High Stall Torque Converters

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	Turbine in its cover	Stator & Turbine	Stator in Turbine	Pump lying @ angle

First of all, what is a torque converter, and what does "high stall" mean?  Basically, a torque converter is the unit that performs the fluid coupling between the engine and transmission.  Below a certain RPM, the converter slips, allowing the output to turn much slower than the input.  At a certain RPM, called the stall speed, the input and output sections of the converter are essentially "locked" by fluid pressure.  This stall speed will depend on the torque output of the engine, the higher the torque, the higher the stall speed.  For this reason, it is not accurate to classify a converter according to stall speed, as this is dependent upon the torque output of the engine.  So, when you see a "2800" stall converter advertised, you have to ask, 2800 rpm behind what engine?  Because, if this stall speed of 2800 rpm was measured using a hi performance big-block V8, this same converter used behind a 4 cylinder would have a very low stall speed indeed.

To avoid this confusion, the OEMs rate their converters with a term known in the industry as the "K factor".  By definition:

K factor = Stall RPM / SQRT(Eng Torque @ stall) 

This formula effectively takes into account the engine torque, making comparisons between torque converters much more valid.  Essentially, the higher the K factor, the higher the stall speed.  The stock factory converters used with the L67 3800 Series II Supercharged engine since 1997 is a 258mm (diameter) unit rated at a K factor of 155.

So what performance advantage is there in increasing the stall speed?  Plenty, if it's done right!  Every engine has a torque curve, meaning the torque output increases as the rpms rise until peak torque is reached.  For most engines, this is around the 4000-5000 rpm range, so ideally, for optimum acceleration, the stall speed of the converter (for street cars) should be in the 2500 to 3000 rpm range.  Most factory stock stall speeds occur in the upper teens for fuel economy reasons.  Low power 4 cylinder vehicles actually have higher factory stall speeds to allow these cars to get out of their own way.  By increasing the stall speed higher than the teens, this allows the engine to operate in a region where it has more torque output to help your acceleration, much like slipping the clutch on a manual transmission.

However - and this is a big however - the real trick is to increase stall speed without negatively impacting the all-important torque multiplication and efficiency!  Otherwise, you'll get the increased stall through slippage, and increased slippage never made a car faster!

Here's a stock factory GTP/GS converter exploded.  From left to right is the Converter Housing Cover, Pressure Plate/Clutch with damper, Turbine, Stator, and Converter Pump.

How does a converter work?  First, a running engine spins the pump.  The pump picks up fluid at its center and discharges it at its rim between the blades.   This fluid force then hits the turbine blades, causing it to spin.  

The stator, located between the pump and turbine, is mounted on a one way roller clutch that only allows it to spin in one direction and not the other.  Its function is to redirect fluid returning from the turbine to assist the engine in turning the pump assembly. thus "multiplying torque".  At low speeds, fluid from the turbine hits the front of the stator blades, the one way clutch prevents the stator from turning in the same direction as the fluid flow, thereby redirecting it to assist the engine in turning the pump.  Fluid leaving the pump now has more force to turn the turbine assembly, and torque is multiplied.  

As engine speeds increase, centrifugal force changes the direction of the fluid leaving the turbine so that it hits the back side of the stator blades instead of the front.  When this happens, the roller clutch overruns and allows the stator to spin freely.  Fluid no longer is redirected to the converter pump, and torque is no longer multiplied.

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	TCC assembly	Turbine Cover	TCC & Cover	fitted together	backside of turbine

Outside of Cover	Turbine & TCC mated

Lastly, the pressure plate/clutch assembly is used when the torque converter clutch (TCC) is engaged and the converter is "locked up", usually at highway cruise speeds.  This hard couples the turbine to the cover assembly, eliminating all slip for better fuel economy.  The spring loaded damper reduces torsional shock during the apply, and reduces irregular torque pulses from the engine or road.

Bottom line:  Were these modifications successful?

Answer:  We're not sure yet, the jury is still out on that one.  This particular converter is currently being tested, and we'll report the results soon.  It should be noted that we had nothing to do with the design and work on this particular converter.  It was purchased by one of our customers from a well-known converter company who advertised they had high-stall converters for the GTP/GS which worked with absolutely no driveability problems.  We borrowed it and cut it open for R&D purposes, and to provide you with this article.  It was reassembled, balanced, and installed on the customer's vehicle.  (Go to his web site, www.c-ya-racing.com.)  Initial observations are that driveability has definitely suffered, and it may not be very streetable for most drivers, but yes, the stall speed is very drastically increased, up to greater than 3500 rpm.  Is this a good thing?  Maybe.  Could this be used as a race-only converter?  Perhaps.  The unknown at this time is torque multiplication and efficiency.  The modifications performed in all likelihood have reduced both, the question is, have they been reduced too much?  The stall speed may be higher, but if the converter is essentially slipping all the way down the track this wouldn't be a good thing either.  The customer's car is not completely finished yet, so he's unable to perform any real tests.  We'll have more conclusive results after mid-April.

 Another (better?) approach for a high-stall converter

As previously mentioned, we've been experimenting with high-stall converters since 1997, so how has our approach been different?  Instead of modifying the production stock factory L67 GTP/GS converter which as shown, has an inherent low-stall pump with very positive fin angles, we've pursued using pumps and stators manufactured for different applications which from the factory inherently have a higher stall speed and thus require little, if any modifications, and thus do not suffer from reduced torque multiplication and efficiency. 

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	Positive fins(left) vs Negative fins (Right)	Exterior view, blue is negative fin one

An example is shown above.  The stock L67 GTP/GS converter is on the left, compared to a converter from another application on the right.  Notice the inherently more negative, high stall fin arrangement on this other pump (the blue one).  You can even see this from the outside, look at the angle of the blades towards the outside diameter of the converter pump.  We feel that better results can be obtained using a pump such as this as the basis for a high stall converter rather than an inherently low-stall one.  The negative side of this would be cost, as it may still be necessary to use other components such as the stator or turbine from the stock unit, or even still another core to come up with the ideal combination.  Naturally, the more components needed from different converters raises the cost, since more cores are needed to make one converter!

We have been experimenting with a smaller diameter unit as well.  The stock L67 GTP/GS unit is 258mm (10.2"), we're using a 245mm (9.65") one.  A smaller diameter converter means a smaller diameter pump, and is inherently a higher-stall unit.  Other advantages are the rotating weight savings, allowing the engine to rev quicker.  As one can see from the photos, it would be highly improbable that any weight could be saved over the stock L67 GTP/GS converter and keep the same diameter.

Some tentative conclusions:

We've had good success in our approach, and should have a high-stall converter ready for release soon.  We'll reserve conclusions on modifying the existing converter as some manufacturers are doing until we've done more testing, although we obviously have reservations on the ultimate success of that approach.  To their credit, the modifications shown in these photos are realistically the only things that can be done using only the stock production converter, and they did a nice job.  However, we have reservations that these modifications produce a proper stall speed for our cars, and without impacting driveability as claimed.

Some things we've learned so far in doing this R&D:

• Converters rarely fail, especially in street applications.  Sure, in real race cars with 700+ HP, that's a different story, but realistically, for our applications of 400 HP or less, there is nothing in the converter to fail.  We're not making enough power to bend fins, shatter hubs or sprags, or balloon impellers.  This essentially means that modifying or combining stock components will produce the desired results with no durability concerns in our hi-performance street machines.  Custom machined billet turbine hubs and covers would be overkill, your money better spent elsewhere.

• When converters used in street applications do fail, it is typically in the bearings and bushings due to debris from the transmission (which has already failed or is in the process of failing). 

• No aftermarket company is actually making new pumps or turbines to their own design - only the OEM have the budget, factories and equipment to actually produce these components.  All aftermarket companies are at the mercy of the OEMs for these parts - their job is to do the R&D to find the right factory components, modify them, and convert them for use in our performance applications.  Believe it or not, many full-race converters use pumps and turbines that originally served duty behind 4 cylinder economy cars!  (Remember they were very high-stall units from the factory, and small in diameter.)  You can bet that 9" full race converter has a pump and turbine that originally lived in some small lightweight car, since only those applications had converters that small from the factory!  The aftermarket racing converter manufacturers that know what they're doing have essentially done enough R&D to find the right units built strong enough from the factory to withstand competitive power levels, or have engineered a way to make them withstand this sort of abuse. 

• The high dollar (i.e. $600+) converters are full-race jobs with custom machined billet turbine hubs, turbine covers, etc. designed for durability to withstand punishment behind 700+ HP machines.  Are they worth the money for these applications?  Of course.  Do street machines need this sort of hardware?  You be the judge.

• To purchase our exclusive Torque Converter, click here.