kevin j

when talking camshafts what are they referring too when they say this?

Kevin Vandevenne

iTrader: (3)

Its the stall speed the torque converter for an automatic transmission is rated for. When you pick a cam, they are usually made to make power within a certain rpm range. Bigger cams make power higher in the rpm range. The way a stock cam and converter work is that it will start to make power off idle, so at 1600 rpm the converter will be engaged and spin the transmission and put power to the wheels. With a converter that stalls at 3500rpm, what happens when you go full throttle is it will 'flash' up to 3500rpms and allow the motor to put power down in the rpm range the cam is designed for.

If you were to run a stock torque converter with a big cam, you'll have a car that won't drive very well. With the big cam, as they are designed to make power up in higher rpms than the stock cam, the low rpm power usually isn't very good. With the stock converter, the transmission is going to start turning when the engine is at 1600 rpms, then you'll need to wait as the car accelerates into the rpm range the cam is made to work in, say 3500+ rpms.

Thats a pretty basic run down of it.

SeanW

This is completely wrong.
Torque converters are torque multipliers. At lower rpm the ratio is low, around 2:1 or lower. At full coupling they approach a 1:1 ratio.
Stall speed refers to the specific rpm your motor will reach in full stall. Full stall is full throttle with the car in gear but the tailshaft is not turning, either locked in place via the brakes or via a transbrake.

After market torque converter suppliers are selling for a transmission type but don't know your engine set up. So they rate their by mininum stall +. The more hp your motor makes the higher the given stall will be with that converter. So they know if that converters stalls at 3500rpm at 300hp anything stronger with stall higher.

AlkyIROC

All torque converters slip. If they didn't, your engine would stall. Anything below the converters stall speed and the converter is slipping. Anything above the stall speed and the converter is in 1:1 ratio.

The idea of a higher stall speed is to quickly get the engine into its best powerband for best performance. An engine that starts making power at 4000 rpm would bog through the lower rpm range if the converter was at 1:1. Driving around below the converter's stall speed means the converter has to slip and this slippage generates a lot of heat in the oil.

A converter needs to be matched to the camshaft grind. Basic rule of thumb is you want a converter that stalls about 500 rpm into the powerband for street/strip use or 1000 rpm for full race only. Too much stall speed can be just as bad as not enough.

A higher stall converter doesn't mean the car won't move at the slower speeds. It will move just fine but the converter will just be slipping. My race car has a converter that stalls around 6200 rpm. I put it in gear and can drive around at 1000-1500 rpm with no problems but if I put my foot to the floor, the rpms will quickly jump to 6000.

Using a transbrake and 2-step, a higher stall converter allows you to launch closer to the camshaft's peak torque range. This allows for faster launches and quicker ETs as you're taking off right where the engine makes it's most torque.

I generally launch around 5000-5500 rpm and all the way down the 1/4 mile, the rpms never drop below 6400.

Building a car for best performance means a combination of parts all matched together. Everything starts with the camshaft. That determines what stall speed to use and what operating range the engine will be running in. The engine needs to be built to work in that range. Transmission and diff also need to be adjusted to work in this range. You can always have mismatched parts but you won't be getting the best performance potential from them. You could build a 500 hp engine that screams to 8000 rpm. If you have 2.73 gears in the diff then dragstrip performance would suffer but you would have a fast car in Bonneville. You would also be idling down the highway with an engine that makes power far in excess of where it's operating at.

SeanW

Good answer just wanted to clarify that a torque converter will never reach 1:1 unless it has a lock up clutch. Thats why lock up clutches were added to improve fuel effeciency at cruise speeds. Without they are at best about 90% efficient.

Stall speed is not the point at which the converter reaches coupling phase either. Kind of the opposite actually. Its the fastest the impeller(input) will turn while the Turbine(output to trans) is held. So its max slip.

AlkyIROC

How much the converter slips above the stall speed depends on a lot of things. My current combination of parts has a calculated converter slip of 6%. When I went from 4.86 gears to 4.57, the calculated slip dropped to 13%. I switched back to 4.86 gears and got the 6% again. Nothing else changed.

SeanW

You are talking about effiency in rotary flow of the torque converter(coupling stage). Thats when the impeller and turbine are spinning the same speed and the stator is unlocked. That doesn't have to happen at stall. Coupling phase happens at cruise speed. You could be near 1:1 at a fairly low rpm as long as you are part throttle at a stable speed.

kevin j

thanks guys so what your telling me is that the converter will slip until the stall speed is met. it does this to rev through the lower rpm's faster to get you into your power band faster without needing to craw threw them lower rpm's?

AlkyIROC

That's the simplest explanation.