Auto trans behavior from shift to 2nd through highway speed?

[GrandpaG]

What is the normal behavior of a stock automatic with the switch pitch? On my 65GS it shifts nicely from 1st to 2nd, and then RPMs just rise with speed.  What kind of RPM's are ya'll seeing at 55, 65? (basically stock motors).   I've read up on how switch pitch solenoid works on initial acceleration and is energized by the linkage/microswitch on the carb.  Does it do anything at highway cruise? What's normal?

[Dr Frankenbuick]

The SP and down shift are not engaged (no power to the solenoid) at normal highway cruise speed with minimal throttle. The engine should suddenly jump a few hundred RPMs and continue to rev at about 70% throttle. This is as the SP solenoid is energized and the converter changes from low to high stall.  The transmission should shift from driver to low at full throttle (assuming you are at normal highway speed or less).  This is as the downshift solenoid is energized and shifts the transmission back to low gear.  It will shift back to high gear at 80 or 90 MPH depending on rear gearing. This up-shift will happen even with the accelerator depressed fully. 

[GrandpaG]

Thanks Dr. F.  Great explanation.  What are typical RPM's at 55, 65? It just really seems like it's screaming high. Maybe I'm just not used to it.

[65GSConv]

Thanks Dr. F.  Great explanation.  What are typical RPM's at 55, 65? It just really seems like it's screaming high. Maybe I'm just not used to it.

What tires are you running on your 65 GS?  And is your rear differential ratio the standard 3.08:1 on your 65 GS (as it would have been built if the buyer did not opt for an optional  ratio rear differential)?

[GrandpaG]

Tires are 215/75R14.  I don't know what the gear ratio is (or how to determine) I'll look on the net for how to do that.

[TrunkMonkey]

What RPM are you seeing at 55-65?

401, with a 3.08 and 26.5 inch tire diameter, will be about 2200-2600 RPM at those speeds.

That will "feel" high since auto gearing and overdrives have made 1200-1800 cruising RPM the "norm".

[Rollaround]

Here’s a handy tool.

https://www.randysworldwide.com/calculators/rpm/

[65GSConv]

The SP and down shift are not engaged (no power to the solenoid) at normal highway cruise speed with minimal throttle. The engine should suddenly jump a few hundred RPMs and continue to rev at about 70% throttle. This is as the SP solenoid is energized and the converter changes from low to high stall.  The transmission should shift from driver to low at full throttle (assuming you are at normal highway speed or less).  This is as the downshift solenoid is energized and shifts the transmission back to low gear.  It will shift back to high gear at 80 or 90 MPH depending on rear gearing. This up-shift will happen even with the accelerator depressed fully.

How does the SP300 act when starting from a stop and acceleration is on a very light throttle - what are the step sequences involving switching pitch and shifting gears?

Is coasting down from highway speed to a slow crawl speed (with no brakes and no foot on the accelerator) exactly opposite that of a very light acceleration from a dead stop up to highway speed?

[Dr Frankenbuick]

The SP trans will be in high stall at a stop and while coasting with no pressure on the accelerator pedal.  There is a switch on the throttle linkage that engages high stall when there is no pressure on the pedal. This keeps the car from creeping at stop lights or at a standstill. It switches to low stall as soon as any pressure is applied to the pedal. 

The up and down shifts under light and medium acceleration are not commanded by accelerator switches. These shifts are determined by the amount of vacuum at the transmission modulator and the rotational speed of the governor in concert with one another.  These components can be adjusted (or adjustable components can be purchased) to change the shift points of the vehicle under both part and full throttle applications.           

[65GSConv]

What I was more looking for was:

• Stopped:  High Stall - Low Gear
• Just starting to move - light pressure applied to pedal:  Low Stall - Low Gear
• Moving a bit faster under light acceleration - what happens next?  Shift from low to high gear or switch from low stall to high stall?
• Moving even faster would be the choice that doesn't apply in the line above
• Steady state highway speed - High Stall - High Gear

[Dr Frankenbuick]

What I was more looking for was:

• Stopped:  High Stall - Low Gear

Check

• Just starting to move - light pressure applied to pedal:  Low Stall - Low Gear

Check

• Moving a bit faster under light acceleration - what happens next? Shift from low to high gear (resulting from governor and vacuum modulator) or switch from low stall to high stall?
• Moving even faster would be the choice that doesn't apply in the line above
• Steady state highway speed - High Stall - High Gear

No: High gear - low stall.

As it comes from the factory, the only control over the converter stall are the two switches activated by the accelerator: one at no throttle to activate the high stall solenoid and the other at roughly 70% throttle to activate the high stall solenoid. High stall is only active when there is no acceleration or when the accelerator position is at 70% or more.   

The aftermarket has some timer systems available that allows you to turn off the high stall before and after the 1-2 shift.  This allows both forward gears to be extended in low stall after high stall had exhausted its useful rpm range in that gear (some just use a toggle switch). These systems usually move the high stall switch for standing stops to the brake pedal from the throttle.  That way you are in high stall at a stop with the brake depressed, and once you lift off the brake it goes back to low stall.   

[65GSConv]

OK, something just doesn't feel right.  High stall is when there is no torque multiplication in effect - sort of like in a higher gear - right?  You want high stall when stopped because low stall torque multiplication and being in low gear makes it tough to hold the car stopped (preventing creeping). But after touching the accelerator pedal from stop, you go into low stall / torque multiplication. Meaning the engine runs faster as the same output speed. And that doesn't change ever until the accelerator pedal is at 70% open?  If barely accelerating at 70% throttle, what ground speed is that? And it doesn't go into high stall (basically output RPM = input RPM - slip) until after 70% throttle (and whatever ground speed that can occur on) happens?  Is this like driving a 3-speed manual in 2nd gear until you reach 70% throttle opening? I'm thinking the engine is running very fast at this point, and ground speed is pretty high as well.  I think I'm missing something - so I'll apologize in advance on that.

[TrunkMonkey]

The high stall is the greater torque multiplication.  (You have the concept, you just are looking at it backward).

When you take off with heavy throttle, the higher stall allows the engine to be at a higher RPM with the stall about 2400 RPM, instead of the "normal" low stall of 1800RPM, so the car accelerates quicker, then as the torque curve of the engine starts falling off, the converter returns to low stall and the car continues to accelerate in effect as if the gear ratio has changed (it is the same effect).

Higher stall means the torque converter is not transferring enough power to the transmission at idle to "pull" the car, but if in low stall, it will transmit power and the car will "tug" or creep if you take your foot off the brake.

The point was to give the "feeling" of smooth idle and no-lurch/tug/creep, and smooth and continual power to get the heavy cars moving.

Again, it "feels" (or should) like a manual car that one is slipping the clutch in a heavy car, or on a steep incline when starting off. Higher RPM.

The effect is like having a variable first to second gear that is smooth and gradual.

The lighter the pedal, the less effect of the high/low stall. The harder the pedal is pressed (and from a lower speed/RPM) the greater and longer the effect.

[Dr Frankenbuick]

There is an excellent Description and Operation section of the 1966 Buick Chassis Service Manual for the ST300 here from Teambuick.com.  Nonetheless, I will do my best to explain it from my drag racing racing experience.  Higher stall converters are less efficient, but have the potential to produce more torque in the same size converter.  A higher stall converter increases the rpm threshold needed to move the vehicle. This allows the engine to slip up to and move the vehicle at a more powerful RPM range that will take advantage of the torque multiplication of the transmission until coupling (when the turbine reaches roughly the speed of the pump).  This is great for racing. It starts and keeps the engine in the right rpm range all the way down the track. 

Now consider all of that slippage below the stall speed when you are not going full speed ahead. Heat and sluggish performance below the stall speed are the result.  Most times cruising RPMs are below the stall of the converter. This means poor mileage and an odd performance below stall speed.  If only you could you could reduce the stall speed to be more efficient and have better low speed performance.  That is exactly what the SP converter does. The direction of the vanes are changed to produce a roughly 1000 RPM difference in stall speed.  Now, you can have efficiency and better low speed performance at low stall, and more torque and a quick trip up to a more powerful RPM range in high stall. That's the beauty of the SP converter!

High stall in no throttle conditions allows the low stall to be lower and more efficient. The car would  creep and overheat the transmission in gear at a stop if there were only the low stall. It could not be used effectively for Buick clientele.  High stall at no throttle allows the practical use of the low stall.       

HTH - Steve  (Michael, I was writing as you posted)

[TrunkMonkey]

...

HTH - Steve  (Michael, I was writing as you posted)

All is good. The Switch Pitch has always been hard for some folks to grasp.
More info/explanations are always appreciated.

I have had four Buicks with them since 1976, and even when I took someone out and "demo" all of it, some folks still shake their head.

Then there is the Dynaflow and "triple converter fluid coupling", and how that one is misunderstood. LOL.

Buick had some really good engineering, but people are fickle, so a lot of those innovations went away.