Torque Converter Basics


A RatStang Exclusive


Here is quick rundown on TC basics.
Number one you need to decide what you want.
Mileage and drivability or Torque and raw acceleration.
You cannot get without giving.
Number two you need to match your cam and TC stall.
You will need to know your Cams torque and Horsepower curves.
(Dyno 2000/2003 is perfect for estimating this)
For "Acceleration" you will need to determine where your peak torque occurs in relation to RPM.

Also you will need to determine where your Peak Horsepower occurs in relation to RPM.
As an example:
In RatStang's 289 and cam the Torque peaks at 5500 RPM and the Horsepower at 7000.
Horsepower will "Pull you thru" the loss of Torque. This equates into acceleration. If you have a cam with a flat torque and horsepower curve then you will not accelerate because the torque is acceleration. If you reach your torque and flatten out you will not continue to accelerate. Your engine will "Bog" at this point because you do not have torque continuing to rise.
(Yes I am going to hear about this)
Here is where the TC comes into play.
All TC's are rated at Flash Stall. This is the RPM that the TC slips to before it starts its lockup. The actual Lockup occurs appx 500 RPM lower after it Flashes to the rated RPM.
So during your shift from gear to gear you Flash to the RPM. What you want the TC to do is to Flash about 1000 RPM below your max Torque.

So in my example I would want my TC to Flash about 4000 RPM.

This way the Torque after the shift will continue to accelerate my vehicle as opposed to flattening out.
The Horsepower will carry me thru the shift until I reach my Flash again.
Now that is Pure continued acceleration.
Here is the give and take portion.
If I have a 4000 RPM Flash Stall then my actual lockup would be appx 3500 RPM.
If I am using the vehicle as a daily driver the lockup of the TC at 3500 RPM would place me at what speed while I am piddling around town? With my setup of a 3.55 rearend in 3rd gear (1:1) with a 26 inch diameter tire this lockup of the TQ (3500) does not take place until 76 mph (use Virtual Engine Calculator to figure this).
This does not equate into mileage and drivability at all.
So I must compromise.
I want my lockup to occur around 40 mph (which is the average speed I will be driving around town). For this to happen I will have two choices.
Lower my Lockup RPM with the TC which will affect my acceleration performance.
Or lower my rear-end gearing.
Or Both.
It is a compromise situation.
The best all around compromise for my example I feel would be a 4.11 rear gears and a 2800 RPM Flash Stall TC.
That places me at appx 40 mph lockup for drivability at the cost of max acceleration.

Give to Get.