Date:         Sat, 30 Jan 1999 23:37:23 -0500
Reply-To:     Martin Jagersand <jag@CS.YALE.EDU>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Martin Jagersand <jag@CS.YALE.EDU>
Subject:      Gearing Was: Re: Fwd: More Q's re Diesel to Gas Conversion???
Comments: To: lvlearn@MCI2000.COM
In-Reply-To:  <199901300102.UAA20366@cayuga.cs.rochester.edu> (message from
              Automatic digest processor on Fri, 29 Jan 1999 17:01:59 -0800)

> From:    Blue Eyes <lvlearn@MCI2000.COM>
> Subject: Re: Fwd: More Q's re Diesel to Gas Conversion???
>
> Integration perspective:
>
> It was suggested that retaining the original atmo Vanagon 4-speed manual
> transmission in a more powerful repowering installation could  be fairly
> described as follows:
>
> "You *can* use the existing transmission just like it is. You will have
> incredible acceleration around town, but the egine RPM's are a bit high on the
> highway."

> Certainly acceleration for the first 100 to 150 feet would be faster by
> retaining the diesel specified overall gearing ratio.  But after that, the
> average horsepower between shift points will govern average acceleration.
> Since the gear staging would be either identical in this comparison, or so
> similar as to be of little importance, and the engine's torque curve would be
> unaffected by which overall ring and pinion ratio is selected, I think the
> statement above overstates the case for the lower overall gearing ratio's
> ability to provide "incredible acceleration around town."  The speeds at which
> peak horsepower is applied in each transmission is dependent on these ratios,
> so we will find speeds for each transmission where it shows an instantaneous
> advantage over the other.  But, for average horsepower between shift points,
> which I contend is the most meaningful around town acceleration predictor,
> neither final ratio has a global advantage over the other after the initial
> start.
> Agree/disagree?
> John

Both yes and no. With only 4 discrete gearings it is of some importance
where exactly the shift points fall. For example if we consider the
"around town" case. Assume we're looking at stop and go up to say 35mph.
With the Diesel 5.43 tranny you use the full rpm range over both 1st
and 2nd gear, while with the Gas 4.57 you would use the full 1st, but only
about 1/2 of the 2nd gear range. Were you to increase final drive
ratio drastically above the diesel, you'd lose the usefullness of
1st altogether, and the analysis will repeat for 2nd 3rd combination.

So while in general you're right, the discretization effect of only
4 gears gives exact shift point speed high importance.

For the 20% difference the above are relatively minor effects
in my opinion. (Not in a hurry around town) More irritating is that
with the 4.57 I run now I lost the "creeper" 1st of the Diesel tranny.
That low 1st comes in handy on logging roads and when starting uphill.
Now I have to do clutch slipping. That wasn't necessaryu with the
1.6 and the Diesel tranny.

The speed-rpm graphs for the two trannys can be found on my WWW pages.

/Martin
--
Westy 1.9l Turbo Diesel
Quantum 1.6l Turbo Diesel

Martin Jagersand                 email: jag@cs.yale.edu
Computer Science Department             jag@cs.rochester.edu
Yale University

        Slow down and visit the VW diesel Westy page:
          WWW: http://www.cs.rochester.edu/u/jag/vw
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