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Author Topic: Compression results/evaluation
Juan Pineda Verified Driver Made Donation to Website
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Checking out my pro-built engine after it's third season, I find that the compression is 190, 175, 185, and 205, cylinders 1-4 respectively, WOT and all plugs out. Why the one high reading? Or is it likely that that cylinder 4 is perfect after three seasons, while the others are degraded? I don't imagine we suffer from carbon deposits. Are there other reasons for high compression readings?

Do these results warrant a motor refresh of some kind? Would the work likely be confined to the head, or would it need the bottom end refreshed in some way? It's much easier for me to just pull the head to send off.

Just trying to understand my options with a mind toward budget.

-Juan

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www.ArtOfRoadRacing.com Race Craft Clinic - Thunderhill - 30 Jan 2011

Kim Hinson
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Sounds like early signs of ring failure. Try a leak down test or the old way to check would be , squirt oil in the cylinder and retest the compression , that would tell you if the problem in the bottom or top end ..

When is time for a rebuild call Bob at RE

Jay Lutz
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Juan,

What Kim said.

Spray a little engine oil in each hole just before spinning the engine over......and repeat the test. That should tell you if the inconsistency is related to rings as the oil should temporarily seal up any ring related leak. Same results... the issue is somewhere else and not necessarily valves....could be a wiped camshaft lobe yielding low lift and low compression.

You should also do this test with the engine hot. Also repeat the first test on the same day as atmospheric variables like humidity and temp do make a difference.


Good luck, Jay

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"In our sport if you go out of bounds you're most likely not coming back" Michael Waltrip

TexasHammer
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Being a 1.6l, your compression pressure should be around 192psi, so you have a high one. #2 & 3 are a touch low. The book says to rebuild at 135psi, but that won't be competitive. Make sure you are using a quality guage that is accurate and repeatable. Test each cylinder twice while the engine is hot. You can also check where there is a leak with a leakdown test. A cheap way to make a leakdown tester is to gut a sparkplug and weld on an air fitting with a tube long enough to reach above the head and valve cover. You won't get any numbers, but it will reveal the culprit.

As for why one is higher, good question. Could be just a guage error, or miscalculated compression ratio from the original build. Won't know until you tear it apart.

Drago Verified Driver Made Donation to Website Series Champ
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Juan
Short of a quick and dirty test, compresion test to me is a waste of time. Warm it up and do a leak down.
Jim

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Jim Drago
East Street Auto Salvage
jdrago1@aol.com
2006-2007 Mid-West Division
07,09 June Sprints Champion

EAST STREET RACING

38BFAST Verified Driver
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I use a compression test to tell me If I have to do a leak down test. kinda like using camber plates to tell me if I have to get out the strings.

Jim is right. Time to do a leak down test. Anything below 5% is not a race motor.

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Ralph Provitz
#38
2008 WHRRI SM Champion
2008 WHRRI Top 10 Overall
V2 Motorsports, Race support, Data Dude

Juan Pineda Verified Driver Made Donation to Website
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I did the compression measurements twice on two separate days with similar results. So I'm confident that they are right.

I'll break out the leakdown tester. So the hard part I find about getting accurate leakdown measurements is getting the rings to seal. If you just position the cylinder at TDC and presurize, I've always found (in other engines too) that there's a lot of leakage, which I attribute to the rings not seating. I've tried is rocking the cylinder several degrees around TDC until I get a low result. I've also heard about turning the air on abruptly to seat the rings. What do others do?

-Juan

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www.ArtOfRoadRacing.com Race Craft Clinic - Thunderhill - 30 Jan 2011

38BFAST Verified Driver
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I rock the piston up and down to get the best number. Get a big braker bar when you do it.

--------------------
Ralph Provitz
#38
2008 WHRRI SM Champion
2008 WHRRI Top 10 Overall
V2 Motorsports, Race support, Data Dude

   

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