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Turn this allen head screw counter clockwise for more pressure:
Do you see the dark square plate that pushes against the orange spring, when you turn the allen head counter clockwise, it threads the square plate closer to the spring and adds more preload. More spring pressure creates more transmission line pressure, each 360* turn adds about 3-4 psi line pressure.
If your truck doesn't downshift early enough when you coast to a stop, you should turn the other allen head screw just to the right of the pressure screw in about 1.5 turns. This screw is a slack-stop for the tv lever. The farther you turn-in this screw, the higher minimum tv lever position or throttle valve pressure regardless of external tv cable slack. If your truck is downshifting perfectly now, I'd go-in 180* or 1/2 turn if you add more line pressure.
I like to measure pressure at the accumulator port on the passenger side of the transmission. This port lets you see line pressure in all forward gears. For high performance 550+ HP, you really should be in the 165-185 range with upgraded internal clutch counts, etc.
The trans is shifting smooth and on time, just not holding peak torque.
So if I add 25lbs of line pressure I should add a half a turn to the tv stop screw?
Am I right on how to check? At idle in drive and at WOT?
I wouldn't really worry about pressure at idle, I'd be most concerned with having good pressure when the truck is physically moving down the road. You'll get the highest pressure reading in overdrive WOT.
My daily driver 98', for example, has about 105 psi at idle and ramps up to around 140 psi in overdrive. I plan to increase the line pressure before any more power mods
If I were you, I'd turn the TV stop screw if you increase pressure at all.
Where should I worry about upping line pressure with my set up and the dvs ill be putting in soon? I lock the tc up in 3rd when someone thinks their car is fast and on the highway in od.
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The answer to your question is: it depends.
Stock max line pressure as measured in the accumulator port on 47rh/re is around 90-96 psi.
In my opinion, for long transmission life, the stock transmission is capable of stock power. For short term border-line slip power, the stock healthy tranny is good till about 250 HP.
The torque capacity of a clutch pack or lock-up clutch in the torque converter is directly related to the coefficient of friction between the clutch disk wet with tranny fluid, and steel it's coupled with and the hydraulic pressure appling the clutch pack. In that scenario, double the line pressure would double the torque capacity of the clutch.
So to answer your question, if your tranny is bone stock and healthy, and you plan to exceed 250 HP or roughly 500 ft/lbs of torque, you need to increase pressure or it will shortly die. If you plan to use 250 HP or less on an infrequent basis, it will probably be fine.
A high mileage stock transmission will not like elevated line pressure due to old tired seals. A healthy stock transmission will not like more than 120 psi with the stock 2nd gear band strut because it will bend and cause 2nd gear to fail.
A line pressure example:
The 3rd gear direct clutch pack is a common failure point in turned up power Cummins trucks with a stock transmission. 3rd gear is a shifting clutch so it takes quite a bit of abuse on the 2-3 gear shift, plus the fact that the motor is up to full power by then.
For example, the stock 3rd gear (4) clutch pack with stock ATF+3/4 fluid with 90 psi line pressure starts to slip at roughly 250 HP or 500 ft/lbs of torque.
If we simply raise the line pressure to 135 psi which equals exactly 150%, this clutch will now start to slip at 375 HP or 750 ft/lbs of torque.
If we also increase the coefficient of friction by say 5% using hydraulic fluid in the transmission instead of ATF+4 in the above case, we would then have a 1.58% increase in capacity to 394 HP or 788 ft/lbs.
Let's say the clutch count was also upgraded to (6) total friction disks which equates to an increase of 150%. Combined with the increased pressure and upgraded fluid, we now have a total capacity increase of 236% for an ultimate slip rating of 590 HP or 1180 ft lbs. Long term useage, that equates to 470 HP 940 ft/lbs of torque.
Unfortunately, there are multiple clutch packs in the transmission and not all of them can easily receive a 50% increase in friction material. However, line pressure positively affects the torque capacity all of the clutches and bands that are prone to slip under extreme torque input so line pressure is by far the easiest and cheapest upgrade you can perform to make the transmission hold more power. This is why I recommend 165-185 psi for 500 HP trucks with upgraded band strut, lever, anchor, and new seals. 200+ psi should only be used on race applications because I suspect components of the transmission will not have a long service life such extreme pressure.
Transmission fluid is another easy upgrade but it is difficult to quantify how much torque capacity is increased with the various fluids on the market. However, in my experience, synthetic fluids such as ATF+4 perform the worst when it comes to preventing slippage in the tranny. Dexron 3 is slightly better than ATF+4, they Ford Type F is about the best red fluid you can run for high friction/low slippage. Hydraulic fluid seems to be the best fluid at reducing slippage although it is generally high viscosity so not friendly at 100% hydraulic fluid for cold climates or daily drivers trying to maintain peak fuel economy. I personally like to have a 50/50 mix of hydraulic fluid and regular Dexron 3 or Type F (whichever is on-sale).
Well that was a lot to take in but I have a lot more knowledge now. Thanks for such a time consuming answer!
I am without a doubt over 250hp so that being said I think ill up my line pressure and change the filter and fluid.
What about tightening bands? I have heard of ppl doing that to get them by short term.
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160hp pump, .025dvs/7x.011 sticks( poor little hx35 , thanks Will! ),racked barrels, 2095 rack plug, AirDog2 165, AFC Mods, AFC FF, Plate MIA, 16.5*timing, 4ks, 60lbers, boost elbow@40psi, stock down pipe to 5" stacks. Built 47rh and hx52 next.
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