Sticky Worthy: Torque Converter Stall-Torque Ratio (STR) Guide
03-27-2013, 10:05 AM
#1
TECH Fanatic
Thread Starter
Sticky Worthy: Torque Converter Stall-Torque Ratio (STR) Guide Great write-up on torque converters, taking into consideration stall speed and STR. Obviously gear plays a part as well but that is external to the actual converter itself. Anywho:
There are a lot of questions about torque converters, or "stalls", especially when picking out an aftermarket high performance unit. One of the major players in this decision is how the converter acts during driving. "Loose" vs "tight" converters and what STR (Stall to Torque Ratio) to get etc. Please feel free to correct me, call me out on bad info or whatever else. I'd like people to be armed with as much info as possible as it expands the enjoyability and results of the hobby
I'm going to come at this from a different angle. Instead of blabbing on about all the technical parts inside the converter that make this stuff happen, I'd rather just explain it in average car guy terms. "What does this mean to me?" style.
Generally, the higher the STR number the more it multiplies torque at the starting line or from a dig at a sacrifice of top end efficiency. The lower the STR, the more it multiplies torque over a longer period of time/longer distance at a sacrifice of short term large torque multiplication. Think of a high STR as a shell of buckshot that hits harder close range/hits all at once off the line and a lower STR like a 9mm that hits decent but effective at a longer range. So if dig racing or strip launching is important to you a higher STR will benefit, if rolls or trap speeds are more important to you than a lower STR will fit. STR range is usually 1.8-2.5.
High STR can also make the car hard to hook up. This is how it works in the first few feet:
Lets say we make 450 ft/lbs of torque at the flywheel @ 4,200 RPM. We add in a 4,200 stall, one with a tight/low 1.8 STR and one with a loose/high 2.5 STR. The STR becomes a mulitplier:
450 x 1.8 = 810 ft/lbs of torque at the trans input shaft when the car flash stalls to 4,200 RPM from a dead stop.
450 x 2.5 = 1,125 ft/lbs of torque at the trans input shaft when the car flash stalls to 4,200 RPM from a dead stop.
See why the higher STR could be harder to hook up consistently? Like stated, the other difference is that the lower STR verter will keep that torque multiplied for a longer time than the big STR. So its all about where you want your power and where you use it the most. If you have trouble hooking up for a reason you cant solve with DRs or suspension, maybe consider a lower STR to get the car off the line.
How about the guy who wants a "driveability" converter in the 3,200-3,600 range that hooks up decent? He may want to still consider a lower STR depending on where his cam makes its power. I.E. lets say we have a 3,400 stall but a big cam that starts the meat of its power band around 4,200-4,400. In that case a lower STR (like 1.8) will still multiply some torque by the time it gets to that engine speed where as a big STR (2.5) combo'd on that same cam will be less efficient and have far less multiplication by the time the car hits that engine speed.
Nitrous and Boost are also better mated to a lower STR converter. You'll often here people saying not to run too large of an STR with nitrous because it will "blow through" the converter. The big STR verters are simply too loose and nitrous would be wasted through the converters slipping action. I.E your going to become good friends with the rev limiter. As far as boost, you want it to build as much pressure as possible in the shortest amount of time so you dont want the engine to flash up so fast before it has a chance to build boost and boosted cars are usually top end cars and low STR verters are better suited for that (once again because of the torque multiplication over a longer period of time). Also boosted applications make a ton of low end torque and a high STR converter could aggravate a traction issue.
Shift extension is another variable to consider. A higher STR converter will give you a higher shift extension (the point your engine RPMs fall to after a shift). A higher shift extension may be more beneficial to those with a monster cam that builds all of its power up high. Example: My 2.5 converters shift extension right now is 5,000RPM. Meaning, when the car shifts the RPMs only fall to 5k and climb back up. Technically, my car is a good example of a mis-match currently since it has the stock cam. However, I spec'd the converter for a big 240+ duration can and dont expect to have the stocker in there forever.
Another thing to consider is high STR converters get their high multiplication from slippage. Slippage causes heat. Heat causes 90% of automatics to fail. I wouldn't recommend a high STR converter for a DD car. Example: I have a ~2.5 STR loose 4,200 stall in my bolt on Goat. Built trans, Derale deep pan w/ cooling tubes, stainless braided -6AN trans lines going to a top of the line Derale Atomic Cool 400cfm fan equipped trans cooler and DEX VI (better heat tolerance than DEX III). My trans cooler fan is auto on @ 180'F and off @ 170'F and lets just say once the fluid hits 180 (and it will) I never hear the fan turn off. Its not uncommon for me to see 200'F temps cruising around town not even hammering on it. Comparatively, my buddies '00 Camaro that is lighter, has a tight 1.8 STR 3,400 converter with a basic B&M supercooler as his only cooling mod sees 170'-175'F in 100'F ambient temps all day long.
So after all that how is a high STR even good and why do people use them? Because they simply PUNISH the street/strip in an all motor scenario from a dig. The high STR converters simply out 60ft the tighter converters all day long for N/A all motor setups. Take my car for example. Its just a bolt on car with maybe ~370whp if I'm lucky. 3,770 race weight, 275 DRs on stock 17s and stock suspension and I routinely pound out high 1.6s and low 1.7 60fts. ROUTINELY. Its not a fluke, nor a "great launch I had one day!" or any of that, its pretty much every time I launch track or street, I run record bolt on GTO stock suspension 60fts. When I blow the tires through the entire first gear I still get 1.9 60fts. This is all in +1,500 to +3000 DA, I could only imagine if I had some of that -1,800 DA east coast goodness.....but I digress. Its all due to the brutal torque multiplication off the line, plain and simple. High STR/stall speed cars are also a blast to drive. It will make your car VERY LOUD if you have a loud exhaust as it is going to be slipping all over the place and causing higher RPMs at lower vehicle speeds. I.E. Off the line and all the way till 4th gear lock up, around 20% throttle my car sits at 3K and sounds like one long gear where you cant even hear or tell if it shifted. People love it. Out of all the bad *** cars at shows/meets mine gets noticed all the time as I roll into the parking lot and the car is at 2,500+ RPM but doing 5MPH as I lumber it into place. Like to rev at people but have an auto? Big STR/stall speed is your ticket. I can throttle pop in gear at a stop a good 500-700 RPM with the car barely even budging and cruising at 40MPH unlocked I can usually rev about 1,500-1,600 RPM without the car moving as well (this will vary with gear ratio, vehicle speed and stall speed...etc). Like to do burnouts? Big STR verters will HAMMER the tires. You've never seen the limiter hit so quick. Learning how to launch a big STR car is a feat in itself. But my point is, with a big, loose converter an auto car is anything but boring to drive.
However, if you have a high STR on a low stall speed converter (2,400-3,200) you probably wont notice much "looseness" in it due to the low stall speed. Also, "loose" and "tight" are subjective like driveability. People have different definitions for what is loose, tight, too big, too small...etc. The best thing to do is try and find a friend with a stalled auto and get a ride and experience the differences.
How do the mfgs figure STR? A good converter shop can give you a close estimate of your STR but actual STR is something only very few builders have access too. Precision Industries claims to have the capability. Other than that it takes a machine about the length of a building and other than PI stating they have a machine, I'm pretty sure only the major auto mfgs have the others. The formula for STR is: exact output torque ÷ exact input torque = STR. Like I said, a good shop should be able to ballpark your stall fairly close. Some may even tell you an exact number but I know Circle D doesn't guarantee a number they just give you their closest estimation. Converter mfgs control STR by modifying the geometry of the turbine and stator blades inside the converter.
So there you have it. Lots of stuff to consider when buying a torque converter for your combo. All the good converter shops know this stuff and your going to want to listen to what your converter builder has to say about your setup. At least now you'll be armed with a little more info to ask them about and I hope you have a better understanding of why they are telling you that you need a certain converter or not. Remember not to take any of my info as gospel and to communicate your goals FULLY to your converter builder as far as driveability expectations, 60ft, trap...etc as the good builders know whats best for your combo. If anyone has anything to add then please do. Also, if anyone has anything to dispute about what I said, please lets hear it so I can edit the post or add in your experience/knowledge. Im a big fan of the spread of good information on this board so I welcome all criticism. Thanks for reading.
There are a lot of questions about torque converters, or "stalls", especially when picking out an aftermarket high performance unit. One of the major players in this decision is how the converter acts during driving. "Loose" vs "tight" converters and what STR (Stall to Torque Ratio) to get etc. Please feel free to correct me, call me out on bad info or whatever else. I'd like people to be armed with as much info as possible as it expands the enjoyability and results of the hobby
I'm going to come at this from a different angle. Instead of blabbing on about all the technical parts inside the converter that make this stuff happen, I'd rather just explain it in average car guy terms. "What does this mean to me?" style.
Generally, the higher the STR number the more it multiplies torque at the starting line or from a dig at a sacrifice of top end efficiency. The lower the STR, the more it multiplies torque over a longer period of time/longer distance at a sacrifice of short term large torque multiplication. Think of a high STR as a shell of buckshot that hits harder close range/hits all at once off the line and a lower STR like a 9mm that hits decent but effective at a longer range. So if dig racing or strip launching is important to you a higher STR will benefit, if rolls or trap speeds are more important to you than a lower STR will fit. STR range is usually 1.8-2.5.
High STR can also make the car hard to hook up. This is how it works in the first few feet:
Lets say we make 450 ft/lbs of torque at the flywheel @ 4,200 RPM. We add in a 4,200 stall, one with a tight/low 1.8 STR and one with a loose/high 2.5 STR. The STR becomes a mulitplier:
450 x 1.8 = 810 ft/lbs of torque at the trans input shaft when the car flash stalls to 4,200 RPM from a dead stop.
450 x 2.5 = 1,125 ft/lbs of torque at the trans input shaft when the car flash stalls to 4,200 RPM from a dead stop.
See why the higher STR could be harder to hook up consistently? Like stated, the other difference is that the lower STR verter will keep that torque multiplied for a longer time than the big STR. So its all about where you want your power and where you use it the most. If you have trouble hooking up for a reason you cant solve with DRs or suspension, maybe consider a lower STR to get the car off the line.
How about the guy who wants a "driveability" converter in the 3,200-3,600 range that hooks up decent? He may want to still consider a lower STR depending on where his cam makes its power. I.E. lets say we have a 3,400 stall but a big cam that starts the meat of its power band around 4,200-4,400. In that case a lower STR (like 1.8) will still multiply some torque by the time it gets to that engine speed where as a big STR (2.5) combo'd on that same cam will be less efficient and have far less multiplication by the time the car hits that engine speed.
Nitrous and Boost are also better mated to a lower STR converter. You'll often here people saying not to run too large of an STR with nitrous because it will "blow through" the converter. The big STR verters are simply too loose and nitrous would be wasted through the converters slipping action. I.E your going to become good friends with the rev limiter. As far as boost, you want it to build as much pressure as possible in the shortest amount of time so you dont want the engine to flash up so fast before it has a chance to build boost and boosted cars are usually top end cars and low STR verters are better suited for that (once again because of the torque multiplication over a longer period of time). Also boosted applications make a ton of low end torque and a high STR converter could aggravate a traction issue.
Shift extension is another variable to consider. A higher STR converter will give you a higher shift extension (the point your engine RPMs fall to after a shift). A higher shift extension may be more beneficial to those with a monster cam that builds all of its power up high. Example: My 2.5 converters shift extension right now is 5,000RPM. Meaning, when the car shifts the RPMs only fall to 5k and climb back up. Technically, my car is a good example of a mis-match currently since it has the stock cam. However, I spec'd the converter for a big 240+ duration can and dont expect to have the stocker in there forever.
Another thing to consider is high STR converters get their high multiplication from slippage. Slippage causes heat. Heat causes 90% of automatics to fail. I wouldn't recommend a high STR converter for a DD car. Example: I have a ~2.5 STR loose 4,200 stall in my bolt on Goat. Built trans, Derale deep pan w/ cooling tubes, stainless braided -6AN trans lines going to a top of the line Derale Atomic Cool 400cfm fan equipped trans cooler and DEX VI (better heat tolerance than DEX III). My trans cooler fan is auto on @ 180'F and off @ 170'F and lets just say once the fluid hits 180 (and it will) I never hear the fan turn off. Its not uncommon for me to see 200'F temps cruising around town not even hammering on it. Comparatively, my buddies '00 Camaro that is lighter, has a tight 1.8 STR 3,400 converter with a basic B&M supercooler as his only cooling mod sees 170'-175'F in 100'F ambient temps all day long.
So after all that how is a high STR even good and why do people use them? Because they simply PUNISH the street/strip in an all motor scenario from a dig. The high STR converters simply out 60ft the tighter converters all day long for N/A all motor setups. Take my car for example. Its just a bolt on car with maybe ~370whp if I'm lucky. 3,770 race weight, 275 DRs on stock 17s and stock suspension and I routinely pound out high 1.6s and low 1.7 60fts. ROUTINELY. Its not a fluke, nor a "great launch I had one day!" or any of that, its pretty much every time I launch track or street, I run record bolt on GTO stock suspension 60fts. When I blow the tires through the entire first gear I still get 1.9 60fts. This is all in +1,500 to +3000 DA, I could only imagine if I had some of that -1,800 DA east coast goodness.....but I digress. Its all due to the brutal torque multiplication off the line, plain and simple. High STR/stall speed cars are also a blast to drive. It will make your car VERY LOUD if you have a loud exhaust as it is going to be slipping all over the place and causing higher RPMs at lower vehicle speeds. I.E. Off the line and all the way till 4th gear lock up, around 20% throttle my car sits at 3K and sounds like one long gear where you cant even hear or tell if it shifted. People love it. Out of all the bad *** cars at shows/meets mine gets noticed all the time as I roll into the parking lot and the car is at 2,500+ RPM but doing 5MPH as I lumber it into place. Like to rev at people but have an auto? Big STR/stall speed is your ticket. I can throttle pop in gear at a stop a good 500-700 RPM with the car barely even budging and cruising at 40MPH unlocked I can usually rev about 1,500-1,600 RPM without the car moving as well (this will vary with gear ratio, vehicle speed and stall speed...etc). Like to do burnouts? Big STR verters will HAMMER the tires. You've never seen the limiter hit so quick. Learning how to launch a big STR car is a feat in itself. But my point is, with a big, loose converter an auto car is anything but boring to drive.
However, if you have a high STR on a low stall speed converter (2,400-3,200) you probably wont notice much "looseness" in it due to the low stall speed. Also, "loose" and "tight" are subjective like driveability. People have different definitions for what is loose, tight, too big, too small...etc. The best thing to do is try and find a friend with a stalled auto and get a ride and experience the differences.
How do the mfgs figure STR? A good converter shop can give you a close estimate of your STR but actual STR is something only very few builders have access too. Precision Industries claims to have the capability. Other than that it takes a machine about the length of a building and other than PI stating they have a machine, I'm pretty sure only the major auto mfgs have the others. The formula for STR is: exact output torque ÷ exact input torque = STR. Like I said, a good shop should be able to ballpark your stall fairly close. Some may even tell you an exact number but I know Circle D doesn't guarantee a number they just give you their closest estimation. Converter mfgs control STR by modifying the geometry of the turbine and stator blades inside the converter.
So there you have it. Lots of stuff to consider when buying a torque converter for your combo. All the good converter shops know this stuff and your going to want to listen to what your converter builder has to say about your setup. At least now you'll be armed with a little more info to ask them about and I hope you have a better understanding of why they are telling you that you need a certain converter or not. Remember not to take any of my info as gospel and to communicate your goals FULLY to your converter builder as far as driveability expectations, 60ft, trap...etc as the good builders know whats best for your combo. If anyone has anything to add then please do. Also, if anyone has anything to dispute about what I said, please lets hear it so I can edit the post or add in your experience/knowledge. Im a big fan of the spread of good information on this board so I welcome all criticism. Thanks for reading.
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99 Black Bird T/A (11-01-2022)
03-27-2013, 08:53 PM
#3
Moderator
WOW! Another fantastic and informative post this week about converters.
I still know 'nuthin' about converters, but now I know ten times as much 'nuthin' as before.
Thank you!
I still know 'nuthin' about converters, but now I know ten times as much 'nuthin' as before.
Thank you!
05-07-2013, 04:22 PM
#6
i asked a trans guy that has been supposedly
"building transmissions for race cars since before you were born sonny"
about this and he got a deer caught in the headlights look. was completely ******* clueless or maybe just forgot decades ago.
idk.
good reading info. thanks. explains alot
"building transmissions for race cars since before you were born sonny"
about this and he got a deer caught in the headlights look. was completely ******* clueless or maybe just forgot decades ago.
idk.good reading info. thanks. explains alot
