So I know the twin cam wet heads run a slightly higher compression ration than air cooled models, but does anyone know what their compression limit would be? Here in Commifornia our pump gas max is typically 91 octane, might find 93 here and there but it is not regularly available. That means air cooled bikes are usually capped at 10.5:1 pistons or less to help avoid summertime pinging. If my 103 wet head needs a top end could I run 11:1, 11.5:1 domed pistons and still be good with 91 octane? I realize tuning plays a part in the pinging as well but if you have to pull all kinds of timing out of the tune to avoid detonation whats the point of running the higher compression. Thats like using a condom with your wife, I paid the money for the full ride, not a safe run down version.

I know the M8s run these 11+ compression ratios without issues but they are different animals and being the twin cam wet heads only ran for a few years there doesnt seem to be much, if any, builder reports out there on them.

 

How well a motor tolerates compression is based on matched cams and a well tuned EFI. What is most important is the corrected compression that the cams influence. A Twin Cam in general can be tuned well with a 10:1 corrected compression. More common for a non hot rod would be in the 9.6:1 to 9.8:1. But as well needs a good tune.

Keep in mind if you're planning head work, the water cooled Twin Cam heads don't flow as much as the air cooled heads do.

 

I live in Socal and have run 11.3 to 1 in 2 TC motors, a 131 fxr and 116 dyna. I'm also running 11 to 1 in a 113 Evo. No issues with any once tuned correctly. All motors did have a late closing intake.. 55 to 57 degrees. All are carbed so they don't control timing based on heat. They run fine even in hot weather but I don't run them in traffic without splitting lanes. My 107 EG with 10.5 to CR gets hotter EFI.. All these are air cooled only..

If it's pinging the summertime, it not tuned right unless you got the motor smoking hot.. Typically if the bike is pinging retarding the timing will make more HP. EFI bikes do a pretty good job of dealing with heat because they can set timing based on CHT or ion sensing.

 

ok so 11+ compression is manageable with the right cams and good tuning.

Why would the wet heads flow less than the air, smaller valves to make room for the water passages or something? Do they offer headwork on wetheads, or has anyone swapped air cooled heads on and just removed the wet system altogether?

 

The ports have less area for the head porter to work with. This is due to the water passages. I'm speaking about TC only.