Most of the 5 speeds which replaced 4 speeds for consumer cars were to allow
for an overdrive gear. That's what the 5 speeds in the Roadsters and Zs did,
although the Roadster 5 speeds are geared differently than the 4 speeds
because of the increase in the size of the engine and consequently the 5
speeds had a higher geared 1st gear and the rest of the gears were also
changed to keep the gearing tighter.
The 6 speeds in the Corvette were to allow an even higher overdrive w/o
loosing acceleration and allow them to get better MPG.
I read an interesting article somewhere that said that trannies are mostly
geared incorrectly because the primary objective of the car makers is to
allow 60 to be reached with only 1 or 2 shifts rather than 3 which would
slow the 0-60 times. Some cars can reach 60 w/o shifting but slower off the
line. Interesting idea and probably right too.
The Miata 6 speed has the same overall ratios as the 5 speeds but Mazda
wanted a close ratio tranny. I did hear that except for bragging rights it
offers not much more performance and several racers I spoke to hate them.
Mike
----- Original Message -----
From: "Gary McCormick" <svgkm@halley.ca.essd.northgrum.com>
To: "Leigh Brooks" <shifty@shiftco.com>
Cc: <datsun-roadsters@autox.team.net>
Sent: Wednesday, January 23, 2002 1:11 PM
Subject: Re: OT - 6-speed tranny
> On the 6-speed deal -
>
> The original applications of 5- and 6-speed transmissions (up from 4) have
been for
> performance purposes, because a larger selection of closely-spaced gear
ratios allows a
> "peaky" engine to stay in its powerband. Closely matching gear ratios to
the powerband of
> the engine is important - it doesn't help to have more if they're poorly
placed relative
> to the engine's powerband (the folks at Mazda probably didn't have
auto-xing in mind when
> they designed the 6-speed box for the Miata). A close-ratio (and more
ratios will
> inevitably mean closer ratios) box can also help fuel economy and
emissions performance,
> again by allowing the engine to work in the most efficient portion of its
powerband.
>
> Gary McCormick
> San Jose, CA
>
> Leigh Brooks wrote:
>
> > Just got an email from Nissan about pre-ordering the 350Z. I'm not
> > going for one, but I did check out the specs. Seems the
> > "performance" model is the only one with LSD. Of course they have
> > the "boulevardier" version as well (geez isn't that what killed the Z
> > in the first place?)
> >
> > My question though, is about 6-speed trannys. They seem to be all
> > the rage these days. Miatas have gone to them, but it puts the 2-3
> > shift in a really bad place for auto-xing. Is this true for all
6-speeds.
> > And is this a case of "one more must be better" marketing vs.
> > engineering?
> >
> > Leigh Brooks
> > BADROC
> >
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