Octane question

[71VERT]

When ever I add racing fuel to my gas the car runs rough, I am adding 4 gallans of 100 octane per tank of 93 octane fuel. If I just use 94 octane Sunoco the cars runs fine. Do I have to adjust the timing in order for the car to run better on the higher octane fuel?
I have been adding the racing fuel because I figured it would be better for the car.

427/435 hp 1969

 

[clar2001]

Just a thought if it runs fine don't mess with it, carefull not to run to hot with your octane.
Is your engine stock if it is I woulden,t run race fuel youl burn something.
If it,s built I would ajust timming and carb.

 

[Ken]

He's right; if your engine is stock, there is absolutely no need for anything higher than premium gasoline (92-93 octane) in your Corvette. As a matter-of-fact, it should run ok if you put 87 octane in it, as long as it's in tune.

Oops, didn't see the 427/435 numbers at first. Yes, you do need the high-test (92-93 octane) fuel for that engine, but if it's in tune you should not have need for racing fuel if it's stock. To take advantage of the higher octane fuel it might benefit you to tweak the timing somewhat, but that's something you gotta play by ear.

 

[SwaveDave]

If it isnt pinging or running adversely on 94 octane...youre only wasting your money on putting higher octane in it. What i would do, is, have some Octane Boost in the back storage bin in case your somewhere where you can only get 89 ; thats what i do.

Dave

 

[yakatak76]

higher octane will not make your engine run hot, and it will not generate more horsepower as some people suspect. if anything, higher octane will help your engine run cooler. octane shortens the duration of the burn time of the gas in the cylinder. the knocking and pinging associated with lower octane gas is caused by it being ignited while the piston is lower in the cylinder causing it to rattle around down there as there is more room in the bottom of the cylinder than the top. so if you can get the thing fired off while the piston is at the top you won't get the knock. i will see if i can fine the website where i read all this and post it so you all can read up on the chemical mix of gasoline.

i used to run 100 Low lead octane mixed with pump gas in my dirt bikes. they ran a little better, and cleaner but it got to be a hassle dragging the five gallon can to the airport to get gas. can't just drive your car up to the pump you know, too many airplanes in the way.

 

[AKRAY4PLAY]

Yakatak76 is right about running cooler. higher octane is just a more stable fuel. running lower octane leads to detonation. the detonation is caused by "dieseling" or better known as preignition. the higher temps in the combustion chamber set off the fuel air/charge before the spark plug fires. the higher the compression ratio, the earlier the preignition. if the preignition happens before top dead center, then the piston and charge fight against eachother resulting in the KNOCK that is heard. knocking will eventually blow your ring lands out or break your pistons. Brian

 

[JohnZ]

Pre-ignition and detonation are two different things, but the bad results are similar. Pre-ignition happens when a "hot-spot" in the combustion chamber ignites the mixture before the spark plug fires, or simply due to really low octane or contaminated fuel. Detonation happens when the plug has already fired and the normal flame front is propagating across the chamber, but end gases on the far side of the chamber auto-ignite due to heat and pressure and the two flame fronts (intense pressure waves) collide with each other - that's the "knock" you hear, also commonly called "pinging".

 

[Rab563]

Brian is right about higher octane is just a more stable fuel. running lower octane leads to detonation. the
detonation is caused by "dieseling" or better known as preignition. the higher temps in the combustion chamber set off the fuel
air/charge before the spark plug fires. the higher the compression ratio, the earlier the preignition. if the preignition happens before top
dead center, then the piston and charge fight against eachother resulting in the KNOCK that is heard. knocking will eventually blow
your ring lands out or break your pistons. Brian

Brian, Octane raises the flash point "Temperature" of the fuel.The flash point is the temperature in which the fuel will self ignite. As you said higher compression equals high compustion temperature. So, the higher the compression the higher the flash point or octane rating of the fuel must be to prevent pre-ignition.

Ron

 

[71VERT]

thanks for the repleys guys I will stick to the 94 octane I was using before it ran fine on that, It was not knocking or pinging from the 100 octane mixture, it just wasnt firing correctly . had a hesistation before accelerating. I added some lower octane to the gas and it was running better tonight. Sure do like the smell of the racing fuel burning though.

Regards,
Jerry

 

[Ken]

Sure do like the smell of the racing fuel burning though.

Me too, but I sure don't like it smelling like bubble gum, or flowers. You smell that stuff yet? :L

_ken

 

[reubenmc]

Ken, At the local drag strip near me, I've smelt some that smell like root beer. Haven't smelt any flowers or bubble gum yet.

 

[Rodney Gold]

High octane fuel should not smell any different when burning to ordinary unless something like naptha or benziene or some other hydrocarbon or wierd substance is added.
A lot of high octane fuels are "old" , IE have been stored a while under bad conditions and the octane rating actually drops or they absorb water - this might be the case if the motor runs rough with them.
As many other posters have said , using higher octane fuel if a lower one works well is a waste of money and depending whats added to raise the octane level , can actually harm your engine - like benziene attacks anything rubber.
Hesitiation on acceleration would not be due to octane ratings , especially if using a HIGHER octane
Its generally a lean condition that causes hesitation (unless it stumbles and emits black smoke) and that might be due to the mix of the 2 fuels and the "vaporising" properties.
As you floor the throttle , intake vaccum drops radically and the engine doesnt "suck" enough fuel , thats where an acceleration pump adds a "booster" shot to stop this.
Changes in the way the fuel reacts with heat etc will change the fuel delivery pattern , for example if the new mix vaporises more readily with heat , then the engine might be starved relative to a fuel that doent vaporise - IE cos you have no vacuum , the better vaporisation is not a desireable trait , a stream of unvaporised fuel might be better to cure the lean condition.

 

[Ken]

Rodney, as for talking about lovin' the smell of racing fuel, I was refering to the wonderful aroma of nitromethane. As for the aroma additives to which we refer, you can buy them at the local auto parts stores; they come in bubblegum and various other scents.

I know about the smell of naphtha, benzene, and all of the other components that go into the manufacture of gasoline - I worked in a refinery for twenty years, specifically in Blending & Shipping, where we combined distilled product into various fuels, and also stored and shipped same. I know what fuel smells like, both when it's fresh and when it's stale.

_ken

 

[SurfnSun]

I can't add anything more here that hasn't been said. Except do NOT waste money on octane booster. A point of octane is measured in .1 or .2 so a 8 point boost is actually .8 not 89 to 97. It doesn't work that way.

But I do love the 114 leaded I run in my Callaway. It does however adversely affect the drivability of the car around town. It tends to stall at stop lights....but when the boost comes on...LOOK OUT!!!

 

[80vette_ca]

I can only get 92 octane at the pump so I picked up VP RACING C5 which is there octane boost, I found the car runs a bit better with the octane boost but not good enough to be putting in a $12 bottle every fill, but I do feel better telling people that I'm running 92 octane with an octane boost, then they look at me like oh wow this guy means business

 

[6shark9]

How about a chemists point of view.

The lower the octane rating, the lower flash point. Low octane burns faster than higher octane. Higher octane is in a way less explosive than low octane.

So when the sparkplug fires there is an optiminal time in the piston stroke when that spark should occur. With high octane the spark can occur earlier in the upstroke (burns slower) so that max power occurs right after TDC. If too low an octane is used the max power occurs before TDC so force gets applied back down (opposite direction of the engine rotation) and into the valves as the full explosion continues to be compressed before the piston passes TDC and the power is then translated to where we want it to go, that's the "ping".

To experiment with lower octane try lowering the timing advance until the engine runs best under a load.

6shark9