Aug 2005 ------------------------ Overdrive transmission / torque converter Lockup / Stall Speed -------------------------------------------- Using a higher stall and no lock up would be a bad decision. The axle ratio in cars that the 200R4 was originally used in was anywhere from 2.56 to 2.76. If lugging wasn't an issue there it certainly won't be with 3.31's. Keep in mind that any cruise rpm below the rated stall could cause the converter to slip, that's why the lock up became an integral part of the overdrive system. At cruise with the high rear end ratios ( low numerically ) the converter would "slip" to it's stall point. So the engine would be turning 2500 rpm even though it didn't have to. Stall is a variable and even though a converter is rated for 2500 RPM the engine it's behind has to develop enough torque to make use of the stall speed. Forgive me if you are familiar with this. But, conceivably, using a 2500 RPM converter means that if you could be moving along nicely at 2100 RPM your engine will always spin to 2500. Creates more heat and wastes fuel. Ernie R2 R5388 Lock up is the key here. If it locks up in 4th then the rpm will be whatever is needed to move the car at 70 MPH. If it didn't have lock up...a 2500 RPM converter will allow the engine to spin to 2500, no matter what. Again, forgive me if I'm repeating common knowledge. Inside the converter there are two vanes that are completely separate and spin independently of each other. The fluid spinning around within the converter eventually gets those two vanes spinning in concert, almost. The engine side will always spin a little faster than the trans side. The converter is designed to slip, as a you would do with a clutch, as you accelerate. The stall speed is, basically, the point where the two vanes spin together and the action of the fluid doesn't allow for much slippage. So, with foot on the brake and trans in gear, if you were able to keep the tires from spinning, at the rated stall speed the engine would stall like you were coming to a stop without pushing the clutch in. A higher stall speed means that the converter is designed to spin faster than stock on the engine side before the trans side catches up in order to allow the engine to get into a good power band before it starts to turn the trans side vane. Important feature with cams that develop power at 2500 RPM or higher and a high axle ratio. I say stall is a variable because if there isn't enough torque in the engine to overpower the trans side vane at the specified stall speed then the converter "stalls" at a lower RPM, like 2200 RPM instead of the rated 2500. Or, conversely, put 500 lb. ft. of torque on a 2500 RPM converter and you might spin it past its rated stall. So if your converter isn't locked, picture your engine side vane spinning to 2500 RPM every time you step on the gas, before the trans side starts to move. But say the trans side really only needs to spin at 1800 to achieve the speed you want to go so, your converter is slipping and your engine is spinning faster than it needs to. That heats the converter up BIG TIME. The reason the modern iron had to go with lock up is that with the axle ratios being used the engine would only have to turn 13-1500 rpm at cruise. If the converter were built to stall at that speed you wouldn't be able to put the car in gear at high idle without stalling. But without lock up and a normal stall of about 1800 RPM at cruise it would be slipping, wasting fuel and getting hot. So in your case, with lock up, your cruising RPM will be determined by the trans gear ratio, rear axle ratio and tire circumference. Without lock up it would never be better than 2500 RPM. In any mode that the converter is not locked whenever you step on the gas the engine will spin to 2500 RPM before it starts to move you. I didn't mean to make a mountain out of a molehill, we're only talking a few hundred RPM. But having lock up engaged whenever you are cruising will be important. Ernie