PlanetMinis Forums banner


1172 Views 17 Replies 5 Participants Last post by  cyclerider57
what are the advantages or disadvantages in getting a high compression piston :?:
1 - 18 of 18 Posts
I only know of a advantage, more power.
advantage: more power

disadvantage: must use high octane fuel, but as long as you dont go over 12:1, 92/93 pump gas will be fine
Not true, my TB kit is 13:1 and I run 93 octane, no problems and it runs just fine.
Go ahead Dezmo explain what octane rating is for and why octane changes performance . :wink:
did the kit say 13:1 on the box or is that what you measured it at? :)

all I can say is youre lucky...really good gas (not everything that comes out of a pump marked 93 is actually 93 octane) or the kit is slightly less than 13:1 or something....just because it works for you doesnt mean it will work for anybody.
Vince I really dont feel up to the typing so try this:

they put it better than I could have :)
Well even if it isnt exactly 13:1, I know its up there, if a Take S-stage is supposed to be 11.5:1 with a dished piston, then this one has got to have lots more with a popup. I was told the compresion was 13:1 by KAOS, who CC'ed the head and all. I believe him.
octane is is not everything! the ron number is the limiting factor of deternation and yes tb kit is 13:1 and runs great on pump gas why?

because of the combustion chamber shape! dezmo do limit your self to black and white thinking,always think out side the lines and use the k.i.s.s. method!

and that is most important in engineering! R.J.P.
if anyone is interested? check the explination!

uhhh rjp: the octane rating on the pump is an average of the RON and MON numbers....look close at one and the small print says something like 'R+M/2 method'

and please explain to me how combustion chamber shape allows low(er) octane fuels to not detonate under high compression...this I gotta hear :roll:

btw I never said anything over 12:1 will detonate, I just used that as a rule of thumb.
Hmmm thats what I was thinking I would like to hear this too

we have played around with combusting chamber design with very little results on these small motors

combustion chambers have angles and edges right? well they do, sharp machined edges get hotter than smoothed out hand blended edges(fact).so when the engine is really hot these parts can cause preignition from these hot points! so if you have a good design you can get away with a higher compression ratio before detination,provided your load on the motor is not to excesive(eg,fat ass) so what do you think? is that a good enough explination? i have many years experience building lawn mowers to touring cars and everything in between i have ama,afm#1 winners with my chassie and suspension, so i guess i have a clue on what works, i don't need smoke nor mirrors to convince people on what they need. R.J.P.

if you do have so much experience building motors, then you should be well aware that compression-induced detonation and preignition caused by a hot part in the combustion chamber are 2 totally different things.

Having a smooth combustion chamber does nothing to 'help' higher to the 'angles and edges' you talk about.....most everything uses smooth, flowing shaped not prone to hot spots that can cause pre-ignnition....if you do get a carbon deposit or something thats glowing and causing pre-ignition, the compression ratio wont do a thing to help it, its just gonna pre-ignite no matter what. If you could just smooth out the chamber and run super high compression, why dont we have superbikes with 15:1 ratios?

I'm not calling you an idiot, but you obviously need some help with the differences between pre-ignition and detonation. And I've been building bike motors for 10 years now, with many race winners using motors I built. As to the smoke and mirrors, I dunno where you got that from as everything I have said is fact.
See less See more
i'm not going to have a pissing contest with you! sorry your the man i how many ama championships you got?guess i will see you at the track and lets see whats up? you in cali? R.J.P.
pissing contest? I'm just stating facts...sorry if that bothers you.

as to your ama said "i have ama,afm#1 winners with my chassie and suspension" ok, thats great...I'm truly happy for whats that got to do with motors? :roll:

also "I'll see you at the track" ok, if that didnt invovle a 3000 mile trip I'd take you up on if you beating me would make you right . :roll:
Heh I've been playing with motors since I was kid everything from lawnmowers radio controlled car motors to big cars this idea that hot spots in the combustion chamber causing predetonation is true but not on these small 4 stroke motors. the compression ratios do effect detonation but little hot spots never. I've played with combustion chamber like I said before and no differance was noted . I've also played with the dome shape of the pistons Hieght ,progressive edge and squash zone no differance . these theorys just don't apply with little 4 strokers from what I have seen .

No pissing matches needed just facts . but like I said these little 4 strokers are being built bigger and better than lets say 10 years ago so we should keep the lines of comunication open and the R&D rolling.

there is still alot we don't know about these little guys so back to the drawling board we go untill we figure it out
See less See more
this isn't extremely accurate but it will due. this is a chart out of an atv magazine. they were doing a test over octane. they found out that more important than the octane of a fuel is how fresh it is. in a bored out 250r with 14:1 compression they found that a lower octane fuel that was two weeks newer performed much better than the higher octane. if you want me to be more specific i can reread artical. both fuels used were race fuel from vp. if you can get away with a lower octane fuel than do it. i would always recommend vp though expecially if you can afford to spend the money on it. i conclude by saying that if you use a lot of fuel in a week or two then get vp. if it is going to sit then you are better off saving your money. i am not looking to get into a fight either. just wanted to make a point for whichever side wants to use this info.

See less See more
1 - 18 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.