- Location
- Knoxville, TN
I have a js 550 that just had the head milled and a new exaust pipe added. Do I need to run higher octane fuel now? The guy took 35000s of an inch off. does any one know the compression ratio?
:kabong:
:kabong:
compression ratio and fuel octane
- Thread starter Maxumflux
- Start date
Mark44
Katie's Boss
- Location
- 100% one place
o
You need to do a compression check on the motor and let us know the number and you also need to check your squish, this is very important.
.035 sounds like a lot.
Mark44
You need to do a compression check on the motor and let us know the number and you also need to check your squish, this is very important.
.035 sounds like a lot.
Mark44
Last edited:
- Location
- Knoxville, TN
Ya thats what i thought. I'll torque the head down to night, and i'll get the numbers out.
should i clean the pistons? they have a little carbon build up. would this make a difference?
Squish>?:notworthy:
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Ericfox
Do it twice?That's DooDoo
- Location
- Akron area --> OHIO!
prob. not... because you will get the carbon buildup back... so you should measure with it on the pistons... I'm no expert though... it might not even matter...
Without very fine details about your cylinder and head, no one can tell you what the compression ratio is. It is a series of equations that gives you ratio. It is pretty rare for anyone to talk much about compression ratios...because what we are truly concerned about is compression pressure, which is something, again, that no one can tell you. You will need to take the readings yourself. 185 and under run premium pump gas, anything over race fuel. On stock 550 heads it was/is pretty common to mill 0.05 without having to make any other changes.
This is true and false in my eyes. I may be off on this but from my experience, the compression ratio is the only thing that matters. We just generalize and talk about cranking pressure because it's easier to get. Compression ratio requires calculation and careful measuring where the cranking pressure just requires a charged battery and a gauge.
chrisdoc0608
insulting
- Location
- rocky mount, nc
measure squish and check the compression, read this http://www.x-h2o.com/threads/2748&highlight=squish
Mark44
Katie's Boss
- Location
- 100% one place
let us know the numbers when you get them.
Mark44
Mark44
Soul Man
X
- Location
- Jacksonville, FL
I've always thought for 2-strokes the pressure was really what mattered the most. Since you aren't compressing against closed valves (like in a 4 stroke) you can't really just compare the volume top dead center v. bottom dead center. You lose volume out the exhaust. Plus any porting would effect it.
I've been told (by Wamilton) that even the pressure isn't completely reliable if it is ported (ie. a stock porting at 185 may run fine on 93 octane, but if ported it may need higher octane). My skis are 185, but need race gas to run WOT because they are ported out the wazzu. I run pump since I never go WOT more than a few seconds.
I know I'm not answering the question, but I would say that at least with a compression number we could give a ballpark octane recommendation.
a key factor is really HOW YOU RIDE.
for example, weve got people running 220psi motors on pump gas, but we dont hold our motors wide open for extended periods of time.
a racer would probably want to run at least a mixture of race/pump gas in excess of 185psi .. however, a freestyler/freerider can get away with straight pump gas if they just ride "throttle blipping" style.
its those extended wide open runs that will cause deto and tear up your motor quick if youre not running enough motor octane for your motor's compression.
.02
for example, weve got people running 220psi motors on pump gas, but we dont hold our motors wide open for extended periods of time.
a racer would probably want to run at least a mixture of race/pump gas in excess of 185psi .. however, a freestyler/freerider can get away with straight pump gas if they just ride "throttle blipping" style.
its those extended wide open runs that will cause deto and tear up your motor quick if youre not running enough motor octane for your motor's compression.
.02
SUPERTUNE
Race Gas Rules
- Location
- Clearwater Fl.
Or use a programmable ignition curve for retarded timing in the upper RPM's
Sorry you think its false. Also, there are different theories on ratio as well, there is corrected vs. uncorrected. How many people do you think know what their compression ratio is? How many people do you think know what their compression pressure is? You don't hear people running around saying I have an 8:1 CR on my motor. No, they say my motor runs at 185 PSI. Same thing when people are asking about fuel octane.
Its like comparing a red apple to a green apple, both are equally important and one is used much more commonly. Bottom line is CP is easier to obtain than CR.
Thought that was almost the exact same thing I said...and I agree with you. :dunno:
But anyway, xkawi550x, have you managed to get hook up a compression tester to your motor yet?
- Location
- Knoxville, TN
no, still working on it. i should have the results posted by sunday. My Torque wrench gear has stripped so the head isn't on yet. :[