Best Way To Raise Line Pressure
Thread Starter
Staging Lane
Joined: Nov 2003
Posts: 50
Likes: 0
From: SW MI.
Best Way To Raise Line Pressure I was wondering by what means does the pcm raise line pressure as opposed to a shift-kit? My idea was that I think a shift-kit is a good idea but instead of using the stiffer spring supplied with the kit to raise the pressure would it be better and offer more adjustability to use tuning to adjust it? My situation is even with my converter (tci-ssf) my 1-2 part throttle shift is really hard but my 2-3 is soft (can hardly feel it). My solution will probably be to buy hptuners to adjust line pressure according to tps input. So my main questions are:
1. How does the pcm adjust line pressure and how does this compare to the way a shift-kit does?
2. Would it be better to leave the stock spring in the tranny and instead use tuning for the line pressure?
3. Does enlarging the holes in the separator plate actually increase the fluid flow thru the trans or does it just help to quicken the shifts only?
I have many more questions but we'll just start with this one. I would like to understand more about the mechanics of our transmissions and therefore learn the best ways to modify them for performance AND durability. I know there are alot of tricks that some might want to keep secret
I just would like more information about that mysterious contraption under the floorboard of my car
1. How does the pcm adjust line pressure and how does this compare to the way a shift-kit does?
2. Would it be better to leave the stock spring in the tranny and instead use tuning for the line pressure?
3. Does enlarging the holes in the separator plate actually increase the fluid flow thru the trans or does it just help to quicken the shifts only?
I have many more questions but we'll just start with this one. I would like to understand more about the mechanics of our transmissions and therefore learn the best ways to modify them for performance AND durability. I know there are alot of tricks that some might want to keep secret
I just would like more information about that mysterious contraption under the floorboard of my car PCM has control of a "force motor" PWM
regulator that varies the line pressure seen
by the hydraulics, downward from the hard
mechanical line pressure. Sometimes by a
lot. The action here is pushed around by a
lot of settings, bumped up during shifts to
meet the shift time requirement, raised to
follow the "load" calculation, etc. Here you
can command the PCM to great extent to
change the transmission behavior. Since
max line is max line, factory, not so much
risk of messing things up mechanically (in
theory). Under some conditions like WOT
shifting / acceleration the applied pressure
goes to max or near-max (98% in some of
my stock tables) anyway.
But if you go raise the mechanical line too
far you can exceed seals' pressure limits and
blow them out etc. and that would be bad.
Seems like there's something about too much
line pressure can mess up a band, punch a
hole in it too. But I doubt this is something
that a stock mechanical pressure setup would
make happen, or we'd all be crawling to the
dealership in 1st/2nd....
regulator that varies the line pressure seen
by the hydraulics, downward from the hard
mechanical line pressure. Sometimes by a
lot. The action here is pushed around by a
lot of settings, bumped up during shifts to
meet the shift time requirement, raised to
follow the "load" calculation, etc. Here you
can command the PCM to great extent to
change the transmission behavior. Since
max line is max line, factory, not so much
risk of messing things up mechanically (in
theory). Under some conditions like WOT
shifting / acceleration the applied pressure
goes to max or near-max (98% in some of
my stock tables) anyway.
But if you go raise the mechanical line too
far you can exceed seals' pressure limits and
blow them out etc. and that would be bad.
Seems like there's something about too much
line pressure can mess up a band, punch a
hole in it too. But I doubt this is something
that a stock mechanical pressure setup would
make happen, or we'd all be crawling to the
dealership in 1st/2nd....
Thread Starter
Staging Lane
Joined: Nov 2003
Posts: 50
Likes: 0
From: SW MI.
Thanks for the onfo Jimmy, So based on your explanation I'm guessing it might be best to still up the line pressure via the shift-kit then use tuning software to reduce line pressure at part throttle to achieve the desired shift firmness but then I would still have all the available line pressure for the WOT shifts. Sound like a good idea? (I hate trying to pull away from a light with a cop next to me and have it bark the tires hitting second even at very light throttle input. Local cops are not impressed :LOL
On The Tree
Joined: Jun 2002
Posts: 155
Likes: 0
From: Doniphan, MO
The PCM varies line pressure via %. It still goes back to; 98% of whatever the Force Motor happens to be set to. The factory spec for the max line pressure is 189 psi @ WOT.
Problem is, without putting a pressure guage on the tranny and running down the street, you do not know what the pressure actually is. We've seen a number that run in the 160 to 170 range, some even lower and only a few that actually hit 185 from the factory.
The force motor IS adjustable and a good tech will take a 'control reading' and know how far he has to adj the force motor to get it to 190 psi. The stock tranny can safely be set to 200 or so without damage occuring.
Higher pressure increases the heat of the fluid, so keep that in mind as well!
Mine is set to 195 psi @WOT. Still not what I'd call a 'chrisp' shift but it does 'chirp' the 1-2 shift.
Lots of luck!
Problem is, without putting a pressure guage on the tranny and running down the street, you do not know what the pressure actually is. We've seen a number that run in the 160 to 170 range, some even lower and only a few that actually hit 185 from the factory.
The force motor IS adjustable and a good tech will take a 'control reading' and know how far he has to adj the force motor to get it to 190 psi. The stock tranny can safely be set to 200 or so without damage occuring.
Higher pressure increases the heat of the fluid, so keep that in mind as well!
Mine is set to 195 psi @WOT. Still not what I'd call a 'chrisp' shift but it does 'chirp' the 1-2 shift.
Lots of luck!
Teching In
Joined: Jan 2004
Posts: 11
Likes: 0
line pressure I thought that ultimately by increasing the line pressure this would reduce the shift times which in turn reduces wear and heat on the internals of the tranny...am I wrong? This is what I thought happens....
Sure shorter shift times reduce wear on the clutches, but can increase were on everything else. Solinoids and pistons that "slam" due to higher line pressures will cause damage. Ideally, you want a combination of increased pressure and increased volume. If the 1-2 shift is too firm, you can punch the apply pin right through the band and also break the sunshell. "Crisp" shifts are ideal...Full race shifts are better left to built TH400s.
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Teching In
Joined: May 2004
Posts: 12
Likes: 0
From: NYC
Technically Iron worker, a shift kit doesn't raise line pressure(with the exception of the pressure regulator spring, which increases Initial line pressure only). What a shift kit can do is increase the "feed" and alter the accumulator rates, to a particular clutch or servo. This will then shorten the amount of time that it takes for a clutch or servo to apply. This can be done even at stock line pressure levels, which if you have ever installed a radical shift kit into even a stock trans, it will chrip the tires! The advantage to raising line pressure is to increase "clamping force" on the clutch or band when the device is locked and under tremendous loads(presumeably WOT). The piece that will do this mechanically is a Boost Valve, such as the .490 Sonnax type. It can be done electronically, or as A Bad T/A says through adjusting the back of the PCS, but also as he said, it shouldn't be done without the aid of a pressure gauge!!!
Punch through of the band isn't really a concern unless your using the "Super Servo" which I'm not a big fan of(Corvette prefered). Ironically, in manual first and second, line pressure is already boosted(as high as 200+ psi) automatically, it's just the way the trans is designed! I know that some people don't like to manually shift, but there are more than one advantages to doing so.
Punch through of the band isn't really a concern unless your using the "Super Servo" which I'm not a big fan of(Corvette prefered). Ironically, in manual first and second, line pressure is already boosted(as high as 200+ psi) automatically, it's just the way the trans is designed! I know that some people don't like to manually shift, but there are more than one advantages to doing so.
Last edited by Jeff406cid; Jun 18, 2004 at 03:45 AM.
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On The Tree
Joined: Jun 2003
Posts: 113
Likes: 0
Ideally, you want to increase the volume of fluid going to the servos/pistons to increase the apply speed, and only increase the pressure enough to prevent slippage. That would best be done by tuning it, either using a PCM editor, or an adjustable pressure regulator (force motor).
Also, if you're going to increase your apply speed, you want to increase the release speed of the releasing gear, so you don't have too much overlap (the overlap is what chews the friction surfaces up).
At the same time, you don't want to eliminate your overlap altogether, or you risk having the tranny essentially go into neutral between shifts, which, at WOT, will cause the tranny to speed up, and then the next gear will grab when the apply surface is moving at high speed (very bad).
You can destroy a tranny, or make it live a lot longer, depending on how you "program" the shift kit.
Also, if you're going to increase your apply speed, you want to increase the release speed of the releasing gear, so you don't have too much overlap (the overlap is what chews the friction surfaces up).
At the same time, you don't want to eliminate your overlap altogether, or you risk having the tranny essentially go into neutral between shifts, which, at WOT, will cause the tranny to speed up, and then the next gear will grab when the apply surface is moving at high speed (very bad).
You can destroy a tranny, or make it live a lot longer, depending on how you "program" the shift kit.
Teching In
Joined: Jan 2004
Posts: 11
Likes: 0
??? Hey all, I just got my Predator tuner and was playing around with it. What is the ideal shift pressure that will shift hard but still maintain the life of the tranny. I'm just scared becasue its getting up there in miles! thanks!
