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IS STALL SPEED THE ONLY
CONSIDERATION IN SELECTING A CONVERTER?
While stall speed is very important it is by no means the
only consideration when selecting a converter. Torque multiplication
at launch and high-end efficiency (lock-up) are equally important.
Stall speed can be attained in many ways that cripple the
converter in other ways. Stall speed can be obtained at the
expense of looseness at low speeds and loss of performance
at higher speeds after launch. You want a converter that
produces the right stall without sacrificing performance
down the street or down the strip.
WHAT ARE FLASH STALL, BRAKE STALL AND
TRUE STALL?
The truest definition of stall speed can only reliably be
determined in conjunction with a transbrake. Locking wheel
brakes and running the engine up against the locked brakes
determine “Wheel Brake Stall”. Drum-style brakes
have more holding power than disc brakes, but usually create
more drag. As discussed above, the engine will usually
overpower
the brakes and wheel rotation will begin before the true
stall speed is encountered. Flash stall is observed by launch
at full throttle and observation of peak rpm at launch.
HOW CAN I DETERMINE THE STALL SPEED OF
MY CONVERTER?
Stall speed is very difficult to determine unless your car
is equipped with a transbrake to lock your drivetrain. Testing
stall speed by holding the wheel brakes and running the engine
against the locked brakes will usually result in wheel rotation
before true stall speed is reached. The engine simply overpowers
the ability of the brakes to hold the car. When rotation
starts you are no longer at stall. For this reason people
talk about brake stall, which is not a true stall at all.
An alternative method of measurement is to launch at wide-open
throttle and observe engine rpm reached at launch. This is
flash stall.
WHY IS STALL SPEED DIFFERENT WITH THE
SAME CONVERTER ON DIFFERENT ENGINES?
“Engine Torque Output” is what really determines
stall speed for a given converter. For this reason the converter
you have been using may not be adequate when you improve
performance of your engine. This is particularly true in
using an improved camshaft. Improving heads, carburetion,
changing airflow with intake manifolds, installing power-adders
such as turbochargers or nitrous oxide can all affect stall
speed. Remember, you want to have stall speed matched to
your particular car combination.
WHY IS PROPER STALL SPEED IMPORTANT?
Stall speed should be matched to the car's general use, engine
specifications and performance, the car weight, tire size and
gear ratio. Proper selection of stall speed will make for quicker
launch, better 60-ft. time and better e.t. It is very important
that these parameters be specified when ordering a converter
to assure satisfaction. In fuel conservation applications you
can actually increase the amount of low-end power the rear
wheels see along with reducing highway cruising rpms.
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