Improving fuel economy for the 1991 200q 20v

[Dan Cordon]

Eric Wrote:
>  I am curious about
> possible causes for lower than expected mileage for the 1991 200q 20v cars.
> Conversely, I am thinking about what changes could be made to improve fuel
> economy. It seems many things are controlled by the ecu, but there are
> still many things that could affect fuel economy. 

There *are* many factors that go in to fuel economy. You wrote a pretty 
good list. But there are some factors that are a *lot* more important 
than others.

On the engine side...the best you can realistically hope for (ECU/sensor 
constraints) is to run a stoichiometric air/fuel ratio. Your ECU uses 
the O2 sensor as the feedback for this. If that's not functioning 
correctly, fuel economy will suffer. On another note, (not sure on the 
specifics of our ECU), the ECU typically disregards the O2 sensor at 
higher throttle conditions. For some ECU's, anything over 70% will go to 
open-loop. The O2 sensor is the main item, but many other things can 
cause your air/fuel ratio to be incorrect. MAF sensor, air leaks....this 
list alone could get exhaustive.

Other engine items....anything that reduces friction (thinner oil, 
synthetic...etc.) will help. Also, running *higher* compression ratio 
pistons will help. Of course, you couldn't run a turbo...but, but the 
fuel economy would improve. Likewise, anything that will improve 
combustion efficiency would help. If everything is in working order 
(plugs, wires,...) there won't be much to gain. But any parts *not* 
working properly can really hurt you.

Transients (not homeless people) in the engine load/speed require extra 
fueling. In theory, if you could maintain a single load and speed, that 
would help.

On the chassis side, (already mentioned) alignment makes a big 
difference. Technically, no toe would offer the least rolling 
resistance. Camber...hard to say. Running zero camber would make your 
tires flat on the road (good for braking/accelerating), but puts the 
largest contact patch down too. Excessive camber would have a smaller 
contact patch, but would lead to higher tire deflection (and horrible 
tire wear too). I believe Bernie prefers to run at zero camber, but my 
memory isn't great...  Also, tires make a big difference. With our HEV 
and EV vehicle projects, we found large improvements when running 
special 'low rolling resistance' tires. The tires last a long time, and 
reduced road load....but they handle horrible. As previously notes, a 
skinny tire at higher pressures will offer less resistance than a big, 
fat, sticky one. Honestly, after the cool winter, your tires could be 
slightly under pressurized...

You didn't mention anything about aerodynamics. At highway speeds, the 
drag force is quite significant. Hard to say what would improve the 
aerodynamics of our cars....but it could be improved upon. Since drag is 
proportional to the cube of velocity (the power to overcome drag 
anyway), cruising at 60 mph will likely get you better fuel economy than 
cruising at 80 mph.

So, are you looking for hardware changes, or driving style changes? 
Unless you have some defective hardware (O2 sensor, plugs, wires, etc.) 
you probably can't justify any cost savings by replacing parts. $1000 of 
aerodynamic enhancements will probably take 400,000 miles of driving to 
pay for itself in fuel savings. But, driving style changes can make a 
HUGE immediate difference. If you're serious about this, I highly 
recommend getting yourself a wide-band O2 kit with a display. Mount this 
in your field of view while driving. When the meter reads around 
14.5...you're doing okay. When it dips to 12:1....not so good. This 
serves as a reminder for the driver, but can also alert you to when the 
engine isn't being fueled properly.

I'm only touching the surface here, but since this is what I do a lot 
of, I thought I should chime in with some input.

-- 
Dan Cordon
Mechanical Engineer
University of Idaho - Engine Research Facility