First Generation Firebird-L Mailing List
.........................................................................
Zak,
Few changes return as much bang for your buck as a gear ratio change.
The lower the gear ratio (higher numerically) you use, the greater the
usefulness
of an overdrive. So if you keep the 2.73 ratio, an overdrive has no
practical value
as your RPMs in high gear are already pretty low. My calculator says
2268 RPM
at 65 MPH with a 26.29 inch tire (205/70R15).
So, if you keep the present gear ratio, upgrade to a TH350 or TH400 as
each has
a better spread of gears, and lower first gears than your 2 speed auto.
The down
side is that the 2 pinion rear is weaker than the 4 pinion used on the
lower ratio
rears such as a 3.42, etc. And that 2.73 ratio will not accelerate as
well as a
steeper gear even with the upgraded transmission. The cost of a gear
upgrade
will have to be your gating factor.
If you choose to upgrade the rear gears, and say you go to the 3.23
ratio you mentioned.
Assuming a 26.29 inch tall rear tire, your RPMs at 65 MPH would be
2683. At this
engine speed, you may or may not want an overdrive depending on how
comfortable
you are with the RPM level. But combine this ratio with either a 3
speed or 4 speed
automatic transmission, you will get much improved acceleration.
TH350s have a lower first gear than TH400s. And when properly built,
they can handle
any engine. They weigh less, and are a direct bolt on in place of your
ST300. You can
even use your original crossmember and driveshaft. A TH400 weighs more,
but are
strong and will last nearly forever if maintained. It will require a
unique crossmember
and driveshaft.
While the TH700R4 is a popular option, it does have some downsides.
None were
built with the BOP bolt pattern and would require an adapter. It has a
very low
1st gear , but the RPM drop between 1st (3.06) and 2nd (1.62) is
dramatic. The
transmission is longer than either the 3 speeds or the 2 speed, and will
require a
shortened driveshaft.
The TH2004R is not as well known, but a number of the list members are
using them
and report being quit happy. Being the same length as either the ST300
or TH350,
you don't need to shorten your driveshaft, and I believe they use the
same output
spline count as your current transmission. They are available in a BOP
pattern
and thus don't need an adapter. It would be advisable to have it
properly rebuilt with
high performance in mind though since in totally stock form they aren't
the strongest
transmissions offered by the General.
TH2004Rs are showing up in more magazine articles as the transmissions
of choice
in Pontiacs and other makes. So don't dismiss this option.
Both the overdrive transmissions use a TV (throttle valve) cable rather
than vaccum
modulator (TH350) or kickdown switch (TH400) to determine shift points.
They can
be tricky to set up, and some folks with Quadrajets seem to have the
most difficulty
getting them sorted out. It's very important do get this adjusted
properly else the trans
won't last very long at all. Some shops attach line pressure gauges to
the trans to
make the adjustment. But this isn't really needed if your patient.
My suggestion in any case is to upgrade the transmission for sure.
You'll get a lot
more driving fun out of the car. And that's the whole point to owning one.
-Bob Cisneros
http://firstgenfirebird.org/show/closeup.mv?CarID=231
San Jose, CA
BTW: Here's a nifty gear ratio calculator with which to entertain
yourself and
the kids! Invite friends over and make an evening of it! As
a bonus,
the second link shows automatic transmission gear ratios.
Too much fun!
http://www.idavette.net/tech/ratioc.htm
http://www.oldengine.org/unfaq/leadfoot/trans.htm
Zak wrote:
First Generation Firebird-L Mailing List
.........................................................................
Hey guys! Which way would be best to wake up the performance of a
400 bird currently
having a 2 speed auto and 2.73 rear end 4bbl and Crane Ram Air IV
cam? Change to a 3 speed auto or one with overdrive and keep the
rear or
change the rear end to maybe 3.23 with the 2 speed or maybe something
different. Looking for best bang for the
buck here. Thanks
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