[Diy_efi] The Hunt effect

Wed Oct 5 15:14:22 UTC 2005

Yes, I do need to watch my wording. I tend to use the wrong phrase and generalize when I shouldn't. I didn't mean to sound as if the test applied to all vehicles. But, the few tests GM performed did show a decrease in some vehicles.

dh at busb.com wrote:Actually, I'm a former GM programmer and I agree that there is an error in trip computer calculations.  But the calibration has nothing to do with repeatability.  If the trip computer says 16.9 this time and 19.5 next time there was an improvement whatever the actual number.  Accuracy and repeatability are not even closely related.

Also, the 'in fact' small decrease is BS.  True, there is more energy in a heavier fuel, but the heavier fuel also requires more oxygen which is provided due to the feedback from the oxygen sensor.  There are many factors mitigating the response of various engines.  Compression ratio, rear end ratio, initial efficiency, driving habits and so forth.  A blanket 'in fact' is not appropriate.  I would agree to a 'in some cases' because that is in fact quite true.

dh
----- Original Message ----- 
From: Bret Levandowski 
To: diy_efi at diy-efi.org 
Sent: Monday, October 03, 2005 8:49 PM
Subject: RE: [Diy_efi] The Hunt effect

Being a Cadillac tech, I can tell you that you can't trust the acuracy of the trip computer. The program for the tc is inherently flawed due to the way it calculates and the sensors used. All Northstar engines are recommended to use reg unleaded. GM tests have shown no gain (in fact a small decrease) in MPG by going to mid or super. I'm not saying it's worthless, but we've noted up to a 15% error on the MPG.  Ski

"Ulmer, James D - Denver, CO" <james.d.ulmer at usps.gov> wrote: My off-hand test with my '94 Cadillac NorthStar yielded just under 2 mpg
difference. 16.9 verses 19.5. For me, the better gas is clearly well
worth the cost. When prices peaked at $3.00 a gallon around here, I
thought I'd save a little money and switch to the lower octane. Went
back to the better stuff and the trip computer showed the difference
within just a few miles of driving.

Big Jim 

> -----Original Message-----
> From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On
> Behalf Of dh at busb.com
> Sent: Monday, October 03, 2005 5:13 PM
> To: diy_efi at diy-efi.org
> Subject: [Diy_efi] The Hunt effect
> 
> Took a trip to Washington a couple of weekends back. Told my Prius
> driving
> Texas friend (From Austin) that he should start purchasing 89 instead
of
> 87
> octane. That's 'cause it would cost him less. Well, partner, he said
he
> would try it if I paid the bill, which I did. We shared driving
duties
> and
> sure 'nuff the car got 51 mpg instead of the previous 48 mpg. We did
this
> averaging over 80 mph (while moving) and with two drivers.
> 
> How does 3mpg save money? Well that is 3mpg divided by 48mpg which is
a
> whopping 6.25% improvement. Big deal? Well yes! You see that gas
cost 10
> cents more per gallon which is 3.22% more. Since I'm an accountant I
> noticed that the 3% increase in cost (because gas is quite expensive
now)
> was giving a 6% increase in mileage. That, folks, is a 3% reduction
in
> gas
> price per mile.
> 
> Don't mean squat in a non-computer controlled car, but in engines that
> have
> high compression (over 8.5:1) and 'bouncing' engine management (where
the
> ignition is repeatedly advanced until the engine pings and retarded
> slightly) you can expect the same gains.
> 
> Please, try this for yourself and report the results. Especially
those of
> you that have a consistent route to work and don't run into a lot of
> traffic
> in-route (both of you). Report your results here, in public.
> 
> In reality, it should also work for even older engines with lower
> compression. I first noticed the effect with an 83 Olds with computer
> controlled carburetor and that was NOT a sophisticated engine control
> system. With that car the engine got about 6% better fuel mileage
making
> it
> breakeven even at $1.50 per gallon. With the wonderful side effect of
> being
> able to pull the hills of North Alabama without pinging.
> 
> I call this phenomenon the Hunt Effect.
> 
> dh
> 
> 
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