This story first appeared in the November 2020 print edition of Drag Racing Edge, but it's worthy of another look in case you missed it.
Bolt-Together Torque Converters Offer The Ultimate In Performance
A typical torque converter used in any vehicle is a sealed unit generally consisting of only four components: a turbine, stator/sprag combination, pump and cover. In some cases today, the sprag has been replaced by a fixed unit making the converter “spragless,” a design which is almost as old as drag racing itself having been developed decades ago and popularly used today in a number of race combinations. However, internally fins direct the oil throughout each component and eventually drive the transmission’s input shaft. The correlation of each of the components to one another is the key to efficiency.
The problem lies in a word in that first sentence: sealed.
The converter isn’t something most racers can take apart, look at and/or adjust. An alternative to the use of a converter would obviously be a clutch. A very simple design, the clutch can easily be looked at and/or adjusted to determine its efficiency, oftentimes done at the track and even between rounds in some cases. Which is really the point. Joe Racer can disassemble his engine, transmission, rear end, etc., and inspect the components. Can’t do that with a converter… Until now with bolt-together technology
Actually, bolt-together converters aren’t something new. There are some industrial applications which use a bolt-together converter, but it may not have been until the mid-‘90s when drag racers saw the benefit. With a converter welded together, the converter become a bit of black magic in that a racer couldn’t simply open it up and see what’s going on. The ability to open up the converter, make a change or to simply clean it out became a lure for some. The idea was simple, but as a lot of racers no doubt found out, the idea wasn’t for everyone.
“Traditionally, it was a converter for Pro Mod/ high HP levels but now we’re finding that all heads up racing and more bracket racers like the ability to clean out the converter in the off season as part of their maintenance scheduling,” says John Lane of ATI Performance. “The bolt together is a benefit to every racer in every class.”
One concern to the unit is the actual weight. Adding a ring with corresponding bolts adds weight in the same fashion as if you were to add a 50-pound flywheel to your engine. A somewhat typical nine-inch welded converter weighs roughly 28-pounds while a corresponding bolt-together may come in slightly heavier, give or take their construction.
Greg Samuel of FTI Performance reports, “Our billet bolt together converters are typically lighter than a welded unit and therefore it is easier to control wheel speed on the launch due to less inertia.
“We have been able to build extremely light bolt-togethers,” said Samuel. “They’re fast, user-friendly, serviceable and they get rid of having to send the converter back to the manufacturer for a freshen up or if you’ve killed the trans and don’t want to put the converter back without cleaning it out.
And of course, if you don’t know what you’re doing inside the converter, you can easily get yourself lost.
Yet finally, there is the cost. Substantially more expensive than a “standard” welded converter, it does have its advantages. And regardless of all of that, bolt-together converters are becoming more popular every day.
“We’re seeing more people using a bolt-together as a tuning tool no different than changing the jets in your carburetor,” adds Samuel. “The downside might be they’re expensive, but it’s really no different than purchasing any other components for your car. Do you want a generic set of cylinder heads or the latest trick heads to build horsepower. We now have one converter which can be used behind a small block right up to a screw-blown alcohol dragster, just by utilizing different internal components.”
Certainly not new when it comes to converters, but the growing list of racers using them has allowed companies to constantly be in development.
“The most infinitely tunable part in your race car is the converter,” says Samuels. “The reason for that is there is no book for it and that’s because it’s always situation-able. Something as simple as camshaft location in an engine can make a big difference in not only an engine but also a torque converter. The guy who keeps data and makes the necessary changes to their combination are the ones who are not only going to go fast but also do it consistently. Those are the guys I call tinkerers, the guys who don’t mind spending a little more time to improve their combination. The guys who might make a simple jet change in only one corner of the carb instead of all four. The guys who would move the stator up or down by 0.010” or move the fin angle by a couple of degrees. The ability to do that in a weekend can be priceless.”
As for an instruction manual, Samuels says, “There is one as to how to disassemble and reassemble, but as far as what to change… we’ll give you an idea as to ‘if you do this, this is what it will accomplish.’ But generally it is once again depending on your particular combination. Changing ‘this’ might equate to a couple of hundreds of a second, whereas someone with the same engine type might change tenths.”
One fallacy heard very often is stall speed. “Once you get people off that fictitious nomenclature of stall speed,” Samuels pointed out, “you realize changes could be made to soften the hit of the converter and/or tighten up the top end or anything in between. Stall speed is ‘how long did you hold it on the floor cooking the fluid in your trans?’ The longer you hold it the higher it goes! That’s all stall speed really is. It’s just making a lot of noise in the pits. Once you get off that, you realize the converter is the most tunable piece in your car.”
Kevin Kleinweber of Hughes Performance concurs. “Stall speed is simply the amount of engine rpm which can be achieved without the turbine; which drives the transmission’s input shaft; physically moving. Stall speed is not a fixed number. It can change based on a number of variables. We can have one converter which may stall at 6,000 rpm and another which stalls at 6,600, yet both converters could act very similar.”
For years, people have learned how to tune a carburetor, or change camshaft timing, gearing, etc. And they learned that by actual doing it. Keeping accurate records when a change is made and what transpires after that.
“Not every move is always for the positive,” says Samuels, “but when you find a move that isn’t in the positive, you go back in the other direction. It’s that simple.
“Your converter is no different than the clutch except that your ‘clutch’ is full of fluid. That’s the only difference. One is fluid driven and the other mechanically driven.,” added Samuels.
As oil flows through the fins within the converter, those fins direct the oil in different ways. Changing the fin angle causes the converter to act differently. One of the most important changes to a converter can be in the use of stators with different fin angles and/or openings between the fins.
“It’s often when a person purchases a bolt-together converter,” says Kleinweber, “that they’ll also purchase a couple of different stator designs. These stators can be either an active or inactive one. An active stator would be one with a sprag; or one way clutch. The sprag allows the stator to turn freely at a certain power input or rpm.
“An inactive stator is probably the most popularly used unit today in drag racing with no sprag at all,” adds Kleinweber. “This enables the stator to stay locked regardless of rpm or power input. Most race converters are built today in that style. It’s a design which we have learned to be the most effective, although there are some instances where a sprag is still used.”
So what is it that can be changed with the use of the bolt-together converter?
It comes back to exactly what Samuels referred to when he spoke about tuneability. If you’re looking for the absolute best to be gained from your combination, it’s best to investigate all areas. Most racers would never think of taking a carburetor out of the box and using it without making a jet or air bleed change to suit their combination. The use of a bolt-together converter can be thought of in the same way. If you’re interested in gaining the most from your combination, the bolt-together is for you.