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I recently had to switch from the 100LL avgas that I've been running, (I forgot to pick it up at work), to some Sunoco 94 I bought on my way to the track. The bike actually seemed to run with a little bit more grunt to it. Like more bottom end, or torque. Anybody have any ideas why? Or do you think I'm just old and senile, and just imagined it.

You only run the octane you need to keep from detonating, race gas in a stock motor will usually make less power unless it is oxygenated. Your after a burn for the fuel to stop burning at a certain degree after it has completed spark.

Race gas burns slower so in the case of a 2 stroke you want all the fuel to be done burning around when the exhaust port is going to open. Because race gas burns slower the energy used to drive the piston down can escape out the exhaust port because it is burning at a slower rate and is to late in completing the burn process and the port opens allowing enregy out.

This is why they have different octane fuels for different style motors, race motors, each motor depending on how it is setup will burn fuel at a different rate and in order to get the proper burn rate speed after spark to stop burning at proper degree from TDC for max performance on a certain engine combo + not to detonate if you are having that problem.

So you will find if you just run the fuel a motor wants to not detonate you are usually making the best power as well for your setup .

If the SG isn't exactly the same the diff could be jetting

nmdesertrider wrote:If the SG isn't exactly the same the diff could be jetting

That is true alot of times race gas will allow you to run leaner because it burns cooler and you can make more HP from the rise in gas temperatures. It is a fine line on tuning between max power and max siezure.

Is it true that lower octane nets more BTU? Or is that a wise tale?

i tried pump gas in my bike once, i wont do that again

I'm gonna run this some more, then try the avgas one more time to be sure of the difference before I finally decide. I want to really know if there is a difference. I did not change the jetting or anything.

If I am not mistaken avgas has deicing agents in it and is not really designed for a 2 cycle sea leval engine?

Slow old Fart wrote:If I am not mistaken avgas has deicing agents in it and is not really designed for a 2 cycle sea leval engine?

Slow old Fart wrote:

nmdesertrider wrote:If the SG isn't exactly the same the diff could be jetting

That is true alot of times race gas will allow you to run leaner because it burns cooler and you can make more HP from the rise in gas temperatures. It is a fine line on tuning between max power and max siezure.

I work at the airport in town here.Usually a few times a year there will be little planes come in to do some exploration or wildlife surveys that run on 100LL Avgas,I've asked these guys lots of questions concerning avgas and they say avgas burns HOTTER than pump gas,I was very close to trying it in my TTR 230 when I had it,I was doing anything I could to make that thing have more juice but it was brand new and I didn't want to fry it,the pilots told me if I was to use it it should be a mixture of pump gas/avgas,they also said it would be more suited to a 2 cycle engine rather than 4 and that it would burn much cleaner than pump gas,with all that said I'll remind you that I have not tried it that I am passing word of mouth,also in my experience with any 2 cycle engines they make better power when the engine and pipe are up to operating temperature & beyond so how could something burning cooler make more power?
also my 03 F7 is a EFI and has a switch for ethanol,when you flip the switch it richens the fuel curve not make it leaner,I'd like to see you run any kind of race gas and lean out your main,see what happens

anybody tried sunoco ? is it any good ?

4Z wrote:Is it true that lower octane nets more BTU? Or is that a wise tale?

True.

Octane and BTU are separate parameters...

AlisoBob wrote:Octane and BTU are separate parameters...

Also true.




But there is more power to be made with lower octane gas, in theory you shoud run as low as possible before it starts pinging.

britincali wrote:


But there is more power to be made with lower octane gas, in theory you shoud run as low as possible before it starts pinging.

No, there is more power to be made by building your motor use the highest octane you can afford to buy.

Race fuel has other advantages too. Octane in pump gas is boosted with butane, which is a gas. If your float bowl is hot it will boil and cause all kinds of problems.

pump gas=shit

Making me break out the wiki



"To a certain extent a fuel with a higher carbon ratio will be more dense than a fuel with a lower carbon ratio. Thus it is possible to formulate high octane fuels that carry less energy per liter than lower octane fuels"

"while engines perform best when using fuel with the octane rating for which they were designed and any increase in performance by using a fuel with a different octane rating is minimal or even imaginary,"


"Burn Rates- That's the ticket!!
If you take that same fuel that worked well in the above V8, and run it in an engine, like a cbr900, with it's lower cranking compression and lower combustion chamber temps, it will, without a doubt, burn much too slowly at those lower temps and pressures and reach maximum cylinder pressure too far after TDC for best power.
Things generally burn slower when they are cooler and vice versa. Peak cylinder pressure will occur much past TDC - decreasing the power produced if you keep the same ignition timing.

You can advance ignition timing to try to recover power, but that will cause the air/fuel mixture to burn earlier in the crank stroke and spend, percentage wise, more of the energy produced by the expanding, burning mixture, pushing back down, trying to prevent the piston from rising up to the power stroke - robbing power.

If you MUST use a slow burning fuel, which USUALLY has a high octane rating, advancing the ignition timing will lessen the power loss, but the best results are usually obtained with the quickest burning fuel obtainable, that, of course, doesn't "knock"."




"Does using a fuel with higher octane numbers automatically make more power?
Not unless they are preventing "knock"."

That's great when you have a closed, pressurized fuel injected system but in a carb butane loaded gas burns like shit.

This is my buddy Gary's Comp motor...

320 C.I. = 970 h.p. ( Really.... not some fantasy number)

18:1 compression ratio, VP C19 ( 116 octane) fuel.

Compression + Octane = Power.

It is semantics, but compression is related to torque and power. The resistance of the fuel to detonation allows greater performance to be achieved without adverse effects. The bottom line is that in most cases running race gas like C-12 in an engine designed for pump gas is unnecessary. The are other VP fuels that have a greater oxygen content and therefore claim to make more power - with jetting changes - but I have only heard of them used with 4-stroke engines.

With these 1200 cc sled motors I ended up with 13:1 cr and the lowest octane possible for the (altitude specific) and timing at 16* @ 8000 RPM. At sea level on the dyno a 50/50 mix with pump and C12 netted 163 lbs of torque peak. By testing at sea level off the dyno, clutching set up confirmed previously stated by others that using higher octane when not needed can hurt performance. In other words, the mix hurt it at elevation where pump gas gave best performace. The highest compression possible without deto made a happy motor. FWW

The BTU of pump gas is the same accross the octane numbers.

If you want to see proof I can show you dyno sheets of several HP INCREASE by running lower octane numbers. My CBR1000RR LOST 2HP by running 100 octane over 92.

Ask VP what the octane of their super nasty hot U4 or U2 oxygenated fuels are...89. You can get 6-8HP just by switching a 4 stroke 4 cyl. sport bike motor over to it, and while much of the gain is in the oxy, why do you think they only mix it 89 octane??