Converter slippage

[sonicbluegt]

What is a normal percentage of converter slip on a 4500 stall converter in a c4

[Jimbo]

The answer when it comes to converters is usually, "it depends", lol. I guess this is a good a place as any to post this info I saved.

Racing Torque Converters
How They Work and How to Select One
In today’s ultra-competitive drag racing game, racers have grown to understand that every detail should receive their attention. Most modern drag race cars use an automatic transmission. Even extreme horsepower cars such as Pro-Mods, Alcohol Funny Cars & Dragsters are finding success with Converter Drive Units coupled with Planetary Transmissions. All of these cars have the engine coupled to the transmission through a torque converter. The torque converter needs to be matched to your combination and application or you might as well take up playing checkers or some other hobby. As with many other components such as camshafts, carburetors, etc, your torque converter can make or break your combination.
In order to get the right torque converter for your race car it is extremely important to communicate with your converter builder. Often times, racers will order a torque converter with little or no conversation with the converter builder. I can’t count the number of times a racer has phoned asking for a 5000 RPM stall speed converter, unprepared to supply any information about their race car combination. We could supply them with a torque converter that stalls at 5000 RPM in a supercharged alcohol combination, but when they install it in their naturally aspirated small block bracket race car it may only stall at 2200 RPM.
In communicating with your converter builder, it would help to know a little about what’s inside a torque converter, and what factors affect a converter’s performance and characteristics. Hopefully this information will help you to understand a little of how a torque converter works, and how to prepare to order the right converter for your application.
Torque Converter Theory and Elements
The torque converter serves two primary functions. First, it acts as a fluid coupling connecting engine rotational power to the transmission’s input shaft; and second, it multiplies torque from the engine when additional performance is desired.
The torque converter consists of three basic elements: the impeller or pump (driving member), the turbine (driven member), and the stator (reaction member). The converter cover is welded to the impeller to create a sealed assembly consisting of these three elements filled with oil from the transmission pump. The converter cover is bolted to the engine flywheel and therefore rotates at engine speed.
When the engine is running, the converter impeller acts as a centrifugal pump, picking up oil at its center and discharging this oil at its outer rim, between the impeller blades. The oil discharged by the impeller strikes the blades of the turbine, delivering a rotational force on the turbine, causing the impeller to try to turn in the same direction as engine rotation. The turbine is splined to the transmission input shaft. At idle or lower engine speeds, the force of the oil discharged by the impeller is not great enough to turn the turbine with any degree of efficiency. This allows the vehicle to remain motionless with the transmission engaged in gear. As engine speed increases, the force of the oil leaving the impeller
increases, resulting in greater force acting on the turbine and therefore the transmission input shaft.
After the oil has imparted its force to the turbine, it follows the blades of the turbine, leaving the turbine at its center, spinning in a direction opposite engine rotation. If the oil leaving the turbine was allowed to re-enter the impeller in this counter-rotating state it would impart a force on the impeller, acting against the engine’s rotation, greatly reducing the effective output of the engine. (This is what can happen when a converter sprag fails). To prevent this from happening, the stator assembly is added.
The stator is located between the impeller and turbine and is splined to a non-rotating stator shaft of the transmission. The stator generally incorporates a sprag or roller clutch, which is a one way clutch element that would allow the stator to rotate in the same direction as engine rotation, but prevent it from rotating in the opposite direction. The purpose of the stator is to redirect the oil returning from the turbine to the impeller, changing its direction of rotation back to the same direction as the engine rotation. The energy of the redirected oil acts on the impeller in the same direction as engine rotation, thereby multiplying the torque output of the engine. This is what is known as vortex flow inside the torque converter.
As both engine speed and transmission input shaft (turbine) speed increase, the oil leaving the rotating turbine acts on the back side of the stator blades, causing it to begin to freewheel (on its sprag or roller clutch) in the same direction as the engine and turbine are rotating. When the stator freewheels, the torque converter ceases to multiply torque and becomes simply a fluid coupling between engine and transmission. This is what is known as rotary flow inside the torque converter.

[Jimbo]

Stall Speed, Efficiency, and the Elements that affect them
The characteristics of a racing torque converter are all too often oversimplified into simply where it stalls. Converter stall speed can be simply defined as the greatest engine RPM that can be achieved while holding the transmission input shaft steady. The means by which the input shaft is held can vary from using the vehicle’s service brakes (foot brake) to a hydro-mechanical means to bind up the transmission so that the input shaft can not turn (transmission brake). It stands to reason that you can much more easily overpower the service brakes, causing the vehicle to start to roll at a much lower engine RPM than you could reach with a transmission brake holding the input shaft still. Thus you could have a torque converter that may stall at 5000 RPM against a transmission brake, but pushes the car out of the beam at only 3000 RPM if you were to try to foot-brake it.
The elements that affect torque converter stall are the diameter of the torque converter, the angle or pitch of the blades in the impeller, the shape and number of blades in the stator, and the clearance between the elements inside the torque converter. It is possible to achieve near identical stall speeds from two torque converters assembled from different combinations of these elements, but those torque converters would exhibit different performance characteristics.
The diameter of the torque converter and the angle or pitch of the blades in the impeller both are relative to the effective force at a given RPM that the impeller
can deliver to the turbine, as well as how much engine torque would be required to achieve that RPM with a stationary turbine (input shaft). The greater the diameter at a given blade pitch, the greater the force imparted by the exiting oil from the impeller (think longer lever or swinging a weight in a longer arc), but a greater rotational force would be required to reach the same RPM with the turbine being held stationary. Also, for the same diameter impeller, the more positive the blade pitch or shape (think of a cupped hand moving forward through water), more force will be delivered to the turbine by the oil discharged by the impeller at a given RPM, and a higher engine torque would be needed to achieve the same RPM with a fixed turbine. As this applies to racing torque converters, a desired stall speed could be achieved with a larger diameter torque converter by using an impeller of less positive or even negative blade pitch. Often, converter builders will build a converter that is too large in diameter for an application, but reach the racer’s desired stall speed by using an impeller blade angle that is less than optimum. This generally results in a converter that is inefficient (too much “converter slip” at the finish line).
The shape and number of the stator’s blades also affect stall speed, as well as the torque multiplication characteristics of the converter. This is the “black science” in torque converters. There are some rules of thumb pertaining to exit angle, etc, but every converter builder has their own theory on which stator is best for a given application.
The internal clearance of torque converter elements also affects converter stall speed efficiency. Particularly, the clearance between the impeller discharge area and the facing blades of the turbine (commonly called “deck clearance”) greatly affect these properly. Additional clearance will tend to increase stall speed at the cost of efficiency. Conversely, reduced clearance tends to lower stall speed and increase efficiency, but also tends to create more heat from fluid shear and increases the risk of these elements colliding (usually with catastrophic results). The clearance between the stator blades and the blades of the impeller also have effect on converter performance, and moving the stator closer to or further from the impeller at a given deck clearance may be something your converter builder can play with for your combination.
Although inappropriate to make generic summations, it is generally more desirable to employ an impeller of as much positive (or the least negative) blade pitch, even if it means using a converter of smaller diameter. Be aware that as racers, we often get hung up on numbers. Moreover, we frequently find ourselves comparing numbers with fellow racers. We are not racing their car or collecting their data. What counts is the performance and repeatability of your car. Example: I was at a race where two fellow racers, both using identical torque converters, were comparing converter slip percentage as recorded by their on board data recorders. One racer’s slip percentage was considerably lower than the other’s. I was asked if one of the converters might be broken, and looked at each racer’s data. The racer with the lower slip percentage was crossing the finish line at 9200 RPM, while the racer with the higher slip percentage was crossing at 7400 RPM. As the converter’s impeller is a centrifugal pump, it stands to reason that the converter would get more efficient as RPM increases. After explaining this to both racers, they understood.
Careful selection of the right components should result in a torque converter that can maximize your race car’s performance.
To Sprag or Not To Sprag
The question of whether or not to consider a “spragless” torque converter, as well as the benefits and drawbacks of them is a topic of much recent discussion and bickering. Knowing a little more now (hopefully) about how a torque converter works, let’s think about it for a moment. Under hard acceleration, the stator should be locked stationary against the sprag or roller clutch for most of, if not the entire run down the drag strip. Every combination of converter elements will vary, but generally the stator will begin to rotate when the turbine reaches 80-90% of impeller (engine) speed. It is safe to say that the stator may only rotate on the sprag or roller clutch at the top of each gear (just before the gear change), and at or approaching the finish line. On the flip side, if the sprag or roller clutch fails to hold and the stator is allowed to rotate backward (counter engine rotation), the torque multiplication disappears, as does stall speed, and the performance suffers tremendously. If the sprag is eliminated and the stator is held stationary all the time, then obviously it can not break. Some have argued that excessive heat is generated by spragless torque converters. This could be true in some applications (such as street driven vehicles), but we have not experienced this in any of the cars we have tested spragless converters in. Another argument is that the torque converter will exhibit a loss of efficiency due to the converter’s inability to change to rotary flow. This argument could have some credibility in some vehicles, however many vehicles experience the opposite; an increase in efficiency due to the assisting effect of the like rotating oil exiting the locked stator.
Recent advances in technology, materials, and manufacturing techniques have resulted in the availability of stronger sprags and roller clutches. However, the horsepower and torque levels we subject these components to have also increased. Other factors such as burnout technique, transmission condition (particularly the converter charge circuit), etc. also play a role in sprag durability.
Although certainly not for street driven vehicles, there appears not to be a significant drawback to the spragless stator in most torque converters. It really comes down to personal preference.
Summary
How do I get the Right Converter?
When ordering a torque converter for your race car, it is almost impossible to give your converter builder too much information. The things that the builder must know include but are not limited to:
•
What type of racing do you do, or how the vehicle is used (brackets, heads-up, throttle stop racing, full or pro-tree, street strip, quarter or eighth mile, etc).
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How the car is staged (foot brake, transmission brake, 2-step, throttle controller).
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Horsepower and torque characteristics of your engine. Dyno sheets are best if they are available. ALWAYS PROVIDE MEASURED RESULTS INSTEAD OF CORRECTED!! (Especially important if you race and have had the engine dyno’d at higher altitudes) ESTIMATES OF HORSEPOWER AND TORQUE USUALLY RESULT IN AN ESTIMATE OF WHAT YOU NEED IN A TORQUE CONVERTER!!!!
•
Engine specifications (displacement, compression ratio, camshaft specifications @ .050 lobe lift, induction system, etc).
•
Power adders (supercharged, nitrous assisted, turbocharged).
•
Vehicle weight
•
Tire Size
•
Axle Ratio
•
Shift RPM / Maximum RPM
•
Mid plate thickness
By supplying the correct information, you greatly enhance your (and the converter builder’s) opportunity for success in getting you the right converter for your application.

[sonicbluegt]

Thx for the info. Think it might be time for a refreshing and new converter.