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Does anyone know why you cant use the SE 11.1:1 pistons with the Twin Cooled heads? PN 21915-08A
 

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The pistons you are referring to are for the MVA heads which have a 95cc combustion chamber so the pistons are domed and will make contact with the bottom of your water cooled heads which will destroy your valves, heads, pistons etc.
In general it will grenade your engine.
Unfortunately there isn't a lot of bolt on piston or head upgrades available for the twin cooled engine right now. There may be builders with machine shops that could do performance head work on the water cooled heads.
In general a 9.5:1 to 10:1 compression ratio is the best for a touring bike IMO.
 

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I agree with Ironmark,when pistons become available for the wet head bikes i would stay under 10 to 1 ,the poor grade of gas could cause detonation issues,although compression can be considered cheap horsepower,its not cheap if it hurts your engine.
 

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It's not so much the compression ratio, but rather the cold cranking pressure being generated by a given combo.
The 255 cam in a OE 110"er at 9.2 cr from the Moco cranks at around 207 and reported cases up to 215.
A 124" crate engine @ 10.8 cr with the S&S .640 cam cranks at about 180-185, for example.
We strive for 195-200 psi ccp here on the efi bikes, and a tick less for the carbed builds.:)
Scott
 
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