say <600 rpm)....meaning the injectors will not spray.
Yup, KISS.
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post #12 of 35 (permalink) Old 09-25-2018, 11:52 AM
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Yup, KISS.


Did a compression test on an Alfa race car in the paddock at Road Atlanta this weekend. I can assure you that it is a darn site simpler just to crack the throttle than get it wide open when you have Webers and double return springs.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke

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post #13 of 35 (permalink) Old 09-25-2018, 03:01 PM
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for instance, injection will go into 'clear flood mode' with WOT whilst cranking (say <600 rpm)....meaning the injectors will not spray.
I'm not sure about that on a Spica car

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post #14 of 35 (permalink) Old 09-25-2018, 05:36 PM Thread Starter
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I'm running Webers, should I clamp the fuel line during the redone compression test with WOT?
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post #15 of 35 (permalink) Old 09-25-2018, 05:47 PM
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Quote:
Originally Posted by spiderserie4 View Post
to me it is just better to stick to a simple procedure
WOT, hot engine, all plugs out.

for instance, injection will go into 'clear flood mode' with WOT whilst cranking (say <600 rpm)....meaning the injectors will not spray.
Amen. I takes seconds to get a reading per cylinder (you only want a max of 3-4 revolutions of the engine per compression check, otherwise you will kill the battery) ... heck I've never worried about fuel.

Just do it as it is supposed to be done. Don't worry about fuel and get your numbers
Pete

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