comp. piston

[nicktrx400ex]

what does a more compression piston do does it make more low end or top end.

[400exstud]

It adds more power throughout the entire powerband. If you get too high you will HAVE to run race gas though.

[nicktrx400ex]

what is the highest with out having to run race gas. which one would be the best to race with and most reliable.

[2004exrider]

Air cooled engine= 11:1 and under 93 octane
Liquid cooled engine= 12:1 and under 93 octane

Jimmy

[zeppelin]

Originally posted by 2004exrider
Air cooled engine= 11:1 and under 93 octane
Liquid cooled engine= 12:1 and under 93 octane

Jimmy
my yz250f comes stock with a 13.5:1 and it runs fine on 89

[nicktrx400ex]

all i run is 93 so would 12:1 be alright ,still which is best. I have a 400ex.

[2004exrider]

Originally posted by zeppelin
my yz250f comes stock with a 13.5:1 and it runs fine on 89

Ok, sorry for the wrong info:)

Jimmy

[400exrules]

Originally posted by nicktrx400ex
all i run is 93 so would 12:1 be alright ,still which is best. I have a 400ex.

do u not pay attention?!.........no you shouldnt run 93 with 12:1, bad idea!

u could get some 110 though, and mix it 50/50 with 93

[bwamos]

400ex is air cooled.

Therefore you should not run more than 11:1 compression if you want to run premium pump.

Regarding the yz250f comment. That is a small motor, with water cooling, and advanced technology that is not availiable in 90% of the atv models out there. There is no atv on the current market that will run a 13.5:1 piston on pump gas safely under heavy loads.

[underpowered]

there are many factors that play into comp ratio to octane rating. a smaller motor can run a higher compresion on pump gas. the smaller motor creates less heat so pre-det is les likely so they can run a 13.5:1 on a 250f water cooled. on teh 400ex with alarger displacement air cooled engine 11:1 is aboutthe limit. you may be able to go slighlty higher, but not much. now get into big motors lik car engines, on a buddies 11:1 360(Bored 350) SBC engine, it requires race fuel because of the great displacement and much more heat created.

[nicktrx400ex]

my next question is what bore size and who to get it from, again i want it reliable.

[JOEX]

Originally posted by nicktrx400ex
my next question is who to get it from, again i want it reliable.
C&D Racing (http://www.exriders.com/vbb/contact.php)

[zeppelin]

Originally posted by bwamos
400ex is air cooled.

Therefore you should not run more than 11:1 compression if you want to run premium pump.

Regarding the yz250f comment. That is a small motor, with water cooling, and advanced technology that is not availiable in 90% of the atv models out there. There is no atv on the current market that will run a 13.5:1 piston on pump gas safely under heavy loads.
i understand, that a 400 couldnt run a 13.5:1 with the same gas that i am, ( the recomended octane for me is actualy 94 btw) i was just tring to say that the numbers that were givin before were a little on the conservitive side and dont apply to everything, realisticly if it were me in put something around an 11:1 and attempt to run 93 and if it runs okay next time try running the next lower grade, higher octane will not give you more power unless you are knocking, infact if you go too high you will actualy loose power not to mention the money you wasted on the higher grade

[nicktrx400ex]

will i have to do any other mods to the motor like stronger parts if i do this.

[pit/wood]

octane is simply a measurement of a fuels resistance to detonation under the pressure and heat generated in the combustion chamber.you can also reduce the tendency to detonate by removing the cause of it(hot spots left by the previous combustion cycle,either embers stuck on the comb.chamber or pieces of metal left from poor factory valve seat alignment and blending with the cast surface of the alu.cyl head)I guess it all depends how you apply a theory.did I realy say all that s%3t rotflmao.what zepp said about losing power from too high an octane is true.you only need a higher oct if you have a det. problem.lower comp.less chance of det.