82z

Alright. So the other day my roomate came back from class and said that his instructor told them that race gas burns slower than regular pump gas due to the higher octane. i.e. more octane makes the fuel burn slower. I could swear that I read a Ray t Bochaz article that said more octane=slower burn was a myth. "The slower the burn the more time for something to go wrong. Wouldn't it make more sense for race gas to burn faster due to the lesser amount of time for fuel to burn at higher rpms. You don't want to burn to be occuring when the exhaust valve opens. The could bend pushrods,and put undue stress on the valve train" I said. My roomate said "you want it to burn slower so that a burn instead of an explosion happens." Whose logic do you guys buy into? Can I go home and say "I was right again!"? Have you guys seen the article I am thinking of. I think it was in a GMHTP in Ray's section a while ago.

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82 z28 350cid, vortec heads, comp 262h cam, Holley 600cfm carb, 2in twice pipes, MSD ignition, turbo 350 trans, 3.73 posi, manly b&m megashifter

johnyIROC

Higher Octane gas needs more heat to set it off.... the purpose of race gas.. or ave gas is to prevent detonation or spark knock

Kat

iTrader: (4)

Also to slowering burning fuel lets ya run more timing without your car being prone to detionation.

Yes tho 87 octane does burn faster than 110 and 87 makes more power than 110 cause of the faster burn.. With 87 tho you are more likely to screw something up tho (or atleast that is what I like to think)

Kat

AlkyIROC

It's the same as gunpowder. Not all gunpowder is the same. Quick burning powder is used for handguns because of the short barrel and burning time. Muzzle velocity is actually quite low. Gunpowder for rifles is a slow burning powder since it has a long barrel to burn off all the powder. Because of that slow burn time more energy is created before the bullet leaves the muzzle creating a much higher muzzle velocity.

Back to gasoline.

Same thing happens. 87 octane burns quickly with very little spark advance. Since the piston is only a few degrees before TDC very little energy is stored up before the piston starts going down again.

Using 110 octane fuel allows more spark advance but doesn't mean there's an unlimited amount you can use. The purpose of the higher octane is to prevent preignition from higher compressions or power adders. Since these things can benefit from more spark advance you don't want the fuel to be completely burnt up before the piston reaches TDC. By slowing down the burn time there's more time to burn the fuel when the spark advance is increased.

There's no more energy in 87 octane as there is in 100 but if increasing your advance a few degrees starts getting spark knock then a higher octane will be reqired.

Also without getting your distributor recurved or having the timing curve changed in the computer chip, advancing your timing may lose top end HP. An engine will only like a specific amount of total timing. Anywhere from 32-38 is usually prefered. Lets say your car likes 32 degrees total timing with 10 degrees base timing (factory settings). If you advance the timing to 15 then the total timing also gets advanced to 37 and the engine may not like that. It's best to figure out what base timing is best for your car then get the distributor reset to only go to the older total timing setting.

But that's an entirly different subject however since fuel octane also is related to timing I thought I'd bring that up.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z SuperPro ET Bracket Race Car
461 naturally aspirated Big Block

Best ET on a time slip: 11.242 altitude corrected to 10.89
Best MPH on a time slip: 121.52 altitude corrected to 125.89
Altitude corrected rear wheel HP: 497.9
Best 60 foot: 1.546

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

Blue_Meanie86SC

which is one reason it burns slower, a higher flash point means more heat (energy) is needed to ignite it. so the time it takes for the needed amount of energy for ignition to pass from one molecule to another increases because of the higher energy requirement. These delays add up and makes the higher octane fuel burn slower.

darbster

wow! now i see why your a moderator stephen...smarty pants.

MikeC.86Roc

iTrader: (10)

Just a little note that is somewhat related to this thread... Most race gas is leaded. Don't forget this when running in a non-race type motor with normal valve seats and such. This gas is also hell on OX. sensors from what I have heard. On another note, I run 104 when I go to the track and it allows my TPI to give the total advance when I lay on it heavy. I am running close to 11:1 compression with a near stock TPI injection system and the knock sensor just loves to retard the hell out of my timing under heavy acceleration. With a computer monitoring, you can watch it pull timing out of the motor when I run 93 octane even.

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Mike C
88 Iroc 5spd
355 TPI, AFR 190s and lotsa other mods
April feature ride at Sethirdgen.org

[This message has been edited by MikeC.86Roc (edited August 09, 2001).]

82z

Why does increased octane rating decrease the propensity for detonation with higher compression ratios like, 11.0:1+?

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82 z28 350cid, vortec heads, comp 262h cam, Holley 600cfm carb, 2in twice pipes, MSD ignition, turbo 350 trans, 3.73 posi, manly b&m megashifter

ATOMonkey

At higher pressures the fuel will try to burn much faster. Example, blow up a balloon of propane, or blow up the tank it came out of. The purpose of High octane is to slow the burn at high pressures so that it does not explode. This cause very high pressure rise rates which hurt bearings and seals. It's the same thing as whacking the top of the piston with a hammer instead of pushing it down.

camaro chick

Beautifully depicted there, Stephen! I love the gunpowder analogy