The converter will make a very small difference to the way the transmission feels when it shifts. Transmissions that seem to shift a little abruptly or harsh usually do so as a result of having a 'shift kit' installed which is a selection of small parts some people install to alter the road speed shifts
occur at and the abruptness of the actual engagement of the new ratio in the next gear. I am not a fan of shift kits but they are very common and popular.

The torque converter is a fluid connection between the engine and the transmission. I'll give a somewhat oversimplified version but it is pretty much the basics. There is no direct mechanical connection between the engine output and transmission input. Instead, the converter is a pair of turbines, one driven by the engine pumping oil to another that drives the transmission input. The oil is recirculated, but the principle isn't much different than a hydro electric turbine really. When the amount and speed of oil being pumped
by the engine turbine is low, there is little power transmitted to the transmission and this allows the vehicle to idle in gear with the fluid sloshing through the stopped transmission input turbine without turning it. As
engine speed rises, the fluid flow from the engine pump turbine increases and eventually becomes great enough to turn the transmission input turbine against whatever resistance there is. The point at which the engine cannot turn faster against this flow or that the transmission turbine begins to turn is 'stall' speed. The magic part of the torque converter is it is able to recapture the energy in the returning recirculating flow from a stalled or slower running (than the pump) transmission input turbine and add this energy to engine pumping pressure, which increases the power (torque) available. That's why its called torque converter.

Stall speed varies with the design of the converter but also with the amount of power available in the first place. As a result, stall speed is not an absolute, it varies a little between different vehicles even with the same converter. The original factory installed GMC converter with a very healthy engine would have about a 2300 rpm stall. A very large increase in horsepower might raise this stall speed modestly, doubling power, it might stall at 2600. With half the power of a GMC, it might stall at 1900.

If the stall speed is very high, the transmission feels unresponsive and loosely connected to the engine, a real 'slushbox'. A very low stall speed converter feels responsive and 'connected' much like a manual transmission but has the disadvantage of having weak 'torque multiplication' characteristics. The switch pitch converter you hear about that had two stall speeds was an attempt to address this situation, but usually what's done is the manufacturer selects a compromise stall speed like the GMC originally had. The GMC factory converter is a pretty good balance between adequate torque multiplication under stall speed and responsiveness in normal part throttle driving. This directness helps keep the converter from
slipping which is where transmission heat comes from as well.

If stall speed was very high, you would find shifts were somewhat softer than usual, and if the stall speed was very low you would find shifts a little more noticeable. They would still occur at the same speeds as ever and there would be little difference in shifting overall. The shift schedule of the transmission and modifications like shift kits are the main influences on shift timing and quality.

If you are not currently having any converter problems I certainly would not waste money installing one as a pre-emptive strike. Its significant that most failing converters will not fail on the road and strand you, they usually suffer a slow deterioration over time and you can generally limp out of trouble. If you have a transmission rebuilt for any reason it might be a good idea to look into replacing the converter while you're in there. Certainly any time the converter seal is accessible it and the support bushing for the converter should be changed (in front pump housing).Brent Covey
 
 

This is a scary Picture

SUMMARY

I will list the references for the Torque converters to be at fault and for the Engine builders to be at fault.  But you will see there is  no clear consensus of opinion and certainly no facts to confirm one scenario over the other.

There are recommendations in the references for Torque Converter  construction , vendors, and setup tips for the thrust Bearings and lower end assembly.  We interviewed mostly engine builders and users, the transmission builders did not answer my email requests so infer from that what you will.

We discussed the topic until we got tired and did not come to a recommendation based upon fact.   So as always, read the material, and make your own decision....  Good luck gene

Before I get too far its important that I remind people 'ballooning' and the *crank thrust failures* from high converter feed pressures ideas are separate things.

When people talk about a converter ballooning, they mean its inflated itself much like a balloon, its distorted and misshapen from extraordinary internal fluid pressures caused by a fatigue failure at high rotational speeds. The weight of the oil in the wildly spinning converter is able to deform it from
the centrifugal forces. This distortion generally would take the form of the hub blowing wider across the converter axis, the converter sort of starts to turn itself inside out. No GMC will ever suffer this failure, its something that takes incredible power at very very high rpm on an already abused and end-of-fatigue-life part.

The thing I am attempting to address is the crank thrust thing. There has been a theory that very high converter fill pressures (perhaps as high as 250 psi) multiplied by the surface area of the converter neck can account for why some presumably improperly rebuilt engines have suffered failures of the main bearing that formed the thrust surface for the engine. The idea is the high pressure filling the converter tends to attempt to propel it off the transmission towards the engine and the crank bearing resisting this
pressure wears out and fails from it.

I have not accepted this as a reasonable cause of the crank bearing failures- I think the problem is lousy workmanship. I also don't think the converter fill pressures tend to get to very high levels in a properly set
up vehicle, which further discounts the transmission induced engine failures theory.

So far my incomplete results indicate the transmission pressures remain fairly low if flow is not restricted, typically between 15 and 40 psi. Completely shutting off flow in 1968 on THM 400's and 425's will cause pressures to rise to mainline. There are a few parts combinations I haven't tested yet, and I also need to determine what degree of restriction is caused by the cooler lines themselves.

I have found it interesting that crank thrust failures are extraordinarily uncommon on factory assembled and well maintained GM engines. GM certainly will have an understanding of their own transmission and I am sure the THM 400/425 does not cause a situation that exceeds design loads for the bearings in the engine, so I need to see if the shift kits (most add a spacer to the pressure regulator pop-off valve) might effect things or any other common modification throws things off balance. It might turn out that
the pressure regulator (which is the circuit the converter is tapped from) can be modified to drop pressures in the converter fill circuit to the minimum necessary. Anyhow I have a good idea what's going on now, but haven't tested everything I wanted to.

I'll be certain to write it up when things are complete, but my feeling is, at least on a stock transmission, theres really no issues with the converter fill pressures. I suspect this will prove the case on the others as well, even with modifications.

As far as converters go, I use very stock factory type replacements in every case, whatever is a match for diameter and stall speed for the vehicle its out of, the GMC for example used a very ordinary GM converter. A standard 13" switch pitch converter is just fine for those. I have quite often used converters from wrecked cars when I had some confidence in their history in fact. Any stock converter can be rebuilt in any metropolitan area by  a local outfit, theres always someone in every city, and costs are very minimal. My absolute preference would be new GM if cost was no object, and the parts were available.
Hope this is interestingBrent Covey

There has been a lot of posts lately about adding auxiliary transmission coolers controlled by a thermostatic valve.  If you do, I would highly recommend that you add a temperature measurement on the line going into the 4-way valve and another one on the line coming from the cooler, before it goes through the valve. That way you can tell if the valve is working and also what your cooler is doing.  I had a thermostatic 4-way valve on my auxiliary transmission cooler and it failed closed and caused all kinds of grief as I thought the transmission was over heating and I couldn't understand why.  I also had a thermostatic control 4-way valve
on my auxiliary engine oil cooler and it also failed closed which caused some more gray hairs before I found out what was going on.  I have sense taken the valve out of the A/T cooler and I now have a manual valve on the engine cooler.  I also have a temperature measurement in the engine pan and one going into and one going out of the cooler.  Also have the same arrangements on the A/T system.

The thermostatic control valves are spring loaded to the closed position and the thermostatic unit opens the valve when the oil gets up to around 170 F.  The thermostatic unit is what fails and the spring holds the valve closed.  The way the valves are designed, there is very little
chance that a valve will fail open unless the spring breaks.Chuck

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After radiator's transmission oil cooler, I've got an 8-tube (16 passes) external cooler, about 10"x18", mounted in front of the radiator below the a/c condenser.

Normal running, level ground, outside temps 90+F, the engine temperature was very steady at what should be the thermostat's 195F.  The engine oil temperature generally ran 25+F more than that.  The transmission pan temperature slowly reached 185F-195F.

Climbing mountains (up to 7%+ @ 90+F) for 5+ miles, the pan temperature would generally top out at about 225F with the converter about 50F warmer. If I really abused it, from necessity or curiosity, such as by holding 3rd gear & full throttle down to 45 mph, they converter temp would go to 300F+ and the pan temp follow right behind by 50F - 60F.  On the worst grades at 95F I could always keep the converter output temp below the 275F onverter/225F pan readings which I consider to be my normal maximum permissible temperatures.

My primary reference for transmission temperatures is Appendix 8-1 of the Chevrolet Motor Home Chassis Service Guide.  It has an excellent 3 page treatment of the subject including cooler comments.  Some quotes:

    "OIL TEMPERATURE MEASURED AT CONVERTER OUTLET TO COOLER   300F is the maximum temperature.  This is the normal place to install a temperature gage or signal.  The temperature in this location will vary significantly with each vehicle start-up or hill.  If the temperature reaches 350F, reduce throttle.  To lower the transmission temperature with the transmission in NEUTRAL, run the engine at 1,200 RPM for 2-3 minutes to cool the oil.  Do
not allow the converter outlet temperature to exceed 350F.  Keep a close check to prevent the engine cooling system from overheating.  350F would be typical of rocking the vehicle in mud, snow, or sand, or a transmission in stall (full throttle, no vehicle movement).  When the transmission is in stall, the transmission will develop heat at a rate of one degree per second
of stall.

    "OIL TEMPERATURES MEASURED IN THE SUMP OR OIL PAN
    150F -- Minimum operating temperature for continuous operation.  It is possible in low ambient temperature to overcool the transmission with oil to air-type coolers; it is hard to overcool if used in conjunction with oil to water coolers installed in most standard automotive radiators.
    190F-200F -- Maximum oil level checking temperature.  Beyond this, readings are not reliable because of expansion.
    285F -- Maximum sump/oil pan temperatures for short duration such as a long hill climb.
    300F -- Metal parts inside the transmission begin to warp and distort in varying degrees, seals melt rapidly, and transmission fluid life is extremely short due to oxidation and distress.

    "AUTOMATIC TRANSMISSION FLUID OXIDATION  Automatic transmission fluid
can provide up to 100,000 miles of service before oxidation occurs under normal operating temperatures of about 170F.  Above normal operating temperatures, the oxidation rate doubles (useful life of the fluid is cut in half) with each 20 degree increase in temperature.  The approximate life expectancy at various temperatures is a follows:
        Degrees F    Miles
             175            100,000
             195               50,000
              212              25,000
              235              12,000
              255                6,000
              275                3,000
              295                 1,500
              315                    750
              335                    325
              375                      80
              390                      40
              415              Less than 30 minutes
NOTE:    THE ABOVE CHART IS BASED ON THE ASSUMPTION THAT OIL TEMPERATURE REMAINS CONSTANT FOR THE MILES INDICATED.  TEMPERATURES WHICH APPEAR FOR SHORT PERIODS, SUCH AS CLIMBING HILLS, ETC., WOULD NEED TO BE AVERAGED AGAINST NORMAL OPERATING TEMPERATURES TO DETERMINE ACTUAL LIFE EXPECTANCY."

 They caution against using Type F vs Dexron IIE because Type F is a higher friction fluid, yielding higher shock loads and harsher shifts.

Also state that odor and color change ARE NOT satisfactory criteria for recommending fluid drain and refill because "With the Dexron II fluid, rapid
loss of the red color and darkening of the new fluids are normal and DO NOT
affect their performance.  Contrary to past performance, the service
technical SHOULD NOT CONSIDER A DARK APPEARANCE OR BURNT ODOR as the signal to change fluid. ... Short of a laboratory analysis the owner's manual drain recommendation should be followed."

 Their position is somewhat neutral on after market coolers, basing the decision on initial cost, need, potential extra leak points, line fatique/failure, and installation quality.  To judge need (in the absence of a gage) estimating temperature rise from dipstick readings.

"NOTE:  After-market transmission temperature gage should be installed in the lower (hot) oil line as viewed from entering the radiator. "After-market external oil to air cooler should be installed after the GM transmission cooler.  The lower (hot) line should go first into the lower
fitting of the GM radiator cooler then out from the top fitting to the after-market oil to air cooler.
"Extreme cold weather may require the after-market oil to air cooler to be covered so not to cool the oil to (sic) much.
"After-market external filter should be installed in the lower (hot) oil line to prevent any debris from reaching the radiator cooler if the filter is being installed in conjunction with a  transmission failure or overhaul."

They comment on the possible need to cover the external cooler in extremely cold weather to prevent overcooling.

They make no mention of reduced pressure/flow caused by external cooler.

SO, there you are!  I don't think it's likely we'll get anything more definitive from GM than those comments.  While the Chevy P-30 uses a different transmission, this seems to be generic information.
Ken Henderson

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If you read the following from the Western States web site, you get both sides of the story:

http://www.gmcws.org/techcenter/99-12tc27.html

Both of the letters above are quite eloquent in their descriptions of the problems, but what is the final answer.  This is an expensive issue, New motor, new torque conv. and lots of labor.
Can we test for this, should there be a procedure for the cranks, should we be looking at the crank movement every trip ??  inquiring minds want to know.

Statkus point is, if the crank was not machined or setup correctly, it would not wait 20,000 miles to wear out it would go out in the first 200 miles..

If the torque converters are doing it, why did the OEM GMCs' go 100,000 miles without this problem.  What do we buy or what specs do we use to keep this from happening ?

Mondello suggests clearly that balooning destroys the Crankshaft:

7150 Torque Converter
heavy-duty furnace brazed Torrington roller bearings, heavy-duty 4140 steel hub Sprague, new alloy spines, this torque converter is for sever towing conditions and motor homes.  These converters have pressure regulating, return oil holes to equalize the pressure internally from front to back so the converter does not balloon and push the crankshaft forward in the engine, which causes the crankshaft thrust clearance to burn up in the engine resulting in low oil pressure and finally engine failure. The stall speed is 1800 to 2000 rpm.  A plus feature is an anti ballooning plate that keeps your converter running for a long time.  This is the most effective , efficient , torque multiplying and strongest torque converter available any where plus it saves your engine's crankshaft from failing.  This converter fits all 400 and 425 Turbo-Hydramatic transmissions and id available in 3 lug or Allison 6 lug bolt patterns.  We also recommend our No. OL-1205-fp, heavy duty billet steel flexplate for this torque converter.  This is the last flexplate you will ever have to purchase for your motorhome.

$325--- my catalog is 1998 so I am sure they have not come down..