Lets Talk torque convertors, based on Cam, Torque, Rear Gears, And Tire height

So when i switched my 47 chevy truck over to a Th400 from the M21 muncie 4 speed. I just swapped the whole running trans and stock torque convertor from my stock 73 3/4 ton truck. The small block in my 47 has quite a bit of lope to it ( im sorry im at work and i dont have the exact specs on it, nor do i remember them well enough to take a stab at it) But it makes it enough i know i should have a different torque convertor.

So what are the general rules for picking out the right amount of stall based on the Cam, Torque, Rear Gears, and Tire Height, vehicle weight?

No im not drag racing it, its just a street driver.

 

Hmmm if it's in a truck, and you want to use it for normal driving, and occasionally hauling stuff, wouldn't it be better to change the cam?

 

It all has to work together, and match what you want to do with the vehicle.

So....what do you want to use the truck for? "street driver" is pretty vague. And what is the rest of the truck built like? You do seem to understand what most of the important variables are, but you didn't tell us the numbers for your truck.

 

I have a not quite so big stinky lopey cam in my 55 with a TH400 and 2.75 gears...I cheated and put a 1500/3000 stall switch pitch converter (and pump and input shaft and stuff) in it.

 

Power range of the cam. You never want the engine pulling below resonable power range AND you want enuf stall you can idle at the lights without having to keep your foot on the throttle. Since you appear not to be race oriented and not in Kill mod anything in the 3000 RPM stall range will do a great job. Buy a brand name converter not a cheapie.
Now someone is going to tell you the car wont move till you hit 3000 qnd if you only have 6000 RPM you will only have 3000 to use. Ignore that it is the voice of innocence. The 3000 will give you UP TO 3000 stall if you want it and a good converter from a well known company will NOT slip like a giant rubber band. It will be very efficient. I use a 3800 on my street piece (1963 Dodge Stg III Max Wedge Nascar style) from Frank Lupo Dynamic Converters. If you rode with me you would not reaize that. The car drives normal and will roll along nicely at 25 to 30 MPH with no throttle in high gear. It allows the engine to idle nicely with the big cam HOWEVER if I wish I can stab the brake , bring it up to 3800 and let er go. Usually at this point my wife is screaming louder than the 295/70R 15 BFGs and complaining about a sore neck. The Launch at this point is BRUTAL!!The rest of the time though it drives like a stocker and you would never know it was any different other than allowing a 990 RPM idle in gear. Once coupling speed (Coupling speed is when all three elements in the converter are turning and is right after leaving the stop light for instance) is reached a good converter becomes an almost solid connection (3% to 4% slip which is about what a stock one does. ) A bad or a cheapie feels like it is always slipping A good one does not. Lupo is in New haven Con
ph is 302 454 something. (small block ford / big block chev is how I remember the ph number) You will be talking to Sean. Trust them should you descide to go that way. You will be happy for sure.
Don

 

Here is what Phil Parsons of Phil Parsons racing has to offer

Selecting The Right Torque Converter
By Phil Parsons
​

The torque converter (TC) is probably one the most misunderstood pieces of equipment in all of automotive. Hot rodders (even experienced ones) often brag about having a "3500 stall speed converter", without even knowing what that implies (as you will find out soon, a 3500 stall speed converter would generally be useless on any street car short of the most radical of Pro Streeters). Some even boast of having just a "stall speed converter", which displays a stunning ignorance, because ALL automatic transmission equipped vehicles have a "stall speed converter" - it's the number that makes the difference. The whole idea behind different stall speeds is to allow the car to launch at or just below the point where the engine makes the most torque. That way, the engine doesn't have to build up to the peak RPM point - all of the power is right there, on tap. If you've ever been to the drag strip, you have probably noticed that the cars rev way up before the light turns green - this is because most racing engines don't make substantial power until they are spinning over 3000 RPM. If these cars were using the stock stall speed, the tires would break loose long before the engine reached it's optimum RPM. A higher stall speed converter allows the engine to rev up to this optimum point without breaking the tires loose.​

So what is stall speed? This is another widely misunderstood term. In the simplest of definitions, stall speed is the engine RPM level at which the torque converter "locks" and overcomes whatever resistance is present to turn the wheels. This resistance is the weight of the vehicle, combined with any other factors (i.e. if you have the brakes on). The old definition of stall speed used to be the engine RPM at which the brakes can no longer hold the wheels still at full throttle. This is not exactly accurate, due to the variations in brake holding power from vehicle to vehicle. In other words, in two cars that are exactly the same weight, horsepower, etc., the one with weaker brakes will display a lower stall speed, even if it really isn't, because the brakes will lose their grip at a lower RPM. The most accurate method for determining actual stall speed on your vehicle is to launch the vehicle at full throttle, and note the rpm at which the car actually takes off (this generally requires a partner watching the tach). This will be quite low on stock vehicles - around 1500-1800 RPM, slightly higher if the engine has been modified. ​

The next question probably goes something like, "If I already know the stall speed (i.e. what was printed on the box), why would that number change in my car?" The answer is that even though all converters have a rated stall speed (based on a fixed set of torque and weight figures), there are variables that affect this figure, mainly vehicle weight and engine torque. If you are really sharp, you may have already figured out why these two variables affect the stall speed. Weight affects the stall speed because it changes the amount of resistance that the converter has to overcome. A lighter car produces a lower stall speed because the amount of resistance (weight) has been decreased. By the same token, a more powerful engine also lowers stall speed because in the simplest terms, increasing engine power has essentially the same effect as decreasing vehicle weight.​

Obviously it is extremely important to know what your vehicle's weight and peak torque is before ordering a torque converter. One of the worst mistakes in all of hot rodding is to buy a converter with a stall speed that is too high. This usually results in a car that is not only slower than it used to be, but also gets horrendous fuel economy and eats transmissions. This is because the converter is slipping all of the time, absorbing power and passing it along as heat to the rest of the transmission. If Joe X. tells you that his TPI equipped 305 Camaro has a 3500 stall speed converter, and you're reasonably sure that he's telling the truth, challenge him to a race and bet large amounts of money on it. Why? At 3500 RPM, TPI in stock trim is pretty much at the end of it's torque curve, meaning that most of the engine's usable torque is absorbed by the converter and passed along as heat. The same exact car with a stock converter would destroy Joe's car off the line, because the stock converter is designed to take advantage of the TPI's excellent low end torque output by using a stall speed of under 2000 RPM, right under the peak torque.​

Probably the most important factor to consider when selecting a torque converter is the camshaft. The connection may not seem obvious, but the fact of the matter is that the camshaft basically dictates the RPM level at which the engine will produce it's peak torque, which will in turn dictate the optimum stall speed. If your camshaft has a duration of 220-230 degrees (@ 0.050" lift) or more, you definitely want to think about a higher stall speed converter, probably about 1000 RPM over stock, because the engine will probably make peak torque at well over 2000 RPM. A general rule of thumb is that most stock small blocks (especially TPI equipped) are designed to make most of their torque at low RPM, while small blocks with high horsepower generally lack low RPM torque. Does this mean that you shouldn't bother with an aftermarket converter if you don't have a radical cam? No. Performance converters are usually designed to accelerate more aggressively than stock, so an aftermarket converter with the same stall speed rating as stock will often be more responsive than the stock unit. You just want to be very careful about the stall speed that you select.​

Here are some general guidelines for selecting a converter:​

• According to B&M, the stall speed should be rated at about 500-750 RPM under your engine's peak torque RPM. If you don't know this figure, be conservative in your estimate. You don't want to end up with a converter that has too high of a stall speed. Don't be too conservative, though - it is possible to get a converter with too low of a stall speed, which will have roughly the same effect as too high of a stall speed.​
• Know your camshaft specifications. If your cam has less than 220 degrees duration (@ 0.050" lift), which most street machines do, you make most of your torque down low in the RPM range, and you probably won't need more than a 2500 RPM stall speed, if even that much.​
• Have a good idea of your vehicle's weight. Remember, lighter vehicles will lower the rated stall speed; heavier vehicles will have the opposite effect.​
• High stall converters generate a lot of extra heat. The installation of an external transmission cooler is mandatory with a higher than stock stall speed converter. Actually, you should have one in there anyway. Heat is the number one killer of transmissions - 85% of all trannies die because of inadequate cooling.​

The best advice I can give anyone buying a converter is to talk to the manufacturer. They know torque converters better than anybody, and can help you to select exactly the right converter for your combination. This article was designed to give you some insight into what is needed to determine the right converter, and to make you familiar with the terms and what you need to know to speak intelligently with the experts. ​

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I have wheeled and dealed in enough of this stuff to come to the conclusion that torque converter salesmen have more bullshit than a congressman.
They want to know tire size, cam size, car weight, motor size, what color your car is and any other piece of important sounding crap that they can come up with and then..guess what! they got just the converter already made and sittin on his desk! And when you get it and go thru the hassle of putting it in and it barks the tire when you drop it in gear so that when you call back he says...gee, you don't have enough torque to make the converter work! It is your fault!
Heres' what i do, i sell a bunch of them, i lie know that they will lie and they balance out. You need a converter so you can get thru a traffic light, right? start and put it in gear without barking the tires? So you need something around 1800ish rpm, very mild. So order one from Boss Hog, get thier converter for around 2200, 2300 rpm and it will work just right. If you order thier 1800ish rpm converter it'll stall out at about stock - they lie remember? If they ask what color car or how many rear tires tell them to send you what you asked for and leave thier calculators alone.
Boss Hog really is very good and affordable, i've been selling them for 10+ years and have had just one problem that they took care of without a murmur. I wouldn't use them for a race car.
I can tell you stories all day long about converter people.