Trans pressure and converter coupling

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rb0804
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Trans pressure and converter coupling

#1 Post by rb0804 » Thu Dec 08, 2011 8:40 pm

Is there an ideal transmission pressure to run? I know that sometimes in an effort to hold more power the transmission guys will up the pressure a bit. What does this do to our torque converter and its ability to couple? I know in some instances it is OK To enlarge the converter feed hole in the glide to .250" and in other applications (tire limited mostly) we may want to reduce the size of the hole a bit with a restrictor. Is it possible to hydraulic lock the converter (make it 1:1) with lots of pressure, or will balloning of the converter result?

HPDRIFTER
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Re: Trans pressure and converter coupling

#2 Post by HPDRIFTER » Fri Dec 09, 2011 11:50 am

Too much transmission pressure will push the torque converter forward, away from the transmission and wipe out the thrust bearing on the main bearing. Everyone has their own ideas about what is the correct pressure and how to limit it.

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wikd69
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Re: Trans pressure and converter coupling

#3 Post by wikd69 » Fri Dec 09, 2011 3:53 pm

HPDRIFTER wrote:Too much transmission pressure will push the torque converter forward, away from the transmission and wipe out the thrust bearing on the main bearing. Everyone has their own ideas about what is the correct pressure and how to limit it.
Shown below is the result of high trans pressure in a TCI built TH400 - originally bench tested between 240 - 250 psi line pressure. I researched this afterwards and read that standard BBC main thrust bearings are designed to handle upwards of 225 lbs of forward thrust. Any more than that and you start pushing through the oil cushion eventually wiping thrust bearings and smoking the crank like I did here. When I caught it, the crank had around .075 end play and the rods where starting to blue the block main journal bosses. Ugly.

My local trans guy (Jim at ATO) told me he typically sets line pressure in a TH400 at around 200 psi. Anything beyond that wears the pump out more quickly and contributes to heat issues in the trans. Without setting something up on a trans dyno, I have no clue how to measure the about of forward thrust resulting from a given trans line pressure. I do trust my trans guy tho and will limit internal line pressures to no more than 200 psi going forward. I belive you set line pressure in a TH400 by adding/removing shims under the bypass valve spring.

I dunno how this might compare to a Glide setup, but converters are converters and I would think they would all behave the same, regardless of which transmission they sit in front of. Having said that however, the '200 psi' rule of thumb for a TH400 may not apply to a Glide setup.

Your own mileage may vary. Hope this helps.

Image
1969 ProStreet Camaro RS Best 9.75@139 1.46 60'
Blown 427 BBC, TH400 w/Brake, Back-Half Ladderbar
Narrowed 12-Bolt, 4.10 Gears, Spool, Moser 33 Spline

http://www.marsh-racing.com/harrys_camaro-1.htm

rb0804
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Re: Trans pressure and converter coupling

#4 Post by rb0804 » Fri Dec 09, 2011 4:39 pm

wikd69 wrote:
HPDRIFTER wrote:Too much transmission pressure will push the torque converter forward, away from the transmission and wipe out the thrust bearing on the main bearing. Everyone has their own ideas about what is the correct pressure and how to limit it.
Shown below is the result of high trans pressure in a TCI built TH400 - originally bench tested between 240 - 250 psi line pressure. I researched this afterwards and read that standard BBC main thrust bearings are designed to handle upwards of 225 lbs of forward thrust. Any more than that and you start pushing through the oil cushion eventually wiping thrust bearings and smoking the crank like I did here. When I caught it, the crank had around .075 end play and the rods where starting to blue the block main journal bosses. Ugly.

My local trans guy (Jim at ATO) told me he typically sets line pressure in a TH400 at around 200 psi. Anything beyond that wears the pump out more quickly and contributes to heat issues in the trans. Without setting something up on a trans dyno, I have no clue how to measure the about of forward thrust resulting from a given trans line pressure. I do trust my trans guy tho and will limit internal line pressures to no more than 200 psi going forward. I belive you set line pressure in a TH400 by adding/removing shims under the bypass valve spring.

I dunno how this might compare to a Glide setup, but converters are converters and I would think they would all behave the same, regardless of which transmission they sit in front of. Having said that however, the '200 psi' rule of thumb for a TH400 may not apply to a Glide setup.

Your own mileage may vary. Hope this helps.

Image

When you say line pressure, you are talking about taking the pressure tap at the cooler ports or did you use another location for your pressure reading? Thanks for the replies guys.

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wikd69
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Re: Trans pressure and converter coupling

#5 Post by wikd69 » Fri Dec 09, 2011 5:21 pm

rb0804 wrote:
wikd69 wrote:
HPDRIFTER wrote:Too much transmission pressure will push the torque converter forward, away from the transmission and wipe out the thrust bearing on the main bearing. Everyone has their own ideas about what is the correct pressure and how to limit it.
Shown below is the result of high trans pressure in a TCI built TH400 - originally bench tested between 240 - 250 psi line pressure. I researched this afterwards and read that standard BBC main thrust bearings are designed to handle upwards of 225 lbs of forward thrust. Any more than that and you start pushing through the oil cushion eventually wiping thrust bearings and smoking the crank like I did here. When I caught it, the crank had around .075 end play and the rods where starting to blue the block main journal bosses. Ugly.

My local trans guy (Jim at ATO) told me he typically sets line pressure in a TH400 at around 200 psi. Anything beyond that wears the pump out more quickly and contributes to heat issues in the trans. Without setting something up on a trans dyno, I have no clue how to measure the about of forward thrust resulting from a given trans line pressure. I do trust my trans guy tho and will limit internal line pressures to no more than 200 psi going forward. I belive you set line pressure in a TH400 by adding/removing shims under the bypass valve spring.

I dunno how this might compare to a Glide setup, but converters are converters and I would think they would all behave the same, regardless of which transmission they sit in front of. Having said that however, the '200 psi' rule of thumb for a TH400 may not apply to a Glide setup.

Your own mileage may vary. Hope this helps.

When you say line pressure, you are talking about taking the pressure tap at the cooler ports or did you use another location for your pressure reading? Thanks for the replies guys.
This is the ATO TH400 I'm running now - I added this short braided line to the case pressure port just prior to installing it - it allows me to tap into and test line pressure while installed. This port carries the working line pressure in the trans. If you wanted (and I considered it) you could add an electrical sending unit here and a 0 - 300 psi gauge in the dash to monitor live trans pressure. For now, it's just normally plugged off. I would suspect both the Glide and TH350 have similar pressure ports on the trans case.

Image
1969 ProStreet Camaro RS Best 9.75@139 1.46 60'
Blown 427 BBC, TH400 w/Brake, Back-Half Ladderbar
Narrowed 12-Bolt, 4.10 Gears, Spool, Moser 33 Spline

http://www.marsh-racing.com/harrys_camaro-1.htm

HPDRIFTER
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Location: High Plains

Re: Trans pressure and converter coupling

#6 Post by HPDRIFTER » Fri Dec 09, 2011 6:11 pm

I spoke with an old friend here in DoDa that has forgotten more about transmissions than I hope to ever know. He said that you want the "out" cooler line on a PowerGlide to have no more than 50 psi. He laso said that the cooling line diameter should be left at the stock 5/16" diameter, with the bypass the same or a little larger. The pressure bypass will prevent damage to the crank thrust bearing. He said to mention that the cooling line pressure is not the same as the internal transmission pressure. For a PG, the 1/8" NPT plug on the servo cover is an accurate tap for the internal oil pressure.

I have a high speed bypass valve for fuel injection to plumb into my "out" cooler line, with a tee, to dump cooling line pressure back into the pan. It's set at 50 psi.

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wikd69
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Re: Trans pressure and converter coupling

#7 Post by wikd69 » Sat Dec 10, 2011 1:51 am

Ok, so without getting *too* overboard with this, I'll provide this excerpt from the research I did. It talks about converter pressure and crank thrust, to your point HPDrifter.

If you'd like to get the entire article, bing me with your email address and I'll send it over - it covers a lot of info on main thrust bearing failures, etc. The excerpt below is specific to converters and converter oil pressure.

I'll follow this with another issue I discovered and addressed: In my case, the use of VM300 billet input shafts - and the loss of the normal converter oil return ports in the stock input shaft.

** ** **

How does the torque converter exert force on the crankshaft?

There are many theories on this subject, ranging from converter ballooning to spline lock. Most of these theories have little real bases and rely little on fact. The force on the crankshaft from the torque converter is simple. It is the same principle as a servo piston or any other hydraulic component: Pressure, multiplied by area, equals force. The pressure part is easy; it’s simply the internal torque converter pressure. The area is a little trickier. The area that is part of this equation is the difference between the area of the front half of the converter and the rear half. The oil pressure does exert a force that tries to expand the converter like a balloon (which is why converter ballooning is probably often blamed), however, it is the fact that the front of the converter has more surface area than the rear (the converter neck is open) that causes the forward force on the crankshaft. This difference in area is equal to the area consumed by the inside of the converter neck. The most common scenario is the THM 400 used behind a big-block Chevy. General Motors claims that this engine is designed to sustain a force of 210 pounds on the crank shaft. The inside diameter of the converter hub can vary from 1.5 inches up to 1.64 inches. The area of the inside of the hub can then vary from 1.77 square inches to 2.11 inches. 210 pound of force, divided by these two figures offers an internal torque converter pressure of 119 psi to 100 psi, respectively. That is to say, that depending on the inside diameter of the hub, it takes between 100 to 119 psi of internal converter pressure to achieve a forward thrust of 210 pounds. The best place to measure this pressure is the out-going cooler line at the transmission because it is the closest point to the internal converter pressure available. The pressure gauge must be "teed" in so as to allow the cooler circuit to flow. Normal cooler line pressure will range from 50 psi to 80 psi , under a load in drive.

Causes for excessive torque converter pressure:

There are two main causes for excessive torque converter pressure: restrictions in the cooler circuit and modifications or malfunctions that result in high line pressure. One step for combating restrictions in the cooler circuit is to run larger cooler lines. Another, is to install any additional cooler in parallel as opposed to in series. This will increase cooler flow considerably. An additional benefit to running the cooler in parallel is that it reduces the risk of over cooling the oil in the winter time—especially in areas where it snows. The in-parallel cooler may freeze up under very cold conditions, however, the cooler tank in the radiator will still flow freely. Modifications that can result in higher than normal converter pressure include using an overly-heavy pressure regulator spring, or excessive cross-drilling into the cooler charge circuit. Control problems such as a missing vacuum line or stuck modulator valve can also cause high pressure.
1969 ProStreet Camaro RS Best 9.75@139 1.46 60'
Blown 427 BBC, TH400 w/Brake, Back-Half Ladderbar
Narrowed 12-Bolt, 4.10 Gears, Spool, Moser 33 Spline

http://www.marsh-racing.com/harrys_camaro-1.htm

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wikd69
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Re: Trans pressure and converter coupling

#8 Post by wikd69 » Sat Dec 10, 2011 2:58 am

Ok, so here was the other related issue I discovered. Note that this will only be of interest to you if you're running an aftermarket billet input shaft like I am with this TH400. I'm not familiar with TH350's, Glides or Flites but I would suspect there may be some parallels with them as well, so who knows.

Also, I run this heavy comp TH400 build on the street - most guys running this hard core stuff are on the track where heat and wear are not such big issues. My concerns were two-fold: keep pressures down in the converter to lessen forward crank thrust and to ensure oil flow out to the cooler to help keep trans temps down on the street. Recall that I'm running around 200 psi constant line pressure on the street with this TH400 and that contributes to a lot of heat buildup, so every bit helps.

If you'll note the first combo photo shows both a stock and aftermarket VM300 billet input shaft. The stock shaft is gun barrel drilled and ported behind the converter splines, providing a means for converter oil to flow back and out to the cooler. You'll note the billet shaft does not provide this passage - it's solid beyond the locator dimple you see in the photo. I came to the conclusion that this was contributing to overpressure in the converter and was a significant issue in rapid heat buildup.

My solution was the second photo. You'll note I had them machine relief grooves in the splines themselves, providing a passage for oil return behind the converter and on out to the cooler. I talked it over with Jim prior to the machine work and he felt it would'nt compromise power transfer under load. You'll note that since I did this modification I've launched this 3700 lb street ride at 4000 rpm on a trans-brake with slicks, picking the front end up and consistently turning 9.70's at 139 mph. So far the spline count seems to be holding :smt003

This trick worked out slick, allowing oil to route out of the coverter. It helped with heat management and I'm certain it has helped lessen converter pressure under load. I found out afterwards that this is a favorite trick with the off-road guys running down in Baja where they need the hard core trans but also need to regulate heat and wear, etc. during sustained high-speed runs.

In this second shot I also point out the Coan CaseSaver - there are several variations of this sort of thing out there, a number of them less expensive than the Coan billet goodie but just as effective.

I hope this wasn't too much info but I felt it worth sharing - I paid for my education in this stuff in long green dollars, wiped cranks and broken transmissions - hopefully my sharing this will help others skip that sort of tuition cost. :smt003 :smt003

Image

Image
1969 ProStreet Camaro RS Best 9.75@139 1.46 60'
Blown 427 BBC, TH400 w/Brake, Back-Half Ladderbar
Narrowed 12-Bolt, 4.10 Gears, Spool, Moser 33 Spline

http://www.marsh-racing.com/harrys_camaro-1.htm

rb0804
Posts: 6
Joined: Sun Jul 24, 2011 3:47 pm

Re: Trans pressure and converter coupling

#9 Post by rb0804 » Sat Dec 10, 2011 10:59 am

HPDRIFTER wrote:I spoke with an old friend here in DoDa that has forgotten more about transmissions than I hope to ever know. He said that you want the "out" cooler line on a PowerGlide to have no more than 50 psi. He laso said that the cooling line diameter should be left at the stock 5/16" diameter, with the bypass the same or a little larger. The pressure bypass will prevent damage to the crank thrust bearing. He said to mention that the cooling line pressure is not the same as the internal transmission pressure. For a PG, the 1/8" NPT plug on the servo cover is an accurate tap for the internal oil pressure.

I have a high speed bypass valve for fuel injection to plumb into my "out" cooler line, with a tee, to dump cooling line pressure back into the pan. It's set at 50 psi.
The cooling line is the 'converter out' where fluid passes to the cooler and then comes back into the transmission and re-enters the back side of the input shaft and lubes all the bushings and misc stuff back there. On some of the early powerglides the converter feed hole was smaller than the rest of the passage and I remember that a common mod was to enlarge it to .250".

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