MOST SPEED MECHANICS have two burning desires concerning their cars: to boil along at good top speed and to accelerate rapidly. Although engine soup-up will provide additional pep, without proper gearing neither desire can be achieved.
The stock cogs in gearbox and rear end are designed to give
a car in stock form acceptable performance. Because the maximum
torque developed by the engine is puny compared to the weight
of the car, mechanical advantage must be obtained through leverage
to move the car and this is accomplished with gears. By multiplying
the torque available at the flywheel through speed reducing gears,
300 pounds feet of torque can become as much as 3000 (or more)
pounds feet at the driving wheels.
So much confusion exist about gear ratios that clarification of
the meaning of terms might prove valuable at this point. Four-to-one
is a high ratio, but a low gearing. When used between the engine
and rear wheels, it reduces wheels speed to one quarter engine
speed and at the same time, (because of leverage advantage), increases
torque available at the wheels by four. Two-to-one is a lower
ratio but higher gearing: Wheel speed is half engine speed and
torque at wheels is twice that available at the engine.
The rear end gear ratio affects the car's top speed and can limit
or expand the engine's potential power output. Top speed however,
is not necessarily increased when a higher-than-stock gearing
is used and many, who substituted a 3.54 rear for a 4.11 for more
speed, learn this the hard way. Acceleration through the gears
is affected by the overall ratio of transmission and rear end
gears.
That it requires more force to overcome the inertia of a body
at rest than to keep it moving is a well-known law of physics.
The additional force required to move a car from a standstill
is made available by transmission gears. Low overall gearing also
permits the engine to rev close to the rpm point at which maximum
power is developed for good acceleration.
Theoretically a perfect transmission is one in which an infinitely
variable gearing is produced, such as in Dynaflow. Unfortunately
power losses usually prove to be a disadvantage that is not overcome
by theoretical advantages. One of the best gear-box bets is a
four speed unit, fitted with close ratio gears, that allows selective
gearing of engine speed to wheel speed under a variety of driving
conditions. In American gearboxes, the use of three forward gears
usually requires a wider spread between ratios and consequently
power is lost during shifts and elapsed acceleration time is greater.
As a general rule gearing changes accomplish very little in the
stock car. Earlier we mentioned that top speed and that substituting
a 3.54 rear, for instance, for a 4.11 for more speed does not
result in a gain of 10 mph or so. Usually the stock car's gearing
is chosen so that top speed is reached at about the rpm point
that the engine develops its maximum hp. Gains in top speed are
made with changes in rear end gearing when the engine is reworked
to pull more power.
The standard Ford-Mercury boxes have a 28- or 28- tooth main driving
gear. The Lincoln Zephyr box carried a 25- or 26-tooth main gear.
The lower ratios available in the Zephyr first and second, produce
a higher gearing and permit a higher maximum speed in first and
second, without changing the speed possible in high (or direct).
In addition, Zephyr ratios are closer than in Ford-Merc boxes.
As a result of Henry Ford's feeling for standardization, Z gears
are interchangeable in most Ford and Merc boxes.
Despite the fact that, among Ford-Merc owners, Lincoln Zephyr
gears have been widely used, it would be unwise to order a set
for your own rig without a thorough study of your needs and your
equipment. Z gears should not be regarded as a cure-all. Unless
you've souped or lightened your rig, they will not appreciable
improve your car's acceleration, for the higher gearing requires
more power from the engine or a lighter wagon to drag. With Z
gears it will take a heavy car longer to get going from standstill
than when equipped with stock cogs.
In a lightweight chassis, however the Z combination usually spells
success and the advantages of Z conversions are most noticeable
when used with souped engines or with stock mills capable of handling
the load. Combining greatly increased engine output with a light
chassis almost always demands the use of Zephyr gears.
It should be obvious that knowledge of your engine's capabilities
is necessary to correct analyze the elements that determine proper
gearing. A tachometer (discussed in August SM) can be helpful
in experimenting with various tire sizes in search of more perfect
gearing. If a higher gear will give you more speed, a larger tire
size will duplicate this effect. If you obtain the same or a higher
reading on the tach, with speed improvement, higher gearing will
be of benefit. As it's less expensive to test gearing with borrowed
larger tires than to purchase a number of ring and pinion sets,
keep experimenting with tire sizes until no further improvement
is found.
Determining proper gear ratio can be done, with some rough accuracy
due to our omission of wheel slip and wind resistance factors
with the formula: Gear ratio = Engine rpm X tire diameter (in
Inches) divide by mph X 336 (a constant *). Double check answers
thus found against specifications of quick moving cars with similar
modifications to those found in your car. The more time you spend
checking for the right gear, the less heartache you'll have on
road or drag strip.
Zephyr gears can be installed in any (except V-8 60) '32-'48 Ford
or in '39-'50 Merc transmission case with little difficulty although
some years are a tighter "squeeze" then others in installation.
It is impossible to change just one gear in Z conversion; in some
models ('40-'48 Fords and '40'50 Mercs) only the cluster gear
with rear thrust washers, main drive gear and second gear must
be replaced. In other models, it is necessary to make an almost
complete parts-changeover.
Boxes built before 1940 were "floor shifters" and later
model were "side shifters." Some but not all, speed
mechanics think it wise to change to a floor shift box if extensive
power modifications are to be made, The floor shift box has a
reputation for greater strength that is increasingly appreciated
as power is upped.
In the '32-'36 boxes only a few of the original parts are retained
in a Z conversion, but in the interest of reliability all parts
are usually replaced. The parts changes required on early Fords
are: the cluster gear and thrust washer; main drive gear; main
shaft; second gear; synchro-mesh assembly (and second gear thrust
washers on some models) low and reverse idler. For the proper
combination, check all parts needed against a late Ford parts
catalog for the current stock numbers.
In '37-'39 Ford and '39 Merc boxes, the above parts (except low
reverse idler) are replaced. The '39 second and high shifter fork
is used in converting all '37-'39 boxes.
Cost for a Z conversion runs approximately $40 for parts in '40-'50
boxes and somewhat higher for earlier models and labor. If you
choose to have the job done, should run at least $30 more. Two
series of Zephyr gears are available: the 26-tooth gears ('40-'41
Lincoln) and 25-tooth gears (up to '48 Lincoln).
If you decide to do the job yourself, these are the basic steps
to follow:
On models up to '48, remove the rear axle-floor boards if necessary.
Remove the clutch equalizer shaft, detach the shaft rods, place
a support under the engine unbolt the transmission. Disassemble
transmission. Remove U-Joint (one bolt holds it in place). Remove
clutch throw out bearing and unbolt front and rear bearing retainers,
and transmission cover. Push countershaft out trough rear, allowing
cluster gear to lie in case. Pull main drive gear through front
of case, push mainshaft to the rear and pull rear bearing. Lift
mainshaft out through cover opening, keeping synchromesh and low
and reverse in position. Do not move synchro assembly; it is spring-loaded
and parts can be lost easily if disturbed. Lift cluster gear from
case and drive out reverse idler gear shaft and lift of gear.
Slide reverse and low gear off the shaft. Remove lock ring from
front of shaft but keep synchro gear centered on the assembly.
Wash parts to be reused and inspect carefully. Worn parts should
be replaced.
The Zephyr cluster is first replaced on reassembly, with other
parts to be replaced in reverse order to that used in disassembly.
The development of the Ford Thunder-Bird almost from the beginning
meant much more to the professional speed mechanic than merely
the appearance of a new American sports car. It didn't take these
professionals long to recognize that the T-Bird synchro-mesh box
provides still another excellent source of good transmission gear
ratios.
Some such as Milby Jones of Exton Pennsylvania have already made
use of the T-Bird's 24-tooth clusters in reworking late Ford and
other gearboxes. Jones, one of the East's veteran "Sultans
of Swap" says he'll be glad to answer any specific questions
concerning installation of T gears.
To take advantage of the new ratios available, it is necessary
to procure the following T-Bird parts:
B5S-7017A-----Input shaft and gear
B5A-7102C-----Second Speed gear
B5S-7113A-----Cluster Gear.
Other modifications depend upon what engine and transmission case
is used, but one thing that must be done in whatever transmission
case is used to line-bore the cluster shaft hole from .750 (or
¾") to .875 (or 7/8") because the shaft that
accompanies the T cluster is larger and uses 50 instead of 44
needle bearings. As the T input shaft is two inches longer than
previous shafts, it is ideal for coupling to '51-'53 Chrysler
engines with a deep bell. Usually the shaft must be cut shorter
and resplined for general conversion use.
To those speed mechanics, seeking the ultimate in a both performance
and reliability in a box that must handle the output of a big
and hairy power plant, a full-scale shift to pre-war Cadillac,
La Salle and Packard brute gear boxes is called for. Attention
auto wrecking yards: Brace yourselves for a boom in good boxes!
Transmission Gears and Ratios
26H Lincoln 25-tooth Low Second High 2.12 1.44 1.00
06H-56H Lincoln 26-Tooth Low Second High 2.33 1.58 1.00
Ford-Merc Lincoln 28-tooth Low Second High 2.82 1.60 1.00
Ford-Merc Lincoln 29-Tooth Low Second High 3.11 1.77 1.00
Ford Thunderbird 24-tooth Low Second High 2.32 1.48 1.00
* The Constant is used to equate inches and miles and minutes and hours, and minutes and hours, and diameters and circumferences.