Date:         Tue, 28 Mar 2000 22:59:02 EST
Reply-To:     Wolfvan88@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Robert Lilley <Wolfvan88@AOL.COM>
Subject:      Re: Why did this mod increase my fuel economy?
Content-Type: text/plain; charset="US-ASCII"

It is quite simple actually...

The engine is getting more air (=more power) to get you to the desired speed
quicker so the throttle does not stay open as long and the vacuum can build
back up, reducing the fuel pressure at the injectors, thus causing an
increase in mileage.  When the Vanagon is at speed, the engine has less of a
load then trying to get to the speed.  As the throttle is opened, the vacuum
decreases slightly.  This causes the fuel pressure regulator to increase the
fuel pressure briefly, then the pressure drops to below idle speed pressure.
On my setup the idle pressure is set at 29 PSI.  When I rev the engine with
no load the pressure increases briefly then drops to @20 PSI. I first thought
that this was a problem with the vacuum port that I attached the new
adjustable regulator (I put it on the same port as the old one), But after
reading and asking, I had indeed had the correct port and it was functioning
correctly.

The engine needs LESS fuel pressure running at 3000 RPMs with NO LOAD then
what it takes to run the engine at idle.  While at idle, the engine has to
over come friction and other forces of motion that decrease as the engine
revs, momentum and other forces come into play here.  I have revved my engine
at 3000 RPMs and saw the pressure drop to below idle pressures.

At idle X amount of vacuum is built, when the engine RPMs are increased with
no load the X amount of vacuum decreases slightly, INCREASING the fuel
pressure, then builds up to greater than X amount of vacuum, DECREASING the
fuel pressure.

When an engine is subject to a load this changes the demands on the engine
and the X amount of vacuum.  As the load increases the vacuum decreases
increasing the fuel pressure to match the demands of the engine.

The cam comes into play and plays a critical role in the X amount of vacuum
that an engine develops, the engineers design the FI pressure differences on
the cam profile.  ANY CHANGES TO THE PROFILE CHANGES THE way vacuum is
developed.
THIS CORRESPONDS TO MILEAGE.

IF the cam/lifters are worn from high mileage, total vacuum is reduced and
the fuel pressure increases and mileage will suffer.
If the lifters are not properly adjusted-too loose when the engine is fully
heated, the profile is altered.  The lift and duration are reduced increasing
the idle pressure reducing mileage.
If the regulator is worn out the fuel pressures increase and mileage will
suffer.

If the throttle is opened too far for the amount of air Vs RPM needs, the
vacuum decreases quickly INCREASING fuel pressure, but because of the high
load the vacuum stays reduced keeping the fuel pressure high, but with
reduced power because the air is saturated with fuel and does not burn fully.


All this to say that the modification increased the mileage because he was
able to use less pedal to get to the same speed.

Along the same line...

When I went from 36 mm dual dual-throat Dellortos to 40 mm dual dual-throat
Dellortos, my mileage went from 19 mpg (US numbers...) to 30 mpg, I even had
a 914 mild hydraulic performance cam in my custom 2L T4 engine (Almost exact
same engine in the air-cooled Vanagon... )  The mileage increased because the
engine made more power which made the 73 VW Bus quicker to speed, using less
throttle which got me to a less actual load on the engine quicker.

I have learned all this by trying to figure out my engine and retune the FI
for it to maximize both the power and economy and work through a lot of bad
(from a short in the wire to the distributor) and slightly out of spec NEW
parts.  I am in the process of retuning the AFM to be more responsive in the
upper RPMs Vs the effects on mileage.

I have learned that the DF FI system is really touchy.



Robert