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High octane fuel burns slower?

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High octane fuel burns slower?

Postby Truckedup » Tue Dec 30, 2014 8:12 pm

I'm talking about pump gas not race gas. Hear it all the time,93 burns slower than 87 to resist detonation. I been looking around for real info on this and only found a few things.
One from Sunoco's race fuel site saying it's not true for racing gas but has some merit for pump gas.. What does some merit mean??
Other technical info just says that higher octane ignites at a higher temperature to resist detonation...
Anyone have technical information on this? Thanks
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Re: High octane fuel burns slower?

Postby mk e » Tue Dec 30, 2014 8:15 pm

Everything I've seen says the burn rate is about the same regardless of octane. The higher octane resitsts pre-ignition better so it burns when you want instead of exploding which you never want....which i guess is "slower" burning in a way ;)
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Re: High octane fuel burns slower?

Postby The Radius Kid » Tue Dec 30, 2014 8:26 pm

mk e wrote:Everything I've seen says the burn rate is about the same regardless of octane. The higher octane resitsts pre-ignition better so it burns when you want instead of exploding which you never want....which i guess is "slower" burning in a way ;)



That's correct.
Octane is basically the measure of a fuel's ability to resist auto-ignition.
It's not it's burn speed.
Different fuels with the same Octane number can actually exhibit different burn speeds,once they're started.
Go figure.
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Re: High octane fuel burns slower?

Postby CharlieB53 » Tue Dec 30, 2014 9:38 pm

Mike and Radius have the right idea.

Burn speed can be measured in Feet per Minute. The speed of the different gasoline are very near the same.

Octane increases the 'flash point of the fuel, the specific temp that the fuel will self-ignite. IIRC it's Bolye's Law, pressure and temp, yada, yada. As compression comes up and pressure rises so does the charge temp. Upon ignition of the charge pressure radically rises, if the octane is insufficient the unburned charge on the far side of the 'cloud' can self-ignite, causeing the knock, or ping from the sudden pressure spike. No longer a curved rise in pressure but a sudden spike.
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Re: High octane fuel burns slower?

Postby Lizardracing » Tue Dec 30, 2014 10:07 pm

Speaking of burn rate....Why is ignition timing static in race cars, yet the time the fuel needs to completely burn is continually shortened with RPM?
On a street car, a low static is set, then the burn process is started earlier and earlier with RPM until 3k or so, then is static from 3k or so to redline?

The fuel will burn at a set rate yet we change the rate but not linear with RPM. Why?
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Re: High octane fuel burns slower?

Postby MadBill » Tue Dec 30, 2014 10:22 pm

Race engines have very poor cylinder filling at low speed, so they need/can use a lot of advance, just as vacuum advance helps during light throttle cruise. Also, they don't get driven much at those speeds, so if the timing is 10° or more from 'ideal' for the conditions, no one cares. As the revs increase, the higher cylinder pressure gives a faster burn, as does the increasing turbulence, so a fixed advance isn't far off for most apps, and can be improved only with a sophisticated spark table and a lot of tuning time.
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Re: High octane fuel burns slower?

Postby Splitter » Tue Dec 30, 2014 10:43 pm

Flame speed increases as rpm increases, but remains the same in terms of crankshaft degrees.
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Re: High octane fuel burns slower?

Postby The Radius Kid » Wed Dec 31, 2014 1:10 am

Lizardracing wrote:Speaking of burn rate....Why is ignition timing static in race cars, yet the time the fuel needs to completely burn is continually shortened with RPM?
On a street car, a low static is set, then the burn process is started earlier and earlier with RPM until 3k or so, then is static from 3k or so to redline?

The fuel will burn at a set rate yet we change the rate but not linear with RPM. Why?


As I understand it,there is a certain amount of "lag" between the rise to max compression pressure and detonation,*if there is a tendency for it to occur*.
As the RPM comtinues to rise,the time for the detonation to occur shortens before the actual spark event happens.
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Re: High octane fuel burns slower?

Postby peejay » Wed Dec 31, 2014 8:46 am

Lizardracing wrote:Speaking of burn rate....Why is ignition timing static in race cars, yet the time the fuel needs to completely burn is continually shortened with RPM?


Racers prefer the reliability of having no advance system in the distributor to stick/fail over having a few extra HP.

At least, this is the rationale I gather from reading the commentary when the subject comes up. I don't do circle track, but apparently having a mechanical advance on a 2bbl engine is worth quite a bit of power.

For digital ignition control, the only excuse for having a fixed ignition timing is laziness on the part of the engine tuner.
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Re: High octane fuel burns slower?

Postby 4sfed » Wed Dec 31, 2014 9:29 am

The Radius Kid wrote:
Lizardracing wrote:Speaking of burn rate....Why is ignition timing static in race cars, yet the time the fuel needs to completely burn is continually shortened with RPM?
On a street car, a low static is set, then the burn process is started earlier and earlier with RPM until 3k or so, then is static from 3k or so to redline?

The fuel will burn at a set rate yet we change the rate but not linear with RPM. Why?


As I understand it,there is a certain amount of "lag" between the rise to max compression pressure and detonation,*if there is a tendency for it to occur*.
As the RPM comtinues to rise,the time for the detonation to occur shortens before the actual spark event happens.

Three things are required for a mixture to auto ignite . . . time, temperature and pressure. Increase any one of them and you may cross the threshold.
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Re: High octane fuel burns slower?

Postby 140Air » Wed Dec 31, 2014 3:13 pm

Truckedup wrote:I'm talking about pump gas not race gas. Hear it all the time,93 burns slower than 87 to resist detonation. I been looking around for real info on this and only found a few things.
One from Sunoco's race fuel site saying it's not true for racing gas but has some merit for pump gas.. What does some merit mean??
Other technical info just says that higher octane ignites at a higher temperature to resist detonation...
Anyone have technical information on this? Thanks


This is generally true for pump gas. The Sunoco site is correct. The reason is that pump gas differs in octane rating based on blend. The higher octane gas tends to have more of slower burning, more stable compounds blended in. I think the difference is small.

Race gas is another matter. Same for non-gaasoline fuels. Octane number does not faithfully follow burn speed.
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Re: High octane fuel burns slower?

Postby Truckedup » Wed Dec 31, 2014 4:10 pm

140Air wrote:
Truckedup wrote:I'm talking about pump gas not race gas. Hear it all the time,93 burns slower than 87 to resist detonation. I been looking around for real info on this and only found a few things.
One from Sunoco's race fuel site saying it's not true for racing gas but has some merit for pump gas.. What does some merit mean??
Other technical info just says that higher octane ignites at a higher temperature to resist detonation...
Anyone have technical information on this? Thanks


This is generally true for pump gas. The Sunoco site is correct. The reason is that pump gas differs in octane rating based on blend. The higher octane gas tends to have more of slower burning, more stable compounds blended in. I think the difference is small.

Race gas is another matter. Same for non-gaasoline fuels. Octane number does not faithfully follow burn speed.


Ok, but how do you know what you are saying is true? :D What does burning slower accomplish? The longer burn gives detonation more time to happen. Yes?
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Re: High octane fuel burns slower?

Postby Mike Croley » Wed Dec 31, 2014 4:20 pm

The term "slow burn" is only relative where race gas is concerned because of the RPM a race engine turns. You want as much of the charge as possible to burn, but at 7000 RPM there isn't much time for that complete burn to happen. Stock street engines don't turn very high RPMs and don't need an extremely fast burn.
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Re: High octane fuel burns slower?

Postby The Radius Kid » Wed Dec 31, 2014 4:22 pm

Truckedup wrote:Ok, but how do you know what you are saying is true? :D What does burning slower accomplish? The longer burn gives detonation more time to happen. Yes?


Not really.
You have to control the combustion pressure rise speed or a too fast moving pressure front in the chamber could cause the light end chain gases to pre-ignite,causing the two pressure fronts to collide creating a situation we all know and love so well.
I think I sad that right. :shock:
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Re: High octane fuel burns slower?

Postby 140Air » Wed Dec 31, 2014 5:11 pm

Truckedup wrote:
140Air wrote:
Truckedup wrote:I'm talking about pump gas not race gas. Hear it all the time,93 burns slower than 87 to resist detonation. I been looking around for real info on this and only found a few things.
One from Sunoco's race fuel site saying it's not true for racing gas but has some merit for pump gas.. What does some merit mean??
Other technical info just says that higher octane ignites at a higher temperature to resist detonation...
Anyone have technical information on this? Thanks


This is generally true for pump gas. The Sunoco site is correct. The reason is that pump gas differs in octane rating based on blend. The higher octane gas tends to have more of slower burning, more stable compounds blended in. I think the difference is small.

Race gas is another matter. Same for non-gaasoline fuels. Octane number does not faithfully follow burn speed.


Ok, but how do you know what you are saying is true? :D What does burning slower accomplish? The longer burn gives detonation more time to happen. Yes?


Actually, I take the oil companies' (like Sunoco) word for it. I think they know what compounds are best (and cheapest) to increase octane rating and what else they do to the fuel. I do know that some compounds, like Toluene and Xylenes, have a higher flash point and I believe a slower burn rate. They have high octane ratings and are used as octane boosters.

As for the time factor, I think compounds that have the effect of octane boosters help to stabilize the rest of the fuel otherwise their mere presence would not slow the pre-flame decomposition reactions in the less stable components that lead to detonation. The chemistry is complicated. All high octane compounds are not equally as effective as octane boosters for low octane gasolines.
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