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

["RoccoPhil" <roccophil@mindspring.com>]

This has got to be one of the best and most productive conversations I've
seen on the List in a long time.

My surmising at this point:

If I stay w/ an AGB I'd add a .80 fifth
If I stayed w/ a 9A I'd add a .75 fifth
If I had wads o' money, I'd construct a 4k w/ an .80 fifth and .71 sixth.
Does that sound like a hot tranny or what?

-PK




> 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
>
>
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