Date: Sun, 22 Aug 2010 14:54:26 -0700
Reply-To: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Subject: Re: Garcia's Ghost doesn't like gettin' high.
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good to read your post.
I have not thought of octane in terms of 'rate of burn' ...just in terms of
resistance to detonation.
if I 'find time' ha ha, I'll check into it.
wikipedia doesn't have all that much to say about it -
http://en.wikipedia.org/wiki/Octane
Scott
----- Original Message -----
From: <mcneely4@cox.net>
To: <vanagon@GERRY.VANAGON.COM>; "Scott Daniel - Turbovans"
<scottdaniel@turbovans.com>
Sent: Sunday, August 22, 2010 2:06 PM
Subject: Re: Re: Garcia's Ghost doesn't like gettin' high.
Well, I did make up that phraseology. It does that, though. You are right.
Higher octane fuel resists premature detonation (pinging), because it is
less volatile. Lower octane fuel is more volatile, thus vaporizes more
readily, and detonates when enough oxygen is available (when the engine
compresses the fuel/air mixture enough). But, since there is less oxygen at
higher altitudes, lower octane fuel is required to get the same burn rate as
at lower altitudes. Actually, "Octane rating" is a measure of the burn rate
of a fuel (at some set of specified conditions that I do not recall)
compared to the burn rate of pure iso-octane under the exact same set of
conditions (this is also a measure of the fuel's resistance to premature
detonation -- higher burn rate, higher premature detonation rate). So, give
pure iso-octane a rating of 100. A fuel with a rating of 87 burns at a rate
comparable to that of 87% iso-octane. Notice that this is faster, not
slower, burn rate than iso-octane. In the test, the other fuel component
would be heptane, which has a 0% octane rating. It would burn at a faster
rate at any given oxygen level than would iso-octane. An actual gasoline
might have a couple of hundred constituents, most of which are products of
petroleum distillation, and are not additives per se. But additives are used
to raise octane rating rather than trying to get to a higher percentage of
iso-octane by the distillation process. At one time, the main additive for
this purpose was tetra-ethyl lead. Today it is various aromatic and
branched compounds.
The actual reason the lower octane fuel doesn't detonate at altitude like it
would at a lower altitude is the lesser oxygen available, not the lower
pressure per se. But I guess that's a quibble. Long ago I knew the details
of the test to determine burn rate compared to the burn rate of iso-octane.
But you are right, it is resistance to premature detonation that is the
reason high-octane fuel is used. That allows higher compression ratios than
would be possible with lower octane fuel.
DMc
---- Scott Daniel - Turbovans <scottdaniel@turbovans.com> wrote:
> Hi Dave and all -
> re
> "make the stuff burn faster than it would at that altitude if it were 87
> octane"
>
> I suspect you just made that up. The reason lower octane is used at higher
> altitude is there is no need for the higher octane level required at
> lower
> altitude.
> "Octane" is a measure of resistance to detonation in the combustion
> chamber. At higher altitude, there is less pressure in the cylinders, thus
> a
> lower octane fuel will suffice.
>
> Scott
>
>
> ----- Original Message -----
> From: "Dave Mcneely" <mcneely4@COX.NET>
> To: <vanagon@GERRY.VANAGON.COM>
> Sent: Sunday, August 22, 2010 7:05 AM
> Subject: Re: Garcia's Ghost doesn't like gettin' high.
>
>
> > Robert, the high altitude mix should actually be lower octane than you
> > get
> > at lower altitude. 87 is usual below 3000 ft. I have noticed on
> > numerous
> > trips out West that once I get above 3000 ft, the octane is usually
> > either
> > 85 or 86. That is to compensate for the lower atmospheric pressure --
> > make the stuff burn faster than it would at that altitude if it were 87
> > octane. But, if your van has any breathing problems at all, as you have
> > noticed, 9000 ft doesn't provide much air, therefore drop in running
> > efficiency. I used to select higher octane fuel at altitude, but that
> > was
> > actually a mistake.
> >
> > Don't know what one does with modern cars to offset the atmospheric
> > pressure drop. I used to, with an old Dodge Dart (with carburetor),
> > take
> > the cover off the air filter (reverse it). The air still had to go
> > through the filter, but there was a greater volume of air. I don't know
> > if that reduced the filtering efficiency, but at the time (I was a kid,
> > 22-25 years old) I reasoned that the air was cleaner in Colorado
> > mountains
> > than in Texas. Maybe. But it did give me enough air to get over the
> > passes. David
> >
> > ---- Greg Potts <greg@POTTSFAMILY.CA> wrote:
> >> Hi Robert,
> >>
> >> Rule of thumb says you are losing 3% of HP for every 1000' altitude.
> >>
> >> The ECU can correct the mixture, but it can't increase the O2 content
> >> of
> >> the intake air.
> >>
> >> Happy Trails,
> >>
> >> Greg Potts
> >> Toronto, Ontario Canada
> >> 197x Westfakia "Bob the Tomato"
> >> 1987 Wolfsburg Weekender Hardtop
> >> www.pottsfamily.ca
> >>
> >> BUSES OF THE CORN - AUGUST 13-15th, 2010
> >> www.busesofthecorn.ca
> >>
> >>
> >> On 8/21/2010 4:31 PM, Robert Fisher wrote:
> >> > My van does not run well at high altitude. When we came here to
> >> > Mammoth
> >> > last
> >> > year the van lost a considerable amount of power after running
> >> > normally
> >> > the
> >> > whole way up here, and despite the fact that I filled up on local
> >> > gas.
> >> > I
> >> > don't remember the octane, but it was one of those pumps where you
> >> > have
> >> > three choices for one hose and I think I'd remember picking something
> >> > other
> >> > than the usual 87... on the other hand you'd think that "regular"
> >> > here
> >> > would
> >> > be a high-altitude formulation. I am going to try to remember to
> >> > check
> >> > it on
> >> > the way back through town.
> >> > This time I filled up on 87 in Bishop with the same results.
> >> > Everywhere
> >> > we
> >> > go here is between 8& 9 thousand feet; in addition to the low power
> >> > it's
> >> > difficult to start when cold, to the point that I actually have to
> >> > goose the
> >> > gas until it smooths out. Is there some point where the ECU can no
> >> > longer
> >> > compensate well for the altitude, or does this point to another
> >> > problem?
> >> >
> >> > On another distantly related note, I've noticed that the wood fires
> >> > here
> >> > seem to smoke much more than at lower elevations... is there anything
> >> > to
> >> > that?
> >> >
> >> > Cya,
> >> > Robert
> >> >
> >>
> >>
> >> --
> >
> > --
> > David McNeely
>
--
David McNeely
| 
