owen, yes i was being 11s specific, but if guys on this board with 12s's can't tell the performance is suffering with a change in fuel, perhaps they should utilize craigslist, get rid of the bike and go do something else.

guess i'm too old school, because i can't remember ever riding around wondering about "peak combustion pressure at the ideal crank angle to generate maximum torque."

acknowledging that this IS a tech forum, it's still worthy of mentioning that if some of you guys spent as much time riding as you do pondering.....

Yeah Brad, it's like taking your bike to the Rockies and touting the better gas mileage; but, on the other hand the performance sucks.;)

Brad, I'm gonna need the complete PSDTA list. You know, for safety reasons...

You're absolutely right. I screwed the pooch on the logic of this one. I guess I really don't know why retarding the timing helps control knock, but a lot of references say it is not related to the speed of the burn although what you're saying makes intuitive sense.

- Mark

I'm not versed in engine mechanics, so I'm a bit confused about the fact that higher octane fuel burns more slowly. I thought that the whole point of using higher octane is to facilitate higher compression ratio. Higher CR in turn should cause the mixture to burn more quickly. In terms of physics this would mean that, although the net energy produced by explosion is the same as in lower CR engine, it happened in less time, so the power of explosion is greater. This would than increase the efficiency at which the expansion of hot gasses is transferred as force to the piston, generating greater torque at the crank and further down the line.

In extreme example I could equate this to the fact that although my arm can throw a basketball (for instance) with far greater kinetic energy than the bullet fired from a gun has, I could never through a bullet so strong that it would kill someone. The power with which I throw a bullet is much lesser than the power of the explosive in the cartridge, because it takes my arm, I'm guessing, a good quarter of a second to swing while the explosion lasts mere milliseconds.

When thinking how could lower octane fuel beget better performance, I'm leaning more toward the explanation that it might have something to do with energy density of fuel which can vary with additives. Although I find it hard to believe such differences would be noticeable.

The literature on this is pretty conclusive - although energy densities of different fuels do vary slightly (again, notwithstanding things like ethanol vs. non-ethanol fuels), this is not correlated to octane rating. While I can't speak to the experience that an engine runs better/worse on higher/lower octane fuels (with no other changes), there really isn't much of a technical footing here. But you can't discuss this subject without noting that a person's perception of how an engine runs is subjective to begin with and then there are tons of confounding influences (ambient temperature, humidity, variations in engine tune, etc.). And "butt-dynos" are notoriously inaccurate. Doesn't one of the motor oil commercials prey on our perceptions here to say that you can just "feel" how much better an engine runs when you use their brand of motor oil? Technically, it just doesn't hold water.

I know for an absolute fact that every one of my vehicles runs much MUCH better after I wash it. Every one.

- Mark

Mine too, why is that?

It's psychological. I feel it, too.

Science facts aside, I think that's the biggest truth in this thread so far.

With 91 in the tank (it's nearly empty now) the engine has provided a nice tight happy buzzing sound and the feeling of available power is more immediate.

Although I am eager to try some 94 (RON 98) again and compare, I clearly recall the slightly loose barking sound and the every so slight delay in power when I roll it on.

Both conditions are satisfying in their own way.

Maybe fuel is different up here (no corn juice).

And, yeah, I think about how my bike works all the time. And, fortunately, I get to ride a lot too.

Cheers,
Ian

see if you can find some of this at the pump and then see how it runs. A gas station near me sells it for about $8.00/gallon, smells great, runs great, but I can't always stop there every 150 miles or so, especially when I am chasing Cageyar around NY State :D



Sunoco 260 GT - Unleaded Race Fuels

After reading this thread, if i feed my hexhead regular gas, it will become a complete retard. Is that correct?:D

Passing gas on regular basis makes anyone retarded ...:D

Au contraire, Mr BB. The average person passes gas about 14 times a day!!!

NOT passing gas regularly would most likely make one an airhead!

Not true!......I has bin doing it fer years and I is still smarter than you's!

PS.........check my sign in name.

PFFFFFFFFFT

the enormous depth and scope....the incredibly valuable things one learns on this bulletin board is....it's just..staggering!

Are we becoming as dangerous as cattle in the development of greenhouse gas?

delete - the quote didn't post correctly so I re-posted. Sorry

I know for an absolute fact that every one of my vehicles runs much MUCH better after I wash it. Every one.



Not mine - it slips into parade mode if it's too shiny, so I rarely wash it.

serious tuners bias their whole tuning setup towards a particular brand and formulation of fuel...with 'spec fuel' you dial in everything towards max efficiency using the type of fuel you have.

visit the VP site and look at all the difefrent formulations...all different for different reasons and applications, yet all 'gasoline'.

If your total state of engine tune, compression ratio, ignition timing, a\f mixture was optomized for say, 87 octane regular and it ran most efficiently that way, then 87 octane is the 'optimal' fuel.

but you can certainly tune for better gas..that's why a 'dual map' addon like the latest PC or a reapidbike is cool, one map can be for pump gas and one map could be for VP110 with a 20% by volume nitromethane load.

if your scoot runs bettr (and has been said a butt dyno is notorously inaccurate) with something beyond standard pump gas, then something is not optomized for pump gas.

of course you can pull more power with premium or specialty fuel..if you tune for it.

my HD streettracker is dialled in for vp u4.2 and 20% by volume price fuels max race (40% by volume nitromethane)...timed and jetted for this particular combo.