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post #1 of 17 (permalink) Old 01-04-2016, 12:51 PM Thread Starter
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Comp. Ratio

Hi guys, I am getting my heads modified by Fuel Moto , going with the level B CNC heads. My question is how high is too much comp. 10.5 seems high for an air cooled engine. Not going for all out top end performance, just a nice drivable upgrade. They said they will cut to order my desired comp ratio.
What are you guys running. I have a 2012 103 RGC with the Andrews 57h cam upgrade with 32 degrees overlap and an intake closing of 38 degrees after. I live in OK and usually ride 2 up.

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post #2 of 17 (permalink) Old 01-04-2016, 03:59 PM
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10:1 with that cam (57h) and a .030" head gasket at your altitude will give you a corrected CR of 9.3 and 190psi Cranking compression. Myself, I'd do no more than 10:1. Hope your getting a good tune also.
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post #3 of 17 (permalink) Old 01-04-2016, 04:07 PM
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With a good dyno tune you can go to around 10.2-10.5 with no problem. Anything more and it's a waste of compression with that cam. If you go to 10.5, you will be set up later on for more cam. I'm running stock compression on a 16 RGS with D&D exhaust and really runs good. The 16's automatically have more compression than your stock 103. I'm going to 10:5-1 this winter with a 1.9" intake valve and some head massaging. If I wasn't doing the labor myself, it wouldn't be worth it for the extra hp/tq it produces with that cam. By the way, I'm a big fan of this cam.
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post #4 of 17 (permalink) Old 01-04-2016, 04:40 PM
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10:1 with that cam (57h) and a .030" head gasket at your altitude will give you a corrected CR of 9.3 and 190psi Cranking compression. Myself, I'd do no more than 10:1. Hope your getting a good tune also.
Good advise for the type of riding you describe.


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post #5 of 17 (permalink) Old 01-04-2016, 08:25 PM
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Don't have the heads cut at tall. Stock heads with a .030 head gasket put you right at 10.15:1 or so. You may want to have them CC the heads and trim to match if they are off, but thats all.

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post #6 of 17 (permalink) Old 01-04-2016, 08:42 PM
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Don't have the heads cut at tall. Stock heads with a .030 head gasket put you right at 10.15:1 or so. You may want to have them CC the heads and trim to match if they are off, but thats all.
If I'm taking the time to take the heads off and new cometic .030 head gaskets, I'm cleaning the ports up and putting new valve guides and seals and the 1.9" intake. That's just me after going to all the trouble.
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post #7 of 17 (permalink) Old 01-04-2016, 09:51 PM
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I meant cut, or decked, for compression. By all means clean up the ports and valves/guides. Just don't deck them for compression. The compression is perfect with what he is doing unless the head porter sets the valves deeper and opens up the chamber volume.
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post #8 of 17 (permalink) Old 01-05-2016, 10:51 AM Thread Starter
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Fuel Moto said they would send me a tune to match the heads. I have the DynoJet powervision .
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post #9 of 17 (permalink) Old 01-05-2016, 03:51 PM
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Tune has to match heads, cams and pipe. Hopefully Fuel Moto has a map with your pipe and the 57 cams. Should run very good if he does.
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post #10 of 17 (permalink) Old 01-06-2016, 06:15 AM
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Originally Posted by Darreldanger View Post
Hi guys, I am getting my heads modified by Fuel Moto , going with the level B CNC heads. My question is how high is too much comp. 10.5 seems high for an air cooled engine. Not going for all out top end performance, just a nice drivable upgrade. They said they will cut to order my desired comp ratio.
What are you guys running. I have a 2012 103 RGC with the Andrews 57h cam upgrade with 32 degrees overlap and an intake closing of 38 degrees after. I live in OK and usually ride 2 up.
I'd place a call to that outfit, and ask to speak with the exact person that is actually performing the work on those heads to get the answers, and yes, 10.5 cr is way to much for that Andrews 57.
We pair that cam up with our, IN-HOUSE, Stage II Heads, that were concieved over 15 years ago, at 10.0 cr.
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