Re: 4K tranny - .75 or .80 5th?? - Mileage

[16v Jason <jason@scirocco.org>]

At 01:35 PM 1/23/01 , thescirocco@juno.com wrote:

>Only to add...
>The higher gear, .75,  would help to overcome the increasing wind
>resistance...

Right.  Gearing is torque multiplication.  So having a numerically higher 
gear would mean the corresponding torque multiplication.

Eg:  Assume a constant 100lb/ft at the engine (for simplification 
purposes), and a final drive of 1:1.

With a 0.71 gear, you have (torque * final drive * gear ratio) = 100*1*0.71 
= 71lb-ft at the wheels.
With a 0.90 gear, you have 91 lb ft, or roughly 30% more torque to 
accelerate the car.

Of course, there is one other big factor:  the torque curve.  Of course, 
with a higher numerical gear ratio, the engine is also turning faster.  Up 
until a point (4500rpm on a 16V), the engine is making more torque to begin 
with at higher revs, so you'r _double_ better off having a 
numerically-higher gear.

etc...


>Did you get better gas mileage with the .75 5th gear?

Kindasorta.  Overall, yeah, about 0.5mpg better.

Why so little?  Well, in city driving, I'm in 4th more than I would have 
been -- so mileage probably dropped slightly... I didn't see it, but I run 
very few tanks on city driving alone.

On the highway, I tend to get slightly better mileage.  Why only 
slightly?  Well, here's my Hypothesis:

Like a torque curve that varies with engine speed, engines also have a 
specific efficiency curve.  Basically, all ya need to know is that at some 
RPMs, the engine is more efficient than at others.  It was my experience 
prior to the 5th gear conversion that my 16V is most efficient at around 
5,000rpm.  Why?  Well, on the highway, you should (if the specific 
efficiency curve was flat), get gas mileage indirectly proportional to 
speed.  Not directly, but exponentially -- but that doesn't 
matter.  Meaning, that the faster you go, the worse gas mileage you get.

As many of you have no doubt noticed, it doesn't always work that way.  My 
Scirocco, for example, got the best gas mileage at 90mph.  In fact, it was 
a _huge_ amount better than 65mph.  "Normal" highway driving at 65-70mph 
with the stock gear would give me about 31mpg.  Insane runs where I blasted 
through a whole tank and averaged 90+ miles per hour, I would get 36-38 mpg 
-- well above the "normal" figure.  Therefore, my guess was that the engine 
reaches peak efficiency at like 90mph which was around 5000rpm.  (There are 
other forces at play, like throttling losses at smaller throttle openings, 
but for this illustration, we'll keep them out of it).

Now, since my engine hasn't changed, my peak efficiency is still at 
5-Grand, but I'm not at that high revs anymore.  So, at lower speeds, I 
have a few things working for me:

1.  The lower RPMs help reduce engine friction and therefore BOOST fuel 
economy (+).
2.  Lower RPMs and therefore lower torque output and lower torque 
multiplication mean that I have to apply more throttle to drive compared to 
the old gearing.  This reduces throttling losses, thus BOOSTing fuel 
economy (+).
3.  The lower RPMs move the engine into a band where it is not operating at 
its most efficiency, therefore REDUCING fuel economy. (-).
4.  The need to downshift occasionally to pass up hills on the highway 
increases the revs beyond that of the old 5th gear, and the temptation to 
then "floor it" and fly by someone is enough to REDUCE fuel economy (-).

As expected, I saw a small gain of about 1mpg on the highway.

However:  At higher speeds (90mph or so), I don't get as good mileage as I 
did before -- because at 90 before, I was at 5000+ and at peak 
efficiency.  Now, I'm at 3800, i.e. not peak efficiency, and get 
34-36mpg.  (Still fucking great, if you ask me).

On the other side of insanity, if I were to average 110mph for a tank (I 
wish), I would probably see mileage in the 30-range, whereas with the stock 
gearing (6000+rpm) I would expect to see low 20s.

YMMV. :)
Jason





----------
1987 Scirocco 16v
1988 Mercedes 190E Sport Euro


http://www.scirocco16v.org


--
Email LIST problems to: scirocco-l-probs@scirocco.org. To unsubscibe send
"unsubscribe scirocco-l" in the message to majordomo@scirocco.org