Tell me about stall speed

Question:

I have a 1965 GTO and I upgraded the engine from a stock 389 to a 500 hp 455. I now want to upgrade the anemic 2speed 300 automatic to an overdrive automatic transmission.

(Don’t worry all you original numbers matching diehards; I’m keeping the old drive train off to the side.)

I heard the 2004R would bolt right in without any modifications. I was wondering if the 700R4 was an easy swap as well.

Also, torque converter stall speeds confuse the heck out of me and I’m not sure which would perform the best. I’m running 15” tires and I believe I have a 3.23 stock rear gear ratio, but I’m willing to upgrade. Any help in my project would be greatly appreciated.

Answer:

Yes, you can use a 700R4 overdrive transmission in your GTO.

In general terms, stall speed is the engine speed at which the torque converter transfers the power from the engine to the transmission. The torque converter has two components that resemble fans. One is called the impeller and the other is called the turbine. The impeller is connected to the engine so it spins at the same speed as the crankshaft. The spinning impeller’s blades propel transmission fluid against the turbine’s blades and when the fluid hits the turbine with enough force it induces it to spin. The turbine is connected to the transmission’s input shaft. So the stall speed is the engine speed at which there’s enough fluid pressure to induce the turbine to turn and thus cause the vehicle to move.

As a general rule of thumb, the stall speed should be matched to the engine’s power curve. Cars fitted with an engine that has a high rpm power band, such as many drag racing engines, will normally perform better with a high stall speed torque converter. Conversely, vehicles with engines that have a low rpm power band, such as most diesels, benefit from a low stall speed converter.

What stall speed is best for you depends on various factors, including the power curve characteristics of your engine and what it is you’re trying to accomplish. If, for example, maximum acceleration is your goal, then the stall speed of the torque converter should match the engine rpm at which maximum torque is produced. So if the engine makes peak torque at 5400 rpm then a converter with a stall speed of 5400 rpm will usually yield the fastest acceleration. Bear in mind, however, that higher stall speeds generally result in higher transmission operating temperatures and lower fuel economy, so you should not use a torque converter with a higher stall speed than necessary.