From: Larry.Mc.Donald@ofa123.fidonet.org
Newsgroups: rec.autos.tech
Subject: Re: old cars, emissions,
X-Sender: newtout 0.02 Nov  5 1992
Date: 09 Nov 92  11:09:00

 wlsmith@valve.heart.rri.uwo.ca 
   (Wayne Smith) sez:

> 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).

   Yes, but all things being equal, probably not by very much. 
Smaller engines will tend to have less internal losses by having 
less friction surfaces, less reciprocating masses and the like. 
However, I've found that 'little' engines usually come from 
countries where gas is and has been expensive for years, so they 
have been producing more efficient engines because of this. I 
had, once, a '69 Chevy with a 350 horse 350 ci engine, considered 
a high output engine by Chevy. At the same time I also had a '68 
Volvo with a 108 horse 109 ci engine, their standard issue. These 
days I drive an '86 Volvo with a 2.3 liter four-banger that 
produces something over 100 horse and averages 30 mpg. A friend 
has an '89 Ford Mustang, basically the same trim engine (efi and 
such) that is 2.3 liter, 86 horse (where are the missing horses?) 
and it averages about 22 mpg (where is the energy going?) for a 
car that is the same drive ratios and the same weight. Now I'm 
comparing a relatively low production engine to an extremely high 
production one, but I understand that the efficiency of the Ford 
motor can be brought up to the Volvo level just by reworking the 
tolerances of the Ford unit. Obviously efficiency is still more 
important to folks who pay $3.50 for a gallon of petrol.
   By the way, from research I've done for electric powered cars, 
I've found that to power a Volvo-sized car down a level freeway 
at 60 mph with no cross -or- head winds takes about 15 hp. 
Ideally, you should carry about an engine that is optimised for 
about 20 horse at rpms required to propel your vehicle at 60 mph 
and use it instead of the power-over-a-wide-range monster. Some 
kind of two-engine system with a transfer gearbox?

> 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.

   Tuning any car is possible but rarely done. You can tune older 
carburetted cars to run cleanly, but first you must temperature - 
stabilise the intake air, since a carb only accurately (sort of) 
meters fuel into the air stream at a certain density. Most cars 
from the '70s had some sort of temp control device for the intake 
air, usually a mixing device that mixes ambient air with air from 
a heater box over the exhaust manifold at some point that is 
higher that one would usually find outside (about 150 f). This 
being done, you can adjust the idle circuit mixture accurately. 
To adjust the power on at speed mixture, you'll need a rolling 
road dyno to load the car down at speed and take exhaust gas 
measurements at several 'cruise' speeds. Then you'll need to 
fiddle with the main jets, emulsification tubes (if any), 
metering shafts (if any), air bleed jets and any other things 
your particular carb has done to try and cure the vagarities 
inherent in carbs. Luckily, the tolerances for adjustment are 
rather relax at cruise rpms, so just keeping a stock carb in good 
shape is usually good enough (Chevy trucks excepted, these seem 
to be jetted too rich during the '70s for some reason).

> 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?

   No, it has to do with carb efficiency. Venturi-type carbs tend 
to correctly meter fuel at *one* air velocity and density, which 
is why you find so many parts in a venturi type carb. They try to 
make it meter accurately over a wide range of velocities, and for 
the most part do an ok job. However, most carbs seem to meter 
better in their mid range. A 350 ci engine revving at 3K rpm at 
100% volumetric efficiency (yeah, right) pulls 303.8 cfm. Most of 
these 350 ci range engines come with 300 cfm rated carbs (2V) 
that seem to provide the best mileage for these engines, so 
without really knowing for sure, I'll guess that the cfm rating 
of a carb is it's most efficient point. Every car that I've ever 
owned with a 350 ci engine (350 for chev and 351 for ford) 
usually cruised around 65 mph at 2500 to 3000 rpm. So if you bolt 
a 500 cfm four barrel carb to your engine, it won't be metering 
as well at cruise as will a 2V carb. A notable exception to this 
is the Quadrajet from Rottenchester, found on many big GM V8s. It 
has a pair of tiny primary venturis and a pair of huge secondary. 
I found on my '68 307 Chevy my mileage actually went up a bit when 
I converted it from a 2V carb to the Quadrajet, as long as I kept 
out of the secondaries. Makes you wonder why people bolt 850 cfm 
Carter carbs to their tired, old 350s.

> 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)?

   Yeah, it's called Fuel Injection. Especially ones with lots of 
sensors and an OX sensor in the exhaust. I converted my '67 Volvo 
from the SU carbs (probably the best kind from a metering stand 
point, constant depression and a metering needle you can taper 
anyway you want) to EFI to get away from the inaccuracies of the 
carbs. I increased both mileage and horsepower and now I don't 
have to retune every time the season changes. By the way, SU carbs 
are a lot easier to tune over their entire operating range than 
are american types and when you retune for idle at different 
temperatures, it adjusts the entire operating range. Too bad here 
in California I have to bolt the smelly old carbs back on every 
to years and make it pollute more to pass the *smog* test.





... Larry.Mc.Donald, N6ZMB, Fullerton, California!  <cough, cough>
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