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Old 07-05-2019, 05:51 PM   #1 (permalink)
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Need Compression Reading in Pounds

I have put in a fueling oil pump, S&S cams, S&S 89 cc Performance heads, and Keith Black 10.6/1 domed pistons. This is in a 2005 Dyna Twin Cam 88. My mechanic asked me to do 1000 mile break in b4 we remap the motor, so I put on about 1300 miles below 3500 rpm. During that time I used just a little better than 2.5 quarts of oil. My mechanic and I did compression test and what he called a leak down test. The leak down wasn't a true leak down, just waiting a while to see if the compression bled off. The compression readings were 160 on the front cylinder and 170 on the rear cylinder. My mechanic, whom I trust, felt these were great readings and, initially, so did I. I have since been presented with the information that, with the equipment we installed, we should have readings more in the 200 and above range.

Any thoughts or comments from some of you experienced builders out there? Where do you feel my compression readings should be and where did that much oil go????
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Old 07-05-2019, 06:02 PM   #2 (permalink)
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where does your mechanic think it went??
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Old 07-05-2019, 06:12 PM   #3 (permalink)
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What cam (grind)?Without that information compression reading readings mean nothing here.
Too much oil usage for some reason.
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Old 07-05-2019, 06:41 PM   #4 (permalink)
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Use this calculator to get close. http://www.bigboyzheadporting.com/TwinCamComp.htm
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Old 07-05-2019, 07:42 PM   #5 (permalink)
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A motor that new should read almost identical front to rear.

And any wrench worth taking a scoot to would not have wasted time seeing how long the cylinder held pressure on a compression gauge. That's real shade tree stuff there.

Also, the correct map should have been in the ecm before you drove the scooter. You might hold off setting the final trims till after it had some miles on it, but unless it was mapped real close, like for a high compression 110, there could already be internal damage.

And as far as the oil use, if its not leaking it out, then its being burned. Camera down the plug holes should tell the tail on that.
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Old 07-05-2019, 11:54 PM   #6 (permalink)
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tall terry is still riding a stock bike tall terry is still riding a stock bike
it's like therm says,,, 2.5 quarts of oil in 1300 miles means tear it down again.. that is alot of oil,,

a proper map should have been loaded before the engine was fired up,,, much less ridden

I'm relatively sure there is internal damage,,, every motor I've built has near identical or identical compression readings... you dont want more than 180....

cam grind?

Last edited by tall terry; 07-06-2019 at 07:07 AM.
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Old 07-06-2019, 04:59 AM   #7 (permalink)
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The cams are S&S 510 C. Is that what you are asking by what grind?
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Old 07-06-2019, 08:00 AM   #8 (permalink)
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tall terry is still riding a stock bike tall terry is still riding a stock bike
I see a rebuild in your very near future...
problems I see from the supplied info:

mismatch of compression and cams
170-160 ccp on a combo that should yield 200#,, also 200# is 20# too high for a good street build
2.5 quarts of oil useage
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Old 07-06-2019, 09:45 AM   #9 (permalink)
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You say your motor is 88". So you put the new pistons in these standard bores? Tell us more about the honing
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Old 07-06-2019, 02:13 PM   #10 (permalink)
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standard cross hatch honing. Thanks to all for responding
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Old 07-06-2019, 02:31 PM   #11 (permalink)
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Originally Posted by GHBiker View Post
standard cross hatch honing. Thanks to all for responding
Putting new pistons in a used bore without going oversized is usually a failure. The bores are not straight and true. To get them straight and true in a fixed hone will result is a loose fit and oil consumption is the result. Best fix is to go up to 95" especially with those heads. It will be a lot happier combination at 10.5:1
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Old 07-06-2019, 04:04 PM   #12 (permalink)
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Thanks Don. That is certainly food for thought.
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Old 07-07-2019, 05:45 AM   #13 (permalink)
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Thank you for all the responses. A lot of that info I do not have. The cams are S & S 510 C, the honing was standard cross hatch honing, The heads are S & S 89 CC performance heads, The plugs are light tan and seem to be burning properly, although the pipes are sooty. Todd, my mechanic, has built a number of performance engines. He admits he cannot explain the oil consumption, but has assured me that, if we go ahead with the remap and dyno, a costly affair, and something fails he will stand behind all of it, rebuild, reremap, and what ever else is necessary. I have ordered a leak down tester and a bore scope as he does not have either. we will employ them before we go forward with the remap.
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Old 07-07-2019, 08:52 AM   #14 (permalink)
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Don Dorfman is reading this now saying WTF is this?
Oil goes by the rings, out the exhaust port and coats the pipes. If boring it costs the same to go to 95" as it does to 88" +.010 OS. Move up, the 95" is strong and proven. At the same time check the guides and valves seals. S&S is not perfect every time.
When a bore and hone is done properly these motors break in in a matter of a few miles. A good dyno shop will just run them at low speeds and upload some AFR fixes in the process turning off and on the motor and with fans this does the heat cycling. After they get some dyno miles on them and the low speed ranges are fixed they can make the full pulls and hand you a fully tuned motorcycle ready to go.
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Old 07-07-2019, 10:20 AM   #15 (permalink)
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What was the piston to cylinder wall clearance after you honed used jugs?

And please tell us you used a precision hone.
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