Switch Pitch Trans Detail

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Many GM full size cars (Buick, Olds, Pontiac, and Cadillac) and intermediate cars with the V-8 used a Variable Stator TH-400 transmission from 1965 thru 1967. The 2-speed Jetaway trans used behind the V-8 [not the L6] in the 1965-67 models also utilized a switch pitch torque converter.

For a transmission built during these three years, you can identify the transmission as having the switch pitch parts by the electrical connector on the side of the trans case. A run-of-the-mill TH-400 will have a single spade terminal protruding from the case (to activate the kickdown solenoid valve). A switch pitch (aka variable vane) trans will have two terminals (one for kickdown, one for vane angle). Now, trans made/used outside of 1965-1967 might have two terminals, one for the kickdown, and one for the TCS solenoid (prevents vacuum advance except in high gear). The TCS is wired to a pressure switch connected to the accumulator piston area, that allows full vacuum to the distributor only when the car is in third gear when the engine is cold. There probably is two wires coming from the thermostatic vacuum switch on the front of the manifold.

The other indicator for the switch-pitch converter would be the switch on the throttle linkage which kicks in the high-stall speed at idle and at WOT (and, I believe, on those years it also controls the kickdown).

So, a single terminal is proof that the trans is not a switch pitch unit. Also if one is certain that the trans is a 1967 or earlier unit (didn't these have the horseshoe-shaped indentation in the tranny pan due to the early-design filter?), then the two-terminal connector indicates switch-pitch. Later (is it 70-up?) TH-400s with two terminals indicate TCS.

[ Thanks to Joe Padavano, Bob Barry, for this information. ]

General Information

How & Why the Switch Pitch Works
The name switch pitch comes from the vanes (stator blades) in the torque converter being able to change their angle (pitch) in order to reduce friction (force) and get the vehicle moving. This is similar to somebody trying to start pedaling a ten speed in 10th gear versus 1st or 2nd gear. You pedal fast for the output you get, but you are able to move.

These trannys have both an internal kickdown solenoid and an internal variable pitch torque converter, activated by an similar solenoid. If the tranny side electric terminal shows 2 spades, it may still have the variable pitch converter. A very neat item BTW. Unfortunately, most cars had the switch pitch feature tossed by a rebuilder.

The 66-67 cars use a control switch which is part of the throttle linkage mounted on the firewall. For variable pitch trannys there will be four spade connectors on the switch. One for 12V, one for the kickdown and two for idle and full throtle switch-pitch converter operation. The switch pitch allows a higer stall speed at idle (to assist old ladies who don't like creep while at the stoplight) and full throttle (when the torque multiplication of the higher stall is helpful). This feature was only available from 65-67 on Olds, Buick and Caddy trans.

Oil pressure within the trans holds the stator blades in the converter at a low angle for more efficient coupling. With a high angle, the stator blades offer a greater oil redirection, increased torque multiplication, increased engine speed, maximum performance, and less tendency to creep at idle. This higher angle is achieved by bleeding off the pressure that held the low angle, by applying a 12V signal to the stator control solenoid, located on the back of the pump. This electrical signal is fed into the trans thru the connector on the left side, along with the power for the kickdown solenoid. The two form a "T", with the stator control terminal being the top of the T and the kickdown being the upright.

              Stator   |--   Detent 

To summarize:
Apply 12V ==> hi angle, high performance
No signal ==> low angle, better economy

The converter changed stall speeds from 1800 rpm to 2600-2800 rpm. A 12" converter was used in the TH-400, and an 11" converter in the Jetaway. You can interchange the converters. You will also see a switch on the throttle linkage near the carb where the throttle position will dictate the change in the two stall speeds via an electrical solenoid located inside the transmission.

In the 1965 Oldsmobile Chassis Service Manual, it is stated that switches on the throttle linkage supply the signal for high angle at idle, and at throttle openings greater than 40 degrees. With this setup, the TH-400 offers a 1st gear multiplication ratio of 2.5 via the gears, and 2 via the converter, for an overall 1st gear ratio of 5.0:1. Second and 3rd gears are 1.5:1 and 1.0:1, and are also subject to additional torque multiplication from the torque converter. Reverse gear ratio is 2.0 mechanical * 2 from the converter, for an overall ratio of 4.0:1.

I'd suggest looking at a 66 or 67 Olds factory shop manual. It will explain all.

Where to find a Switch Pitch trans or converter
Many GM large cars (Buick, Olds, Pontiac, Cadillac) and intermediate cars with the V-8 used a Variable Stator trans 1965 thru 1967 (they might have been used in 1964 as well). All such TH-400's & Jetaways share the same bellhouse bolt pattern except Chevy. Unfortunately, most TH-400's were in big cars, and are the long-shaft version, with a tailshaft about a foot long. Luckily, these transes are "easily" [to a trans repairman] converted to an A-body-friendly short shaft trans by simply swapping the output shaft & rear housing, which can even be donated from a Chevy short-shaft trans.

BE AWARE: These early big-car versions of the TH-400 often have no provision for a speedometer drive, as the cars used the left front wheel to drive the speedo. So, you might need to round up three transes to make one short shaft switch pitch TH-400: The '65-'67 SP for the converter & workings, a later BOP TH-400 for the correct bellhouse pattern case with speedo drive accomodation, and a short shaft (even a Chevy type will do) to supply the short output shaft & tail housing.

Due to a one-year-only, hard to find filter configuration, the 1965 TH-400's are slightly less desirable than later versions, though they will certainly work just as well. The "early" hard to find filter was actually used through 1967.

The 2-speed Jetaway trans used behind the V-8 [not the L6] in the 1965-67 models also utilized an SP torque converter- these may make good and fairly plentiful converter donors. The matching fit has not been checked yet, but chances are very good that they will interchange. Previous version of this guide indicated that Jetaways up to 1969? might have had the variable pitch system- however, according to the 1968 Chassis Manual, that feature was no longer available on the Oldsmobile TH-400 or the Jetaway after 1967.

How To Identify a SP trans or Converter by Examination
The trans is very easy. Look for TWO electrical terminals in the LH side connector. This is not a *guarantee* that the trans is SP, as later units used a wire from the trans to help control spark advance. However, if there are 2 electrical feeds, and the year of the trans is '65 thru '67 [see tag on RH side], then it almost surely indicates an SP trans.

To be certain, simply examine the trans input shaft. You will see a smaller 1.0" fine-splined shaft protruding furthest. This is the same for both types. You want to examine the larger 1.5" or so coarsely splined protrusion that surrounds the 1" shaft. The difference here is obvious.

This is the key: the SP trans has a *short* splined section, 1.40" diameter by dial calipers, about *1/2"* long, behind which is a *very smooth* section of shaft, slightly *larger* in diameter [1.43" by calipers], and about 1-1/4" long, with a 0.160" oil feed hole in the bottom. Behind that, the shaft necks down to about 1.270" diameter for some 3/4" and after that enters the trans.

In contrast, the standard constant-angle stator transmissions have similar shaped splines on that larger input shaft, but these splines are much longer, about 1-1/8" along the shaft, and behind the splines, the shaft is SMALLER in diameter [1.270" or so] and NOT finely machined.

Another way to identify a switch pitch TH-400 is by the pan. You will see three dimples and a shoe heel impression. The shoe heel impression is where the filter picks up fluid and the pickup is shaped just like a shoe heel.

How To Identify a SP Converter by Examination
In making this guide, an SP converter from a 1965 "OA" code Switch Pitch trans, and "normal" converters from an early 1970's TH350 & one from, though not necessarily original to, a 1968 "OW" TH-400 non-SP trans were examined.

On the back side of both TH-400 converters, near the shaft that goes into the trans, about 2" from the converter's snout, were seven plug welds about 1/2" diameter. The TH350 converter did not have these welds. A previous version of this guide indicated these welds might be a clue as to whether the converter is SP or not, but it now looks like more of an indication as to whether the converter is TH-400 vs. TH350 caliber.

The best SP ID tip involves looking down into the converter. You should remove most of the oil from it in order to see the details. This is easily done by vacuum pump or siphoning with a small hose [1/4" or so] overnight. Matching the trans shaft, of course, farthest down in will be the splined hole for the 1" trans shaft. Above that will be the more coarsely splined section which mates with the trans's larger shaft.

KEY: in the SP converter, there will be virtually NO GAP between these two splined sections, with the larger coarsely splined hole, about 1" long [deep?], beginning at a depth of about 3-7/8" down from the edge of the converter snout.

With the standard converter, there *is* a gap of about 1" between the smaller fine-splined section and larger coarsely splined section. [Aside: Into this gap, you can guide a stiff 1/4" hose to reach even more oil when siphoning the oil from these units.] While this unit also has a coarsely splined hole depth of about 1", these splines start about 2-7/8" down from the edge of the converter snout, some 1" higher than the SP converter.

You are now an Expert at identifying SP transes & torque converters while perusuing the piles at the scrap metal dealers.

[ Thanks to Chris Witt, Bob Handren, George Nenadovich for this information. ]

Torque Multiplication
The torque multiplication is 30% greater at stall in high stall than low stall (2.6 fold vs 2.0 fold). It makes it damn near impossible to hook up a healthy big or small block engine even with street gears without some very sticky tires. However the slip at all engine speeds in high stall is much greater and for best results it is best to leave high stall around 2/3 to 3/4 way through 1st gear. The launch is dramatically improved if you can hook up.

A car with 2.93 gears will accelerate from a standstill like the same car with 3.42-3.55 gears and a fixed pitch converter matching the stall speed of the lower stall setting of the variable pitch. Therefore you can sort of use it as a poor mans over-drive, gear the car higher and let the converter in high stall make up for the lack of a low first gear. If you switch back to low stall towards the end of 1st gear you'll be able to stay in first gear longer as the rpm's will drop 500 due to the converter tightening up. There is some information in the book How to Work With and Modify The Turbo 400 Transmission by Ron Sessions. 10 Years ago Kenne-Bell sold lots of variable pitch parts plus had lots of free info on them. I'm not sure if he still does or not, he seems to have moved more to the mustang market now.

I have always thought (through reading and explanations of how hydraulic dynamics work) that a higher stall converter has torque multiplication over a tighter converter, period. So I don't understand the comment that the high position doesn't involve any multiplication over a lower stall because it's a switch-pitch converter. Seems like a contradiction.

As to the time it takes for the switch to take place, the vanes are fighting the fluid and only kept in the high position by the fluid pressure applied through the front pump solenoid valve which controls pitch angle. It changes pretty quickly. The other way may be different. I've played with mine some but it's a while since I did the following.

When I tried applying the high pitch from low pitch at WOT it seemed to me the pitch did not change until I let up. In other words application of the fluid pressure by activating the solenoid on the front pump to increase stall didn't happen very quickly under WOT conditions, if at all. No drag race reason I can think of you'd want to do that but as best as I can recall that's what happened.

Stall speed on the Jetaway high pitch is 2800-3000 and 2300-2500 on low. The Jetaway converter was stock for the '67 W-30. One inch or so smaller diameter than the TH-400 convertor. Works great.

The whole point of the switch pitch design originally was to provide the added torque multiplication for a "passing gear" effect. The pitch which gets "switched" is the angle of the stator vanes in the converter - this same angle determines the torque multiplication. More pitch=>more multiplication. Since you can't get something for nothing, this torque multiplication is at the expense of "slippage". I suspect that what you're seeing is the hydraulic equivalent of a gearing down; in other words, the input to the converter (what is that, the impeller?) must turn faster to flow more oil through the stator and onto the turbine wheel due to the more severe angle of the stator, but this more severe angle provides the multiplication. I'm sure that there's some inefficiency involved, as the angles of the impeller, stator, and turbine blades are probably optimized for a certain rpm and changing one without matching the others will likely result in additional slippage.

As for real world driving with a switch pitch, I had one installed in a 71 Cutlass with a built 455. At the track I would launch at high stall and switch to low stall about halfway through first gear. It's been quite a while since I drove the car, but I don't recall any noticeable delay in the time it took to change pitch. Perhaps this is due to the opposite effect from the one Bob H. noted; I was trying to go from _high_ stall to _low_ stall under WOT conditions (including the use of nitrous). If Bob's theory is correct, the WOT conditions would actually contribute to moving the vanes to a low stall position. I should note, though I doubt that it matters, that this was a 69 H/O trans to which I added a switch pitch front pump and converter from Kenne-Bell. I had also rebuilt the tranny using a B&M rebuild kit.

[ Thanks to Phil Penketh, Bob Handren, Joe Padavano for this information. ]
I just wanted to add that it is possible to convert any TH-400 to a switch-pitch configuration with the appropriate pump, converter, and solenoid. Apparently the passageways are cast into all the valve bodies to allow the system to work. Kenne-Bell used to (and may still) offer just such a kit.

GM made switch pitch TH-400s from 1965-67 and also switch pitch Super Turbine TH-300 Jetaway two speeds from 1964-68. The parts to convert a TH-400 to a switch pitch can be acquired from these as the necessary parts are inter-changeable between these models. You need a variable pitch torque converter, the special front pump and the input shaft. Tou also need a oil restricter to place in an oil channel in the case, some wire and a switch. The rest of the internals are identical to a regular TH-400.

You may use a kickdown/converter switch at the carb linkage. If not, you may be able to adapt a later kickdown switch to your gas pedal or linkage, as the later cars had.

When the downshift switch is activated, it sends 12V+ to the solenoid, which kicks down the tranny.

[ Thanks to Phil Penketh, Bob Barry, Joe Padavano for this information. ]

Torque Convertor
A high stall converter does have a bit more RPM loss than a low stall converter. The GM 12" converter which has a stall speed of about 1800 RPM in most applications has about 100 RPM loss when it is fully coupled (i.e., at RPM well above its stall speed). The GM 12" converter which has a 2400 RPM stall speed in most applications (the P7) has about 200 RPM loss when it is fully coupled.

The RPM loss is the difference between the crankshaft RPM and the transmission's input shaft RPM. This loss is a direct power loss - multiply the RPM loss by the torque at the input shaft, and you have the power loss. So, a higher stall speed converter puts a little less power to the wheels.

Usually, the best way to drive one of these is to start in high stall, then when the road speed is high enough that you won't pull the engine below its power band, switch to the low stall speed. Since it takes the converter a noticeable amount of time to change, you may wish to anticipate the speed and switch it a little early.

[ Thanks to Bob Hale for this information. ]


Buildup Examples


Torque Convertors
Aftermarket converters are available and several companies can make a converter to order.


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