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Re: [Tigers] Gas mileage

To: <CoolVT@aol.com>, <tigers@autox.team.net>
Subject: Re: [Tigers] Gas mileage
From: "Smit, Theo" <Theo.Smit@dynastream.com>
Date: Thu, 20 Dec 2007 07:51:43 -0700
For a highway test it's not about the horsepower rating of your engine,
it's how much power you need to get down the road at 55 mph (or whatever
the speed is), plus driveline friction and other incidentals.
- The Corvette probably has half the drag of a Tiger, especially if the
top is down on the Tiger.
- The gearing makes a difference because the ring friction, etc. is a
function of engine speed.

Other things probably make less difference. A late model 5.0 can be had
(if you build it that way) with low-tension rings, full roller
valvetrain, efficient water pump, alternator, windage tray etc., all of
which will help reduce drag and parasitic power loss, so that would
likely be equivalent to the best you can do in the 'vette. Similarly,
you can dial in a carburetor to be equally lean as a fuel injection
system; it's just a lot harder to do, and won't be as adaptable. When I
get the BossEFI system on the road I'll let you know how that pans
out... It has full-on closed loop capability as well as timing control
for engine speed and vacuum.

I have an Echo as my commuter car, and as a factory-built mileage miser,
they've done some pretty trick stuff.
- 0.29 drag coefficient. Not great, but not bad for a small 4-seater
where 6-footers can fit both front and rear. The trunk is huge.
- About 2100 pounds curb weight. Pretty light for today's standards
where every car has good sound insulation and interior trim.
- Non-return fuel line. The thinking here is to prevent heated fuel
going back to the tank and causing undue evaporation (it's a sealed tank
anyway, but every little bit helps)
- Stainless fabricated header for lighter weight and to reduce heat loss
going to the catalytic converter. This helps get the system into
closed-loop sooner, thereby saving fuel.
- Composite intake manifold for reduced heat conduction to the intake
air, and the injectors
- Offset crankshaft (relative to the cylinder bores) to reduce side
loading during the power stroke
- 10.5:1 max static compression with variable valve timing - the intake
cam can be retarded up to 60 degrees via a hydraulic control system, to
optimize actual compression and broaden the torque curve.
Nothing that, by itself, will make a big dent, but all the little things
do add up.

Best mileage I've achieved is 760 km on about 40.3 liters of gas,
driving my normal commute-to-work route (which involves a 2 mile, 8%
uphill grade on the way home).
That's 43.3 miles per US gallon. Since the engine is rated at 108 hp,
it's not exactly a rocket, but it's got better power-to-weight than an
early Miata.

Theo
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