From: mark@wdcwdc.sps.mot.com (Mark Shaw)
Subject: Re: Old cars, emissions, engine size and carb tuning...
Organization: Motorola Western MCU Design Center, Chandler Arizona
Distribution: na
Date: Fri, 6 Nov 1992 16:49:04 GMT

In article <1992Nov6.071001.22666@julian.uwo.ca>, wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:
|> Given that to cruise at x MPH you need y Horsepower, is it true that
|> an engine of smaller displacement can produce y horespower more
|> economically than an engine of larger displacement? (I'd like to
|> keep the comparison limited to 196x circa engines and technology, no
|> fuel injection or multi-valved cylinders).

Generally speaking the one with the lowest fuel consumption will be the engine
that is running near it's torque peak with the widest throttle angle.  This
is the best conditions for minimum specific fuel consumption (which usually
occurs at engine speeds near the torque peak).  The specific fuel consumption
is in lbs/hp-hr.  In practice this is hard to match over the entire speed range.  
The reason that a larger throttle angle is needed is because of poor volumetric
efficiency at small openings.

With the proper gearing a smaller engine should give better fuel economy at a
given speed, as long as it can supply the power.

Past experience in fuel economy competitions shows that you get your best
fuel economy occurs near 40-45 mph regardless of your engine/drivetrain
setup since this is usually where the engineers place the torque peak.

|> A lot is said about "tuning" an older vehicle to reduce its emissions.
|> But unless I'm mistaken, the only tuning that can be done on a carb is
|> the idle mixture.  If so, how can a carb be tuned for off idle mixtures?
|> I would think (or hope :) that most of the time, the car is not idling,
|> but is actually in motion.

You can also tune the driving portion, it's just that this requires changing
jet sizes and/or needles.  It requires more disassembly and messing around
to get it right.  Also, if you don't know what each jet is for you can
actually screw it up (some jets adjust fuel flow and some adjust air bleeds).

|> If you "enhance" your engine by going from a 2-barrel carb to a 4,6,or
|> 8 barrel carb, you generally seem to decrease fuel economy.  Is this
|> because a 2 barrel carb makes the engine run leaner than a 4 or 8
|> barrel carb?

This is a good example of poor volumetric efficiency and mixture
formation at part throttle. The smaller carb will be running near more
open.  Also, I suspect that most larger carbs are jetted for higher
performance and may tend to run rich like you say.

|> Does the ideal carb exist?  (ie a carb that will run an engine at
|> what ever stochiometric ratio the designer wants, over a given range
|> of rpm and load)?

Holley makes some throttle body carbureator replacements which are
electronically controlled.  Supposedly they will take your old carb
are install their injector in it.  I'm not sure if they also use an O2
sensor for closed-loop stoichiometric trimming and emissions reduction.
I talked to a Holley rep at the IEEE/SAE Convergence last month and 
the number he gave me for their HRPD group in Michigan is 497-4000.
He said to talk to Steve Allor.

Mark
Newsgroups: alt.autos.antique