Stall speed is essentially the engine rpm at which a torque converter transfers the torque of the engine to the transmission (see our earlier post on how a torque converter works). When choosing a torque converter, it’s important to properly match its stall speed with your vehicle setup.
According to Nelson Gill, President and CEO of ACC Performance, there are three factors to consider when matching stall speed to vehicle setup: engine setup, weight of the vehicle, and rear gear with the run out of the tire. Gill uses these three factors to help people select the right stall speed when buying ACC Performance’s Boss Hog torque converters, and he has written extensively about stall speed in a series of posts on the company’s website.
The first part of Gill’s equation is engine setup, including the number of cylinders. Gill says you can make some mild changes to an engine and in some cases no longer be able to use a factory-style 12-inch torque converter.
Due to its diameter, it’s hard to get additional stall speed from a mild 12-inch converter. Generally, Boss Hog 12-inch converters stall 500-800 more rpm than stock. If your converter is affecting the idle, then the 12-inch torque converter may not work for your application.
Gill offers a specific example of how changes can affect stall.
He uses a 350-cubic-inch motor with 9:1 compression that has a standard 750 cfm carburetor that is still on pump gas. The carburetor is bolted up to a 1-inch spacer plate that is then bolted up to an intake. The 1-inch open spacer plate will give you a couple hundred more rpm at the red line or at the top of the rpm range. The intake in his example is an Edelbrock Performer RPM dual-plane with an advertised operating range of 1,500-6,500 rpm. It is just over 4 inches tall — taller than most factory intakes.
“When you factor in the intake’s powerband and the 1-inch open spacer plate, the airflow really begins to flow well at about 2,600 rpm,” Gill writes. “While the operating range of the intake indicates that it will start to die out at 6,400 rpm, the 1-inch spacer gives it an extra couple of rpm at the top end, putting the redline at around 6,600-6,700 rpm.”
The powerband has essentially moved up in the rpm range, making the bottom-end weaker. This produces less resistance against the fluid pressure inside the torque converter in order to move the car at idle. Because of the slight changes to the engine, the stall range of the converter is now below the engine’s powerband.
You’d need to step up to higher stall range to match.
Because torque converters operate using centrifugal force, we know that these small changes will affect a larger diameter torque converter more so than it would the smaller diameter torque converter. The drop in actual stall will be greater in the larger diameter converter because its pressures are much greater.
NOTE: In some cases, you may find that the converter is stalling low or you can feel the transmission engage once the transmission has been put in gear. If you let your foot off of the brake and the car begins to move, then it is most likely you will have to upgrade the torque converter. (If the vehicle creeps forward after it has warmed up that is OK.) If you have to fight the brakes at a red light or stop sign, you will need to upgrade the torque converter. If you have idling problems with the AC on or have to fight the brakes again, this is a stall issue that comes from not properly matching the torque converter to your vehicle’s set-up.
As you can see, engine setup plays a major role in choosing stall speed. Even small changes in your setup can necessitate a change in torque converter stall range. This can be accomplished by manipulating the fins inside the existing converter or may even require a change in torque converter diameter.
In the next installment, we’ll look at how your cylinder heads and exhaust can affect stall speed.