How to reduce 4L60E line pressure in Reverse
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From: Ann Arbor, MI
How to reduce 4L60E line pressure in Reverse When a 4L60E (4L65E, 4L70E) is shifted into Reverse, the "Reverse Input" circuit applies pressure on the Boost Valve which increases line pressure; presumably because the Reverse clutches need it to engage properly.
My question is: Is it possible to reduce the line pressure in Reverse, and how?
How the obvious question is: Why would I want to do that?
Reason is to reduce load on an engine when Reverse is first engaged. Line pressure is created by the pump and the higher the pressure, the higher the load on the engine.
My engine has a large cam and therefore has very little power and stability at idle. Shifting into Reverse often stalls the engine. My problem is worse because I also have Hydroboost braking which uses PS pressure for power brakes; therefore pressing on the brake also adds some load to the engine. As a result, if I press on the brake and shift into Reverse, it very often stalls the engine. I suspect many others with wild cams also have this problem.
Finally, since I built my 4L60E with a larger boost valve and other mods which always increase line pressure (and load on the engine), I suspect my default line pressure will be enough for Reverse. In any case I am willing to try it as I have spent endless hours changing tunes to stop the stalling without much success, short of idling at 1200+ RPM. Hopefully I only need to drop the valve body to make such a change.
My question is: Is it possible to reduce the line pressure in Reverse, and how?
How the obvious question is: Why would I want to do that?
Reason is to reduce load on an engine when Reverse is first engaged. Line pressure is created by the pump and the higher the pressure, the higher the load on the engine.
My engine has a large cam and therefore has very little power and stability at idle. Shifting into Reverse often stalls the engine. My problem is worse because I also have Hydroboost braking which uses PS pressure for power brakes; therefore pressing on the brake also adds some load to the engine. As a result, if I press on the brake and shift into Reverse, it very often stalls the engine. I suspect many others with wild cams also have this problem.
Finally, since I built my 4L60E with a larger boost valve and other mods which always increase line pressure (and load on the engine), I suspect my default line pressure will be enough for Reverse. In any case I am willing to try it as I have spent endless hours changing tunes to stop the stalling without much success, short of idling at 1200+ RPM. Hopefully I only need to drop the valve body to make such a change.
TECH Junkie
Joined: Dec 2012
Posts: 3,615
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From: Iowa
First things first what ECU are you running? Stock ECU are actually quite capable at adding transitional airflow for a shift into reverse from park or neutral.
I've had similar issues with my cammed vehicles so I feel your pain.
As for reducing reverse pressure, there are a couple ways around it. First, I would log your EPC amperage and see what it's up to. If you can do your adjustments here, they will be the easiest!
Mechanically, you can block reverse boost entirely. This can be done in the pump. You'll also need to then vent the rev boost land to avoid pressure buildup during leakage that can cause runaway pressure. I wouldn't recommend this for anything that may need to back up a trailer. For just a coupe or something it will likely be OK but still not the best fix
You could also just run a small feed/bleed system in the reverse boost circuit.
Do this by restricting the feed and introducing a bleed to exhaust. At low pump volume/pressure (aka idle) the bleed will be enough to match the small feed, and you'll have no reverse boost.
Increase throttle and let the pressure/volume come up and the feed will "overpower" the bleed and then you'll get the reverse boost effect (albeit slightly reduced from normal)
I've had similar issues with my cammed vehicles so I feel your pain.
As for reducing reverse pressure, there are a couple ways around it. First, I would log your EPC amperage and see what it's up to. If you can do your adjustments here, they will be the easiest!
Mechanically, you can block reverse boost entirely. This can be done in the pump. You'll also need to then vent the rev boost land to avoid pressure buildup during leakage that can cause runaway pressure. I wouldn't recommend this for anything that may need to back up a trailer. For just a coupe or something it will likely be OK but still not the best fix
You could also just run a small feed/bleed system in the reverse boost circuit.
Do this by restricting the feed and introducing a bleed to exhaust. At low pump volume/pressure (aka idle) the bleed will be enough to match the small feed, and you'll have no reverse boost.
Increase throttle and let the pressure/volume come up and the feed will "overpower" the bleed and then you'll get the reverse boost effect (albeit slightly reduced from normal)
On The Tree
Joined: Feb 2022
Posts: 196
Likes: 72
From: Grand Rapids, MI
What techniques have you tried to strengthen the idle characteristics? I usually get the AFR reasonable, then play with ignition timing until I get the most vacuum(lowest KPA...), then start fiddling with Injector end angle until the engine sounds happy, again watching the manifold pressure. My car is much smoother and sounds stock even with a decent cam. There is also idle spark control and IAC control but I'm not super familiar with how that operates, I just leave it alone. Haven't learned PID control yet.
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From: Ann Arbor, MI
Thank you for the helpful feedback and suggestions.
I am running a Holley Dominator. I'm pretty sure it does not respond to shifting from Park/Neutral to Reverse/Drive.
The Dominator allows tuning the EPC "Duty Cycle" according to TPS. (Low duty cycle = higher pressure). Even at Zero TPS the Duty Cycle is a low 35%; which kinda means 35% short of full pressure. Since at idle my TPS is 6%, I will set it for much lower pressure and see how that works. Bit inconvenient to measure EPC amperage, but that is a good suggestion.
I hadn't thought of needing to vent the boost land. I was thinking a boost valve without the Reverse boost land would be nice, but I don't have the ability to make such a precision part.
I like your idea of adding a feed/bleed to the reverse boost circuit. Just not sure how to do that. If I decide to go ahead later this year, I will drop the trans, remove the pump and ask for more detailed instructions.
Hopefully the tune change will help. I will try that today.
I am running a Holley Dominator. I'm pretty sure it does not respond to shifting from Park/Neutral to Reverse/Drive.
The Dominator allows tuning the EPC "Duty Cycle" according to TPS. (Low duty cycle = higher pressure). Even at Zero TPS the Duty Cycle is a low 35%; which kinda means 35% short of full pressure. Since at idle my TPS is 6%, I will set it for much lower pressure and see how that works. Bit inconvenient to measure EPC amperage, but that is a good suggestion.
I hadn't thought of needing to vent the boost land. I was thinking a boost valve without the Reverse boost land would be nice, but I don't have the ability to make such a precision part.
I like your idea of adding a feed/bleed to the reverse boost circuit. Just not sure how to do that. If I decide to go ahead later this year, I will drop the trans, remove the pump and ask for more detailed instructions.
Hopefully the tune change will help. I will try that today.
Thread Starter
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Joined: Aug 2010
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From: Ann Arbor, MI
What techniques have you tried to strengthen the idle characteristics? I usually get the AFR reasonable, then play with ignition timing until I get the most vacuum(lowest KPA...), then start fiddling with Injector end angle until the engine sounds happy, again watching the manifold pressure. My car is much smoother and sounds stock even with a decent cam. There is also idle spark control and IAC control but I'm not super familiar with how that operates, I just leave it alone. Haven't learned PID control yet.
I have AEM Widebands and a Vacuum gauge on the dash. The Dominator uses separate 1 or 2 Widebands. I tune the AFR in VE mode; the Dominator uses it Widebands to create Error/Learn tables which can be merged in the VE table while running.
I run a flat 24 degrees advance in the entire idle area. Plus a fence of 30 degrees at 800&900 RPM to help reduce stalls as 30 degrees give max vacuum. Idle Spark control is on. The Dominator will calculate a good injector end angle after entering the cam's opening/closing angles. Its Drive-by-wire which automatically does IAC, but not very quickly. I need to work more on accelerator enrichment as it generally goes very lean which doesn't help the IAC prevent a stall.
My trans mods, the Hydoboost and the dual throttle bodies make all this more complex than even usual. My current cam is a 233/248-113 with 14 degrees of overlap. I previously had a 239/251 which gave me too much tuning trouble with the GM PCM.
I'm hoping this will help others as I know I'm not the only one with this stalling problem.
On The Tree
Joined: Feb 2022
Posts: 196
Likes: 72
From: Grand Rapids, MI
I have followed many of your transmission threads so I knew you weren't a newbie. Also saw the moderator tag, but we all have stuff to learn right?
I think you may be leaving a lot on the table by not tuning the DBW IAC control. I have only tuned 1 DBW vehicle(my own, I'm not good enough to do it for a customer), but I believe the IAC is still adjustable, under the "CUSTOM" advanced idle control tab.
Once you select it, there are several PID control things that pop up, way above my pay grade. All I know is that when I turn up the "P" on the idle spark control it sounds like I have a big cam.
My engine did not like the injector end angle values that the Holley software calculated by entering my cam specs. Was better on the base tune. Base tune was -90* everywhere, when I changed it to -70* injector end angle at idle it sounded like a load was lifted off the engine. I suspect this due to the 80lb injectors I am using, they only need to be open for a very short amount of time to get the fuel needed to run compared to factory ~24lb injectors.
I think you may be leaving a lot on the table by not tuning the DBW IAC control. I have only tuned 1 DBW vehicle(my own, I'm not good enough to do it for a customer), but I believe the IAC is still adjustable, under the "CUSTOM" advanced idle control tab.
Once you select it, there are several PID control things that pop up, way above my pay grade. All I know is that when I turn up the "P" on the idle spark control it sounds like I have a big cam.
My engine did not like the injector end angle values that the Holley software calculated by entering my cam specs. Was better on the base tune. Base tune was -90* everywhere, when I changed it to -70* injector end angle at idle it sounded like a load was lifted off the engine. I suspect this due to the 80lb injectors I am using, they only need to be open for a very short amount of time to get the fuel needed to run compared to factory ~24lb injectors.
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Thread Starter
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From: Ann Arbor, MI
@wlink14 - I think your Holley Terminator and my Dominator have pretty much the same tuning options. BUT: The only DBW tune I have is adjusting the TPS according to pedal position.
Perhaps you meant Idle Settings. The "P" and "D" settings only apply when Idle Spark is enabled; I try to get the tune right without it enabled. I only enable it when done as a last resort. I don't have any kind of "CUSTOM" tab.
I have the EFI V6 software.
Interesting that you did not like the Holley calculated Injector end angles. I may have to play with this again. I have 60lb injectors (@43psi, or 69 lb at 58 psi). I current have -73* in my idle range; seems similar to yours and that is the calculated value.
Perhaps you meant Idle Settings. The "P" and "D" settings only apply when Idle Spark is enabled; I try to get the tune right without it enabled. I only enable it when done as a last resort. I don't have any kind of "CUSTOM" tab.
I have the EFI V6 software.Interesting that you did not like the Holley calculated Injector end angles. I may have to play with this again. I have 60lb injectors (@43psi, or 69 lb at 58 psi). I current have -73* in my idle range; seems similar to yours and that is the calculated value.
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Thread Starter
Moderator
Joined: Aug 2010
Posts: 7,323
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From: Ann Arbor, MI
Damn, I had no idea this "Custom" was available; I thought "GM LSx" was the only reasonable option.
Your screen shots show the same "Idle Control Parameters" values I get now. Can't find any doco on this. How did you change them?
To others, sorry we are going a bit off topic here, but solving a transmission "problem" with tuning, beats removing and modifying it.
Your screen shots show the same "Idle Control Parameters" values I get now. Can't find any doco on this. How did you change them?
To others, sorry we are going a bit off topic here, but solving a transmission "problem" with tuning, beats removing and modifying it.
