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TRANSMISSIONTHERAPY
by Larry Frash
Success is Measured in
Inches…Not Miles!
Hollow type, 3lbs - 3oz
Solid type, 4lbs - 9oz
Figure 1: Output Shafts
C
ircle track racing is a very
competitive sport, with races
often won or lost by less than a
tenth of a second. With this in mind,
even the smallest improvements to the
racecar can take you to the winner circle.
Because many classes have strict
rules about engine modifications, we
sometimes need to look elsewhere to
find these small performance gains. In
this edition of Transmission Therapy,
we’ll discuss modifications to the
TH350 that can shave a few hundredths
or even tenths of a second from your lap
times.
Taking load off the engine is like
adding horsepower. Building a transmission with lower drag, less static
weight and less rotating weight can do
just that. However, with weight reduction comes possible durability problems. The key is to find a happy medium. This depends on the engine horse20
p ow e r,
vehicle weight, and type of track (pavement or dirt).
In circle track racing, there are two
ways the TH350 is used. For short
tracks, only first gear is used; for longer
tracks, only second gear is used. In this
article, we’ll cover both. If you’re building a second-gear transmission, all of
the modifications in this article will
allow a manual low, first gear start. This
function is something you may need, to
get up to speed quickly during the first
lap or after an accidental spinout.
Static Weight
Transmission static weight is the
weight of all non-rotating parts of the
transmission. Static weight reduction is
often not practical. One reason is that
all classes have minimum weight rules.
Another reason is that static weight
reduction doesn’t affect the performance nearly as much as rotating weight
reduction.
Not to mention, if you need to lose
a static weight of 5 to 10 lbs, it might be
easier just to ask the driver to go on a
diet!
Rotating Weight
Rotating weight is all transmission
parts that rotate. Unlike static weight,
reducing rotating weight can greatly
improve performance by reducing the
load on the engine. Depending on
whether the racecar runs in second gear
only or first gear only, there are areas
where you can lower the rotating weight
easily, using parts that were produced
by GM when gas mileage first became
an issue.
The solid output shaft for the sixinch tail housing weighs 4 lbs., 9 oz.,
while the hollow output shaft weighs
only 3 lbs., 3 oz. (figure 1). Although
the hollow shaft isn’t common, you can
GEARS May-June 2003
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Success is Measured in Inches…Not Miles!
Solid type
5lbs - 8oz
With holes
3lbs - 5oz
Figure 2: Drive Shells
still find one if you search.
The solid drive shell weighs in
at 5 lbs., 8 oz., while the drive shell
with holes in it weighs a mere 3 lbs.,
5 oz. (figure 2). Keep in mind that in
a second-gear transmission, the
drive shell doesn’t rotate, so replacing the drive shell would only reduce
static weight. In a first-gear transmission, the lighter drive shell would
reduce rotating weight, and offer
greater performance gains.
Rotating weight can be reduced
further by drilling holes and machining excess metal from rotating parts,
such as grinding
the park lugs off
the rear ring gear
(if park isn’t necessary).
Washer Drag
Drag is unnecessary friction that
creates heat and
robs horsepower
from the engine.
One way to reduce
drag is to replace
Figure 3: Need bearing style rear ring gear.
Figure 4: Needle bearing style front planet.
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GEARS May-June 2003
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Figure 6: Rear planetary with two pinion gears removed.
For second gear transmissions only.
Figure 5: Bearing style pump
thrust washers with needle bearings.
Always use the needle bearing-style
rear ring gear (figure 3) and front planet (figure 4). These two areas will
reduce drag in both first-gear and second-gear transmissions.
Using a needle bearing-type of
pump (figure 5) will reduce drag on a
first-gear transmission, but isn’t as
important on a second-gear transmission, because the direct drum doesn’t
rotate.
in second gear. So in a second-gear
transmission, you can completely
remove two of the pinion gears (figure
6).
Gear Drag
Clutch Drag
The rear planetary transmits power
in first gear and reverse, and freewheels
Clutch drag can rob you blind. One
way to reduce clutch drag is to remove
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Success is Measured in Inches…Not Miles!
0.116" Thick by 0.350"
Length piece of o-ring
Figure 7: Direct drum with low drag modification.
To reduce the direct
clutch friction even
more, install pieces of
o-rings that are 0.010"
to 0.018" thicker than
the frictions, between
the steel plates.
as many clutches as possible.
Whether you have a first-gear or a
second-gear transmission, the direct
clutch is only used for reverse. Use a
three-clutch direct piston from a 6
cylinder, and only install two frictions.
To take up the clearance, install an extra
steel plate just below the backing plate.
Warn the driver not to abuse reverse:
It’s not as strong with only two clutches.
To reduce the direct clutch friction
even more, install pieces of O-rings
between the steel plates that are 0.010"
to 0.018" thicker than the frictions. In
our transmission, we used 0.100" frictions with 0.116" thick strips from a
KM175 pump O-ring, cut into 0.350"
lengths. We placed them on every other
tab of the steel plates, on the outside of
both frictions (figure 7). As the vehicle
accelerates and decelerates, the steel
plates no longer drag against the frictions. But when you select reverse, the
direct piston supplies enough pressure
to compress the rubber spacers and
fully apply the clutch.
In a first-gear transmission, the
low-reverse clutch has to hold for
reverse and engine braking in first gear.
Use an 8-cylinder piston and install all
five frictions.
Direct drum
Removable
Inner race
Outer
sprag
race
Sprag cage and
roller assembly
Removable intermediate sprag assembly.
Figure 8
24
Figure 9: Subassemble sprag before welding.
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Success is Measured in Inches…Not Miles!
Weld the sprag races in four places
Complete servo must be installed on all transmissions.
Cut the center
out of the sprag
retainer and
install it here.
Snap ring
Figure 10
Figure 11: Intermediate servo
Complete 2-3 accumulator must be installed
in all transmissions.
Complete 1-2
accumulator
must be
installed in all
transmissions.
Figure 12: 1-2 Accumulator
A second-gear transmission, however, only uses the low-reverse clutch
for reverse. In this case, use a 6-cylinder piston and only install two frictions.
To make up the clearance, add as many
steel plates as necessary. Always add
the extra steel plates to the piston side
of the low-reverse clutch pack. Under
hard acceleration, the extra steels will
press against the piston and not against
the frictions.
In first-gear applications, remove
the second-gear piston, clutches and
steels. You can also remove the band
and the intermediate sprag assembly.
This will reduce static weight, rotating
weight, and drag.
Burnt Intermediate Bands
On second-gear transmissions, the
band is used for engine braking. In
most applications, this function isn’t
critical. But in circle track racing,
26
Figure 13: 2-3 Accumulator
engine braking is very important.
Some racers lower mainline pressure to reduce the horsepower necessary to turn the pump. It’s not a bad idea
as long as the clutches can survive the
lower pressure.
If the forward and intermediate
clutches last but the band burns, there’s
an extreme measure you can take:
• Find a direct drum with a removable inner race (figure 8).
• Use the sprag cage and rollers to
center the outer race (figure 9).
• Weld the outer race to the inner
race in four places.
• Remove the complete assembly
from the drum and remove the
sprag cage and rollers.
• Weld the back of the sprag races.
We suggest grinding deep grooves
so that the weld can get as deep as
possible.
• Cut the center out of the sprag
retainer and use it as a shim under
the snap ring (figure 10).
With the sprag welded, there’s no
need for the band; the intermediate
clutch handles engine braking. The
durability of this weld depends on good
penetration. We only suggest this modification if you’re having problems with
the intermediate band burning.
Hydraulics
Line pressure is still fed to both
accumulators and the intermediate
servo, so you still have to install the
complete intermediate servo assembly
(figure 11), the second accumulator
(figure 12), and the 2–3 accumulator
(figure 13).
To reduce rotating weight further,
you can remove the governor. To do
this:
• Countersink the governor feed
GEARS May-June 2003
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Success is Measured in Inches…Not Miles!
Counter sink governor feed
holes 3/8" deep with a 0.365"
to 0.370" drill.
Plug these holes with
3/8" cup plugs.
Drill an 1/8" hole through the wall that seperates these
two circuits.
Figure 15: Cross drill line to governor.
Figure 14: Plug the governor feed holes.
holes about
3/8" deep
with
a
0.365"
to
0.370" drill
(figure 14).
Add a 0.550" pin inside the 2-3 shift control spring
to block the 2-3 shift valve.
• Plug these
two holes
with a 3/8"
cup plug.
2-3 Shift valve
2-3 Shift control valve
2-3 Shift control spring
2-3 Shift control sleeve
Cut 3 coils off of the 2-3 shift
valve spring and install it here.
• Drill a
1/8" hole
Figure 16: 2-3 Shift Valve
through the
wall
that
separates the two circuits (figure
15).
This puts line pressure into the
governor circuit, making it a manuallyshifted transmission, which requires
the following modifications.
2–3 Shift Valve
1-2 Shift
valve
1-2 Shift
control valve
1-2 Shift
control
sleeve
1-2 Shift
control spring
Because third gear is never used,
you should block the 2–3 shift valve
(figure 16). To do this, install a 0.550"
pin inside the 2–3 shift control spring
(figure 16). You no longer need the
large spring that normally fits between
the 2-3 shift valve and the 2–3 control
valve. But don’t throw it away… you’ll
need it later.
Figure 17: 1-2 Shift valve (Second-Gear Transmissions Only)
28
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1-2 Shift control
valve
1-2 Shift control
spring
1-2 Shift control
sleeve
1-2 Shift
valve
Add a 0.780" pin inside the 1-2
shift control spring to block the
1-2 shift valve.
Figure 18: 1-2 Shift Valve (First-Gear Transmissions Only)
For both first-gear and
second-gear transmissions,
you’ll need to block the
manual low control valve
toward the deepest part of
the bore. To do this, add a
1/4" steel checkball inside
the manual low control
valve spring.
Manual low
control valve
Manual low
control spring
Note: may need to grind stem.
Add a 1/4" steel ball inside the
manual low control spring to
block the valve.
Figure 19: Manual Low Control Valve
30
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1–2 Shift Valve (Second-Gear
Transmissions Only)
For second-gear transmissions, the
1–2 shift valve still has to function. But
now that governor pressure equals line
pressure, you’ll need to add a spring
between the 1–2 shift valve and the 1–2
shift control sleeve. Cut three coils off
the large end of the spring you left out
of the 2–3 shift valve, and install it as
shown in figure 17.
1–2 Shift Valve
Transmissions Only)
(First-Gear
For first-gear transmissions, you
should block the 1–2 shift valve. To do
this, install a 0.780" pin inside the 1–2
shift control spring (figure 18). There’s
no need to reinstall the large spring in
the valve lineup.
Manual Low Control Valve
For both first-gear and secondgear transmissions, you’ll need to block
the manual low control valve toward
the deepest part of the bore (figure 19).
To do this, add a 1/4" steel checkball
inside the manual low control valve
spring. It may be necessary to grind the
stem of the valve to install the end plug.
Before you perform any modifications, always check the rules in your
specific class to make sure you don’t
do anything that will disqualify your
car.
In the next performance edition of
Transmission Therapy, we’ll cover
modifications to the THM200 for circle
track racing. Until then, get creative
and have some fun!
GEARS May-June 2003
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31
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3:32 PM
CORRECTION
Page 83
Reference Technical Article in the May-June 2003 Issue of GEARS
Magazine “Success is Measured in Inches.... Not Miles”
In the May-June issue of GEARS Magazine, there is a correction needed in the Transmission Therapy article Success is
Measured in Inches…Not Miles. Never block the 1-2 shift valve. Blocking the 1-2 shift valve will cut off apply oil to the LowReverse clutch causing a condition of no engine braking in manual low (Figure 1). For both, second gear only and first gear
only transmissions, simply cut three coils off of the 2-3 shift valve spring and install it as shown in (Figure 2).
1-2 Shift
control valve
1-2 Shift control
spring
1-2 Shift control
sleeve
1-2 Shift
valve
Add a 0.780” pin inside the 1-2
shift control spring to block the
1-2 shift valve.
Figure 1: Blocking the 1-2 shift valve will cause no engine braking in manual low.
Cut 3 coils off of the 2-3 shift
valve spring and install it here.
1-2 Shift
control sleeve
1-2 Shift
valve
1-2 Shift
control valve
1-2 Shift
control spring
Figure 2: This modification will work on both first gear only transmissions and second gear only transmissions.
GEARS August 2003
83