Brand of Gasoline

[Grant Lenahan]

Yes, completely right. That's why i noted that there is only a finite amount of advance possible.  So the idea that we'll just keep advancing and get more economy is fallacious.  At high vacuum you are already at optimum, likely.

Advance is (mostly) dealing with the elasticity of air, and the finite speed of the flame front expansion, and that especially at high rpms, you actually need to begin well before TDC to apply maximum expansion through the majority of the power stroke. Start at TDC and the piston is well down its stroke before the expansion applies much force.

however, do it too soon and you begin to oppose the travel of the piston - reducing economy and power.

This in fact is what knock (aka pre-detonation) is,and why it is potentially harmful.

Grant


On Sep 21, 2012, at 1:34 PM, Cody Forbes wrote:

> I think there may be some misinterpretation possibly. Timing advance has an optimum number which you get ZERO benefits for surpassing. This is easily to see on a braked Dyno (not an inertial) with a tuneable ECU. I can, and have, watched torque numbers increase to a point as I advanced ignition timing and then at a certain point the torque will start to go back down. Typically on race gas or E85 I see peak torque in the mid RPM range 2-4 degrees before I see knock. At higher RPM the spread is greater. On 93 octane it depends on boost level and the engine. The 1.8t powered S3 race car I tune will easy run 15-18 PSI on 93 with no knock at peak torque. Along with watching peak torque rise and fall you can also see EGT fall until about the same as when you make peak torque then more advance causes EGT to start increasing again.
> 
> I'm sure that BMW ECU tries to keep it advanced, but only to the mapped optimum. Going beyond that is DECREASING efficiency. If its running on a lower grade fuel than required sure it'll keep increasing as much as it can, but on the proper grade of fuel it shouldnt be the case.
> 
> -Cody Forbes
> 
> On Sep 21, 2012, at 12:41 PM, Louis-Alain Richard <5145727795 at bell.net> wrote:
> 
>> Spent the night with my good friend Lucian, a mechanical engineer that is a
>> tech/sales trainer at BWM Canada. He also translate and writes the teaching
>> material from Munich. He knows a bit about cars, let me tell you.
>> 
>> I've asked him about knock sensing in BMW engines and he said that knock is
>> monitored all the time. Even on the highway at part throttle. More, he said
>> the ECU will try to advance timing all the time, until knock events are
>> detected, and then back up a bit. It will do that again and again and again.
>> That way, the ECU knows that it extracted the last bit of energy from each
>> liter of fuel for any conditions (temp, speed, gradient, atmospheric
>> pressure, etc). 
>> 
>> This fact do confirms the theory that Premium fuel is not only for WOT
>> events, but is beneficial all time on an engine that is mapped for it. 
>> 
>> And regular gas is detrimental for such engines... 
>> 
>> Picture the G37 IPL convertible that I drive this week. I'm SURE the
>> preceding journalist didn't fill her up with premium. At low speeds, 25 mph,
>> I can clearly hear knock with a steady throttle when the 7-speed auto trans
>> gently upshifts. And every time I accelerate the tiniest bit... Sounds like
>> an old Tercel from the 90's.
>> 
>> Fuel economy is appalling : 17l/100km in town, that is 14 mpg.
>> 
>> Will empty the tank this weekend and report back with Premium.
>> 
>> Louis-Alain
>> 
>> 
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