When we initially began to do the research for this article, we had conventional wisdom in mind – the less horsepower a car makes, the smaller dimensionally the torque converter can be, but as we covered the topic with our four specialists, we discovered that wasn’t necessarily the case. We spent plenty of time discussing torque converter theories with some of the best in the business – TCI Auto‘s Jeff Reed, Pro Torque‘s Joe Rivera, ATI Racing‘s David Caine, and Neal Chance Racing Converters‘ Marty Chance, and hopefully by the time you’re done reading you’ll have a better understanding of how torque converters work and what goes into their parts selection.

Check out this cool video from the US Army on the principles of torque converter operation

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Torque Converters: The Basics

In order to help those of you who might not understand exactly what the torque converter does, here’s a semi-quick explanation of the video above:

A torque converter is designed to transfer the torque produced by the engine, and convert that into usable power to be applied to the ground through the transmission, rear-end gears, and ultimately the tires. There are four main parts to a converter – the turbine, the pump, the stator, and the front cover. The cover is the part that bolts to the engine (via the flexplate) and has the pump welded to it externally.

Stators are selected with specific characteristics - the number of windows and the angle of the fins are important depending on your application.

The pump (which has fins welded at precise angles inside it) and the front cover are welded together, so they turn at the same speed as the engine. Opposite the pump is the turbine, which is the part that actually connects to the input shaft of the transmission and turns independent of the engine. The pump hub, which is welded to the pump cover, engages the pump in the transmission.

The turbine has fins welded inside it just like the pump does, but at opposing angles from those in the pump. The transmission fluid enters the torque converter from the transmission, fills the converter, flows through the fins in the center of the pump, exits the fins at the outer ring of the pump, and flows back across to the outer section of fins on the turbine, driving the turbine.

After that, the fluid exits the turbine through the fins in the center, and moves into the stator. The stator is probably the most important part of the whole equation and is often called the “brain” of the converter, as it is the part that helps to multiply the torque from the engine. After the aforementioned fluid flows out of the center of the turbine, it flows through the stator.

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Sizing Guidelines

There are a few basic guidelines to think about when selecting a torque converter. You want to use a torque converter that is designed to keep your engine in its powerband, and not fall below the engine’s peak torque RPM. When using a transbrake, stall speed should be selected to be somewhere around 300-400 rpm higher than the peak torque RPM range, since brake-equipped cars tend to drop a few hundred RPM upon release of the brake. Although we do recommend working with an expert to design the one that’s right for you, the following list is a good place to start:

– 7-inch Converter: From 6,000 to 9,000 rpm stall speed
– 8-inch Converter: From 4,000 to 7,000 rpm stall speed
– 9, 10, and 10.5-inch converter: Typically used in power-adder applications. Stall speed will vary according to your particular engine combination.

These particular guidelines are only a starting place, and if there’s anything we learned during the creation of this article, there is no perfect torque converter for all applications – every car, engine, and transmission combination will require something different, so do your research and buy accordingly.

What happens when the fluid flows to the stator is nothing short of amazing.

The fluid flows through the center of the stator, which also has angled fins on it, and back into the pump. The stator has a one-way clutch inside it that allows the fluid to turn it only in one direction, which is the direction it needs to reverse the fluid angle as it flows back towards the pump. This is where the torque multiplication comes from as the fins in the stator redirect the fluid and speed up its flow as it reverses the direction of the fluid.

By increasing the fluid flow, the stator pushes the fluid through the pump faster, which drives the turbine harder, and attempts to make the turbine catch up to the pump in speed (engine speed). At idle, the stator is not moving, and actually blocks the flow from driving the pump so that the car will stand still with only light pressure on the brakes.

Now, oddly enough, once all of these fluid direction changes have occurred, and the stator has done its job of helping to multiply the torque to get the car moving, the car reaches higher speeds and the stator stops moving, directing fluid where it needs to be without actually doing anything. That’s the point of the one-way clutch in the center of the stator, so that the stator can freewheel once the turbine speed reaches nearly the same speed as the pump (and essentially, the engine) does in high gear, to minimize slip and maximize performance.

Does Size Matter?

Now that you’ve got the basics down of exactly what’s happening inside your transmission’s bellhousing, let’s bring our experts to the table. One of the common threads we discovered during our conversations was that size is not necessarily the most important thing about a converter, but on the same level, size plays an important part.

Basically, what you’re trying to achieve during converter selection is to ensure that the converter itself has enough volume to handle the power level, while still retaining a physical size that will not be too large – a delicate balancing act that is at the forefront of what each company does to provide the proper unit for power transfer.

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“We do tons and tons of Stock and Super Stock converters – most of the cars in NHRA have an ATI converter inside. Torque converters are very sensitive to horsepower; if you’re making a lot, you need to have a converter to harness it all,” said David Caine from ATI Racing.

Left - Rich Bruder has been knockin' 'em dead in X275 with an ATI torque converter. (Photo by Todd Dziadosz). Right - One of ATI's new lockup converters designed for use with one of the company's new Turbo400 SuperCase transmissions.

Caine continued, “The absolute basics we need to know are how much power the customer is making, along with the weight of the car, and from there we can figure out what the best solution will be for them. For example, an X275 car will typically use one of our Outlaw Series converters – guys like Dean Marinis, Al Marlow, and Rich Bruder use our heavy-duty piece to handle the big power they are working with. The cars are very heavy, and we need to make sure that they aren’t driving through the converter.

As the car goes down the track, if you are making too much power for the converter selected, the converter will slip, and you’ll need to run a tighter converter. - David Caine, ATI Racing

“Getting away from X275, a typical NHRA Stock or Super Stock car will use a converter that is on the small end, typically an 8-inch diameter, because that is all the car requires to transfer power effectively. On the other end of the scale, if you’re making 2,000 horsepower, a 9-inch converter simply won’t be able to handle the power.”

TCI Automotive’s Jeff Reed also weighed in on the subject, “A naturally-aspirated combination needs as loose a converter as you can possibly get to get the car up into the powerband – a smaller converter has more stall. We couldn’t use a 9.5-inch converter and stick it into a small-block combination and have any sort of success. Stall speed is determined by the combination of stator and pump that you’re using.”

“The more power you have, the harder the engine will try to drive the converter, and a smaller converter would be very inefficient in an application like that.” According to Reed, the stator will be locked up while the car is on the brake, and at some point in the run, depending on combination, it freewheels (unless it is spragless, in which case it will never freewheel), trying to catch up with the pump and turbine at that point to direct the fluid where it needs to be internally. The fluid will lock the stator and keep the windows in the stator where they need to be in order for the fluid to flow properly and the converter to be efficient.

Left: TCI's 9.5 and 10-inch Pro-X converters are used in classes like X275, where Jason Lee and Bob Kurgan have used them with great success. Right: One of TCI's lockup converters designed for use with the 6L80E transmission found behind many LS engines. It uses a fluid-activated clutch to assist in lockup.

Power Adder Combos

Power-adder combinations will all require a different physical converter due to their inherent manner of power application. For example, Reed says, “In a supercharged X275 car, the converter will typically be a 10-inch size, but a turbocharged car might use a 9.5-inch converter. The key to making horsepower in a blown car is getting the supercharger spinning into its happy place, somewhere around 7000 to 7500RPM. But in an X275 turbo car, you’ll want a lower stall speed, and the flash will typically be somewhere around 5700RPM. It all comes down to efficiency and getting the engine in the place where it needs to be in order to apply the power.”

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Reed continued, “Turbo and nitrous combinations want to be lugged along a little bit to help load the converter up, and today’s progressive nitrous systems apply the power much like a turbocharged combination. A small cubic inch supercharged engine flashes somewhere around 7700, and shifts at 9000, but on the gear change, the converter needs to fall right back into the powerband of where the blower wants to operate. A turbocharged or nitrous car, on the other hand, will need a little bit more of a drop on the gear change in order to help load the turbocharger and spin it harder.”

A Different Approach

Pro Torque’s converters rely on a billet cover to minimize weight while maximizing strength.

The conversation with Joe Rivera from Pro Torque went in a completely different direction. His company’s approach is that the physical size of the converter itself doesn’t matter one bit. He said, “Size doesn’t make a bit of difference – I take a completely polar opposite position. Many people believe that size means everything when it comes to vehicle application, but my approach is completely different. Sizing is not as important as overall capacity. I can get more capacity out of smaller diameters because of the way we construct our parts.

“Traditional thinking is that the larger the diameter, the more the capacity but the lower the stall for bigger engines, and the smaller the diameter, the more slip, for the lesser horsepower and high-revving engines. Our 10-inch converter actually has more capacity than our 10.5-inch converter – so that line of thinking is absolutely untrue. To debunk the myth that size has everything to do with stall speed and efficiency, we have smaller converters that have more capacity than some of our larger converters.”

Our interest was piqued when Rivera brought up the word capacity. We then talked a bit about the differences between how he’d select a torque converter for something like a naturally-aspirated combination versus some of the higher-horsepower, drag-radial style cars competing today.

I use the smallest possible diameter torque converter for any given application that meets the capacity needs of the engine and vehicle combination. - Joe Rivera, Pro Torque

“A naturally-aspirated small-block Ford requires less capacity than a twin-turbo piece, so we’ll use the smallest converter we can and still harness the power. Think of it this way – you wouldn’t use a 12-inch clutch in an application where a 6-inch clutch would be effective – the 6-inch clutch has enough capacity for the engine,” said Rivera.

Continuing Rivera explained, “The smaller the diameter, the less the rotating mass, and generally the smaller the diameter the lighter the converter. Less rotating mass – it’s the same reason someone uses titanium wheel studs. I’m more interested in getting the converter to have the right capacity for the application, rather than the size of the box. Generally speaking, the smaller the diameter, the lower the capacity – you can’t get away from that, but there are differences depending upon application.”

Marty Chance of Neal Chance Racing Converters was happy to discuss the subject of torque converter selection with us. Chance explained, “Conventionally, what most people do in a limited-tire application, is to set the engine up so that the converter lessens the shock value, or the inertia, in the driveline at the release of the brake, and use a converter that grabs the engine at a lower RPM than what would normally be optimum.”

One of Neal Chance’s full-billet racing converters is designed to lessen rotating weight while retaining strength in all the right places.

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“The best way to run a car, in a perfect world, is to leave at nearly peak power RPM, and accelerate from there to bring the driveshaft to the engine RPM. The technology that has brought us to the point where we are today, helps to put the power to the tire less aggressively, without losing the G-meter, which is your most valuable bit of data. What we’re working on now is some new stuff that lets us leave at a higher RPM and control the tire, to help bring the driveshaft to the engine faster and accelerate the mass more quickly,” he says. Putting that into practice has always been the challenge, and Neal Chance is working to design a converter that only has as much flash as what the suspension and tire combination can take at the hit, and then bring the power in as quickly as possible.

The R&D Edge

Chasing research and development for all of these companies is what keeps them at the forefront of the industry; it’s a constant, ongoing process that doesn’t allow for any downtime. One common thread that we found during all of our conversations is that the manufacturers rely on their racer contingent to constantly test, test, test, and provide them with data both before and after the converter’s installation, in order to better analyze what’s happening with each of the particular torque converter combinations they’re using. That’s what allows for the constant improvement in elapsed times, especially in today’s ever-quicker racing world.

Left: All of the manufacturers surveyed for this article furnace-braze the internal fins for strength. Here is a before-and-after shot of a turbine after undergoing the brazing process. Right: Balancing is necessary regardless of what type of torque converter you have - unwanted vibration must be kept to a minimum.

Each company has customers that have been very successful with their products, across all different styles of heads-up racing. Pro Torque’s team includes racers like Pro Mod record-setter Jose Gonzalez, Outlaw 10.5 superstars Tim Lynch and Mike Murillo, and also eight-time NMRA Super Street Outlaw champion John Urist. Neal Chance has been setting records for years with racers like Charlie Booze, Jr., Frankie Taylor, and Frank Manzo, while TCI’s converters are behind the engines of X275 players Jason Lee, Shawn Ayers, Bob Kurgan, Dan Pharris, and even in Reed’s own machine. ATI’s pieces are making noise in X275 with the aforementioned Bruder Brothers, Dean Marinis, and Al Marlow.

Regardless of application, they are all trying to maximize the benefit to the end-user, and those R&D efforts are what allow them to do so. Each manufacturer was adamant about the fact that there is no one-size-fits-all torque converter, and each application is completely different. The more data you can provide them on the front side, the better your chances are of getting the correct product when that pretty box shows up on the big brown truck.

Each also stressed the idea that the first converter you get may not be the best torque converter, especially if you’re a heads-up racer looking for that last bit of elapsed time to keep up with, or be better than, your competition. Providing your manufacturer of choice with as much data as possible is the single most important element of selecting the proper torque converter for your application, and maximizing its performance on the track.

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