You call a converter shop and ask for a price on a 3500 rpm stall, he answers with a price; you have just been scammed! 100% no ifs no buts, he is a scammer and he has just conned you. Why? Because unless he knows how much torque your motor makes and at what RPM it makes it there is no way he can tell which converter you need.
Have you been down this road before? Well if you think you haven't then you probably have but you just haven't realised it yet. Stall speeds are something that most converter builders don't even understand, let alone the people purchasing them. A torque converter of a particular diameter with a particular angle on the lid fins and a particular stator design will have a natural reaction to the torque that the engine is making.
In Australia there are very few torque converter manufacturers that have ever owned a race car, torque converter dyno or even done testing with their own vehicles. Most take a stab at what the customer needs and then worry about it if they ever return and complain. A hell of a lot of converter shops, some that even call themselves "Performance Specialists" get a marker pen and write the number that you want to see on the converter and that's their way of making a 3500 rpm stall. Sound hard to swallow? Well that's how a lot of them operate, they don't care if it works or if it maximises your performance, they just care about how much money they have made, or at least how much money they have just scammed you for!!
So I want to arm you all with some information so that you know if you have been had or not, so here goes:
Stall Speed: Most people think that Stall Speed is either how much RPM they can hold with one foot on the foot brake or how hard they have to rev the engine before the car moves; it is neither. Stall Speed is the RPM that can be achieved with the engine at wide open throttle and the input shaft locked solid. If you have Trans Brake then it's is an easy test, Tran Brake on, foot to the floor, what the engine revs to is your stall speed with that engine with that tune on that day. Remember it's the engine torque at that RPM that makes the converter go to that stall speed, if the engine makes more torque, it will stall the converter higher and visa versa. You can see this effect on turbo cars, they will go to an RPM and then as the boost increases the RPM will increase till it holds a stable boost and a stable RPM. Please don't do this for more than a couple of seconds because you will brake your converter or transmission or both. 8 seconds at 6500 RPM in a 8" converter will melt the stator, how do I know, because I have done it!
Foot Brake Stall Speed: As the name suggests, this is the stall speed that the car will hold on the brake; now there are a lot of factors that effect this, what type of brakes, what condition they are in, the diff ratio, the suspension set up, and the list goes on. As far as the converter goes, the STR or Stall Torque Ratio has more of an effect then the Stall Speed. STR's is something that most converter manufacturers don't understand, not just here in Australia but world wide, some can modify a converter to lower the STR to give you more Foot Brake Stall Speed but they don't know why they are doing what they are doing, they just know that if I do this, it will do that.
What Stall Speed do you need?: For drag cars, the idea is to keep the motor where it makes it's best average Hp, in the old days people would go on about peak torque etc, that's all B.S. Physics tells us that the higher the average H.P the quicker and faster the car will cover the quarter mile, no ifs no buts. It's simple maths. Off road cars and circut cars are different, they might need to use peak torque but in-liners want average HP. If you have an Engine dyno sheet then it's easy, look at the curve and work out where your best average is, if you don't you need to work out where it's going to be. We do this with dyno simulation software, it's not perfect but it's better than a guess. Either way the more info you have the closer to perfect it will be. I won't tell you how much RPM the average spread will be, but I will tell you that it depends on how many gears you have.
What about STR?: If you have a foot brake launched car then STR is very important, if you don't (have a foot brake launched car); than as much STR as you can get. Too much STR will pull the motor down, and you will see that in your data logger or on the tacho. Not enough STR will just make the 60' times slower than they could be.
The bottom line: If you ring a converter shop and they don't ask you a million questions your going to get scammed. What questions should they ask? I'll let you think about that, they need to know how much torque your motor makes at every RPM so what should they ask you? EVERYTHING!!!!!! After they finish they should be able to tell you what diff ratio you need, what tyre size and how quick and fast your car should go, they should also be able to pick up on flaws in your combination. If they can't then you rang the wrong shop!
Happy Racing
