Octane and mileage and perceived power

[EenyBear]

Until last year, I had been putting Chevron 94 in my 2007 BMW R1200S sport bike. One day, the best I could get was 91 so I filled up. I immediately felt that the throttle response was more immediate and that the power was available lag-free. Unexpected. I soon chalked it up to imagination and wishful thinking. The temperature had dropped and of course there would be perceived performance due to denser air... etc. etc. etc. blah blah

However, I ride year round. Hot days and cold days. Dry days and wet days. After alternating between 94 and 91 octane over several tanks in varied conditions (in as much of a controlled experiment as I can devise), I can honestly say that the 91 performs better in my R1200S. I have an "adjustable chip" and I've tried dialing in performance with both. 91 wins.

So... I wondered what the effect might be in the Z4MC.
Over the last few months, I have put several tanks of 91 though the car in all weather conditions. While I do not see any major difference in performance (same acceleration etc. although a more immediate throttle response) my fuel consumption has gone down (e.g. better).

Converted to mpg, I'm getting 3-4 more mpg on 91 octane according to the calculator and my manual fuel/odo records. Just for fun, I tried two tanks of 94 octane in a row and... boom... consumption went up and perceived performance dropped. In fact, it kinda feels like the car is running lean on 94. Same sensation experienced on the bike.

The funny thing is, I can wring it out with some truly spirited driving using 91 octane, especially in sport mode, and consumption over a couple of tanks doesn't even come close to what I spend on 94 octane with tame sedate, careful driving.

I was always told that the higher the octane the better it is for the engine (all other things being equal, like cleaning additives etc.)

My experience suggests otherwise.

In the end, for both my R1200S and my Z4MC I save money on 91, and I get better mileage, and I get better performance.

Now that I've confirmed it's both vehicles, over several tanks, in all temperatures, with fuel from several different stations (all Chevron).... I think there might be something to this.

Any fuel engineers want to comment?
Feel free to ask questions to ensure I've explained myself clearly.

By the way, both vehicles are otherwise perfectly tuned. Oddly enough, they are both 2007.

Best regards.

[gferil]

One thing I learned in engineering school was "higher doesn't always mean better". If something is designed to run at a certain spec, follow the spec, don't do more or go higher just to do it.

Now, I'm no superstar mechanic, and I don't recall what our cars are designed to run on, but I'd imagine it's 91 since that's all I'm able to get here in California, and it would make sense for the automotive engineers to dial the spec to something that all drivers can access...not just the ones in regions that have higher octane.

I've heard if you're running an aftermarket tune or race engine, higher octane can help since that's what the tune or system is spec'd for, but again, I'd imagine a stock street legal Z4M wasn't designed to run on that stuff out of the box.

[Caddyshk]

In Canada I believe most 94's (Sunoco/Petro Canada) contain high levels of ethanol. The reduced mileage and performance of ethanol enhanced fuels is well documented. 91 Octane throughout Canada I believe has no Ethanol.

[davesz4mc]

The diffrent octane gasolines burn at diffrent rates.

Lower grade gas burns faster and is harder to controll combustion/timing with by a small degree.

Something that revs really fast like a bike will like lower grade gas to a certain degree of performance levels. For anything short of a track/big tune bike the lower the better. Something with a mild tune would prefer mid grade which you have found out.

Now when it comes to cars i.e a standard car they are going to run a knock sensor in most cases that advances timing untill knock is detected and then it pulls it back out. It will do this varying on models up to a few dozen times a second. There fore it is useless to run a higher grade when the car will run fine with a lower grade.

Now a sports car with a min octane requirement like my z4mc will not like retarding the timing that much as it is a huge performance robber. Would it run a low grade yes but thats not what it was designed to do. US rated 91 is the stock octane requirement and is designed to get the best performance based on that fuel requirement.

Now a higher grade gas will give you more performance if the car knows to look for it or is specifically tuned for it. Just adding higher grade gas with nothing to take advantage of it could indeed hurt performance (small).

Now take an ESS supercharged car or similar, they are tuned into looking for the higher octane levels and that is why they make the most power on high grade gasoline 100 and higher. They run lots of timing and controll the detonation very well.

As for ethanol it is a high octance fuel (E85 is 110 octane) the price you pay with E85 in particular is that it requires 30% more fuel to achive the same performace levels. It is also a cooler burning fuel and does not create the same heat levels as gasoline another advantage of running it!

If you get 100 miles per tank on gasoline, you will get 70 miles per tank on Ethanol because of the 30% fueling increase. The first thing you have to do to run Ethanol is put in bigger injectors to get that extra gas into the cylinders. The second thing is find someplace close that currently sells it lol.

If what caddyshk said about 94 containing ethanol and 91 not in Canada that is more than likely your fuel milage differance and the performance aspect is all in your head =-)

[johanness]

Originally Posted by Caddyshk

In Canada I believe most 94's (Sunoco/Petro Canada) contain high levels of ethanol. The reduced mileage and performance of ethanol enhanced fuels is well documented. 91 Octane throughout Canada I believe has no Ethanol.

94 Chevron in Canada - ZERO ethanol.

[EenyBear]

Originally Posted by johanness

94 Chevron in Canada - ZERO ethanol.

This is true. Ethanol is not a factor in this analysis. But it was a reasonable assumption.

I would have to say that the general idea of running 91 "...because that's the way it was tuned..." is probably the best explanation for the experiences I've been having.

Regardless, it makes for interesting discussion.

As for performance (apart from the cost savings) ... the only time I can detect a difference is in second gear roll-on. There's a bit of a noticeable delay with 94 and none with 91. Other than that, I sense no difference.

The sensation is like the body of the car tries to overtake the chassis with 94 and the chassis pulls the body along with 91. Best I can do to describe it.

Anyway, I thought the community might be interested.
Cheers.

[Caddyshk]

We don't have Chevron in this part of the country. When we changed the plugs on two identical mileage S54 motors last year, one that used Ethanol blended fuels and one without, the plugs had markedly different appearance; damp and rusted, clean and dry with slight oxidation. Talked to the folks at Engineered Automotive and NGK and apparently they've known about this since the first High Performance high octane fuels with Ethanol hit the market? Explained to me as leftover moisture as a result of the Ethanol?

[EenyBear]

Spot on.

My other (motorcycle) forum is full of reports of the water, alcohol, and acids that accompany ethanol in typical "enriched" fuel. This causes untreated aluminum to corrode (like in Bosche fuel pumps) and the micro-dust from the corrosion easily gets through the fuel filter and lodges in the injectors.

Simply put, ethanol is an excellent SOLVENT (dissolves plastic, rubber, fiberglass and more), and unlike MTBE (the current gas additive), ethanol will ABSORB WATER, which can cause serious problems. On the other hand, it OXYGENATES the gas, allowing it to burn a little hotter and more completely. And, MTBE can be a source of groundwater contamination to it's not all rosy.

Chevron 94 in British Columbia has no ethanol and also includes the detergent Techron (double benefit). However, the stock tune of the Z4MC responds as well or better to 91 in my experience. For fellow left-coast dwellers, there is also quite a bit of E10 available so check the pump label if you have any concerns. Ontario... it would be hard if not impossible to find pure gas anymore.

To round things out, here's an interesting website where you can locate "pure gas" if you want to.

In a similar vein... when we bought our Nissan Pathfinder we were quietly advised by the Service Manager to keep trying lower octane (cheaper) gas until the engine knocked... and then raise it a notch. We got down to 87/89 ("regular" around here) in the first 3-4 tanks with no knocking and have had no issues since. That was 10 years and 200,000 miles ago. Engine still purrs.

I realize the point (above) about ethanol is a little off the specific "octane topic" but it's worth mentioning. Ethanol enriched fuel just seems to be a bad idea for performance engines.

Airplane fuel contains no ethanol (as far as I know) due to the very risks presented by the potential for corrosion and consequences of engine failure.

[davesz4mc]

Regular cars nowa days will have knock sensors so octane does not matter and if flexfuel will run on any combo of gas or ethonal up to e85.

E10 is not corrosive enough to eat out the rubber lines on your car anytime soon: e85 is a diffrent story and you have to do some mods to switch over to that fuel.

Notice that the added ethonal increases octane rating on cheaper gasoline, the major downside is water absorbtion will retard timing in performance cars (may cause damage). Slightly decreased fuel milage should be the only real indicator that you have e10 in the tank. If it is a more drastic differance find a better station pronto...

[Caddyshk]

Shell V-Power and the other 91 Octane brands in Ontario have had no ethanol, although I hear this will be changing...