Volumetric Efficiency does not encompass Ignition Timing, Spark Control
and
stuff like that. Volumetric Effciency is a measure of how well your engine
is breathing. As in if you have a 500 c.c. Cylinder, how much of that
cylinder is being filled with a charge of Air and Gas. Gas Flowing a
Cylinder Head will increase Volumetric Efficiency, Valve lift and overlap,
and scavenging Exhaust Headers and free breathing Air Filters along with
Carbs contribute to it as well. Variable Choke (Variable Depression)Carbs
like SU's are less of a factor as far as matching size being crucial as
long
as they're large enough.
Compression Ratio, Ignition Timing,Spark Control and Running Temps
all fall under the category of Thermal Efficiency.
You can have an Engine in a High State of Tune (High Volumetricly
Efficient) with long Valve Overlap in Cam Timing, but will give worse
mileage in the real world when some of your Intake charge is flowing out
the
Exhaust Valves(helps cool Exhaust Valve though) Ideally of course You'd
want
the beat of both worlds, ( hence 4 -5 Valve Heads, Direct injection) of the
modern Engines.
While we're having this discussion, I'm curious what others are getting
for mpg in their 6's. I run 2 SU HS6's with BAE Needles and K&N Air Filters
and Ansa Exhaust on my 75 TR6 with Stock C/R of 7.75:1 and Stock Cam. I
have
a Pertronix Module and Lucas Sports Coil and getting nice Spark Plug
readings. Also run 10 degrees BTDC with Vacuum Advance plugged, along with
non EGR manifold and Air Pump disabled.
I get about a 23-24 mpg average driving on country roads daily during my 40
mile a day commute using 87 Octane. I think that could be inproved by
bumping up the C/R a little, but then I might need to use 89 octane for
another $.15 a gal. I'm not ready to adapt a knock sensor and change out
my
distributor yet. Also, I don't have an O/D tranny, but I'm not on the Hwy,.
much, But, I do run 205/75-15 Yokohama Avid's which are a little taller
than
what my odometer's calibrated for too.
Cheers,
Wayne Lee
Douglas,MA
58 TR3
64 TR4
75 TR6 Daily Driver
Lola T598 Track Day Car
>
>
> Bob Lang Wrote:
> "But to your point, changing the carb size does not help fuel economy.
> What you need to do to get better fuel economy is to increase the
> volumetric efficiency, That means optimize the ignition timing and sprak
> control and stuff like that".
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