Highest compression on pump gas?

[tar]

So just like the title says, what's the highest compression piston to run on pump gas?

[tayyo789]

It depends on the pump, as some pumps can have 95+. I would never go over 11:1 on 92 octane, but it can also depend on your running temperatures, altitude, jetting, etc. Proper octane is a highly debated topic, which is why I say 11:1 on 92 octane since its a pretty safe place to be and still get lots of power

[deathman53]

generally.....12:1 on a liquid cooled motor and 10.25:1 on air cooled motor. This is assuming 92-94 octane. w/ an advanced timing motor, you might be over w/ pump gas. On a 2 stroke, 190psi is close to the line, porting, port timing, squish band, timing advancement all play a big part.

[Drewltr450]

At sea level with cams valves intake exahust and a 13:1 piston i ran 93 pump gas with no problems whatsoever.

[tar]

I will probably just go back to 12:1. I'm really not worried about losing a little power.

[justin.hughes]

I have been curious on this as well. I have a stage 2 hot cam in my raptor 700 and would like to goto a 12:1 if I could still use pump gas. I live at sea level. a 11:1 would be ok but if I could get away with 12:1 that would be nice. I can get 94 octane from Sunoco here in Indiana.

[muddzilla94]

i ran 93 in my 450r with a 13:8:1 compression with no problems and am still running it.

[justin.hughes]

I just got done talking to a engine builder and he said nothing higher than 11:1 for pump gas. This is because of the issues with the pump gas not being exactly the octane it should be. We should be able to get away using 93 or 94 octane with 12:1 but if the gas station is screwing us then that could be a problem.

[sixer3]

so far on a stock cam set up, most water cooled bikes are able to run 91 octane at 12:1 this may also depend on your jetting. The bikes I'm referring to that I ride with are at sea level as well.

Its best to check your actual compression numbers. this will get you closer to your answer. Generally more aggressive cams also leave a little more overlap as your valves open and close, causing you to lose compression. The reason behind wanting to go up to a higher compression piston when moving to a stage 2, 3 or higher cam set up. The change in combustion chamber volume just combats the effect of the lost compression in cam overlap.

[ridehonda400ex]

would it hurt to run 100 octane gas in a 11:1 engine? im thinking about rebuilding my engine over the winter and wanted to know.

[fearlessfred]

Originally posted by ridehonda400ex
would it hurt to run 100 octane gas in a 11:1 engine? im thinking about rebuilding my engine over the winter and wanted to know. running to high of an octane wont hurt the motor ,but might hurt performance slightly. the higher the octane,the slower the fuel burns. so to much octane and its kinda like u are retarding the timing.the whole idea is for the flame front to meet the piston,at the very moment it starts its downward travel,if it meets it to soon,it knocks or pings.if the flame meets it to late u have a loss in power.

[ridehonda400ex]

Originally posted by fearlessfred
running to high of an octane wont hurt the motor ,but might hurt performance slightly. the higher the octane,the slower the fuel burns. so to much octane and its kinda like u are retarding the timing.the whole idea is for the flame front to meet the piston,at the very moment it starts its downward travel,if it meets it to soon,it knocks or pings.if the flame meets it to late u have a loss in power.


okay so what if i run like 91, 92 or 93 octane? my engine air cooled too

[fearlessfred]

Originally posted by ridehonda400ex
okay so what if i run like 91, 92 or 93 octane? my engine air cooled too i am no expert on what your bike needs ,but from what other members say on here 93 octane should be good. if it pings knocks or rattles under a load then up octane until the noise goes away,but u should not have to do this.if u are going to rebuild and can afford the higher price of race gas,u should concider going higher on the compression jmo

[f4iracer]

if you have a high compression piston, you should run some race fuel. Especially if it's an air cooled high compression piston.

i would say starting at 12:1 you should try to run something higher than 93. This comes from all of my mechanics and racers from around my area. If the mechanics and racers do it with their high compression motors, then why would i believe a thread on the internet over real life.

It's sold at every track in my radius......why would they sell it?

110 and 112 sunoco is what the local track sells.

[fearlessfred]

Originally posted by f4iracer
if you have a high compression piston, you should run some race fuel. Especially if it's an air cooled high compression piston.

i would say starting at 12:1 you should try to run something higher than 93. This comes from all of my mechanics and racers from around my area. If the mechanics and racers do it with their high compression motors, then why would i believe a thread on the internet over real life.

It's sold at every track in my radius......why would they sell it?

110 and 112 sunoco is what the local track sells. they sell it to make to make money and they are not going to tell a fool that doesnt need it otherwise. to do something because everyone else does it is not good advise. to run 110 octane in a motor that only needs 100 is a waste of money and horse power. there is no magic race gas that make more horsepower,however higher octanes will allow a motor that needs it,to make more horsepower. you only need enough octane to prevent pinging,detonation,plus a little extra for insurance. race gas SLOWS THE BURN TIME.

[2-330s]

Originally posted by justin.hughes
I have been curious on this as well. I have a stage 2 hot cam in my raptor 700 and would like to goto a 12:1 if I could still use pump gas. I live at sea level. a 11:1 would be ok but if I could get away with 12:1 that would be nice. I can get 94 octane from Sunoco here in Indiana.

we have ran a 686 raptor (660 with a 102mm bore) at 12.5:1 with stage 3 hot cam for a few race seasons no problems.

last year went to a 720 added a +4 stroker crank and a 13:1 piston and custom ground cam still on bp 93 oct no ethanol runs great

[fearlessfred]

Originally posted by 2-330s
we have ran a 686 raptor (660 with a 102mm bore) at 12.5:1 with stage 3 hot cam for a few race seasons no problems.

last year went to a 720 added a +4 stroker crank and a 13:1 piston and custom ground cam still on bp 93 oct no ethanol runs great i just wanted to add to your comments.the bigger the cam (the more valve overlap) the less compression you have. this is why you always raise compresion when running larger than stock cams.so if a person were to raise compression without making cam changes than he may need more octane,than the person who raised compression and larger cams.

[sixer3]

Originally posted by fearlessfred
i just wanted to add to your comments.the bigger the cam (the more valve overlap) the less compression you have. this is why you always raise compresion when running larger than stock cams.so if a person were to raise compression without making cam changes than he may need more octane,than the person who raised compression and larger cams.

exacttly also the raptor mentioned is water cooled which keeps combustion chamber temperatures much lower. An air cooled motor with a stock cam set up will usually require race gas on anything over 11:1

[fearlessfred]

Originally posted by sixer3
exacttly also the raptor mentioned is water cooled which keeps combustion chamber temperatures much lower. An air cooled motor with a stock cam set up will usually require race gas on anything over 11:1 sorry man didnt mean to copy your post .i went back and reread all the posts and seen were u had already said the same thing,but i guess its worth saying again

[sixer3]

Originally posted by fearlessfred
sorry man didnt mean to copy your post .i went back and reread all the posts and seen were u had already said the same thing,but i guess its worth saying again

no worries man as long we're keeping everyone on the same page, i think my post got looked over anyways haha, its all good though. Just tryin to help :)

[fearlessfred]

Originally posted by sixer3
no worries man as long we're keeping everyone on the same page, i think my post got looked over anyways haha, its all good though. Just tryin to help :) right on.i get the impression that a lota folks think that you can just pour in race gas and have instant horse power.when often, the race gas could lessen horsepower. most engine builders suggest octane and jetting that will make there motors retard proof,but often there is a lotta room for gains.most people want an instant answer to what octane and jetting and are not willing to do the work that it take to get to max horsepower. u sound like someone who knows the difference

[sixer3]

Originally posted by fearlessfred
right on.i get the impression that a lota folks think that you can just pour in race gas and have instant horse power.when often, the race gas could lessen horsepower. most engine builders suggest octane and jetting that will make there motors retard proof,but often there is a lotta room for gains.most people want an instant answer to what octane and jetting and are not willing to do the work that it take to get to max horsepower. u sound like someone who knows the difference

yeah, most builders just play it safe unless you tell them specifically that you know what you want and how octane ratings actually effect your combustion. I find a lot of people are also thrown off by the actual octane rating of a given fuel and how its defined. i don't know where it differs but in the u.s. the standard "displayed" octane rating is (mon+ron)/2

I think another good point is that in our engines we are not looking for our fuel to "explode" even when it receives spark, we want it to burn. higher octane usually correlates to a higher resistance to explosion. I think i remember reading something though that "activation energy" is also hightened depending on the chemical used of course. Activation energy is the amount of energy necessary to start a chemical reaction. Which makes sense it would rise with resistance to explosion.

one last thing i'll let someone else elaborate on is, static compression vs dynamic compression. and obviously other factors increasing the possibility of detonation

the science behind the proper fuel really is interesting and there is definitely some power to be had with the right information

I know gpracer knows a lot about this subject

[fearlessfred]

wanted to say ,one last thing,i am in no way against race gas. i am all for it when it is needed and using the octane needed for each motor