Octane and Compression

[hercalmighty]

Ok here it goes. I'm looking at bumping up my compression on the shee and my question is why does my 125 run at 170 psi on 92 octane but I cant run the shee any more then 150 psi on the same octane? What is the differance between the two? I know that alot of people have brought up the fact that their dirt bikes run much higher compression then the shee but still run on pump gas yet know one has giving a clear answer as to why this is.

[4stroker]

have you actually tried the 92 in the banshee with 150+ psi? or are you just going off what you read/hear?

 

if you have tried it and it deto'd, but it doesnt in your 125, then its probably because your 125 has a much better combustion chamber design, and/or less ignition timing...

[hercalmighty]

Thats what I'm trying to get answered. Is it that the new dirt bikes have better ignition or better combustion chambers? I'd like to hear from some motor builders on here

[4stroker]

your probably better off ringing one of them.

 

i would ring jim from passion racing.

[TeamRealtreeHD]

It all goes by bore size... the 125's I think are like a 54mm bore or something. Stock banshee bore is 64mm I believe. That might be the difference

 

At least is is logical with pressure and Octance requirements...

Edited August 5, 2008 by TeamRealtreeHD

[hercalmighty]

I know 250 have higher compression then the shee and they have bigger pistons. What about 250Rs what kind of compression do they have? Someone has to have an answer to this

[TeamRealtreeHD]

My honda 250r had a standard bore of 66mm. Higher than the banshee...

[hercalmighty]

Did you ever do a compression test on it? By the way glad to see your got the shee running and that it runs like a raped ape. Have to chalk another one up for Kevin

[TeamRealtreeHD]

I had a strong running bike, that I regret I sold a few years ago... I shoulda kept it and got a banshee too...

 

I used to have anywhere from 170-195 on a strong stock stroke motor... but after alot of work, and a ESR Big bore kit, head, port job and timing, I was running well over 200 PSI in that thing, closer to 210-215 PSI on a fresh top end.

 

And since the banshee has 2 cylindcers, it is hard to tell with the kicker. But you sure as heck could tell with 1 cylinder trying to kick that much compression.

 

I am a firm believer in the bore size making this happen. The four strokes are a totally different story, but a few buddies have kickers on their yfz 450's, and man, they have to get up on the seat and give 'er hell. The cylinders and pistons on those puppies are like bowling balls.

 

I am not going to say that is what makes the compression happen, and making you need race gas.... but it sure is logical and I think may have a;lot to do with it with my experiences.

[4stroker]

i think the banshee will run fine with 150+ psi and 92 octane

 

we accidently ran a buddies banshee the other day on 87 octane, and it has 19cc domes, and a 66mm bore, at sea level. no signs of deto

 

the same bike has been run for years like this on 93/94, no problems what-so-ever

[hercalmighty]

I'm not asking if I can run 92, I'm asking why you have to put race gas in a Banshee well before any other bike or quad. I hear you on the 4-strokes. I had a YZF426 and I'm only 5'6" so you can imagine what it looked like seeing a midget try to kick start that monster. I think the seat came up to just about my chest without any weight on it. I had to get the thing lower 2" front and rear.

[TeamRealtreeHD]

Yea, the single cylinder big bore bikes have alot of compression. Even the quadzillas and like we discussed, the 250's, have alot.

[TeamRealtreeHD]

Although a guy I know runs a CR500 and that thing is piped, ported and has a few other mods and only runs 175 PSI