Originally Posted by customharvester2011
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).