Interesting!!! Thanks
Dan
-----Original Message-----
From: mike f [mailto:[EMAIL PROTECTED]
Sent: Saturday, December 03, 2005 7:04 PM
To: The Chevelle Mailing List
Subject: [Chevelle-list] Drive train possibility for Dan
Dan, with that big motor and your Ring and Pinion
posts in mind, I happenned by this little gem. The guy
says use a TH400 called a "switch Pitch". He got his
from a Texas company. http://hotrodhomepage.com/?p=65
See below. Any one hear of this before?
mike
The Switch Pitch TH-400
Summary:
The reasoning behind installing an overdrive
transmission is running a low rear gearing to get off
the line, but still retain manageable RPMs at freeway
cruising speeds. This seems like the best of both
worlds, but this article will examine some reasons why
an overdrive transmission is not always be the best
solution.
Traditionally GM overdrive transmissions such as the
700-r4 and the 200-4r were viewed as anemic pieces for
gas sipping transportation appliances. The last
several years have revealed that these transmissions
can be modified to handle significantly more power
then in base configurations. A well-built and properly
adjusted overdrive transmission should be able to
handle around 450 ft/lbs of torque. Is that enough for
the engine combinations that people run in their hot
rods? In many cases the answer is no and I for one,
have been through enough "built" 700-r4s for a
lifetime.
So what then is the solution?
The solution is the tried and true TH-400, more
specifically the variable-pitch version of the TH-400.
The variable-pitch, or switch-pitch TH-400 is nothing
new as it has been around since 1965. Originally found
in Buick, Olds, and Cadillac the switch-pitch offers
two stall speeds, a low stall and a high stall.
Applying a positive 12-volt signal to the appropriate
terminal on the transmission case makes the selection
of low or high stall. The high stall gives you the
advantages of torque multiplication to get off the
line with a taller rearend gear and the low stall
decreases slippage for cruising or on the big end of
the track.
The variable-pitch stator is not the only advantage
that the TH-400 holds over its more modern overdrive
counterparts. A TH-400 will generally cost less then
an overdrive and it can be built to handle some
serious horsepower numbers. The TH-700 and TH-200 also
require a T.V. cable to be installed and adjusted
correctly or serious damage and shifting problems will
result, and getting it correct is often harder then it
seems at first glance. Another advantage of the TH-400
is a small spread between gears. First lets put some
gearing numbers out there for reference:
Gear Ratio Comparisons:
1st 2nd 3rd 4th
TH 400 2.48 1.48 1.00
TH 700 3.06 1.63 1.00 0.70
TH 200 2.74 1.57 1.0 0.67
It is clear that the two overdrive transmissions in
the comparison chart have lower first gear ratios as
well as have the advantage of overdrive, but let us
look at the numbers that are hidden between these
ratios.
Between Gear Gap Comparisons:
1st-2nd 2nd-3rd 3rd-4th
TH 400 1.00 0.48
TH 700 1.43 0.63 0.30
TH 200 1.17 0.57 0.33
Notice the large gaps between gear ratios. The TH-700
has a particularly nasty gap between 1st and 2nd. In
fact if you combine the 1st-2nd and 2nd-3rd gaps on
the TH-400 you get roughly the same gap as the 1st-2nd
on the 700-r4. The result of this is that your first
gear in the TH-700 may launch you like a rocket, but
the huge gap is going to lug your engine and drop
RPMs, hurting performance. The TH-200 has a more
favorable gear separation then the TH-700, but the
TH-400 still has a slight edge.
Now to address the flaw in my argument that I am sure
many of you are thinking about. The TH-400 does not
have an overdrive gear and if you don't run an
overdrive then how can you possibly expect to have a
respectable freeway cruising RPM?
The solution to this is easy, run a taller rearend
gear. I argue that the many hotrods rarely, if ever,
see the drag strip and spend a lot more of their time
on the road. So why then do we put in a low rearend
gear that makes freeway driving unpleasant and limits
out terminal velocity to 115 miles per hour. An
overdrive transmission will make up for our low rear
gearing, but this introduces several problems.
Overdrive was never intended to be an acceleration
gear, which it is being forced to become if you are
running a deep rearend.
High drive shaft speeds
Running a 4.11 gear with a TH-700 sounds like a good
idea until you consider that with a 275/40R17 tire at
100 mph your drive shaft is spinning at somewhere in
the neighborhood of 5383 RPM for comparison a TH-400
with the same tire and a 3.08 rearend gearing will
turn 4034 RPM. The same comparison at 150 mph yields
8075 for the overdrive versus 6051 for the TH-400.
Spinning the drive shaft that fast takes horsepower
and can have other problems like magnifying the
effects of driveline alignment problems, drive shaft
imbalance, and driveline component wear.
With the right setup, a switch-pitch TH-400 provides a
viable alternative to the modern overdrive.
Controlling the converter:
There is a multitude of possible methods for
controlling when and how the converter changes from
high to low stall. Use a delay box to have the
converter go to low stall after you leave the starting
line. An RPM switch could have the converter lock up
once your car is into its power band. Wiring a relay
to the brake switch would allow you to put the
converter in high stall for stoplights in a car with a
radical camshaft. A simple switch can be used to allow
full driver control of the system. For most those who
want to have the most control of their driving
experience this probably the best option. [article on
wiring a switch pitch transmission]
Where do I get my hands on a switch pitch TH-400?
I bough mine from PAE Enterprises in Texas. They offer
both Chevy and BOP cases for a resonable price. PAE
also sells conversion kits so those of you that
already have a TH-400 are in luck. PAE's contact
information can be found in the sources section of
this article.
Sources:
How to Work with and Modify the Turbo Hydra-Matic 400
Transmission
by Ron Sessions
ISBN: 0-87938-267-8
P.A.E Enterprises, Inc.
4401 Turf Rd. Bldg E
El Paso, TX. 79938
915.855.6009
915.857.4727 -> Tech Line 8-10am and 4-5pm MST M-F
www.paeenterprises.com
--- Dan Mascheck <[EMAIL PROTECTED]> wrote:
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