The Mysteries behind the Torque Converter Revealed: Part 6

Now we have covered the engines set up in part 1 In part 2 we have started on how the engine breaths and how that can affect the torque converter. Then in Post number 3 we checked out the heads and exhaust to finish up how the engine breaths. In post number 4 we talked about actual gear ratio Vs run out of the tire and how that affects the torque converter. In part 5 we touched on the cam and its effect on the converter. If you missed the earlier posts, take a few minutes to catch up and go through them. We will be putting all of this information together with a test set up. We will also explain the differences in each item showing how everything effects the stall in the torque converter.

Our test vehicle is a 1987 G body Oldsmobile 442.

Actual curb or shipping test weight of 3,599 Lbs.

Transmission; It had been replaced with a TH350

350 cubic inches

650 cfm carburetor

No spacer plate between the carb & intake

Stock intake

Cast Iron heads

1.94 intake valve

1.50 exhaust valve

Manifolds

Y pipe to a single muffler, split back to 2 pipes out the rear of the car

3.42 gears

215/65/15 tires that measures out to be 26.00 inches tall in diameter

.403 intake / .415 exhaust

.202 intake duration / .206 exhaust duration @ .050 lift

.115 lobe separation

With this car we had run several “Foot Break” tests. This is where we held the brake pedal while pressing the gas pedal to see how high the RPM will go while forcing the car to stand still with the factory torque converter. The first test that we performed was a test on back pressure. We had heard that the exhaust was very restrictive on this car. We unbolted the exhaust at the manifolds. This means there was no back pressure or restrictive mufflers / catalytic converters. The only back pressure was the size on the exhaust port diameters from the cast manifolds and size of the collector where the exhaust manifold meets the exhaust pipes. In this Foot Brake test with the factory torque converter we were only able to achieve 1,450 RPM. When we tested the car with the exhaust system bolted up and in place we were able to get the RPM’s just over 1,800 RPM. (Remember the torque converter’s stall come from the Fluid pressure Vs. the Resistance against moving the vehicle). There is a difference of 350 RPM less than what it was from the factory set up. This is because there is less back pressure. That lack back pressure is less resistance against the motor and will begin to move the car at an earlier RPM. See Post 3 in the differences in exhaust back pressure. All of these different types of exhaust systems will offer an array of outputs in combination with the rest of the engines set-up & weight of the vehicle. Open headers will need the most amount of RPM stall to properly support the lack of back pressure and so on.

Another test that we performed on this vehicle was an intake swap out. We had a GM performance intake that was designed to operate between Idle to 5,300 RPM & went to a 1,500 to 6,500 RPM Dual Plane type intake for this test. When we did the foot brake test with this one change to the vehicle we were only able to get just under 1,350 RPM. The taller / longer runners on the intake changes the torque curves to that extent. The taller intake from 600 to 1,100 RPM the torque output from the motor is less than factory. From 1,100 to 1,600 RPM it is about the same as factory output. You can see the torque begin to increase above 1,600 RPM. The weaker torque at idle is what causes the lower effect in the factory 12 inch torque converter. (Remember that the larger 12 inch diameter torque converter will be effected the most by each change made to the set-up because the fluid pressure is higher) Each step up in RPM operating range of the intake will require more stall to help overcome the bottom end of the RPM range and support the higher RPM rev. The higher the RPM the intake can reach, will also weaken the bottom end of the RPM range or have less torque to support that stall. Like the exhaust, you can also use spacer plates for slight adjustments for where the torque curve begins with the air flow. A 4 hole spacer plate is also known as a re-stricter plate. It will limit the amount of air coming into the intake. Whereas the open spacer plate can add on to the higher RMP as much as an additional 400 RPM. Be careful of this because it will also make the bottom end of the RPM range weaker and may require more stall.

We kept the intake and put 1 & 5/8ths inch headers on the car with the 2.5 inch collector. We did run 2.5 inch exhaust pipe with true duals. We had some kind of short generic turbo muffler. At this time the stall in the torque converter effecting the idle when we put the car in gear. We also now had noticed a bog in the takeoff or launch of the car. The car began to pick up momentum when the motors RPM revs got to 2,000 RPM. In order to fix this problem the torque converter needs to be changed. This does not have to be a high stall torque converter. By going to an 11 inch torque converter at this stage would fix these issues and allow the stall to get back between 1,800 to 2,000 RPM. But, we wanted more take off in this car so we added a 3.73 rear gear ratio. The ability to move this car became much easier with a lower gear ratio. With a 215 / 65 / 15 size tire which is 26 inches in height. Which gives us a true run out with the gear. For every 1.49 inches over 26 inches you can take 1 whole gear set out for your final drive output. If went with a 27.5 in tall tire, this would give us a final out put equal to a 3.55 gear ratio. The opposite is true also, if you have a 24.5 inch tall tire with that same 3.73 rear gear the run out will give you a 3.90 gear output. The set-up we have will tighten or lower the stall in the torque converter again due to less resistance in moving the car. With the gear change however, the 11 inch diameter converter will have to have a lot of work done to it to keep the stall in that same 1,800 to 2,000 RPM range. If the converter is stalled too high for its diameter the torque converter become inefficient and sloppy. This is known as slipping because the torque multiplication drops. It will also create more heat. You will lose the pull or feel of torque the car does not pull as good. That is another reason it is so important to get the correct torque converter to match your set-up. Even if a step over stock cam is introduced in to this set-up then a 10 inch converter will be needed to have a decent idle and take off. This is not a racing style or set-up. This can be driven everyday on the street with good performance.  You also must have enough gear ratio to support the stall at the same time. For example; with the set-up above, if we were traveling at 60 mph our RPM revs will be at 3,000. I could match this with a higher stall 10 inch converter. That would actually foot-break at 2,600 RPM. Flash is to quickly mash the gas peddle to the floor from a stand still. The engines RPM will get ahead of the amount of fluid being pumped into the torque converter. This will give anywhere from 300 to 600 more RPM at launch.  This is not a subjective method of picking a stall. We must have all of the information and it must be correct. If we just guess or assume, we may as well throw darts in the dark. (I did have a guy tell me that he had a step over stock cam. After talking with them I then found out that the cam had a .714 lift on the intake. That is a very large lift for any car) NOTE: There are low stall 10 inch torque converters that would match up with this set-up. We do build a low stall 10 inch converter that has an advertised stall of 2,400 to 2,800 RPM that can actually stall about 2,000 RPM with the right set-up. With the set-up before a cam is added, this torque converter will stall 2,400 to 2,600 RPM in the car. The most important question to ask is how the vehicle is going to be used or driven. You can not assume anything!

   WARNING: Keep your Transmission Cool!

Even OEM cars are under cooled in order to maximize gas mileage.

• 14 million transmissions fail every year.
• 9 out of 10 transmissions fail because of heat!
• Heat causes the fluid to break down prematurely and wears out internal components, Bearings, Clutches, & the Seals.
• Every 20odrop in transmission temperature can double the life of the transmission!
• More power equals even more stress & heat in the transmission.
• Must use the fluids designed for that transmission or failure is imamate.
• Shift kits are better for the life of the transmission. The slower shifts in an OEM set-up makes the transmission slip which creates heat. Improving the quickness of the shift can extend the life of the transmission. (With a stall converter the transmission can be shifting faster and you not feel it. The slip in the stall will absorb the shock of the shift.
• Transmission cooler built into the radiator warms the transmission fluid up fast so that you will get better fuel mileage. There has to be a 47.9 degree temperature difference for good cooling properties to be there. In some cases, I suggest removal from the radiator and use external transmission coolers in front of the radiator.

Using the combined knowledge of all of these posts on linked in will help you to understand that actual stall range is variable and will directly depend on the entire setup of the vehicle.  All of the information about the vehicle is needed in order to match the set-up. The torque converter is by far the most complicated component you can purchase for a vehicle (Engine set-up Vs. Weight of the vehicle Vs. Rear gear with the rollout of the tire).  If you do not give all of the information or asked all of the info. There is a good chance that you might not get the correct torque converter.

Do not forget the most important piece of the puzzle. How YOU want to drive the car.

Important; Make sure that all RPM ranges match up to how you want to drive the car. The intake & the cams RPM should be the same or real close. (like 1500 to 6500 RPM for the intake & 2000 to 6500 RPM for the cam) There are thousands of different set-ups that can be achieved so do your homework on what it is you are looking to get out of it.

For more information and videos on this subject, please visit our   website www.accperformance.com.  We have a method of matching up stalls that will pinpoint the stall range of your application with an accuracy of 99%. Let us help you pick the correct stall for your application and most importantly for how you want to drive your vehicle.