A SWitch Pitch transmission won't solve all problems. It won't even make a huge difference, but it will make a noticeable one properly used. If you have a 3 speed auto, you could change to a 4 speed, or you could change to a SWP. More gears are always better, but these 2 approaches want a different axle ratio for near same results.
Do you want a racer, or a street car? Want a little more performance, or a little more mileage, or a really smooth shifting car? Maybe just not having the trans need to shift so much. Maybe a SWP will do it. Like all converters, there are SWP modifications for different stall speeds. A 68 up THM400 or 425 trans uses a 3 element torque converter, which at stall (vehicle not moving yet) puts out about 2 times the engine input torque. As rpm and output speed increase, that gradulally reduces from 2:1 to 1:1, stretching the useful range of each gear. So the 400/425 will easily outperform the old 4 speed hydramatic or a 3 speed manual, by providing a wider overall range of gear ratios. The switch pitch (SWP) THM400 and 425 of 65-67 had a torque converter 3rd element (stator) with VARIABLE VANE angle, changeable between performance and economy (instead of a compromise) as you drive. This gave 2.5 times torque multiplication, a 25% increase. It also gave a bit better mileage in the other mode. In a heavy vehicle this can provide a reduction of 0-60 time of up to 10%, and also give smoother shifts. The switch pitch is most effective at getting engine rpm into the power range at low speeds. At maximum rpm all torque converters become 1:1 torque transmitters, so switch pitch makes no difference here. It is very effective at max throttle, low to medium rpms, with up to 25% more torque delivered. The beauty of a SWP is ability to give a little more performance, a little more economy, and a little more smoothness in the strongest car trans ever made, for a dirt cheap price. Note that the stock 65-67 setup may not do all this. But a touch of technology used to control a SWP can considerably improve over the original. My first SWP project was a 4000 lb Delta 88. That low compression 350 would cruise all day so nicely at 60 mph and 2000 rpm. But the 2.41 axle felt like starting in the wrong gear, 0-60 in 10 seconds. The SWP brought it down to 9 seconds, what an improvement (no other change)! Performance at no cost. At idle and high stall mode, the SWP put less drag on the engine. So I used less gas idling, and the car had less creep. For part throttle through the gears, the high stall gave up to 25% more torque off the line, and was generally more snappy below 2500 rpm. And the shifts were MUCH smoother. Cruising in high gear and low stall mode, the SWP is tigher than a fixed converter, so I pick up a couple points on gas mileage. Cruising at 60 mph I want to move ahead a little. Touching the gas causes a manifold vacuum drop, the SWP goes to high stall and pulls the car ahead. Engine rpm may increase by 500, without a gear change, and then back, smooth as silk. Going into a corner you might brake, and the SWP goes into high stall. Pulling out, you get extra torque to recover speed. A few seconds later, you are smoothly back in economical low stall. Next car was a 5.7L diesel Cutlass. What a difference the SWP made, it no longer felt like a slug. Same 2.41 axle, several hundred pounds lighter than the Delta, and it still got up to 30 mpg. Eventually I got tired of standard 3 speed autos, and replaced them all with SWP. Many motorhomes have been converted with good results. If you are racing, the above may not be of much use to you. Above 3000 rpm the SWP has no advantage. Big power doesn't need it. If you drive an old car 30,000 miles a year, you might really like it. The original mechanical control won't do it all, I use electronics to sense torque and add time delays. Bruce Roe 26 Oct 2006 John Lee <[EMAIL PROTECTED]> writes: > The th400 in my 67 has a switch pitch, and its behavior > is new to me, so much that, the one time I drove the car, > I could not tell if the > transmission was behaving properly or not. > > Could I lure you fine people into a discussion of when > a switch pitch works, what it does, and how that 'feels' > in various types of driving? The specific transmission > here is a stock 67 442's th400. > > Thanks, > John
