Torque Converter choice / math for stall

Posted By: BDW

Torque Converter choice / math for stall - 12/05/22 01:10 AM

From my research, HP/torque, car weight, and gears all affect converter stall.
Looking for rule of thumb on what a given converter will stall when the parameters are changed.
Given the following, correct me if wrong:
- More HP/torque will increase stall speed
- Lower gears, such as a 2.73 or 3.08 gear, will increases stall
- Heavier the vehicle, the higher it will push the stall of the converter

My goal is to buy a stock rebuilt converter, $150, not interested in spending $600 or more on a "custom" converter.

Assuming gearing is a wash, my HP is up (75-100%), but weight is down (10%)
Double HP won't double the stall, but is there a rule of thumb?
Given the numbers below, I'm leaning towards the 1350 stall, and thinking/hoping it works out to ~ 2000-2500.

70 Challenger, 5.7, 518 OD/LU, 3.73 gears. Mainly a cruiser, not out for max performance
HP/Torque = 400/400
Weight = ~3700

Converter choices:
95 B3500 Van, 318
HP/Torque = 230/300
Weight = 4000
Stall 2100-2400

78 Charger, 318
HP/Torque = 140/150
Weight = 4000
High Stall 1700
Low Stall 1350

Posted By: Guitar Jones

Re: Torque Converter choice / math for stall - 12/05/22 01:38 AM

There is no rule of thumb that I am aware of. I can tell you there are limits on how much a converter will stall dictated by many variables including size. Generally though the smaller the converter, the higher the stall speed.

Posted By: Cab_Burge

Re: Torque Converter choice / math for stall - 12/05/22 02:46 AM

I hate to discourage you and not add anything positive to your quest for help but I would make double dang sure which converters will work properly with your tranny and motor (is this a 5.7 gen 3 hemiroid motor?) If it is a 5.7 Hemi motor make sure the flex plate and starters you're wanting to use will work correctly on your motor

There are a lot of things that can be learned from changing parts between years and car body styles, some of them lessons are not fun or enjoyable

You don't win if you don't try.

Have fun and keep us in the loop please

Posted By: A727Tflite

Re: Torque Converter choice / math for stall - 12/05/22 03:18 AM

Horsepower is not in the calculation.

Torque is.

Problem is you need to know what the K factors are for each converter and the torque output of the engine.

Posted By: TJP

Re: Torque Converter choice / math for stall - 12/05/22 04:09 PM

Originally Posted by Transman

Horsepower is not in the calculation.

Torque is.

Problem is you need to know what the K factors are for each converter and the torque output of the engine.

Always been like black magic to me

What does one do if they do not know the torque output of the engine?
Second question: If one does know I ASSume you would use peak torque ?
Third question: Would the stall speed be determined by the peak torque RPM?
Fourth question: How does one establish whether they will get a tight or loose stall?

Not trying to be a pest and no need to answer if the questions are "black magic" LOL

Posted By: mgoblue9798

Re: Torque Converter choice / math for stall - 12/05/22 05:58 PM

Just my two cents, but for a bucks down stockish converter I would look for one that combines the old high stall 340 stall speed with a lock up if such an animal exists. 340 is the best old school mopar comparison to the 3g hemi in terms of road manners and torque curves.

Posted By: A727Tflite

Re: Torque Converter choice / math for stall - 12/05/22 07:59 PM

Originally Posted by TJP

Originally Posted by Transman

Horsepower is not in the calculation.

Torque is.

Problem is you need to know what the K factors are for each converter and the torque output of the engine.

Always been like black magic to me

What does one do if they do not know the torque output of the engine?
Second question: If one does know I ASSume you would use peak torque ?
Third question: Would the stall speed be determined by the peak torque RPM?
Fourth question: How does one establish whether they will get a tight or loose stall?

Not trying to be a pest and no need to answer if the questions are "black magic" LOL

You can’t effectively compare two converters without knowing the K factor for those converters unless you try them both in the same car, same conditions.

You could back in to the numbers (K factor) if you had an engine with a given torque rating and tested both converters under the same conditions (weather and same car).

The more torque you make the higher the stall. Period. That’s why engineering uses K factor.

But there are other things that affect E.T. And starting line performance. Stall torque ratio is one of them.

Posted By: BDW

Re: Torque Converter choice / math for stall - 12/05/22 09:26 PM

Originally Posted by Transman

Horsepower is not in the calculation.

Torque is.

Problem is you need to know what the K factors are for each converter and the torque output of the engine.

Thx, that is exactly the info I needed! There are several sites that give the formula for K.
- The mathematical constant that decides a converter's stall speed (in rpm) is called the "K" factor,
- This relationship is expressed mathematically is: K=rpm/torque.
- Once the K factor for a particular converter is determined, the stall speed can be accurately predicted for different torque values
- K is derived from the observed stall speed of the converter divided by the square root of the applied torque.

I understand this won't be "exact", but at this point, if I get accurate numbers for the torque, it will be close enough.
I put together a spreadsheet for the calculations, and it looks like the Low Stall Charger converter fits the need.

Vehicle..........Torque...........Stall...................K
B3500............230................2200.................145.06
Charger.........150................1700.................138.80
Charger.........140................1400.................118.32

5.7.................400................ 2901................145.06
5.7.................400.................2776................138.80
5.7.................400.................2366................118.32

Any comment on the torque numbers used for the 78 318?

Thx

Posted By: A727Tflite

Re: Torque Converter choice / math for stall - 12/05/22 10:41 PM

Originally Posted by BDW

Originally Posted by Transman

Horsepower is not in the calculation.

Torque is.

Problem is you need to know what the K factors are for each converter and the torque output of the engine.

Thx, that is exactly the info I needed! There are several sites that give the formula for K.
- The mathematical constant that decides a converter's stall speed (in rpm) is called the "K" factor,
- This relationship is expressed mathematically is: K=rpm/torque.
- Once the K factor for a particular converter is determined, the stall speed can be accurately predicted for different torque values
- K is derived from the observed stall speed of the converter divided by the square root of the applied torque.

I understand this won't be "exact", but at this point, if I get accurate numbers for the torque, it will be close enough.
I put together a spreadsheet for the calculations, and it looks like the Low Stall Charger converter fits the need.

Vehicle..........Torque...........Stall...................K
B3500............230................2200.................145.06
Charger.........150................1700.................138.80
Charger.........140................1400.................118.32

5.7.................400................ 2901................145.06
5.7.................400.................2776................138.80
5.7.................400.................2366................118.32

Any comment on the torque numbers used for the 78 318?

Thx

Other variables will have a greater effect on stall than being off 5 or 10 lbs. ft. In your estimate.

Posted By: GODSCOUNTRY340

Re: Torque Converter choice / math for stall - 12/06/22 03:06 AM

No matter what the torque is you need to know what RPM it peaks at. If it peaks at 4200, get a convertor that locks up around 3500. Don't be fooled into a convertor that locks up at 4500 or higher, why have a convertor locking up after your torque has already peaked, has dropped and it still dropping. Torque is what accelerates weight, you want your convertor locked up before the peak torque RPM to take advantage of the highest torque moving your car forward.