found this interesting. dave posted sometime ago about the use of 91 oct and it not being ness with a relatively stock engine. the other day i put 89 (midgrade) in, just to see what would happen, and the exact opposite of what i expected occured. the tach showed a slight increase in rpm and i could note it by ear as well. nothing was touched on the linkage. no change in perfromance(seat of the pants). next time i'll put some 91 back in and see if the idle drops. i sure don't mind saving a few bucks every fillup.

k

cylinder pressure makes power...

please explain. if anything i expected an idle drop.

k

dunno if this has anything to do with it...but higher octane fuels burn slower correct?

it's because with lower octane fuel, yout timing was to advanced. you must retard timing so you dont get preignition with the lower octane fuel.
your running higher octane fuel at a particular degree of timing.
you may get a higher idle wit cheaper fuel but you will also get ping.
if yo don't get ping with cheap fuel then you didn't have your timing exactly right while using the premium stuff. it needed to be advanced more... MORE POWER. DARN.

higher octane means slower burning fuel. think about alcohol... =-) very slow burning stuff, right?

gas never explodes. it's controlled burn thru the power stroke of a 4 stroke cycle. lower octane means very quick burning stuff. maybe the quicker "bang" of that power stroke increases the idle? i don't really think so....

all i know is a chemist/hot rod buddy of mine suggested many years ago to put 1 gallon Xylene with 10 gals premium gas and get substantial results. i got the results.

or do like my dirt bike race buddys do and just drop by your local airport and fill up on some 115 aviation gas. i used this stuff on a Yamaha street bike just to see what would happen. after adjusting the timing for the increased octane, was blown away by the results. seat of the pants told me a +/- increase of 20%!

QUOTE (rhodyguy @ May 27 2005, 11:02 AM)

please explain. if anything i expected an idle drop. k

with no other changes, lower octane increases the cylinder pressure. (lower it too much and you get detonation - that's too much cylinder pressure...)

slow-burning fuel increases your HC levels. IOW - fuel you could have turned into power has left the building. burn it all before the exhaust valve opens and you have more power.

QUOTE (ArtechnikA @ May 27 2005, 09:23 AM)

QUOTE (rhodyguy @ May 27 2005, 11:02 AM) please explain. if anything i expected an idle drop. k with no other changes, lower octane increases the cylinder pressure. (lower it too much and you get detonation - that's too much cylinder pressure...) slow-burning fuel increases your HC levels. IOW - fuel you could have turned into power has left the building. burn it all before the exhaust valve opens and you have more power.

detonation is the ignition firing/timing too advanced for a particular octane rating fuel at a particular throttle position. compression plays a great part in this.

HC's (un-burnt fuel) are not necessarily more with higher octanes. (ask all the rice burners about ULEV's...they wold go higher if they could...) if fuel burns efficiently there's very little left over (HC's). actually with higher compressions there is a greater ability for fuel to be utilized completely. add a swirl (chevy vortec) or a hemi combustion chamber (most motorcycles, dodge, ford, and many others) and you've got even more of a complete burn.

you can have just about any pressure you want as long as it's just past TDCenter. (that statement will stir up some *hit...)
slow burning fuels such as pure alcohol or nitro methane, at very high RPM's, will not be completely spent during the exhaust stroke.
more advance= more power.
advance up to just before pinging is the best anyone can do. period.