Topic: Best way to REDUCE compression ratio?

[paulages]

do you really need to run hi octane or hi oxy fuels with 12:1 ? i run a 13.5:1 and run the 91octane pump gas???

in what bike and setup?

[c91x]

oops i should of clarified.... my crf250 race bike CP 13.5:1 it just seems detonation is detonation.. maybe i'm wrong but anyways  i think hondamans idea of makin the head hemispherical woudl give you the best of both worlds still retain a higher than stock compression ratio but still have the bennifits of the hemi head

[Ricky_Racer]

Wow, it's interesting that this thread has lasted as long as it has.  When I post a question like this, partly I do so because I doubt that only one person in our large worldwide group has the same issue.  Anyone who purchases either used or NOS vintage pistons is going to encounter the same issue.  CR's (and Octanes) used to be higher, and there are many vintage piston sets that are still usable with some modifications.

Agreed, Sam, that the simplest and best solution is to only buy new pistons and specify the CR.  But again, this question was posted for a larger audience that will frequently encounter a "deal" on vintage pistons... or other vintage used parts for that matter.

In my case, I wanted to use the pistons for sentimental reasons: they were one part of a complete NOS vintage Action Fours 836 kit: pistons, rods, cam, springs.  It just seemed "the right thing to do" to try to make them work rather than replace them with a set of new Wisecos.  And it seemed like a good question to pose to such an experienced group of CB750-lovin' brothers as the members of this board.

I think we have found and discussed a number of usable solutions. I'll probably end up playing with combustion chamber volume and, maybe, a Cometic copper gasket.  I really won't know what'll be necessary until the burette comes out.

But for me and for anyone else who has been interested in this rather technical thread, thank you all for your input.  You're the primary this board can be so inspirational. RR

ps. Not sure about boring small holes in the crown of the piston, but I know for a fact that BIG holes reduce the CR like RIGHT NOW!!!   

[aptech77]

I'm running 11:1 in my 400F and 93oct is fine. It might/should be fine IMO.
My triumph is running 12:1 and they recommend 89oct. It IS however an oil/liquid cooled beast....

Right....big difference. Oil cooled Gixxers we ran at 11:1 no problem. Better chamber, valve angle, piston etc, etc. My FJ is about 11:1 and it gets HOT. The 400F is a very small bore....you can get away with more.

Actually my 400F is punched out a bit. Small engines are more susceptible to slight changes in air, fuel, pressures, temps, etc....especially after all the other changes that I have made. This thing is hyper to say the least.

[paulages]

my 64mm 750 pistons wouldn't even in my 650 head (negative compression ratio?  ). after machining them to fit and dialing in the clearances and CR, i only paid my machinist for about an hour of work ($70). granted, you have to find a good, fair machinist, but it should be fairly simple to turn down some material on a lathe if the machinist has the mandrel to hold the piston. i had mine balanced afterward with the entire crank assembly.