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Discussion Starter #1 (Edited)
I'm in CA, so we only get CA91 octane gas here at the regular gas station. I've been recommended to mix half a tank of 100oct with half a tank of 91oct when I'm on track (HPDE). Or even use a full tank of 100oct. I know it's good for safety by means of avoiding detonation and etc. (coming from turbo AWD vehicles) but will it increase hp and tq? I've been told that yes, it will. And that surprised me as my past experience tells me an opposite. Unless vehicle is equipped with octane sensor there shouldn't be any significant increase. Or it should be specifically tuned for 100oct.
 

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While its not really needed I generally keep my fuel close to the 93 octane rating (mixing 3:1). I am not really daily driving the car atm, so filling up expensive fuel once a while is not a big deal. ~4 gallons of 100oct = ~$30 bucks
Here is the mix chart. In the bay area I can get 100 octane in redwood city king's union 76. I used the same when going to the track few days ago. But then I am a noob track driver, other experienced people will chime in.

 

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Discussion Starter #3
Thank you, Noobies! Do you feel any power increase though? Yeah 3:1 sounds very reasonable. I'm not even sure that 50/50 mix (half 100oct and half 91oct) is needed for our vehicles (by means of detonation safety and etc.) if there is no power increase. And full tank of 100oct could be just a waste.

I sometimes get 100oct in San Jose, used Redwood city 76 station only once. Most of the time I used 100oct available at the track. I do full tank of 100oct when I track my Subaru. And it's $$$ especially purchased at the track pump.

BTW, what club do you track with?

While its not really needed I generally keep my fuel close to the 93 octane rating (mixing 3:1). I am not really daily driving the car atm, so filling up expensive fuel once a while is not a big deal.
Here is the mix chart. In the bay area I can get 100 octane in redwood city king's union 76. I used the same when going to the track last weekend, though had to fill some on the track, so probably went higher than 93 then out of necessity. But then I am a noob track driver, other experienced people will chime in.

 

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I'm sure we can all agree that there will be a power advantage up to 93 octane. That's what they designed the engine for, and if it didn't make a difference then it would be easier if they called for 91.

That said... does anyone have the slightest idea of how long it takes for the engine to figure out that you've stepped it up from the 91 and take advantage of the difference?

I'm wondering whether it's worth while putting the higher grade stuff in right before the track day, or whether you'd need to run through a full tank or more of it beforehand for the engine to slowly ramp up the performance?
 

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Yeah same questions here...if people mix it, how do they "mix" it. How does the car figure out it's a mix unless we actually manually mix it before putting it in the tank.
 

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Discussion Starter #6
My limited experience with ECU tuning (non-Porsches) tells me that it's all about ignition timing advance and air-to-fuel ratio. I.e. ECU sense some detonation on 91 oct and retards timing and/or makes mixture richer decreasing the power output of the vehicle if the ECU programming was initially optimized for 93 oct (more aggressive timing).

So you put some 93 oct in (or mix). ECU sees no detonation and starts to increase the timing back. How long will it take for a full effect is a big question and depends on ECU design and programming. I guess the fastest way to "stimulate" it is to start from scratch, meaning do ECU reset. Any "legit" way of doing ECU reset on 981S?

Yeah same questions here...if people mix it, how do they "mix" it. How does the car figure out it's a mix unless we actually manually mix it before putting it in the tank.
 

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Discussion Starter #7
50/50 mix will be ~95oct? If ECU is optimized for 93 then there should be no power increase running 95 vs 93? I guess same with pure 100oct in the tank. I really doubt that ECU can adapt in that wide of a range if there is no octane sensor in the vehicle. Doing that only based on registered detonation is very dangerous for the engine. 93->91 is OK (using detonation sensor) but 100->91 is dangerous.

But maybe 981S has an octane sensor in the fueling system?
 

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Yeah but my question is how is one "mixing" the fuel. Does one put it in a container, shake it to mix it, and then put it in the tank?
I mean we don't have a spoon to put into the tank to mix the fuels...
If one is putting in 1/2 tank for 100 octane, then 1/2 tank of 91 octane then the engine is not really seeing a mix, it's seeing two distinct octanes?!?
I don't know what the density difference in the octane levels, does one sink while other floats, can it properly mix?
questions questions
 

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Discussion Starter #9
Noobies, I thinks fuels mixes fast in the tank with the natural shake of the vehicle. That shouldn't be a problem. I usually just put additional gallons of 100oct directly in the tank and let it self-mix there. I don't think you need to pre-mix it manually in a jag or something like that.

For example my current track car is tuned to run on ~97oct. That's 3/4 of 100oct and 1/4 of CA91. I make sure that by the time I get to the track I have 1/4 or less of 91 in the tank. Then I pump 100oct until the tank is full. Go and drive first session still using 91oct ECU tuning and let it mix and get to the fuel line. After the first session I pump in some more 100oct (making it equivalently higher than 97oct) and reflash the ECU for a specific track "high octane" tune to take the full advantage of the gas mixture for the next session. By the end of the track day I have almost a pure 100oct in the tank (several refueling with 100).

But my 981GTS is stock and I wasn't sure if using 93 oct or equivalent mix will make any difference at all. :)

Yeah but my question is how is one "mixing" the fuel. Does one put it in a container, shake it to mix it, and then put it in the tank?
I mean we don't have a spoon to put into the tank to mix the fuels...
If one is putting in 1/2 tank for 100 octane, then 1/2 tank of 91 octane then the engine is not really seeing a mix, it's seeing two distinct octanes?!?
I don't know what the density difference in the octane levels, does one sink while other floats, can it properly mix?
questions questions
 
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I really doubt that ECU can adapt in that wide of a range if there is no octane sensor in the vehicle.
That's probably true, though a Turbo powered car with an after market ECU might be able to eek out some more boost. But factory you are probably right.
 

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I use VP's MS103, no mixing involved.
car runs perfectly, exhaust sound is noticeably louder and you can feel the difference in power.
having said that, biggest difference in power noticed is when i use 103 i always get that 'backend going out of control' feeling even on straight line while shifting from 1st to 2nd and 2nd to 3rd.
 
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I'm in CA, so we only get CA91 octane gas here at the regular gas station. I've been recommended to mix half a tank of 100oct with half a tank of 91oct when I'm on track (HPDE). Or even use a full tank of 100oct. I know it's good for safety by means of avoiding detonation and etc. (coming from turbo AWD vehicles) but will it increase hp and tq? I've been told that yes, it will. And that surprised me as my past experience tells me an opposite. Unless vehicle is equipped with octane sensor there shouldn't be any significant increase. Or it should be specifically tuned for 100oct.
In California the street gasoline mix is pretty conservative. It's 91 octane with no MTBE, a powerful (carcinogenic) oxygenate. They blend about 10% ethanol into it, give or take. It's not a terrible idea to top off the tank with some pump 100 octane at the track. Stable fuel is good for the motor under high heat and pressure as seen on the track. You're not going to make excess power unless it's a forced induction application, where they can advance the ignition timing and take advantage of that fuel.
 

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No need to use 100 octane. If your motor comes tuned for specific fuel. The increase in performance may be negligible, and it may also hinder performance to have gasoline outside the octane levels that your motor ecu was designed for.

With 100 octane you may run a bit rich, and from my experience you may get some flames come out of the exhaust if the car is really really hot!

But really its not necessary IMO
 
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Just to offer another perspective. My observations after 10 years at the track and a few PCA TT championships:

>Turbo or vintage high compression NA car that ran leaded gas= run high octane.
>Modern Porsche with adaptive ECU> No benefit at all, save your money for tires.

YMMV
 

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Well, here is a ringer for everyone to think about. A little chemistry, physics, and common sense???

Our gas here in Dallas is a 10% ethanol blend by law, which is what most metropolitan cities are mandated to have. Ethanol is only 61-68% as efficient as gasoline depending on whether he you use BTUs or joules to measure the energy in the gas. At this mix E 10 gasoline is 96% of the energy of pure blend gasoline. So if you use racing gas you will not only get higher octane which should be worth a few horsepower, but you also get a denser more energetic burn of up to 4% with pure race gas. At say even just a 1% gain in power for the increase in octane from a blend of race gas, you can have a potential increase in power of about (4+1) 5% with 100% race gas, or (2+1) 3% with a 50-50 blend. Now I know the exact amount of energy in the gas may not exactly translate into the same amount of the increase in power, but it should be close and or at least give a general idea of what might happen if we use it.

So 3% of the Cayman S horsepower is worth about 10 hp, and 5% could be worth as much as 15 hp. That is basically GTS horsepower for the weekend solely from a more expensive tank of gas. I think that is a pretty good deal. I think a 50-50 blend of 91 and 100 octane gas would yield 95.5 octane with half the power robbing ethanol in the mixture and likely yield about a 3% increase in power or about 10 ponies. Again the power increase comes from reducing the percentage of ethanol in the gasoline, not so much from the increase in the octane. If you can find pure gasoline in your neighborhood without the ethanol, then by all means fill up with that as often as possible and you don't need the race gas. Overall, there is not a huge amount of horsepower to be gained as compared to a turbo, but not something that should be readily dismissed as not being able to help our cars power at all. It is also something that can be achieved quite easily on an as needed basis whenever one desires that extra boost of power.

I think we would all love to take 100 pounds off our cars and agree that it would be a meaningful benefit in the horse power to weight ratio. As we all know, chasing that equation can get very very expensive. Losing one hundred pounds is a 3% weight loss or an effective 3% increase in power from permanent and most likely costly modifications. Assuming there is a race gas pump at the track like we have, one can try this mix for $25-$50 depending on whether you do it for a half or a full tank. I think this is an extremely inexpensive mod to try, and is of course completely reversible once you burn up that tank of gas. There are no parts to buy, no cost to install them, and it doesn't wear out. In the end I don't think we are really adding horsepower to the factory numbers but rather getting back the loss in power that we are forced to accept by running 10% ethanol in our gasoline.

I will be trying a 50/50 blend on a half tank of gas at some point in my next DE to see how it works.
 

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Besides the energy content, IF the ECU can adapt and give more ignition timing than with street gas, you should pick up more HP. However, I have been told the car has to run 100 miles on race gas before it can fully adapt. Anything less yields partial results.
 

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There is a lot of good information on here. In the past, Sunoco 98 GTX has caused these cars to run a bit fatter, which will make more power in some parts of the rev range and allow you to generate slightly cooler combustion temps. The car needs to sample though and adapt and it can take as much as a full tank's worth and truth be told, it may even negate all the differences so that it can properly target its mandated Air Fuel Ratio.

The throttle response feels better; the car feels stronger but I haven't been able to quantify it ever though so I can't say with any certainty that I know that it ultimately changes the AFR of the car with stock mapping. It might do it for a few hundred miles and then revert right back to where it was at the beginning of the day.

When it comes to tuning the car though and when you are using 98 octane unleaded, you are using a much better fuel than the 10% ethanol based stuff you get at the 93 pump so the 98 would allow you to make more power with tuning.
 

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I create my own 'mix' using a specialized octane additive for racing only - just have gas in the tank, know how much you add to get the right amt - and pump away
 

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Discussion Starter #20
The single biggest unknown is whether or not the car's factory AFR of about 12.9:1 drops to 12.6:1 with the 98 octane
Hmmm... Why should the AFR go richer with higher octane? I thought it's an opposite. On higher octane ECU sense no or less detonation and start returning some timing (more timing advance) and make the mixture a bit leaner. Leaner ARF -> more power. But again my limited experience with non-Porsche small turbo engines :) Could be different.
 
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