<VV> Fuel Economy
Padgett
pp2 at 6007.us
Thu Dec 29 23:13:01 EST 2005
Fuel economy has been something I have been playing with since my teenage
daze when economy rallys were common and measured on a ton/mpg ratio (and a
cement truck went off scale).
Do know that for any engine there is a sweet spot - highest load in highest
gear that does not trigger extra enrichment. Frankly I have never seen this
sweet spot anywhere over about 2500 rpm and under 2000 is better.
Case in point: when I hear that a 3.55 gear gives the same mpg as a 3.27
then my first thought is that either there is something wrong with the 3.27
or it is being driven into the PE zone more often.
Similarly a PG engine at steady state should not give as good MPG as a four
speed because even with a tight converter like the corvair (mine appears to
have a 1600 rpm stall which is pretty tight), there is still gong to be
3-5% slip at cruise.
IMNSHO even the 3.08 with stock tires is too low a gear, about 2.56 with a
switch-pitch torque converter for better off-the line or 2.73 with a lockup
would sound better.
The 1964 "Stock Engine" test corroborates this in that "road load" mpg is
pretty flat up to 2000 rpm (about 40 mph with a 3.55) and then drops off
significantly to 2/3 that value at 3200 rpm (about 64 mph) - numbers are
not that important, the ratios are.
So if you want max mpg for a given drivetrain, drive 200 miles at 2000 rpm.
Want more speed, gear for 2000-2400 at your cruise speed. MPG will be less
than for the lower speed but higher than the same speed at a higher RPM.
Now drop the revs far enough and the load will get high enough to put you
into PE so you need to juggle things a bit. Lets just say that my 92
TranSport van with 3800 (231 cid) turns 2k at 70 in lockup o/d and gives
24-25 mpg of 87 octane at left lane speeds. With the dual a/c on. Keep in
mind that while it does have feedback FI, it also is 1000 lbs heavier and
has a lot more frontal area (and a roof rack). It is also 1990 technology
and a 50% larger engine.
So 30 mpg at Interstate speed from a Corvair on pump gas should be possible
if we could get the revs down. Unfortunately, with a 2 speed non-lock-up
trans, it ain't gonna happen. Geared for even 2400 at 70 would require a
2.42 rear gear (though it would be able to get to 70 in low, take off would
be a tad slow) .
Are options, since I have some 215x65x15s, going from a 24" to a 26" rear
tire would be equivalent to swapping from a 3.55 to a 3.23. so is the first
thing to try but first I need everything running right and am not there
yet. Hopefully next year I'll get instrumentation fitted and try some
things. And maybe the horse will learn to sing.
Padgett
More information about the VirtualVairs
mailing list