Selecting the right torque converter
05-18-2010, 04:15 PM
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Selecting the right torque converter
A lil' info for those who may not know.
Adrian
Adrian
Selecting The Right Torque Converter
By Phil Parsons
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:
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.
05-19-2010, 12:33 AM
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Re: Selecting the right torque converter
Always good to see that stuff posted. Even though it might be in archives most don't look for it or even know to think about it.
Thanks
Thanks
05-21-2010, 01:29 PM
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Re: Selecting the right torque converter
A lot of meat & potato's for a stew of a tranny! 
I know.... 
Adrian
I know.... Adrian
05-22-2010, 12:17 PM
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Re: Selecting the right torque converter
Good article. I realized this years ago the first time I put 4000lbs in the bed of my truck and my stall speed went up to 7800prms 
The things you learn when driving a work truck with ever changing payloads- this is one of many.
The things you learn when driving a work truck with ever changing payloads- this is one of many.
10-16-2010, 09:37 PM
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Re: Selecting the right torque converter
So then with this info , what would be a good stall speed for a 327 with what I assume is the stock cam?
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10-16-2010, 11:31 PM
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Re: Selecting the right torque converter
The fastest car in our club has a stall converter of 4000 RPM.
He ran his best of 12.09 and 113.11 mph at LA Invasion this year, with a Yank PT4000 converter.
He ran his best of 12.09 and 113.11 mph at LA Invasion this year, with a Yank PT4000 converter.
10-17-2010, 09:22 PM
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Re: Selecting the right torque converter
Whatever rpm you will never drive under. Case in point- if you plan on driving down the freeway at 2200 rpms and you have a 3000stall covertor you will build up too much heat and kill it.
never put a stall speed higher than what cruising speed you have to drive at.
10-17-2010, 09:43 PM
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Re: Selecting the right torque converter
So should I just cruise down freeway for a while and see about what rpms I am averaging then choose one 500 or so RPMS less?
10-18-2010, 04:20 PM
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Re: Selecting the right torque converter
You guys need to brush up on new technology advancements in converters.
They are "not" the same old things.
They are "not" the same old things.
10-18-2010, 06:26 PM
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Re: Selecting the right torque converter
You're right, so don't assume......
Who around here runs a 700R4 with no lockup?
Last I checked, that's what 3rd gens come with.
So, if you haven't driven one of the new ones ...take a spin, you will be amazed.
Who around here runs a 700R4 with no lockup?
Last I checked, that's what 3rd gens come with.
So, if you haven't driven one of the new ones ...take a spin, you will be amazed.
10-18-2010, 11:08 PM
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Re: Selecting the right torque converter
Around here out of the 20 people you're probably referring to, I dunno. I know plenty of people who do though, and just because the car came with it does not mean that is what someone puts back in. Hell people still swap in powerglides, for whatever reason. I wouldnt personally run a non lockup, but plenty of people do.
10-18-2010, 11:46 PM
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Re: Selecting the right torque converter
Everyone is also assuming you are in overdrive on the freeway cruising in moderate traffic. Wrong. Tooling dowen the freeway caught in about 40-50 mph average speed in "drive" can modulate in and out of OD and lock and unlock overheating the trans with too high of a stall speed.
Last edited by Vetruck; 10-18-2010 at 11:49 PM.
10-19-2010, 10:25 AM
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Re: Selecting the right torque converter
OP's thoughts......
Re: Selecting the right torque converter
Maybe this will help
http://www.converter.cc/
Re: Selecting the right torque converter
Maybe this will help
http://www.converter.cc/
10-19-2010, 02:54 PM
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Re: Selecting the right torque converter
When I originally posted this I had 2 reasons.
1st - To provide useful functional information for those like myself who have very limited knowledge of torque converters and how they work in tranny's.
2nd - To educate myself and see the opinions of others in the know, so when I come to that time to swap my tranny I can hopefully make the correct decisions on which direction to go.
Considering there are very informed individuals in this region who have beefy TPI powerplants. I'm also aware that without the proper trans & torque conv. The car is an oversized paperweight. And with that said it would be useful to have information on how to put the 2 together for an efficient combination
Adrian
1st - To provide useful functional information for those like myself who have very limited knowledge of torque converters and how they work in tranny's.
2nd - To educate myself and see the opinions of others in the know, so when I come to that time to swap my tranny I can hopefully make the correct decisions on which direction to go.
Considering there are very informed individuals in this region who have beefy TPI powerplants. I'm also aware that without the proper trans & torque conv. The car is an oversized paperweight. And with that said it would be useful to have information on how to put the 2 together for an efficient combination
Adrian
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