High compression piston + pump gas safe?

Hey,  I'm looking at a bike, 13 kx250f that has a high comp piston in it, it was broken in with the stock one and then changed right away.  The bike was only used for 12.X hours (i know who's it is) and never raced, but always ran race fuel (he raced, but not with this bike as it was brand new and he didnt get a chance before needing to sell it).  


My question is is it safe to run a high comp piston with 91 pump gas?  I won't be using race fuel and from what I have heard i'm a little worried about running normal gas with the high comp piston in it now...  I'm nothing more then your average weekend warrior, put maybe 30 hours on the bike a season and nothing hard, the only time i'd hit the rev limiter is when i panic rev to not land on my nose! heh


Any advice would be awesome, I was excited to get the bike but now a little worried. Basically, it's an amazing deal on the bike but the high comp piston has me kind of backing away now.




Engines should be run at the lowest octane level that doesn't cause knocking. Higher octane gas just makes it harder to start and runs the same if not worse. I have a Z400 engine that's now a big bore stroker 462 with 14.8:1 and I run 91 octane. I tried 87 once and it sounded like crap, like banging on a simmer pot with a wooden spoon. Put 91 back in it and its perfect.

why dont you just put the stock piston back in? i doubt it will run on 91 with a hc piston. 

Through a great mistake I have learned a lesson. My son has a 250f with more than 14;1 compression ratio and will not run for long on pump gas. I tried to mix race and pump gas and ended up with a cracked $250 piston. The issue here is pre detonation. If the fuel and air in the cylinder ignite before the piston comes up to where it is suppose to ignite, it causes horrible force on several components, piston, crank, bearing,etc. It was explained to me that race gas has detonation inhibitors that will not allow the gas to ignite prematurely. Im sure there are other factors like cam and spark timing that are to be considered in the equation but Im not well versed enough to speak on that. I would call the manufacturer of the piston to see what they recommend. Make sure you tell them if there are any other mods to be considered. Im sure if you keep this post fresh there will be some other experienced builders chime in.

It really comes down to how high of a compression piston are you using and what cams you are running. If you are running long duration cams, they will actually decrease the dynamic compression ratio. This is why really big cams need a higher compression piston just to get the dynamic compression ratio back to where it is stock. I have run 14.5:1 pistons on pump gas with really large cams, but if I ran the stock cams with that piston and fuel it would detonate.


The KX250F comes with pretty high compression already (13.5:1), and 91 octane is about the bottom limit with that motor stock. If you are running stock cams then I would not recommend running higher compression than stock without going up on the octane of the fuel. You could probably get away with mixing 15-20% xylene in with your pump fuel to get the octane level up, but you may just be able to get race fuel even cheaper and cut it with pump gas to save cash.


Another thing to note, in my experience, most 250F's start to lose power with high compression pistons and stock cams. If you don't change the cams it seems to be a downward slope much above 13.5:1 static compression. You may be better off from a performance standpoint, and from a money standpoint to just go back to the stock piston.

I don't want to let this prevent me from buying a killer deal, but having to get the shop to swap it for say 4 hours labour will suck after buying the bike


so the 13 stock is 13.8:1... so guarantee the hc one in it is 14.5:1 and ill have to swap it out. *frown*.  could a high comp one have potentially done any damage to it (on vp110) in the like 8-12 hours of use that I should worry about and rethink it perhaps? or should swapping back to the stock piston be a safe bet and be ready to roll then good old pump gas style, esseiantlly being good as stock/new. I do not believe there was any other engine work done other then the HC piston so that would be the only factor along wiht the 110 it ran on it that has me concerned.  Oh, and any advice on what else ot look for to try to verify the hours on it, th meter is at 12 or so but tht coudl be added anytime, frame has boot stuff marks which seems weird for 12 hrs but if you ride hard probably normal (i dont push my bikes im not good enough lol)


Sorry for the dumb questions, i'm just a little nervous now and don't wanna have a brand new grenade because it had a high comp piston, ran 110 and got swapped back to stock piston to run on pump 91+ gas

Edited by iggle

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