Just wondering.


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1988 Pontiac GTA
Loaded
350 700R4

 

All torque converters are stall converters. A typical factory converter stalls at around 1200 rpm. This means at rpms less than 1200, the converter is slipping and not transmitting full torque through it. A factory converter will move a car at idle. If you could completely stop the output of the transmission with something like a transbrake (vehicle brakes are not strong enough) then put it in gear and go to WOT, the engine would stop building power and stall out at 1200 rpm because the converter is transmitting 100% of the power through it without any more slippage. A higher stall converter, lets say 2500, will slip more and allow the engine to build up to 2500 rpm. Higher stall converters because they slip so much also build up lots of heat and require aftermarket coolers to keep tranny oil temperatures down. The higher stall converter also allows the engine to climb into a higher power range quicker since at the lower rpms it's slipping. The proper converter stall depends on many things such as cam selection, vehicle weight, engine torque etc. A much higher stall race converter is usually over 4500 rpm stall speed. A car with a converter like this usually won't move at idle since there isn't enough torque going through the converter to move the car at low rpm.

Proper selection of converter stall makes a car perform as well as a manual transmission. Stall speed should be at the bottom of the engines power curve. Usually a performance street car is around 2200-2800 rpm.

My race car has a 3800 rpm converter. I launch around 2200 rpm, while holding the brakes. When I shift at 6400, the rpms drop back to around 3500-3800 in the next gear. The converter is now at it's stall and I start to build power at the bottom of the power curve. If the converter stall is too low, lets say 2200, the engine has to waste power building from 2200-3800 rpm to where it starts to make it's best power again.

700R4 trannys are also bad for this since the rpm drop from 1st to 2nd is so far that the engine falls way below it's power curve.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
LS6 Big Block buildup now in progress

Best results before the engine blew up
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!

Member of the Calgary Drag Racing Association

[This message has been edited by Stephen 87 IROC (edited October 08, 2000).]

 

For a thorough and accurate description of what a torque converter does go --> <a href="http://www.racer-x.f2s.com/proc/tcexp.htm">HERE</a> <--

Until the converter stalls, it is multiplying torque(which is why auto trannies launch better at low engine speeds than manual boxes). While the converter is slipping it allows the engine to rev freely. Once the converter overcomes the frictional forces opposing it, the converter stalls(almost no slippage). This makes it more difficult for the engine to increase it's rpm's. For this reason, you want to pick a converter that will stall approximately 400 - 500 rpm below your torque peak, to gain ultimate performance. For instance, if your torque peak is at 2800 rpm, you would want a converter that stalls around 2400 rpm. If your engines torque peak is at 3400 rpm, you would want a 3000 rpm stall speed.

Picking too low a stall speed, means you have to spend time building revs to the torque peak(which is where the best acceleration occurs). Picking too high a stall speed, will cause the engine to rev past the torque peak, past the point where the best acceleration occurs.

One other consideration - heat. As long as the converter is slipping, it is building up heat in the tranny fluid. This heat is the main enemy of your tranny, in terms of longetivity. If the majority of your driving is done below 2500 rpm, it makes no sense to get a converter that stalls at 2800 rpm as the converter will spend the majority of it's time slipping(building heat). You would be better served to get a converter in the 1800 - 2400 range(where the converter will stall a lot more often). The decrease in performance from picking a slightly low stall speed will be paid-off by increasing the life of your tranny. As the stock 700R4 converters are designed to stall at around 1600 rpm, the 1800 - 2400 rpm converter will still give you a noticeable improvement in acceleration, and will be a lot better in the long run for the life of your slush box. At any rate, whenever you increase the stall speed over the stock converter, you should add a transmission fluid cooler. This will help dissipate some of the heat generated by the extra slippage caused by the higher stall speed.

For the best advice on selecting the right converter, talk to a rep at one of the torque converter manufacturers. They don't want to sell you a product you will be unhappy with, and will go out of thier way(in my experience) to sell you the converter that meets your needs and expectations.

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He who hesitates,... is lost!