high octane gas

[CrazyDave]

Feeling a little confused here....

As I understand it compression creates heat..more compression= more heat...more heat creates the likely hood of the fuel/air igniting on it's own before the spark and a higher octane prevents this...no ??

So when exactly does the higher octane play it's roll..? Before or after ignition...? Or both..

 

[CrazyDave]

Another question, perhaps silly, ...does the carbon build up on the Pistons, of a car for instance, create higher compression and therefore profit from a higher octane gas.?

 

[best]

Usually there is not much volume to the carbon deposits, so it is the hot spots that create a problem that might require higher octane fuel due to spark knock and preignition.

 

[Markblaster]

I think you may have good a little over excited on the posts there. 3 post the same. lol

 

[best]

Ha! One of us must have been drinking the high octane if you are seeing triple.

Crazy Dave, the higher octane works by being a bit more slower burning and resistant to ignition (resistant to sudden flame front propagation due to spontaneous ignition in several locations). So to answer your earlier question:
"So when exactly does the higher octane play it's roll..? Before or after ignition...? Or both.."
The answer is both.

Pre-ignition is caused by:
1) the fuel/air being too ready to ignite due to the heat, pressure, and/or ignition source, and
2) once ignited the fuel/air burns too fast due to
- a) the burn rate of the fuel under rapidly rising heat and pressure
- b) multiple spontaneous ignitions due to the rapidly rising heat and pressure
- c) the volume not increasing fast enough due to where in the stroke the ignition started
- d) mixture droplet size, velocity and homogeneity supporting rapid burn.

A perfect stoichiometric mixture of completely vaporized fuel tends to burn explosively (overly fast burn rate), especially when heated to near or past ignition temperature and subject to high pressures. Our tools to resist this explosively fast burning is:
1) higher octane fuel (slower burning and harder to ignite under pressure)
2) less cylinder pressure (lower compression and/or cylinder filling)
3) increase to volume faster or with less resistance (later timing, better crank angles)
4) increase fuel droplet size (carb, intake or inlector selection or alteration)
5) cooler temperatures (thermostat, head material, quench area)
6) mixture ratio (jetting slightly rich)
7) mixture swirl velocity (port layout and squish design)

As you can see, octane is only one tool in your toolbox to battle detonation.
Almost all of these tools pit fuel (and power) efficiency against detonation resistance.
It is a delicate balance.

 

[CrazyDave]

Ha! One of us must have been drinking the high octane if you are seeing triple.

Crazy Dave, the higher octane works by being a bit more slower burning and resistant to ignition (resistant to sudden flame front propagation due to spontaneous ignition in several locations). So to answer your earlier question:
"So when exactly does the higher octane play it's roll..? Before or after ignition...? Or both.."
The answer is both.

Pre-ignition is caused by:
1) the fuel/air being too ready to ignite due to the heat, pressure, and/or ignition source, and
2) once ignited the fuel/air burns too fast due to
- a) the burn rate of the fuel under rapidly rising heat and pressure
- b) multiple spontaneous ignitions due to the rapidly rising heat and pressure
- c) the volume not increasing fast enough due to where in the stroke the ignition started
- d) mixture droplet size, velocity and homogeneity supporting rapid burn.

A perfect stoichiometric mixture of completely vaporized fuel tends to burn explosively (overly fast burn rate), especially when heated to near or past ignition temperature and subject to high pressures. Our tools to resist this explosively fast burning is:
1) higher octane fuel (slower burning and harder to ignite under pressure)
2) less cylinder pressure (lower compression and/or cylinder filling)
3) increase to volume faster or with less resistance (later timing, better crank angles)
4) increase fuel droplet size (carb, intake or inlector selection or alteration)
5) cooler temperatures (thermostat, head material, quench area)
6) mixture ratio (jetting slightly rich)
7) mixture swirl velocity (port layout and squish design)

As you can see, octane is only one tool in your toolbox to battle detonation.
Almost all of these tools pit fuel (and power) efficiency against detonation resistance.
It is a delicate balance.

I do believe I'm a few points smarter for having read that..

Thank you very much..

 

[best]

I went through my "hot rod phase" in the 1970s. The fuel crisis eliminated High octane fuel so we had to figure other ways to keep the old high compression motors going. Some guys used 2 head gaskets and retarded the timing, turning a 10:1 high performance motor into a fuel thirsty slug. I read the likes of Harry Richardo, Ak Miller, Larry Widmer and Smokey Yunick to learn how to make power without detonation. Larry Widmer is a genius. By packing all the mixture into a small pocket under the exhaust valve he could run unheard of compression ratios. I was able to run a 10.5:1 351C on regular fuel (all that was available then).

You can do it with a Blaster as well. A rechambered head with careful squish and chamber design, late timing (because you've sped up the burn you don't have to give it so much lead)and consistent sized ports to keep the big droplets in suspension. You can make Banshee/Raptor style 35hp out of a Blaster on regular octane fuel, no race gas needed, if you know what you are doing. High test 91-93 octane is wise protection for little extra cost if running it hard.