Sounds like it. Something else to keep in mind when I start doing performance work on the car. All that will come after repairs and restoration tho...

First suspension and handling, then brakes, wheels and rear axle at the same time

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Yup, I've bought running cars for less then I paid for that new converter but it was time to get a decent one.
I did some math and saw that my current "budgit" converter is only about 88% efficent.

I hope to swap the engine in the Dragcar in the next few weeks, I'm going to put the 209 "Low Block" back into the car, along with the new converter and the spray set-up. Best ET so far with that engine is a 14.24. (no spray)
DD

A lot of the information on how torque converters work seems pretty vague - both on the Internet and in print. The best description I've seen of them was in an engineering textbook, but most of the material there isn't for the general public. I'll see if I can write translate it into plain English. Let me know if any of this is too difficult to follow - if it is, it's probably my fault and I will see if I can explain it better.

What is stall speed?

This is not a simple question. The stall RPM of a converter is not only different for each converter, but different depending on how power is applied to it.

Imagine a working model of a torque converter that is sitting on a stand with a crank attached to an input shaft driving the converter. The output shaft of this conveter is clamped so it can't turn. Now imagine trying to turn the input shaft by hand. If you can turn the output shaft with 20 lb-ft of torque, you will be able to get the input shaft spinning at a particular speed - let's assume this particular one will spin at 50 RPM if you do this. In this case, the torque converter would be stalled at 50 RPM. To get the input shaft to spin faster, you will have to apply more torque. Theoretically, to get twice the speed for a given torque converter, you will need four times the power. With the model in this example, if you wanted to spin it at 100 RPM, you would need 80 lb-ft of torque. In reality, torque converters may not always follow this law, but usually only the people who build them know exactly how the input torque varies with stall speed. This rule of thumb should be pretty accurate, though.

The design of the torque converter influences how fast you can spin it with a given amount of torque. The outside size is one factor; smaller converters tend to spin faster for a given amount of torque. The other factors are mostly of interest to the people who build them.

Putting it in a real car

A model torque converter may be interesting for engineers to play with, but for most racers, you'll want it in a real car, bolter to a real engine. Most of the people here have seen a chart of a torque curve for an engine, showing the amount of torque an engine makes at each RPM. Take that chart, and add another curve for the converter that shows how much torque it takes to turn the input shaft at each RPM. These curves will intersect at a point, and unless you have a very unusual engine, they'll probably only cross once. At this point, if you've got the engine running at full throttle and the torque converter's output shaft clamped firmly, the engine just won't have any more torque availible to spin any faster. This RPM is the stall RPM for this engine and torque converter combination. If I ever make an article out of this post, I'll have to include some pictures here.

Now, if you want to move the stall RPM, you can either change the converter or change the engine's torque curve. The former is widely known, but the latter often is less talked about. Imagine if your slant six stalls at 2,000 RPM with its converter. Now suppose you add a supercharger that improves torque by 50% at every RPM. This would move your stall point up to around 2,450 RPM.

Since it's in a real car, you also have real brakes and a real drivetrain to deal with. Real brakes may not be able to clamp your output shaft firmly in place, making the transmission start to turn before you can hit the stall RPM. A transbrake, on the other hand, usually can clamp the output shaft so it can't turn, no matter what the torque applied to it. You also have the option of revving the engine hard from idle, which may make the engine rev up to a bit faster before things start moving.

Launching the car

The key to an effective launch is being able to rev the engine up high enough so that you're applying maximum torque, or close to it, to the rear wheels when the light goes green. With a stock converter, your engine may be stuck down in the low RPM range for a few valueable tenths of seconds before it can get into the best part of its powerband. A high stall converter will let you have the engine revving at its peak torque before you let off the brakes (or transbrake). This makes for a harder launch and wastes less time in the lower RPM range.

Driving it on the highway

Does a torque converter transmit power below its stall speed? Absolutely, if you don't have the brakes holding it back. Some people seem to think a converter just freewheels below its stall speed, but this is simply untrue. It continues transmitting some power even at nearly zero RPM, although not much. However, the smaller the converter, the less power it can transmit at a given RPM. So, loose converters with small engines can slip a lot if you're trying to cruise at 2,000 RPM with a 3,000 RPM stall converter. The power a converter can transmit rises dramatically with RPM; you can transmit about eight times the power if you double the RPM. So, numerically higher gears will make your converter slip considerably less at the same speed.

Hopefully this'll take some of the mystery out of torque converters!

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Good explaination Matt. I drew a picture which I think illustrates what you're saying.



The solid lines represent a stock engine with a stock converter. The intersection of the curves ( point A) represents at what rpm the COMBINATION will stall at. Change the combination, say add a higher stall speed converter, and the operating point shifts to point C. Note that since the stock slant has a relatively flat torque curve, even tho the stall speed has increased, there's not much of an increase in torque to launch the car. Therefore, not much of a payback.
Now leave the converter stock but increase the torque curve of the engine with a blower as you suggested. The new combination now has a stall speed at point B. More torque is available for launch but its not at the engines max torque. Add the hi-speed converter to the modified engine and now the stall speed matches the rpm at which the modified engine creates max torque (point D). Max torque translates into quickest launch.

Note that the hi-speed converter creates more of a gain with the modified engine because the engines torque curve rises more dramatically than a stock slant.

In summary, its the matching of the combination that's key to results.

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Cheers!
Al T

So say I got a 2000 RPM stall rated TC, I could still leave my gearing prety much stock (I might be WAY wrong here, but I'm going off my truck which cruises at ~70MPH 2300 RPM with 4.11 gearing), get good highway RPMs and mileage, but in addition to that, get a much stronger launch off the line than with a stock TC?

Seems to be well worth the money to me...

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Stock stall is usually 16-1800, so 2000 wouldn't get you much. Best to go for 2500 (mild 10" converter) to see a real difference.

Lou

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Home of Slant6-powered fun machines since 1988

Yeah I figured it'd be higher than 2K, I was just using my slow slow truck for an example :p

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Just put a 3500 in it and be done.

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Official Cookie and Mater Tormentor.

A word a caution here more is not always better like every thing else your combo should be balenced. That is I plan a converter after I have done intake, exhaust, head, rear gear and cam. Then I try to match the converter to the motor. Have fun

My bigest F*$k up was in 1974 I pulled my stock 10" converter in my STOCK 70 challenger RT 383 mag. and installed a B & M Hole Shot 11" converter. Switched the motor for a 413 with a wild racer brown cam kept my 3.23 gears. It stalled lower and bogged but when it hit the power band hold on!!! The shop told me to keep the 10" I told the local chy repair shop to toss it "I HAD A B & M" When I went back he had sold it.

love it,

dig up an 3 year old thread and

spaminize it..............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

At least it was a good thread

Hey, since this thread has been "Doug" up (pun intended ) and I have been driving the Super Duster more regularly lately, do any of you slant racers have a good used converter for sale (904, of course) that will stall around 2800 RPM behind my mild /6? If not, what have you? Remember, I've got a TCI "breakaway" in it now, it runs mid 15's at around 90 mph, and is a turd in the first 60 feet - what have you? anything nice, second-hand that would work good with my combo?

Thanks,
D/W

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If it ain't broke, fix it!

Aw, c'mon gents, it's Crimmus! ...anybody got one?

D/W

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If it ain't broke, fix it!

Doug - is that TC for an older push button tranny???

I'm looking for one too for my 63...

I'm turning about 3k at cruise (60 to 65 MPH), with my 3.55s.

So I guess a 3k stall would make sense?

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1963 Dodge Dart, motor by Doug Dutra, Offenhauser two carb intake, Pertronix, Clifford 6x2, Flowmaster 40's, Erson RV15/295 RDP, head work. Addco anti-sway bar, urethane suspension bushings, KYB's, racing leafs, SSBC discs. Need ZDDP? Get STP.

I have the 68 and newer "large hub" crank so the converter i ordered are different then the 60-67 "small hub" units.
Chris at Continental can make a small hub converter just as easy, so can many others so do some shopping. A 2800 to 3000 RPM stall should do the job for your 3.55 geared "street performance" set-up.
DD