Selecting the right torque converter for your automatic transmission can make all the difference in how your drag race or street/strip vehicle performs.
Picking the right parts often requires the advice of an expert. The problem is that everyone seems to be an expert, from your dad to your best friend to the guy in line at the local parts counter with the rusty water pump. In the case of selecting a torque converter for your race car or street/strip vehicle, it is important to follow a “logical” process to help in selecting the right components. It all begins by determining your performance “target.”
Where to Start – Transmission Selection
How fast do you want to go? It’s an honest question with an honest enough answer. Luckily, most drag racers have a performance target in mind. In the same manner, they generally already have the racing vehicle as well. But when you whittle down what really counts in terms of final vehicle performance, it simply comes down to a matter of weight vs. power. If you select a target performance figure (elapsed time in the quarter mile) and you have a heavy car, then it stands to reason that you will need a lot more power to achieve your goal. Likewise, if you reduce your performance target or the weight of the vehicle, the power required will be reduced as well.
Let’s begin with transmission selection. For most race car applications, selection of the right transmission is basically a case of strength vs. weight. For many drag race vehicles, the old GM Powerglide transmission is a popular choice based on weight savings, adequate strength, low parasitic drag and the seemingly infinite number of gear choices available. But for street/strip applications, the choice of transmissions can be a more difficult question. Fuel mileage is often an additional consideration.
With street/strip vehicles running steep rear gearing, car builders are forced to turn their engines at high rpm when driving at highway speeds, which not only hurts fuel mileage, but also wears out the engine more quickly. For any part-time, street bound performance vehicle there are GM, Ford or Chrysler overdrive transmissions (700R4, AOD, etc.) available that can deliver both performance and lower cruising rpm numbers. In addition, the rpm point at which full converter lockup is achieved can also be selected, further improving performance and mileage. When designing your dual-purpose vehicle drivetrain, we strongly recommend that you work closely with an experienced and reputable company to get the right parts, the first time.
Real World Converter Selection
To further explain the process of proper torque converter selection, we decided to work with a real world example. In our case, we outlined a fairly typical platform - a 3200 lb vehicle with a Big Block Chevy engine and Powerglide transmission running 9.90 seconds in the quarter mile. After speaking with Westech Performance Manager Steve Brule´, we created a hypothetical engine based on their engine test data, specifically a 566cid engine that churned out 871 horsepower at 6500 rpm and 750 lb/ft of peak torque at 5300 rpm. Having made our basic engine and transmission choices, it was time to focus on proper torque converter selection. Here we opted to talk to the experts at TCI® to find the best torque converter for our race car. Without hesitation, they recommended a few simple guidelines.
“The key is the engine’s peak torque figure,” stated Stanley Poff of TCI®. “Once you have determined that rpm figure (in our situation, a hypothetical 750 ft lbs at 5300 rpm) you should select a torque converter with a stall speed 300 to 400 rpm higher than that number. We’ve found that upon the initial starting line launch with race cars running a trans brake, the rpm drops about [300 to 400 rpm] so the stall speed should compensate for that change. You never want the rpm to drop below that peak torque figure.”
For our hypothetical application, Poff chose a 9-inch TCI® Super StreetFighter™ Torque Converter. The stall speed – 5700 rpm. Poff also felt that the Powerglide transmission, outfitted with a number of TCI® performance components, would be more than able to handle this kind of engine torque. In addition, if further engine torque increases were initiated at a later date, he felt we would be able to handle that condition too.
Size Matters
. . . even with torque converters. But which one is right for you? TCI® offered a few simple sizing rules that should work for most race car and street/strip applications. Remember, the best way to make certain you have the right converter for your vehicle is to make the call to the TCI® tech line and ask the experts. Make certain you give them the complete details as to your engine and vehicle specifications and a clear view of what you ultimately want it to do so they can fit you properly.
The following is a short list and description of the most common torque converter (measured on the diameter) suggestions according to TCI® product manager, Stanley Poff.
7-Inch Converter - Ultra-high performance from 6000 to 9000 rpm stall speeds. Up to 11,000 rpm engine application compatible.
8-inch Converter - High performance with stall speeds from 4000 to 7000 rpm as dictated by your engine combination.
9 and 10-Inch Converters - High performance applications, especially for engines with power adders, such as nitrous injection, super and turbocharging. Generally for engines with 1200 hp. Stall speeds can be designed according to application.
So what have we learned? A number of things: 1) Select a transmission that fits your needs for weight, mileage and strength.
2) Select a torque converter, with the help of a TCI® expert, that fits your vehicle specifications.
3) Select a torque converter with a stall speed 300 to 400 rpm higher than the peak torque output of your engine if you are using a trans brake.
4) Your torque converter and transmission must be in top condition to net top performance. Send them both into TCI® at least every two years for a freshening to ensure they are not only working properly, but that they are correct parts for your application based on your vehicle and engine combination.
So there you have it, the proper way to select a torque converter for your vehicle. Remember, if you have questions or just want advice, don’t be afraid to call the TCI® tech line at 1-888-776-9824. It’s well worth the time and potential frustration avoidance!
The Most Widely Misunderstood Torque Converter Concept The stall speed rpm of your torque converter is the point at which your vehicle starts to move, regardless of throttle input.
Amazingly enough, this lone misconception is the belief of many car builders today. True, when you mash the throttle, the stall speed of the converter does come into play before the vehicle begins to move. But at any throttle level less than wide open, your stall speed is reduced. Case in point, at half throttle the stall speed is cut in half. If this were not the case, it would stand to reason that there would be no way to effectively drive a high stall speed converter-equipped transmission on the street.
Working Backwards?
Is putting the converter before the cam a good idea? The camshaft, as has been said countless times in the past, is the brain of the engine. It is also the engine’s personality. It determines not only the peak power and torque of the engine, but also the shape of the dyno curves. A common COMP Cams®/TCI® tech line question involves folks who want to know the correct camshaft choice after having recently purchased a race car which did not have an engine as part of the sales package. Often these folks have what is claimed to be a racing Powerglide transmission with a high performance torque converter. So the question arises – can you work backwards and pick a camshaft to fit an existing converter?
Says Poff, “It’s possible but not advisable. Determine what you want the race car or street/strip vehicle to do and build an engine, including selecting the correct camshaft, to generate the targeted power and torque. Then you should send in your converter to TCI® so we can check two things; first, that it is working properly, and second, that it is the right converter for the job. We offer a free “inspection-only” service. If the converter needs work, we can fix it after sending you an estimate on the cost of repair or upgrade the converter. If it is the wrong converter, we can advise you on the right converter to purchase for the application. Working backward and building an engine to match the converter is just too difficult and probably won’t deliver in the long run.”
Torque Converter Tech 
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