acceptable compression???
Road Captain
Joined: Mar 2008
Posts: 601
Likes: 6
From: Aruba
Anything over 90 psi is considered acceptable, but the girl is starting to get old at 110 psi. 125 - 145 psi is generally considered OK for a stock or slightly modified motor. 145 - 165 psi is considered excellent for a stock motor. Sometimes more important is that the difference between the front and rear cylinder needs to be less than 10%. You're at 8.3% difference, so this is still pretty good. My opinion is that the readings you have are well within the acceptable range but I would start thinking about a top end rebuild sometime in the future although this could be years out with no issues.
Did you run the compression test with the throttle wide open and run through at least 5 - 7 compression strokes? Without the throttle wide open, your readings will register low by 10's of psi's. Was this when the bike was warm or cold?
If the reading starts off low on the first stroke and builds up on each additional stroke but never gets up to a desired reading, this points to rings. If it doesn't build up much on each additional stroke, this points to valves or head gaskets. Another tests involves repeating the test after pouring about a 1/2 ounce of engine oil into the cylinder. If the readings go up a lot, this indicates worn rings.
To put these in perspective, my '93 had a Harley reman program rebuild done by the previous owner back in 2005 and he put about 2000 mi on the bike afterwards. I've put another 6000 mi on the bike in the last 1/2 year. My readings are 150 psi on the front and 139 on the rear with about 120 psi on the first stroke. These were done with the bike cold and I've never gotten around to running them when hot. As of now, the engine is pretty much stock with a stage one.
Another factor that will lower your readings is with an aftermarket cam along with an otherwise stock motor (pistons, head clearance). Something like an EV27 really wants the heads milled or higher compression pistons to realize its full potential because the cam overlap and point of intake closure lowers the corrected compression ratio. Without these mods, test compression readings will be theoretically lower, but I couldn't say how much.
Did you run the compression test with the throttle wide open and run through at least 5 - 7 compression strokes? Without the throttle wide open, your readings will register low by 10's of psi's. Was this when the bike was warm or cold?
If the reading starts off low on the first stroke and builds up on each additional stroke but never gets up to a desired reading, this points to rings. If it doesn't build up much on each additional stroke, this points to valves or head gaskets. Another tests involves repeating the test after pouring about a 1/2 ounce of engine oil into the cylinder. If the readings go up a lot, this indicates worn rings.
To put these in perspective, my '93 had a Harley reman program rebuild done by the previous owner back in 2005 and he put about 2000 mi on the bike afterwards. I've put another 6000 mi on the bike in the last 1/2 year. My readings are 150 psi on the front and 139 on the rear with about 120 psi on the first stroke. These were done with the bike cold and I've never gotten around to running them when hot. As of now, the engine is pretty much stock with a stage one.
Another factor that will lower your readings is with an aftermarket cam along with an otherwise stock motor (pistons, head clearance). Something like an EV27 really wants the heads milled or higher compression pistons to realize its full potential because the cam overlap and point of intake closure lowers the corrected compression ratio. Without these mods, test compression readings will be theoretically lower, but I couldn't say how much.
Thread Starter
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Intermediate
Joined: May 2009
Posts: 28
Likes: 0
From: White Rock, BC Canada
Thanks for the indepth response 95yj.
Thank you. I was concerned about the differential between the cylinders. I left the gauge in for about 5 minutes after the test and the compression went down about 5-10 lbs over the course of that time on each cylinder.
I forgot to test with the throttle wide open. I did let her cycle between 5-7 compression strokes each. The bike was cold when I tested (putting new plugs in).
I didn't watch the build up progression. I will do it again this evening giving careful consideration to how/what readings I am getting on each power stroke. Haven't done the oil test as of yet but that's another good recommendations as I just had all new valve and head gaskets put in (due to an oil leakage problem).
Sounds like something I need to start saving the pennies for. Would love to have those compression readings. Top end reman deals with what:
heads, rings, valves, springs, push rods, lifters?
No aftermarket stuff on this ole girl. Previous owners just put tons of mileage and lots of pretty lights and chrome on her.
Anything over 90 psi is considered acceptable, but the girl is starting to get old at 110 psi. 125 - 145 psi is generally considered OK for a stock or slightly modified motor. 145 - 165 psi is considered excellent for a stock motor. Sometimes more important is that the difference between the front and rear cylinder needs to be less than 10%. You're at 8.3% difference, so this is still pretty good. My opinion is that the readings you have are well within the acceptable range but I would start thinking about a top end rebuild sometime in the future although this could be years out with no issues.
Did you run the compression test with the throttle wide open and run through at least 5 - 7 compression strokes? Without the throttle wide open, your readings will register low by 10's of psi's. Was this when the bike was warm or cold?
If the reading starts off low on the first stroke and builds up on each additional stroke but never gets up to a desired reading, this points to rings. If it doesn't build up much on each additional stroke, this points to valves or head gaskets. Another tests involves repeating the test after pouring about a 1/2 ounce of engine oil into the cylinder. If the readings go up a lot, this indicates worn rings.
To put these in perspective, my '93 had a Harley reman program rebuild done by the previous owner back in 2005 and he put about 2000 mi on the bike afterwards. I've put another 6000 mi on the bike in the last 1/2 year. My readings are 150 psi on the front and 139 on the rear with about 120 psi on the first stroke. These were done with the bike cold and I've never gotten around to running them when hot. As of now, the engine is pretty much stock with a stage one.
heads, rings, valves, springs, push rods, lifters?
Another factor that will lower your readings is with an aftermarket cam along with an otherwise stock motor (pistons, head clearance). Something like an EV27 really wants the heads milled or higher compression pistons to realize its full potential because the cam overlap and point of intake closure lowers the corrected compression ratio. Without these mods, test compression readings will be theoretically lower, but I couldn't say how much.
Road Captain
Joined: Mar 2008
Posts: 601
Likes: 6
From: Aruba
Run them again with the throttle open. I think you'll probably get readings up in the 130's if you got 110-120 with the throttle closed.
Tourer
Joined: Nov 2007
Posts: 334
Likes: 2
What is your elevation? I live at 6500 feet and in my experience stock evos will make 100-140 PSI at that elevation.
BTW, your numbers are very close as a percentage. That's a good thing, run the compression test with a warmed up engine and open throttle and you'll get a clearer picture.
BTW, your numbers are very close as a percentage. That's a good thing, run the compression test with a warmed up engine and open throttle and you'll get a clearer picture.
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Intermediate
Joined: May 2009
Posts: 28
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From: White Rock, BC Canada
Arealinvestor - what do you feel it should be more like?
Jim
Jim
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Intermediate
Joined: May 2009
Posts: 28
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From: White Rock, BC Canada
What is your elevation? I live at 6500 feet and in my experience stock evos will make 100-140 PSI at that elevation.
BTW, your numbers are very close as a percentage. That's a good thing, run the compression test with a warmed up engine and open throttle and you'll get a clearer picture.
BTW, your numbers are very close as a percentage. That's a good thing, run the compression test with a warmed up engine and open throttle and you'll get a clearer picture.
Jim
Thread Starter
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Intermediate
Joined: May 2009
Posts: 28
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From: White Rock, BC Canada
Will this evening. Thanks 95yj.
Seasoned HDF Member
Joined: Jun 2010
Posts: 9,320
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From: Upstate New York
Compression test should always be done on a warm engine, and with the throttle held wide open. Done this way, my last compression test on my stock Evo (a year ago) was 150 psi front and rear. Not bad for 65,000 miles.
It's about time to check it again.
It's about time to check it again.