Torque converter stall speed vs vehicle

[Gaugster]

Any torque converter Guru's out there?

I'm using a MY14 6R80 and OEM converter that spec'd at 2400-2700 RPM stall. I am trying to anticipate the stall speed in a completely different vehicle with a completely different motor. Currently the converter works like a standard passenger car but it's way down on power compared to the 5.0L Coyote motor that normally mates to the 6R80.

My 1968 Cougar will be at least 300lbs lighter than a MY14 Mustang with something like 200ft-lbs more of torque. Gearing would be similar.

2400-2700 RPM is the 5.0L specification. I assume this is for both the Mustang (~3675lbs) and the 2WD F-150 (~5345lbs) that both use the 6R80. Engine torque ratings are similar at 390ft-lbs and 380ft-lbs. Rear end gearing is similar too.

[FrozenMerc]

I am not a guru by any means, but I can offer another set of data points.

1962 Mercury Monterey Wagon - Broader AOD conversion behind the 352. - Stall: approx. 1800 rpms, perfect for highway cruising.
352: 325 hp, 375 ft-lbs of torque
9.5:1, Elgin 966P cam
3.6:1 9 inch rear axle, 28" tires

1975 Ford F250 Camper Special - Broader AOD conversion behind the 390.  Stall: approx. 2400 rpms, bad for highway cruising as the cruise rpm is below the stall speed, and it heats the fluid due to the excess slip.
390:  360 - 380 hp, 400 - 425 ft-lbs
9.5:1, Howards dual pattern cam.
4.11:1 Dana 60 axle, 36" tires

As far as I know, the torque convertors were near identical for these two builds.  They both were "as tight as possible" in order to have good highway manners since the AOD's lock-up feature was deleted.  The bellhousing on the AOD to FE conversion limits the convertors diameter and makes getting a low stall convertor behind a high torque motor (or heavy vehicle) difficult.

[Joe-JDC]

Usually, and I say "Usually", the more torque an engine makes the higher the converter will stall.  There are always exceptions.  You put your foot on the brakes and hold the engine against the converter until the vehicle either moves or stalls(stops increasing rpm).  More torque will overrun the converter to a higher engine rpm(stall).  Joe-JDC

[Falcon67]

Just FWIW - the brakes, etc have more to do with "foot brake stall" than the converter.  I run a race converter, 10", that flashes to 4600.  If I foot stall it, the car creeps at 2600 RPM.  The dragster would push about 3000, converter flash is 6000.  Point being that rating a converter + engine torque with brakes is useless. 

Yes - the more torque applied to a converter, the higher the stall within limits.  In my experience it's not a great amount, maybe 200~400 RPM depending.  The 10" example flashes 4600 against a 460 HP 355 engine and 4400 behind a 300 HP 302.  I'll expect the 6K unit from behind a 393 to flash around 6200~6400 behind the new 427.  That's a 9" and it's maxed on stall, per the builder.  If I need any more, I'd have to change to an 8". 

I'll also add that having worked with an expert for several years, there is certainly a "black art" to converters.  Especially in race cars there is sometimes a bunch of "try this" rinse and repeat to get it right.  Most times my guy nails it right out.  The 4600 again - it used to be about a 4K unit.  Crashed a C4 so I took it in for a checkup.  Michael asks about the current car config, how it's running, 60's, etc.  I tell him it's perfect, leaves like a scalded cat and pulls hard on top.  I go pick it up "I tweaked it a bit".  What?  Put it in - .3 ET and 2 MPH faster 1/8 mile  Well, OK then. 

[Gaugster]

Great insight Guys! At this stage the motor, in particular the camshaft hasn't been sorted out yet. Since I am not purchasing a converter I can't access the folks familar with this "Black Art". I just want to avoid communicating a bogus stall specification. FrozenMerc has some good examples. It's logical that the lower (2400 RPM) range of the specification applies to the lighter Mustang whereas the higher (2700 RPM) range is for the heavier F-150. Not a huge difference in stall even though the vehicles are very different. Weight, tire diameter, gear ratio, foot brake vs trans brake etc.... all play a role. The Quick 6 transmission controller I use does have a trans brake option but I'm a long ways away from dealing with that.

Oh, and stall speed is just one of many variables in camshaft design.... lol

[cleandan]

A possible mental visual aide for your converter question.
Think of your converter as a rubber band.
Think of your engines power as your hand.
Think of the vehicle, including the amount of traction, as the weight.

Place the weight on a line drawn on your bench.
Draw another line that you must cross in order to finish the test, but not too far from the start/weight line.
Put the rubber band around the weight and stretch it just tight enough so it stays on the weight without falling down....Assume it all stays together for demo purposes.

Now pull the rubber band slowly to begin stretching it until it just begins to move the weight and note the distance the rubber band stretched.
This represents the stall speed.

Reset everything and once again pull the rubber band, but this time as hard as you can, and note the distance of stretch at the point the weight moved.
This represents the flash stall speed.

As you can imagine, changes in the weight will alter the stall speed.
The "weight" must include the rolling resitance, traction, and suspension action that all come together to make the final result the converter sees when moving the vehicle.

The way the converter sees the power makes a lot of difference too, more horsepower, causing the engine to reach higher torque faster, along with higher rolling resistance and traction will cause a higher stall.

In your instance you have a few things to consider with the combo.
I assume you have better traction than the tires on the original modern vehicle the engine came from...The modern performance rubber bands on rims are great for corners, but not so much for off line traction.
Traction is higher....more stall.
Weight is lower.....less stall.
Suspension may be equal in weight transfer, but unless specifically engineered it is likely less than the modern car....less stall, but likely not by enough to matter.

I would guess the stall in your lighter, but better traction vehicle, will realize a higher stall, but probably not enough to make much difference unless tuning for the ultimate race specific performance.

[Falcon67]

Yep!  When I need a cam now, I call Brent.  Give him the best info I can but usually he knows what I'm messing with because he sent the parts.  I let him pick the profile for the engine, then I take that info to Michael at Freakshow and a converter is born.  Or tweaked.  Working with a stocker you'd at least have some decent ballpark that you can target along with the other parameters.  They all work together. 

Having played with this stuff for so long, taking time to deal with this minutia is the key to having the best you can get.  It separates you from other "catalog" builds pretty quick.  The hassle time is huge, lots of testing and trying but when it's dialed in and running well it's guaranteed all worth it.   I worked close with the Freakshow and Lykins on the old dragster motor.  I was shooting for a decent low 6.xx in the 1/8.  Reall small motor by Super Pro standards at 393 CI, parked with all these 598 BBCs and such.  4th checkout pass lifted at 500' and clocked 5.97, wellllll how about that.  When it went 5.3x at 126 all the BBC people stopped laughing and pointing. 

[Gaugster]

Thanks again! The analogies are helpful. Brent is for sure getting a call when I need a camshaft. By then I should have figured out and installed all of the chassis upgrades which are this winter's focus. Currently working to confirm if a 275/50/17 (28") drag radial will fit and tuck up nice in the Cougar. Then I'll be able to finalize the gear ratio. It's looking like a 3.70:1 with 28" tall tires. It's rewarding to do as much up front research as possible when matching all these items together. Having the 6 speed and it's converter already sort of changes the order of things. It is was it is....

[Gaugster]

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