I would like to know your results.

 

Getting real close to putting the 551 in to see how they compare to the se255.

 

Are you running flat top pistons only?
Reason I ask is I talked with S&S and they are planning to re-label the 551 as a bolt in to stock motors (less than 10:1) cause the compression the cam adds, it may have detonation issues. but they would not tell me not to use the combination...
what is your current cam?

 

Cams don't add compression. Long duration cams bleed off compression at lower r.p.m.s.

 

that's not what I understood from S&S; they said that the 551 was designed to close sooner adding to the compression. something about machanical vs running. they did not want to have the machanical above 10:1, the added compression caused by the cams would cause excessive pressure, potentially causing detonation.

 

Look at my signature and you will see the results of a 103 with the SE255's

 

Not exactly into a 103 but I did turn a 96 into a 106 w/570s. I haven't made it to the dyno yet but I can assure you she runs like a scalded dog.

I am a very happy camper.

Big Red

 

Did you do anything to the heads.

 

I think you'll find that "mechanical" compression simply means the ratio of volume at bottom dead center versus top dead center. Cam overlap causes the exhaust valve to remain open while the piston is moving up and pushing some of the volume of fuel air out of the exhaust. At higher r.p.m.s that works well and allows for better cylinder fill and getting the exhaust out. That is real world compression or "running" compression. Shorter duration cams allow less to escape and are good at low r.p.m.s. High compression is achieved using high compression pistons or reduced volume combustion chambers. Using short duration cams with raised compression will result in detonation and hard starting.

Cams don't increase compression. Shorter duration cams just allow for less loss of running compression.

 

I still can't get it through my pointed head how cams with "built in advance", like Harley's SE204, increase static compression. Team Lattus had a Forum answer long ago on how the 204 cams' 4 degree advance (intake is 22/34, vs. Andrews 37 with intake 18/38), resulted in higher static compression and more power on stock motors. A friend who used to manage a local Harley dealership ran them as high as 10 to 1, but beyond that pre-ignition was a problem. I am running a set at 9.9 to 1 in my trike, but am about to change them out for S&S 551 torque cams, as I pull a camper. I just read that S&S is recommending that they not be ran above 10 to 1 because of the static compression that the 551's make (intake 17/19 with intake centerline at 91). -Just trying to understand, thanks!