Leakdown tests.
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Cruiser
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Leakdown tests.
I did an S&S 106 kit, the cranking compression is 178lbs in both cylinders, the calculator comes up with 187lbs for my combo. Its a little lower than I was hoping so I did a leakdown test and I got 6% in the rear and 7% in the front at 100psi, I have 7000km on it. Would this be considered good. When I build a high performance street strip motor I aim for about 4%. Am I expecting to much for an air cooled Harley? Experts I need your input please 
HDF Community Team
Joined: Jun 2006
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From: san antonio
I did an S&S 106 kit, the cranking compression is 178lbs in both cylinders, the calculator comes up with 187lbs for my combo. Its a little lower than I was hoping so I did a leakdown test and I got 6% in the rear and 7% in the front at 100psi, I have 7000km on it. Would this be considered good. When I build a high performance street strip motor I aim for about 4%. Am I expecting to much for an air cooled Harley? Experts I need your input please
Everyone would like to see 2%-4% on these vtwins but unless the top end is built that way, it is not possible. 6% and 7% is nothing to worry about; most will tell you that anything less than 10% with a measurable variance between the two cylinders less than 5% is good. Even it the leakage is 20% or greater and is not leaking past a valve, freshening up the top end won't help performance much; probably not even enough to register on the butt dyno.
I assume that the 100psi was line pressure and the left guage was set to 90%? Did the right gauge read exactly the same as the left gauge before the cylinder was pressurized? Did you secure the brake to prevent the piston from moving when the pressure was applied? Where was the air escaping? Probably by the rings, which have gaps so normal.
Put the leak down tester away and don't test again until you have about 50,000KM rolled up.
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Cruiser
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Thanks for your input, yes all the testing was done properly, I even went as far as to cc my compression guage adapter, it was 2.5 cc which drops 5 lbs off of the readings. I know my ring seal is good but I would prefer better, has anyone run gapless rings in their Harley? Its something I might try when I do another build.
HDF Community Team
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From: san antonio
I haven't run gapless rings but a well know and respected builder in my area does not recommend them for street build. I have never asked him why, so I can't speak to the pros/cons of gapless rings but if Mike says they are not a good idea, I believe him. I know what you are saying about wanting to see better. I have 2000 miles on an all bore 107" motor and just checked leakdown; got 7%-9% and was disappointed but the difference in performance between 2%-4% and 7%-9% would most likely not register on a dyno. My bike runs to good to worry about leak down results that don't really mean much.
Like I said, if the bike runs good, put the leak down tester away and ride.
Like I said, if the bike runs good, put the leak down tester away and ride.
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Thanks for your input, yes all the testing was done properly, I even went as far as to cc my compression guage adapter, it was 2.5 cc which drops 5 lbs off of the readings. I know my ring seal is good but I would prefer better, has anyone run gapless rings in their Harley? Its something I might try when I do another build.
You might take a look at a set of Speed-Pro rings, if manufactured for your bore size.
Used to run those in our AHDRA racer, and have used them in many street bikes with great results.
Scott
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