Is RON97 too high for a M40?

[evilnickwong]

I was reading an article today here:

http://www.vitekperformance.com/technical_sparkplugwires_combustion.asp

One part, it mentions this:

Fuel Characteristics
Low compression engines usually run well on low octane fuel because they have relatively low charge densities and the burn rate within these confines is usually predictable. A low compression engine switched to 118-octane race fuel will always lose power unless the ignition advance is increased to compensate for the slower burn rates. Even then, a low CR engine may lose power with the timing optimized for high-octane fuel.

From that section, what I understand is that running a higher octane fuel in a car with low compression might actually be detrimental to power?

In my country, we only have 97 and 92 octane fuel and 97 is what I typically run in my E30 318i M40, so this bit of reading got me wondering about it as I have never tried the 92 octane before.

[Beach Bum]

That sounds like it could be true if the gasoline was diluted to add octane boosting chemicals, thus decreasing power density.

[rThor432]

Not sure about the M40, but I can add a bit to the topic. My eta's have always run better on low octane fuel opposed to the high octane. But they have a relatively high compression ratio... probably similar to the M40's. I'm sure that the M40 would run just as well on RON92 as it would on RON97. I dont think that engine would see benefits from the increased octane.

Just my opinions/thoughts here^^.

[evilnickwong]

Hmm OK, I guess I'm just being too paranoid after reading these sorta articles.

I would be curious to know what other M40 drivers are using, but I think very few people in your part of the world had them, you guys were lucky and had the much better M42 instead. :D

[rThor432]

You guys were lucky and had the much better M42 instead. :D

Yes, maybe... but look at all of the other great things we missed (and still miss) out on-- E30 M3 Evo's, real S50 motors, modern BMW diesel motors, etc. Those are just a few random points. For the most part we missed out on some pretty cool stuff that the rest of the world gets as standard equipment.

[DuHast]

The differences with the gasoline types as I learned about it in school was that the lower the octane the quicker it burns. So in other words with a lower compression motor you don't need a fuel that burns for a long time because by the time the piston goes down on the power stroke and comes back up on the exhaust stroke, you may be pushing unburnt / just burnt fuel right out the exhaust. Basically with a lower compression motor, using high octane fuel is wasting gas. High octane fuel on the other hand, burns longer and allows a longer power stroke to get the engine to run at max. efficiency (of power and economy). These motors sometimes will also have a longer stroke so that the fuel can completely burn by the time that the piston comes back up for the exhaust stroke.

Another thing, the reason "Premium Fuel Only" cars have no power and ping and run shitty on lower octane fuel is because under the high compression (which increases combustion chamber temp.) the fuel may ignite before the spark plug which is pre-ignition aka detonation. It also doesn't allow the engine to have enough force to push the piston down all the way on the power stroke because the gasoline / air mixture basically flashes and the power needed to push the piston all the way down on the power stroke for max power/torque can not be achieved.

[eta]

In every car I have ever run;
Chrysler Neon
Chrysler Pt Cruiser
BMW 528i E28
BMW 525e E28
VW Mk2 Golf 1.6
Ford Ka

99 octane RON has always given better fuel economy. So not believable to me. Run it 97 RON the difference will be smaller.

Also the octone rating does not change the energy density of the fuel. It changes the speed of the explosion front by changing the activation energy of the the combustion reaction. Thus increasing resistance to knock/detonation....

Quite how high octane fuel has lead to better fuel economy, I don't know but the effect is very real.