Octane limits?

[IndianaRoadster]

I normally put 93 octane gas in my S52 m roadster. There's a station that sells 110 octane racing fuel close by. My question is will 3 or 4 gallon of 110 octane mixed in be of any advantage? I am forced induction.

[rf900rkw]

The correct answer is to ask your tuner. But the practical answer is probably not. Tuners aim the ignition tables for 93. BTW, you do know that the 93 and 110 are on different rating scales.

[IndianaRoadster]

The correct answer is to ask your tuner. But the practical answer is probably not. Tuners aim the ignition tables for 93. BTW, you do know that the 93 and 110 are on different rating scales.

I did not know that. Will have to do some reading up on that.

[cyberman]

The correct answer is to ask your tuner.

Thanks, I did not know that. I would have assumed the knock sensors would just keep advancing timing and increase power. But thinking about it, of course, there may be some limit. I've only seen 91 Octane fuel here - wonder it the stock engine tune can use 93 to its advantage, or if it would be a waste of money.

[joelrg]

It's my understanding that higher octane is only necessary and a benefit in higher compression engines (which a turbo/supercharged engine would be). "Supreme" isn't necessarily a better gasoline then regular, unless your engine needs that octane to prevent detonation. If you have a stock engine, run what the manual recommends, otherwise you're just burning money.

Here's a good short article: https://cars.usnews.com/cars-trucks/...d%20to%20occur.

Joel

[BimmerBreaker]

For stock US Z3's, only the S54 can make use of higher octane than pump gas - it, and the S62, advance timing to account for about up to 96-98 octane. So you can mix 91 with race fuel to get some more performance

As joelrg touches on, the reason this is a benefit is because the S54 and S62 are very high compression motors and can make use of the extra compressibility of higher octanes. Higher octane fuels dont contain more energy/power though

[rf900rkw]

Damn common core...


Alright, everyone gather around. Back in the olden days there were two different octane scales based on different tests. The tests were pretty much the same, but the operating parameters were different. Research octane (RON) and Motor octane.(MON). The motor octane test is much more representative of real world operating conditions. But the research test gave numbers that were 10-15% higher. So *everyone* published the RON number. Then, in those dark ages when Pintos and Vegas roamed the plains, there was an energy crisis of extreme and man-made proportions. Much like toilet paper today, hoarding and rationing was the rule of the land.. Fears abounded that the unscrupulous fuel vendors would start skimping on the fuel quality, so, in an early example of government overreach, they mandated that the octane number be clearly posted on each pump. But the government could not stop there. No, with two different octane scales to choose from, they went and invented a ^&$*@ing third one. PON (now called AKI), Pump octane number. It's an average of the two. Ever notice the octane sticker includes the cryptic (R+M)/2 formula? Now you know what it means.

So thus, in 1973 the octane numbers posted on the pumps went from 99 and 100 to 93 overnight for the same exact fuel... and every illiterate hotrodder screamed about MUH OCTANE... even to this day.


The S52 has a timing table that is the upper limit, and the normal operating point. It only backs the timing down when knock is detected. It did not advance the timing beyond the table point until knock was found. Good tuners changed the timing tables to be correct. Bad tuners left the tables alone, and hoped the knock sensor based retardation would keep them out of trouble. Without knowing more about the tune, I would say the best you can do is run 93 E10. Unlike NA cars, the alcohol helps in FI.

[rf900rkw]

To over simplify, higher octane is harder to light off and burns slower. Thus it can stand higher compression pressures without self-igniting. This is not always beneficial. Back in the days before Mercedes destroyed Formula 1, you had engines that turned 18-20K RPM. To get the fuel to burn in that extremely limited time, they were running fuel with octane ratings that would detonate in a lawn mower.

[cyberman]

Alright, everyone gather around. Back in the olden days ...

Thanks for jogging my memory about that - I remember the Sun Oil brand pumps where you could dial a custom blend Octane up to something north of 100. My little M44's manual recommends 91 Octane, so I think it benefits a bit with that as opposed to 87. It does have fairly high compression (10:1 I think).

[BimmerBreaker]

I think the easiest way to explain it is that all fuels have the same energy density, just some are harder to ignite by compression. Allowing them to be compressed more allows for more efficient extraction of the power potential in the fuel. Higher octane means more compression before ignition. Using a high octane fuel in an engine designed for low octane means the burn rate will be suboptimal and there is danger of damaging the engine since you are compressing something that doesnt want to be compressed, past the point of normal ignition. Use a low octane fuel in a high octane engine and the fuel will ignite before it is fully compressed which is predetonation, very bad. Knock sensors detect this and adjust ignition timing to account for this. S54s will be pulling timing on any pump gas for example and can advance timing up to about where 96 AKI octane will be useful. Higher octane fuels do cause engines to run a little hotter though

[nevan]

Hi IndianaRoadster,

I'm no expert in the subject, but from what I've observed by trials and errors of tuning myself, I've learned that forced-induced s52 works "really well" with ethanol-based fuel. If you plan on raising up the boost in the future, I would especially give the e85 a serious consideration.

I think the easiest route would be to install the ethanol fuel adaption kit. Or another option is to retune the dme to run e85. I think that either route will cost about the same (if you get it done by a tuner).

Zach