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

[thescirocco@juno.com]

On Tue, 23 Jan 2001 16:04:48 -0500 "RoccoPhil" <roccophil@mindspring.com>
writes:
> 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
They are both 3.67:1...

> 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

I'd prefer a .90 5th and .71 6th...

Peter

> 
> 
> 
> > 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
> >
> 
> 
> 
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> 

Peter
'79 Scirocco 16V project car
'83 GTI "Caged Rabbit" 
'84 Scirocco, RD/FF
'80, '88 Scirocco rollers

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