Measured Compression?
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Intermediate
Joined: Feb 2012
Posts: 42
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From: North Carolina
Measured Compression?
I've got a 103", 211 cams, 10.5:1 pistons... What should I be getting for readings on a compression test? (I'm trying to be careful how I word this, I don't want to open a can of worms). Engine warm, closed throttle I measured 175 psi on rear cylinder, 180 on front. I'm happy they were so close but realized I didn't know what kind of numbers I should be seeing if everything is healthy and happy....I think I'm good.... Anybody know?
Tourer
Joined: Jan 2006
Posts: 323
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From:
Throttle needs to held wide open to get good readings.
6 to 8 cranks with both plugs out and grounded. You can take a wild *** guess but if you don't know what the cc of the heads are and deck height of the pistons and head gasket thickness. It will be shot in the dark. IMO
6 to 8 cranks with both plugs out and grounded. You can take a wild *** guess but if you don't know what the cc of the heads are and deck height of the pistons and head gasket thickness. It will be shot in the dark. IMO
Road Warrior
Joined: Mar 2010
Posts: 1,505
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From: Vancouver Island, British Columbia
I went to the Big Boyz site and plugged some flat top numbers into their calculator using the 211 cams intake close of 45 and it told me 181.9 so if you're using pistons that are rated at 10.5:1 I would think you are down on compression for some reason. I got a 103 put together over the winter and I am going to do a CCP on it as soon as the top end gets broken in and before it gets on the dyno. Can't wait to find out the numbers.
Tourer
Joined: Aug 2008
Posts: 364
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From: pennsylvania
Last year when I got my build running, I did a compression test and I got 195 for each cylinder. After a few thousand miles I checked it again and got around 205 for each cylinder. My overall compression is 10.7-1 and the heads are cc'd to I think 86.
My SWAG for you should be around 180-190.
My SWAG for you should be around 180-190.
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Intermediate
Joined: Feb 2012
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From: North Carolina
Thanks for the input guys. I did forget to open the throttle (brain fart), but it did hit max pressure (180 psi) in just 2-3 gulps. I could redo it but don't think I'll bother based on what I saw. I'm OK that I don't have a serious ring problem. The bike has started using a bit of oil- I suspect my valve seals again (had the infamous 05 valve seal problem and the dealer probably replaced them with the same junk they took out). I think I've got 27K on my build and didn't want to see it using oil already. (maybe 3/4-1 qt/1,000 miles of hard riding). I'm going to keep riding and just watch the dipstick for a while. Maybe open it up next winter.
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