Brand of Gasoline

Wed Sep 19 16:34:43 PDT 2012

Hi Mark-

I agree with most of that, but don't see how its at all at odds with what i said.

the rapid ability to re-map is exactly why a motor would not get significant improvement from higher octane fuel - it would only have to change the map under WOT.  The faster it can change its parameters, the less time it must be compromised.  in fct that's why its now fast, of course.

I also agree that ethanol free gas would increase fuel economy. But i also noted that its unlikely today to get ETOH free fuel.  And since ETOH is at or under 15% in normal gas, the benefit can only be as high as 15% if the ETOH had no energy density at all.  But it does ( i don't recall, but between 70% and 85% of regular gas). So the benefit is likely 5-10% (which i note myself when i can compare, which is increasingly rare at least here in the Northeast).

I'm just railing against assumptions that because higher octane allows more advance, that it will ALWAYS be more advanced. It won't (assume you agree). In fact I'd wager it will only affect < 0% of the driving cycle, especially on a long run as this story told of. Most cars us like 10 hp going down the road at 60 mph or whatever.

So - all true, all in agreement, but where's the question? I simply dont believe its possible for octane to tell the difference. I also dont believe its possible for ETOH to make as big a difference as i interpreted (mid drive fill up vs gas left after trip - sounds big to me)

Grant

On Sep 19, 2012, at 7:23 PM, Mark Rosenkrantz wrote:

> Grant, modern engines adjust timing VERY rapidly, to lower emissions as well as improve fuel economy.  Today's engines aren't static like a "single map" or a "jet" in a carb.  It's dynamic, within parameters.  Modern "maps" are ranges, not a static line.  As such, it's certainly plausible to get a SMALL percentage of improved MPG under normal driving conditions.  Your PV=NRT (I love the ideal laws!) and logic is only accurate if we squeeze all of the power out of the incoming fuel.  No variation between slightly rich as a "fudge factor to stave off detonation as an engine wears" or running lean of peak, something which is used to get the combustion chamber hotter, faster.
>  
> Now, the likely scenario is that she got ethanol-free gas.  As stated, ethanol have less BTU's per volume, and will decrease your fuel economy in ANY engine.  Another thought, which may be quite wrong.... but if the engine maps can't handle it because it's so far out of parameters, this would severely limit fuel economy by reverting to a very "safe" map in the ECU.  Honestly, I forget what car, what altitude (air density), etc.
>  
> Do I believe 50% better economy in a "decently running car," no, I don't.  But if you told me by switching brands and going to ethanol free fuel... I'd absolutely believe double-digit improvement as a percentage. 
>  
> As an aside- get this-  In airplanes where we can adjust the mixture manually, there is a growing trend to run the engine lean-of-peak (significantly so).  GAMA has all the write ups you need if it interests you, but the premise is this- high quality injectors and (this is key for an air cooled engine) individual cylinder head temperature (CHT) monitoring.  Over 10% increases in economy are the norm.  Pretty awesome.  Of course, this is in a constant RPM engine... but the ECUs in modern cars are adjusting multiple times per ignition cycle and are quite accurate.
>  
> Mark Rosenkrantz
> 
> On Wed, Sep 19, 2012 at 6:55 PM, Grant Lenahan <glenahan at vfemail.net> wrote:
> No, not exactly. Read all my posts again
> 
> Grant
> On Sep 18, 2012, at 8:29 PM, John Lagnese wrote:
> 
> > Exactly!
> >
> > -----Original Message-----
> > From: quattro-bounces at audifans.com [mailto:quattro-bounces at audifans.com] On
> > Behalf Of Jeremiah Curry
> > Sent: Tuesday, September 18, 2012 7:02 PM
> > To: 'Jeff Waterstreet'; quattro at audifans.com
> > Subject: RE: Brand of Gasoline
> >
> > I am not expert, but isn't it feasible that with low octane gas in the tank
> > you go WOT, maybe to merge on the freeway, leading to detonation.  The
> > computer decides to retard timing.  Then the whole time you are on the
> > freeway the timing is retarded, leading to less efficient combustion.
> > Whereas if higher octane fuel had been used, that would not have happened?
> > My understanding is that the decision to retard timing will stay in affect
> > for some time.
> >
> > I am a convert to premium in vehicles that require it (my Allroad, MR2
> > turbo, and Triumph TR3) after having trouble with my 30v Passat and not
> > using premium.  However I agree it won't help on engines that don't need it
> > (my 80 series Landcruiser)
> >
> > Jeremiah
> >
> > -----Original Message-----
> > From: quattro-bounces at audifans.com [mailto:quattro-bounces at audifans.com] On
> > Behalf Of Jeff Waterstreet
> > Sent: Tuesday, September 18, 2012 4:17 PM
> > To: quattro at audifans.com
> > Subject: RE: Brand of Gasoline
> >
> > This discussion reminds me why the snake oil sellers and "magical magnetic
> > fuel savers" find people to buy their stuff.    Most people don't know how
> > to properly monitor their fuel usage, which is evident by many responses to
> > this topic.  I especially like those that say "my car just feels peppier"
> > when they run premium.    Two of my 8 vehicles require premium (one is a
> > 2001 A6Q 2.7T), and there is no difference in mileage between regular and
> > premium, unless driven very hard. (like towing a boat, with my other
> > "premium" eating vehicle, a Toyota Landcruiser)
> >
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