George Helser

I measured the acceleration in my 1997 993 C2 coupe with stock engine fueled by street gas and then with a blend of street/race gas. The blend of 30/70% street/race fuel was faster but only by 0.01G which is roughly 2% higher.

Here are the details:
- Measured acceleration using Valentine Research, g-analyst, 3-axis accelerometer with 1% accuracy and 0.01G resolution
- Tested at an altitude of 2200 feet and 65F ambient temperature
- The car was at normal operating temperature
- Street gas was Texaco 91 octane and 10% ethanol (NOV-MAR street fuel has ethanol in the Phoenix area)
- Race gas was Union 76 brand 100 octane, “Official fuel of NASCAR” (not for use on the street in the Phoenix area NOV-MAR)

Ran three times on the same stretch of road at full power in 1st & 2nd gears looking for
peak G:

91 OCTANE
1st gear = 0.58 G max each time
2nd gear = 0.36 G max each time

91/100 OCTANE BLEND (2 gal 91 Octane & 5 gal 100 Octane mixed)
1st gear = 0.59 G max each time
2nd gear = 0.37 G max each time

The 91 octane runs were with 2 gallons in the tank and 5 gal of 100 octane in a container in the car. After pouring the 100 octane fuel into the tank I drove for 6 miles to mix the fuels and added water bottles to compensate for weight of the fuel burned.

While I was careful to eliminate any variables, I cannot guarantee there was a real difference between the fuels since the measurements varied by only 0.01 G. However,
the readings from run to run with the same fuel gave the same acceleration 3 times in a row so I can say I am inclined to believe there was a small improvement with the higher octane fuel. I also recognize this simple test is not a complete dyno map of the engine performance. I was just looking to see if there might be a major difference justifying $5 per gallon fuel.

I will try this again in the future at a Drivers Ed to see if the higher octane fuel is a significant advantage under higher engine temperatures at the track.

Regards,
George
In sunny Arizona

Jean-Marc

It really shouldn't be and advantage in a car that wasn't tuned for it. All you'll get is knocking avoidance insurance
JM

KC993

George,
I for one am glad that there are someone like you who do all the R&D for all of us.

Keep up the good work.

tom_993

Good work George, but my question would be what does a 2% gain in G really mean in turns of time or speed? A 2% gain on a 5 second 0-60 time is about a tenth of a second. While that's not huge, it's still considerable. A 2% gain on a quarter mile time or speed would probably also be significant. I don't know that a 2% gain in G means a 2% gain in 0-60 time or 1/4 mile time, though...

Pete Lech

I believe that at say a 90 degree ambient temperature and at sea level, that you would see a greater difference. Under these conditions, the knock sensors would dial in a lot more ignition retard with street fuel than it would using race fuel.

Mdrury

Fun research project George, but doesn't 0.01G qualify as a statistical error? ( Or fuzzy math, to quote g dubya ). That means you have to do it 998 times more!
I did a related experiment to upgrade the appalling gas in California ( 91 ) using a 2::1 mix with Union 76 race gas ( 100 ) = 94. Result? Temporary delusion of grandeur followed by permanent lightening of the wallet. I reckon that it would be an excellent precaution for that DE event tho'.

Flying Finn

Do you guys know what brand sell the cleanest fuel in US? And if differences, here in East coast?

I haven't found any tests anywhere so I'd like to know if anyone knows what fuel cleans the engine best?

Baron

George, One thing you forgot. The brain learns how you drive and you need to remove power to the brain so that the Motronic brain will lose the previous readings. The brain will then continue to advance the engine ignition curve until it pings. Porsche says that it take the better part of a day at the track for the car to reprogram the motronic if yo do not remove power to the motronic brain

George Helser

Jean-Marc,

Years ago a fuel maker (Sunoco?) ran ads that a car (I think it was a Corvette)
accelerated faster with their high octane street fuel. It was true but perhaps misleading.
The car had a knock sensor (rare in those days) which altered timing to get more power from the higher octane fuel.

With the 11.3:1 compression ratio and knock sensor, I expected some power benefit from higher octane fuel in a 993. I was not expecting much increase because if a big increase was possible it would be well published already.

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tom_993,

A 2% improvement in acceleration (Gs) could potentially reduce time to reach a speed by 2%. The 0-60 or quarter mile time would be improved by less than 2% due to launch and shift times which are not dependent on power.
This may not be significant unless you are in a competitive event and the difference between first and second place runners is a fraction of a second.

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Pete,

The octane here is lower because of the altitude. Less octane is required for less dense air. At sea level, I expect you get higher octane fuel to compensate.
I have experience engine knock in different cars over the years when they got very hot from extensive high engine output. I expect you are right about seeing greater benefit from high octane fuel at higher temperatures. I hope to test this in the future.

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- Michael -

0.01G a statistical error? My college chemistry teacher once accused me of “fudging” a lab experiment because the results were too close to theoretical. The other students wanted to be my partner on lab experiments.
My octane results are close to the noise level but my careful tests show a small increase in power. Is it worth running race gas for autocrosses & Drivers Ed? No, not to me, but I like to know I am not overlooking a big advantage.

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Baron,

Can you point me to some documentation showing it will “take the better part of a day at the track for the car to reprogram the motronic if you do not remove power to the motronic brain”?
Interesting point. I did power off the ignition between low and high octane tests. Does this qualify? If the Motronic will not adapt to higher octane for a long time, will the Motronic adapt to a low octane right away to protect the engine?

Regards,
George
In sunny Arizona