It runs faster on low octane fuel?

[Truckedup]

I race bikes and get into stories with other guys...So a guy has has a newer bike and says he was running 91 E10 pump gas and then in a pinch filled it with 89 E10...He claims the bike seems faster...Then the usual comments on how higher octane pump gas burns slower and reduces power....It all sounds like wive's tales... But...
Has anyone have proof than an engine can make more power on lower octane fuel?

[mag2555]

It's no wives tale, if the motor does not need the octane your feeding it then less octane burns the charge faster ( heats and expands the air faster ) and the motor makes more power and perhaps just as important rev's up faster.

It can also be in part that the particular lower octane fuels composition is better suited to the average temp that the motor runs at when racing, especially in terms of air charge temps and fuel to vapor conversion.

[GRTfast]

To echo what mag said, I have always been told that the lower octane fuel has a faster burn rate. If you keep the ignition timing the same, and the charge is fully burned sooner, it is like advancing the timing. If the engine timing was more retarded than it needed to be on the higher octane fuel, and it can tolerate the lower octane fuel without detonating, it seems logical that it would make more power on the lower octane fuel.

[cjperformance]

I have come across this before. Many years back we had an engine failure, 460bbf, in a drag car. We were running Shell RF100. We threw in a near stock 460bbf at only 8.6:1 CR and ran a couple meetings on the leftover RF100 fuel. We then switched it to pump Super(leaded) fuel that we had here in Australia at the time, this was around 1995, iirc 94/96RON, Anyhow the low comp engine was slow, 10.30 in a rear engine rail ,ex old mild steel Top Alcohol Dragster chassis, but we were racing still so all good. The fuel swap in this case was worth 1.3/1.5 tenths in favour of the lower octane fuel, which surprised us enough to re try the RF100 and yeah it slowed down!

[jacksoni]

I race bikes and get into stories with other guys...So a guy has has a newer bike and says he was running 91 E10 pump gas and then in a pinch filled it with 89 E10...He claims the bike seems faster...Then the usual comments on how higher octane pump gas burns slower and reduces power....It all sounds like wive's tales... But...
Has anyone have proof than an engine can make more power on lower octane fuel?

This has been argued a lot. See the threads below ( a nice feature of the new site). Couple easy explanations that have nothing to do with octane. 1) his butt dyno is inaccurate. 2) the Sg of the fuels are significantly different so the tune up was wrong with the former fuel if nothing else was changed.

[Truckedup]

I race bikes and get into stories with other guys...So a guy has has a newer bike and says he was running 91 E10 pump gas and then in a pinch filled it with 89 E10...He claims the bike seems faster...Then the usual comments on how higher octane pump gas burns slower and reduces power....It all sounds like wive's tales... But...
Has anyone have proof than an engine can make more power on lower octane fuel?

This has been argued a lot. See the threads below ( a nice feature of the new site). Couple easy explanations that have nothing to do with octane. 1) his butt dyno is inaccurate. 2) the Sg of the fuels are significantly different so the tune up was wrong with the former fuel if nothing else was changed.

Yes, I have asked this question before along with a few others....most recent a engine design enginner on this site said the single biggest influence on combustion burn is intake velocity and combustion chamber design....All fuels burn at about the same rate..And I believe, maybe wrong that SG and evaporation rates of pump gas are regulated by gov't standards....

[plovett]

I think it is possible to run faster on lower octane fuel. I don't know if it is the burn rate or time of ignition, though. Or both. I think a low compression engine with high octane fuel will want different ignition timing than a high compression engine with low octane fuel. And there are infinite variations in between.

I once had a 9.3:1 compression engine on the dyno with race gas. I don't remember exactly what it was. 104 octane maybe? 110? Anyway, it made best power with 45 degrees total timing! I am pretty sure the ignition timing and dampener were accurate.

Later, I rebuilt the engine with and true 10.5:1 compression and ran it on the dyno with 92 octane pump gas. With the same cylinder heads and dampener, it made best power with 37 degrees total timing. So all this is related in my opinion.


JMO,

paulie

[Truckedup]

My land speed race bike uses a lot less fuel than car...I open a fresh 5 gallon pail of 108 MOT octane leaded, use about two gallons....I use the rest blended into my street bikes than run fine on 91, about 50/50 with pump gas....I notice zero difference in performance...but the exhaust exits do get white from the lead....

[Dirtybob]

My iron headed - stock cammed LT1 saw no ET/mph improvement from switching from 87 octane to 93. I started adding in timing 2 degrees at a time (at the dragstrip, my butt dyno is broken) until I saw spark knock on the data logs. Was able to run 6 degrees more timing (knock free) on the 93 octane but no change in 1/4 mile times/speed at any increment.

[BlitzA64]

This has been argued a lot. See the threads below ( a nice feature of the new site). Couple easy explanations that have nothing to do with octane. 1) his butt dyno is inaccurate. 2) the Sg of the fuels are significantly different so the tune up was wrong with the former fuel if nothing else was changed.

This..... I gained between 5-10hp through the curve on a 630hp engine using a lower octane racing fuel on the dyno at the end of a session. The specific gravity difference was fairly substantial richening the mixture. Basically we had switched carbs after spending a lot of time tuning the original set up, that gave nothing in fact lost a little below peak. For ha ha's I wanted to see what the lower octane more common fuel would do so we switched up. Kind of pissed me off, we were out of time so all that did was muddy up the water. Was it the fuel or the carb tune? Another dyno session to come in the future I guess.....

[gunt]

i'll try to remember to dig out a graph that will answer all

[David Redszus]

It's no wives tale, if the motor does not need the octane your feeding it then less octane burns the charge faster ( heats and expands the air faster ) and the motor makes more power and perhaps just as important rev's up faster.

Not true. Fuel octane has nothing to do with burn rate...nothing. Two fuels with the same octane ratings can easily burn at different rates. Burn rate is principally a function of inlet air temperature, compression temperature, squish velocity and engine rpm. Suppose we use a 100 octane, unleaded fuel on the dyno optimally tuned, and then add some TEL or TML to the fuel while it is running. There will be no difference in performance. The two fuels are identical except for octane.

Of course, using the higher octane fuel would allow the engine to be tweake to a higher performance level without destruction due to detonation.

It can also be in part that the particular lower octane fuels composition is better suited to the average temp that the motor runs at when racing, especially in terms of air charge temps and fuel to vapor conversion.

Getting closer. Two fuels with the same rated octane values could be substantially different in terms of composition, heating value, SpG, stoich, RVP, autoignition point, etc. Then a difference in performance would be easily noticed.

If we are speaking about pump gas, then all bets are off since we do not know what the actual octane values, RON and MON, actually are. Neither do we have any idea regarding the actual composition and properties of the fuels.

There is a reason why OEMs use very expensive certification fuels when performing engine tests; consistency.

[GRTfast]

It's no wives tale, if the motor does not need the octane your feeding it then less octane burns the charge faster ( heats and expands the air faster ) and the motor makes more power and perhaps just as important rev's up faster.

Not true. Fuel octane has nothing to do with burn rate...nothing. Two fuels with the same octane ratings can easily burn at different rates. Burn rate is principally a function of inlet air temperature, compression temperature, squish velocity and engine rpm. Suppose we use a 100 octane, unleaded fuel on the dyno optimally tuned, and then add some TEL or TML to the fuel while it is running. There will be no difference in performance. The two fuels are identical except for octane.

Of course, using the higher octane fuel would allow the engine to be tweake to a higher performance level without destruction due to detonation.

It can also be in part that the particular lower octane fuels composition is better suited to the average temp that the motor runs at when racing, especially in terms of air charge temps and fuel to vapor conversion.

Getting closer. Two fuels with the same rated octane values could be substantially different in terms of composition, heating value, SpG, stoich, RVP, autoignition point, etc. Then a difference in performance would be easily noticed.

If we are speaking about pump gas, then all bets are off since we do not know what the actual octane values, RON and MON, actually are. Neither do we have any idea regarding the actual composition and properties of the fuels.

There is a reason why OEMs use very expensive certification fuels when performing engine tests; consistency.

Great explanation. I've been suspect of the "lower octane burns faster" claim, but it seemed reasonable, and I have heard some pretty reputable engine builders regurgitate it. Goes to show that being an expert in one area doesn't mean much in a different area.

[nitro2]

It's no wives tale, if the motor does not need the octane your feeding it then less octane burns the charge faster ( heats and expands the air faster ) and the motor makes more power and perhaps just as important rev's up faster.

Not true. Fuel octane has nothing to do with burn rate...nothing.

Well when the fuel octane is too low it has a BIG effect on "burn rate"

Somewhere south of adequate octane and north of too low octane, is a sweet spot.

[MadBill]

FWIW, a number of years back we did a chassis dyno session with a 12.4:1 306" SBF road racer. After optimizing spark and fuel on straight C12, we did a couple of pulls with a 50-50 mix of C12 and Petrocan 94 (R+M)/2 pump gas. We had no time to tune further but it picked up 4-5 HP, peaking with 530 HP @ 8,000. We ran it for a full season with no issues until the block cracked. The replacement 12.6:1 380" is now in its 5th season, with no combustion issues to date.

At $25/gal. for C12 in Canada, the mix saves us almost $10/gal., a total of over $1500 for a full season..