Date: Mon, 16 Apr 2012 21:33:01 -0600
Reply-To: Richard A Jones <Jones@COLORADO.EDU>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Richard A Jones <Jones@COLORADO.EDU>
Subject: Re: Fuel consumption in different gears - how does the energy /
fuel, work?
In-Reply-To: <201204080419.ARQ60472@Colorado.EDU>
Content-Type: text/plain; charset=ISO-8859-1
I followed this thread and decided to run some tests with
my Syncro Westy with a Scan Gauge. My idea was to run
a piece of highway in third gear and in fourth gear and
compare numbers. I also ran a test at various speeds in
4th gear to see how speed changed things.
I picked TPS, LOD, MAP and MPG for the Scan Gauge to
report. The first three, throttle position, engine load,
and manifold pressure are reported from the ECU. The
last, MPG, is computed by some (unknown) algorithm by
the Scan Gauge.
I could do this because I have a Subaru 2.5 (OBD II). I
also have, for reference, stock gearing and 215/75-15 tires
(27.8" diameter) and a heavy Westy.
First the speed test. Resistance goes up by the cube of
the speed, or something like that. Nothing here contradicts that:
-----------------------------------------------------------------
Varying speed test in Nevada (S of Boulder City) on April 8, 2012
S G RPM TPS LOD MAP MPG
75 4 3800 33 53 11.3 13.0
70 4 3600 23 43 10.7 14.7
65 4 3400 21 37 9.8 16.0
60 4 3100 16 31 9.8 16.3
------------------------------------------------------------------
Now the comparison of speed and gear. Two runs, two directions,
50 mph in 3rd and 4th gear and 65 mph in 3rd and 4th gear.
Three or four readings were recorded and these are the averages:
-------------------------------------------------------------------
S G RPM TPS LOD MAP MPG
out 50 3 3900 8.7 21.7 4.6 20.5
out2 50 4 2800 9 18.7 6.5 21.7
-----------------------------------------------------------------
back 50 3 3900 14 31.3 6.5 13.9
back2 50 4 2800 16.3 28.3 9.1 15.6
-----------------------------------------------------------------
out 65 3 5000 19.7 39.7 6.3 13.8
out2 65 4 3150 14.5 32.5 8.2 17.7
-----------------------------------------------------------------
back 65 3 5000 27 57 8.5 10.2
back2 65 4 3150 45.5 52.7 10.9 12.4
-----------------------------------------------------------------
Out was slightly downhill and back was slightly uphill. With
one exception, it takes more throttle to go a speed in 4th
than 3rd, but the load on the engine is less. Mileage is
better in 4th than in 3rd.
Again, these are averages from one test run. The raw data
is at: http://jones.colorado.edu/VW/ScanGaugeTest.txt
I wouldn't start a chip tuning company on these--I'd run at
least one more test--or maybe a dozen. ;-)
Richard