speedgene
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posted February 04, 2006 09:41 AM
Compression Testing: Before-During-After
What is everyone's take on reading compression...and why?
Do You:
1. Take a cold reading and these are your numbers?
2. Warm up the bike to get the correct "book" readings?
3. Not rely on compression at all, and use the Leak-Down method?
a. Take readings cold?
b. Take readings with a warm engine for determination?
4. Set the shims a certain way (as in: advance/retard) before taking a compression test?
5. Look for any old valve lash, and then proceed?
6. Watch the compression needle:
a. At the first few revolutions and stop?
b. Wait till the needle stops moving to it's max?
7. Check compression after "each run" down the strip?
8. Have a certain "running" number you use to begin teardown?
Don't think I haven't noticed the many capable, and knowledgeable talent here on this board. I'd be curious to know your approach?
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entropy
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posted February 04, 2006 03:03 PM
I do compression test (not leak down) on ea cyl after the build, just to check that the psi is in line with the CR and that ea cyl is more or less the same.
Last weekend i got the motor back together, did a pressure test and got 290psi which is on the money for my 15.2:1 CR.
I will do a test after it has run a bit, but with a 12 it is such a PITA to remove the plugs, those tests are few and far between.
I have never done a leak down test.
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speedgene
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posted February 04, 2006 03:37 PM
Thanks entropy. I will assume you took that test with a cold engine on the bench before you installed it.....correct? If so, that is a very interesting number if that was a cold test?
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entropy
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posted February 04, 2006 09:24 PM
 Edited By: entropy on 5 Feb 2006 05:49
it was a "in the bike" cold test, spinning the engine on the starter. I have done this test after each build just to assure all is OK before I hook all the bits up. Also, it gets the oil system full before starting it. I got 20psi oil pressure spinning on the starter (24v).
Just did another test after 3 heat cycles, engine stone cold, got 300-310psi (compared with 290 before firing it up)
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ninja12
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posted February 06, 2006 08:11 AM
Karl do you use new rings with each rebuild?
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entropy
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posted February 06, 2006 09:00 AM
nope,
i re-use the pistons and rings now that i SEEM to have solved the lil wear problem i was experiencing. Those with non-offset blocks shouldn't ever experience my issues 
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speedgene
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posted February 06, 2006 05:35 PM
Interesting. Start the engine up a few times and you increase the C with used parts. You know what, I believe it.
I did though, stopped believing in things like 120deg ring stagger on assembly. Add your example entropy, and I also stopped believing in warm C tests too. It's obvious the heat will expand the valve face in proportion to it's 45 seat angle, and the piston grows, the wall expands, etc... so, why bother checking it warm? It sure is holding C well cold. I never heard of an engine start up cold, and then shut down from loss of compression warmed up? Am I making any sense thinking this way?
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entropy
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posted February 07, 2006 01:53 AM
speedgene,
you are certainly making sense to me about cold cranking compression being a solid test, no need to do it warm, ESPECIALLY with the 12 being such a PITA to do a compression on once the bike is fully put back together.
BTW, after every build i do the cold cranking test, then back it up with a later test after the motor has run a bit. It seems I always get a 10-20 psi increase after running it.
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ninja12
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posted February 07, 2006 09:34 AM
Do you think the increase is caused by oil in the rings?
A simple test would be to do a cold test, add a few drops of oil and repeat test.
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frEEk
Administrator
ummm... yeah
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posted February 07, 2006 10:24 AM
how significant is leak down in these engines? specifically, is there enough leak to cause significant differences in compression at cranking RPM and at redline? or is this something noone has ever tested?
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ninja12
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posted February 07, 2006 01:45 PM
For a riding bike, probably not very important unless you the problem is getting worst.
Commuter can ride a long time down 5-10 hp and may or may not even notice.
For a race bike it's the loss of performance, and we want every pony.
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speedgene
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posted February 08, 2006 12:11 AM
quote:
Do you think the increase is caused by oil in the rings?
A simple test would be to do a cold test, add a few drops of oil and repeat test.
Good question ninja12. On assembly, I'm sure entropy assembled with some type of oil to the rings and valves, be it moly base, or plain oil. This will surly be washed off by the intakes, plus burned off the ex. valves, along with any laying on the upper ring surface. Therefore, on the second C check, the chamber was pretty much dry. What caused the C increase?
I'd love to see a few drops of oil, and see if there is any change? I would only be taking a guess who (valves or rings) caused the increase in C.
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entropy
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posted February 08, 2006 01:48 AM
i slop assembly lube & oil on the pistons etc when putting the motor together. But i don't really worry about the source of the increase after running it, shade tree mechanic, me.
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