According to a engineer at BMW M, you will loose 10 - 12 hp if you run the M5 on 95 octane instead of 98 octane ( 98 is the recommended ).

Of course I hope you asked him if you would GAIN any HP with higher octane??

I'm guessing not, but we can hope...

Of course I hope you asked him if you would GAIN any HP with higher octane??

I'm guessing not, but we can hope...
[/quote]

That is a topic that has been hotly debated in the main forum. I say no, and think the science supports me (i.e., the engine is designed to work at a specific compression ratio and you cannot receive any additional benefit merely becasue the gas you use can, in a different engine or environment, be compressed even more). Others, including Iceman, insist higher is better. I have still never seen any empirical evidence that higher than recommended octane provides a benefit.

Of course I hope you asked him if you would GAIN any HP with higher octane??

I'm guessing not, but we can hope...
[/quote]

That is a topic that has been hotly debated in the main forum. I say no, and think the science supports me (i.e., the engine is designed to work at a specific compression ratio and you cannot receive any additional benefit merely becasue the gas you use can, in a different engine or environment, be compressed even more). Others, including Iceman, insist higher is better. I have still never seen any empirical evidence that higher than recommended octane provides a benefit.
[/quote]

You can't change the compression ratio, but you can change the ignition timing and air/fuel ratio with a higher octane fuel.

From a good site on combustion chemistry:

"If the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition." Pre-ignition can damage an engine before you finish reading this sentence.

What we're really concerned with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug). "

Higher octane fuels can allow a more advanced ignition timing, because they are more resistant to 'knock'. This is one of the reasons that aftermarket chip manufacturers can achieve higher HP numbers with very high octane fuels. They're not changing the compression ratio, they're able to change the A/F ratio and the ignition timing.

I agree with you, and doubt BMW has programmed the M5 to take advantage of a higher octane fuel, but it's possible.

The same engineer told me that you don?t gain any hp?s from higher than 98 octane on the M5! :nono:

The only time higher octane fuel ( SHELL V-power 100 octane is now introduced in Sweden at 0.1 euro/L more than the old 98) would give you any advantage is during high temperatures ( 25 - 30 deg C and higher ). This advantage is ofcourse only that you don?t loose any hp?s! :|

Of course I hope you asked him if you would GAIN any HP with higher octane??

I'm guessing not, but we can hope...
[/quote]

That is a topic that has been hotly debated in the main forum. I say no, and think the science supports me (i.e., the engine is designed to work at a specific compression ratio and you cannot receive any additional benefit merely becasue the gas you use can, in a different engine or environment, be compressed even more). Others, including Iceman, insist higher is better. I have still never seen any empirical evidence that higher than recommended octane provides a benefit.
[/quote]

You can't change the compression ratio, but you can change the ignition timing and air/fuel ratio with a higher octane fuel.

From a good site on combustion chemistry:

"If the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition." Pre-ignition can damage an engine before you finish reading this sentence.

What we're really concerned with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug). "

Higher octane fuels can allow a more advanced ignition timing, because they are more resistant to 'knock'. This is one of the reasons that aftermarket chip manufacturers can achieve higher HP numbers with very high octane fuels. They're not changing the compression ratio, they're able to change the A/F ratio and the ignition timing.

I agree with you, and doubt BMW has programmed the M5 to take advantage of a higher octane fuel, but it's possible.
[/quote]


I undersatnd the basic of the combustion engine and the role of compression, but I am in no way an expert. So some quesyions:

If you change the ignition timing to delay ignition, will the engine create higher/more compression and if so, do you run the risk of damaging the engine if no other mods are made?

If dealy does not create higher compression, is there a benfit?

I think I undersatnd the benefit of changing the A/F ratio, and that very high octane fuels can combust with less air, thereby creating more power. But agin, if you tamper with the mixture without making other mods, do you run the risk of engine damage?

Of course I hope you asked him if you would GAIN any HP with higher octane??

I'm guessing not, but we can hope...
[/quote]

That is a topic that has been hotly debated in the main forum. I say no, and think the science supports me (i.e., the engine is designed to work at a specific compression ratio and you cannot receive any additional benefit merely becasue the gas you use can, in a different engine or environment, be compressed even more). Others, including Iceman, insist higher is better. I have still never seen any empirical evidence that higher than recommended octane provides a benefit.
[/quote]

You can't change the compression ratio, but you can change the ignition timing and air/fuel ratio with a higher octane fuel.

From a good site on combustion chemistry:

"If the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition." Pre-ignition can damage an engine before you finish reading this sentence.

What we're really concerned with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug). "

Higher octane fuels can allow a more advanced ignition timing, because they are more resistant to 'knock'. This is one of the reasons that aftermarket chip manufacturers can achieve higher HP numbers with very high octane fuels. They're not changing the compression ratio, they're able to change the A/F ratio and the ignition timing.

I agree with you, and doubt BMW has programmed the M5 to take advantage of a higher octane fuel, but it's possible.
[/quote]


I undersatnd the basic of the combustion engine and the role of compression, but I am in no way an expert. So some quesyions:

If you change the ignition timing to delay ignition, will the engine create higher/more compression and if so, do you run the risk of damaging the engine if no other mods are made?

If dealy does not create higher compression, is there a benfit?

I think I undersatnd the benefit of changing the A/F ratio, and that very high octane fuels can combust with less air, thereby creating more power. But agin, if you tamper with the mixture without making other mods, do you run the risk of engine damage?
[/quote]

Here's a website that gives a good overview of combustion chemistry, and the effect of octane, temperature, and ignition timing:

http://www.faqs.org/faqs/autos/gasoline-fa.../section-1.html

Here's the point I was making earlier:

"The tendency to knock increases as spark advance is increased. For an engine
with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93
octane fuel, retarding the spark 4 degrees lowers the octane requirement to
91, whereas advancing it 8 degrees requires 96 octane fuel [27]."

Basically the earlier you can start the spark, the more complete your combustion will be, and the more power you will get, so you want your ignition timing advanced as much as possible, to the point just before you would get 'knock'.

So, considering all that has been mentioned so far, does BMW have to retard the ignition for all the M5's sold in California (and other states) where the highest widely available octane is 91? If so, doesn't this mean less than optimal engine performance (i.e. <500HP)? :think:

The reason I ask is because of this quote:

[/quote]


To the best of my knowledge, BMW does not adjust the timing based o the region. New engines will automatically adjust to prevent knock and damage, and there will be less than optimal performance.

[/quote]


To the best of my knowledge, BMW does not adjust the timing based o the region. New engines will automatically adjust to prevent knock and damage, and there will be less than optimal performance.
[/quote]
This is my thought too. :thumbsdown: