1970 Hi-performance TH400 questions [Archive]

Aug 28th, 10, 7:47 PM

some torque converter, stall guidelines info .......

Stall is measured in three different ways. TRUE, BRAKE and FLASH Stall.

True stall, is measured with the transmission locked (transbrake) and the drive shaft still. The throttle is then fully depressed and held to obtain the holding RPM of the torque converter. This is the true stall speed.

Foot brake stall, is attained with the transmission in gear and the brakes depressed fully. Now slowly apply the accelerator. The RPM the engine reaches the moment before the tires spin is known as the brake stall. Brake stall is not a consistent way to measure your stall converter and has many variables. This method will be lower then true stall; often by at least 500 RPM

Flash stall, is checked while the car is in motion. While rolling at a slow speed fully depress the throttle to see where the RPM will flash (or peak) to. This method will not be true if tires spin or the car is rolling too fast. Due to the turbine already turning in the torque converter (it is the part the input shaft is splined into) you end up with less torque multiplication. This will give you a lower stall reading than the same converter's true stall rating; often by at least 200 RPM and sometimes quite a bit more depending on the converter design.

STALL SPEED
Torque converter stall, is a commonly used term and is commonly misunderstood. Stall is the speed at which the converter will hold the engine speed and not allow further gain (i.e., the engine "stalls"). The key word here is engine. The speed at which stall occurs with a given converter is a function of engine peak torque. It is clear that a mildly modified big block engine will produce more torque than a mildly modified small block engine thus creating more "Stall" with the same torque converter. When comparing stall speeds it is important to account for the engine that drives it.

True converter stall, can best be determined when a Transbrake is used. Testing for stall value by locking the wheel brakes generally does not produce a true stall value because the engine power can often cause wheel turn by overpowering the brakes. This is known as "Foot-Brake Stall." The speed determined by this method should be identified as such when discussing stall speed determination. Launching at full throttle and observing the peak RPM of the engine just as the car starts its motion forward at launch determine "Flash-Stall". This is a more accurate test. Stall speed test will have better results with a "crisp, responsive, nicely tuned engine."

Selection of the right stall speed for your street vehicle should be matched to the engine's peak torque, engine's torque curve shape and vehicle weight and general use.

In general street applications, the best matched stall speed for your engine would be one that would flash close to the "peak torque range" and then lockup efficiently at part throttle or highway driving. When selecting stall speed without having prior experience to go by, it is better to conservatively estimate the engine torque than it is to over estimate it. If you over estimate the torque output you will have a converter with a stall speed too low, making your car slow off the line and have slow ET. A properly selected stall speed will give you better launch and better ET. You can see why it is important to consult with professionals prior to making a stall speed selection.

Within the converter, stall speed is balanced off against inefficiency after launch. Getting desired stall at the expense of performance after launch is just as costly as improper stall speed to begin. The optimum converter has careful selection and design of changes, specifications and tolerances to the impeller, turbine and stator.

First, the converter must have slippage otherwise your car wouldn't idle in gear ... Basically the stall speed is what rpm can the engine's torque make it slip too.

Even a stock converter has a 1,500 - 1,600 stall speed so that you can sit at the traffic light with your foot on the brake.

The idea behind the higher stall speed is to not load the engine until it is at a higher rpm level.
This provides less drag on the engine at idle, and lower rpms.

Stall speed, since it is based on carefully engineered turbine and stator fin angles and spacing inside the converter as well as voodoo magic also varies with rpm and engine characteristics.

The converter will still move the car, at lower rpms but will have a lot of slippage below the stall rpm. When it gets to its stall speed, the fluid flow is such that slippage becomes minimum.

The idea behind a high stall coverter is to let the engine freely and quickly accelerate into the rpm range that it makes power in, then couple it to the drivetrain. Since you get slippage, and due to the fluid flow setup, at the lower part of the stall are, there is torque multiplication happening in the converter also. This gives you the hit or launch ...

The problem with more stall and street driving is that at cruise RPM, if the stall is above it, it will generate more heat. That being said .... for most acceleration ... the stall should be at peak torque (+-).

A 2,500 stall converter does not feel a hole lot different than a stocker (IMO). Call ATI, BTE, Coan, Hughes or any "GOOD" converter company and get their recomendation.

If you have never experienced a high stall converter, in normal driving, it is not much different from a stock convertor. The is a little more slippage from a normal start, but it's not that noticeable.
It's only when you lean on it that you will see the rated RPM.

Just like the camshaft is the "brain" of the engine,
............ The torque converter is the "brain" of the drivetrain.

You want to put the most torque down to the tires possible (remember! torque converters multiply torque, not Horsepower) The one with the most torque (on the street) wins!!!.

You want to match the torque curve of the engine to the converter, otherwise you have a hodge podge of parts that may or may not work to your liking.

You will still have traction problems.

Usually, the smaller diameter for a given stall speed is more efficient.

However, be sure you don't end up with too much stall speed.

In my experience, on a mostly street driven car,
..... it is better to give up that tenth in the 1/4 than deal with a too loose converter.

That said, the converter has to work with the combo even on a street car.

What are you dealing with for approx HP/TQ, rpm range, gearing, and usage?

I'm guessing a solid street/strip ride from you 2,500-3,000 guestimate.
2,500 is a very reasonable stall speed, 3,000 is where they start to get not so fun to drive ...

A 2,500 converter works fine on a 12" core,
....... but a 2800-3000 unit should be more efficient on a 10 or 11" core.

I have a 2,500 stall converter in my deuce, and it works fine on the street.
If you get much above a 3,000 stall converter, you may not like the street driving characteristics.

Just an FYI.
There's actually 2 different dual pattern flexplates depending on the diameter and starter used:
471529 - Small Block Chevy- 1969-1985 Internal Balanced 2 Piece Crank Seal 12-3/4"-153 Tooth Auto Trans Flexplate - With 9-3/4" Or 10-3/4" Converter Bolt Pattern
and
471598 - Small Block Chevy- 1969-1985 Internal Balanced 2 Piece Crank Seal 14"-168 Tooth Auto Trans Flexplate - With 10-3/4" & 11-1/2" Converter Bolt Pattern

So pick your diameter and bolt pattern diameter.
Apparently, GM used the larger [11 1/2"] bolt circle pattern with the stock 1986 3/4 ton, 2wd/ 454/Turbo-400 w/ 13" convertor.

According to the above post, Small Block Chevys used 9-3/4" & 10-3/4" & 11-1/2" bolt patterns.

[ I was told by Art Reed at Coan Engineering, you needed a machinest rule, to measure the torque convertor bolt pattern, a simple ruler would not work !!! ]

Stall speed is where the converter theoretically is locked and not slipping. Slipping produces heat, therefore, you realistically want the converter to be at the locked rpm or above, while you are cruising, so that you aren't generating a bunch of heat. This is mainly why everyone says to use an external cooler with a high stall converter. now, granted, all converters that are not of the lockup variety, still have some slip, even at high rpm. But usually, it is not any more with a stall like 2,400, than a stock converter. This is why I could run a 2,400 stall in my LS5, and not need an external cooler.
Now that I think about it, I even used a 2,400 stall in the LS6 that I built with 4.10 gears and the solid lifter cam. It was a good package in that car as well.

The converter doesn't "know" what stall speed it is. It doesn't really even care. :D
The interior design of the fins is what determines alot of things in a converter.
Weight has an influence on stall speed as well. The torque curve of the engine has an influence too. If you're not going to the track, then just enough stall to get the car to idle in gear is probably best.
Here's another example. My friend had a built 355 in a 70 Z28. He used the 292 comp cam. It had a radical idle, and we put a 3,000 stall converter in it. It really needed 3,500, as it still wanted to creep in gear and not idle as well as it could've in gear had we put in a 3,500 stall.
I ran a similar cam in my LS5, and I had 3.31 rear gears. Actually, the .050" duration was the same, 224*. I used a B&M 2400 stall converter and it worked perfect. My RPM at 60-ish was 3,100, which would put the converter stalled at about 42 MPH, but I never had a problem with it overheating around town. Your cruise RPM will be a tick higher with the 3.55 gears, so you should do fine with the converter you have listed.
I 2nd the 2400 Holeshot
works great with my 454 using a 223*/231* @.050 cam and 3.31 gears
I use a #20210 Coan 11" pro street converter, in a low compression 8.2:1
1970 Chevelle SS454, turbo 400, 3.31 posi, XE274 cam 230*/236* @.050 ...
It don't act like the same car, "the 2,600 stall made it fun to drive" ...

call and talk to Art Reed @ (765)456-3957 (arthurr@coanracing.com)
tell him your specs, you will be impressed with the knowledge, service, workmanship and performance the Coan converter provides .......
I believe you'd be happier with one of these, than with a shelf converter .....

I had a boss Hog 3,500 that calculated 18% slip through the lights.

Changed to a Coan 3,000, it calculated 4% through the lights

ET went from 14.0 to 12.75 and mph from 94 to 107.

The calculation formulas may not be perfect, but they can sure get you in the ball park and let you know when you have a total piece of crap for a converter. You could hardly tell the difference in my two converters on the street, but it was night and day at the track.

It you are going to use your car at the track some and want to ET to your cars potental .... get a VERY good converter. If all you want to do is brag about how far you can spin the tires in front of your house and guess at your ET, just about any old piece of junk will work.

I used the word YOU as people in general, not as a specific person.

Everybody with an automatic tranny runs a 'stall' converter on the street, some just stall the engine at higher rpm than others. A good 2800 stall converter will act very much like a stock one until you apply some real torque to it.
Like I said, a well made 2,800 rpm stall converter will act very much like a stock converter until you apply some serious torque. I have a 9.5" converter that stalls my engine at about 3,500 rpm that will accelerate the car to highway speeds along with the traffic without exceeding 2,500 rpm. It creeps along when the car is idling just like a stock converter.

If I lower the pedal excessively, it quickly spools up to 3,500+ and away we go.

4,200 RPM stall means nothing. It's what it'll do behind your engine that counts.

I'd want to know where your torque peak is at, as well as what the engine speed falls to after the shift points before just saying to loosen it up 'x' rpm.

With a big motor and a broad torque curve coupled with a 3 speed auto, you will generally find stall speed works best if right about at, or as much as 300-500 over the peak. It all depends on the exact combo.

Generally you'll run the quickest with the smallest converter you can run that still retains a good fin angle and the right stator for the setup.

My setup is as follows:
ZZ572/720R GM Crate Motor specs:
720HP @ 6,250 RPM
685LB @ 4,500 RPM (GM)

Rear Wheel dynoed:
561HP @ 5,600 RPM
571LB @ 5,100 RPM
TH400 w/brake, 91/2 inch 4,200 stall converter, 4.10 gears w/ 29.5x10.5x15s M/T E/T DragsI'd run about 4,800-5,000 RPM for stall speed and shift at 6,500 looking at those numbers. Your converter is waaaaaaay too low now for stall speed it appears. The move to an 8" could certainly be of benefit to you. Something along the lines of a CCX200+ give or take a tad in fin angle would be what I'd recommend.I would say if your motor makes peak torque at 4,500, something in the 4,700-4,900 range will be in order. Maybe even a 5,000 converter. For this you will probably be better with an 8 inch converter.

I think you need a larger diameter converter for either low stall or a lot of torque, like a 1500 hp motor.

Since you are new to the use of automatic transmissions in high performance cars, here's some basics for you on torque converter applications .....

But first let me state that I agree that you should call a vendor (I'd call ATI). But FWIW, you definitely need a 10" converter made to handle the use of a transbrake. I also agree that with 3.73's and that cam you have, a 3,000 RPM stall speed is all you'll need. The converters made for Nitrous are nice and tight, and made to handle a lot of torque without flashing at an RPM that's too high. If that's a roots blower you have, then you definitely will want a nitrous converter since the roots blowers make a lot of low end torque. I'm sure that you'll get power up to 6,500 RPM though.

For my car, even though it's a naturally aspirated motor, because it has big displacement, and a lot of low end torque, I called TCI, and bought the tightest competition 10" converter they have, and had it adjusted to the lowest stall speed that they can get it to for my application. It's one of their "PRO-X" models, and it's made for big nitrous engines, and will handle transbrake use. (TCI part#251050). this is what they recommended for my car, and the stall speed should come in at about 3,200 RPM on the transbrake. It needs to be used with a flexplate that has a dual mounting pattern, because instead of three mounting legs it has six. It also uses 7/16" diameter mounting bolts instead of the usual 3/8" diameter. But unless you have a roots blower with over 10 PSI of boost, you won't need all of that.

In general, when you see the better torque converters for sale, they're listed with a range of stall speed RPM rather than a specific RPM, because the manufacture will want you to give them a call so that they can get all of your vehicle specs in order for the converter to be adjusted to the desired stall RPM. Usually when you see any particular converter listed as being a SPECIFIC stall speed RPM, it's a cheaper converter made for low power applications, or it's of a lesser quality. The fact of the matter is, that any given converter will not exhibit the exact same stall speed for every vehicle that it's used on.

For instance, if you had a brand new torque converter in your car that flashed to 3,000 RPM, and then without even touching the torque converter nor the transmission, you turned up the boost on your roots supercharger from 8 PSI to 15 PSI by installing a smaller blower pulley, your torque converter stall speed would then flash to a higher RPM than the previous 3,000, merely because you've added more torque to the engine output. The same thing would also happen if your car was stripped down to cut out all the weight for racing, and you then added 500 lbs to the vehicle weight with things like a full interior, power steering, hydroboost brake unit, full exhaust, etc., etc

The increased vehicle weight would also cause the converter to flash to a higher stall speed RPM during full throttle launches due to the extra vehicle weight causing an increase of resistance to the engine torque. Even a rear end gear ratio choice can alter the stall speed RPM to a certain extent. The more resistance that is applied to the drivetrain, the higher that the torque converter stall speed RPM will flash to. The level of resistance can come in the form of either vehicle weight, or engine power level, etc....That is why any torque converter will flash to a lower stall speed RPM during part throttle and cruising, than it will at full throttle inputs. The more engine torque applied, the greater the resistance is being placed on the drivetrain, and in turn, the higher the torque converter stall speed RPM will climb.

So a manufacture that offers quality torque converters for moderate and high HP vehicles, will always require the customer to submit all of their specific vehicle specs before stating any specific stall RPM claims for any of their higher end models intended for big power.