Some LT1 4L60E transmission tuning observations (8051)
I promised lionelhutz I'd start a thread on what I'd learned about 4L60E tuning in 8051 LT1 cars, specifically B-cars. Some of this is my own learning, and much of it is what I've gleaned from others who wish to remain anonymous. To all those who have helped me, thank you.
Let's start with some hydraulic theory:
1. The 4L60E builds pressure via a variable displacement vane pump located in the front of the trans, just behind the torque converter, and driven by the torque converter shell. If the engine is running, the pump is running.
2. Line pressure is regulated via a balance circuit. Two small springs push the vane pump's outer ring (the "slide") offcenter which increases the displacement. Line pressure pushes back on the other side, trying to put the ring back on center, reducing pump displacement. The Electronic Pressure Control (EPC) solenoid (also known as a force motor) runs on a duty cycle (pulsewidth-modulated) which, when deactivated, allows leakage out of this circuit which then lets the springs win and more line pressure is generated. At idle, the EPC solenoid is fully on, fully closed, and the trans is making min line pressure except for in reverse, which has a separate boost valve to give more clamping power. It should be noted that the stock pump slide springs tend to give way to centrifugal force around 5500rpm, softening the shifts at or above this RPM. I think this is a built-in safety feature for stock vehicles but would not be good for peaky motors making lots of power above here. Sonnax makes a stronger spring, and for racing applications, some builders will physically block the pump slide all the way over for max pressure no matter what.
3. GM built the 4L60E so in the event of an electrical problem or electrical power failure to the trans, the transmission automatically defaults to max line pressure and still has manual 2nd, 3rd, and reverse. People call this "limp mode" but the only time I can ever find that GM used that lingo was with the Northstar engines during a catastrophic coolant loss event. Pull the codes and fix the problem and you're back in business.
4. The 4L60E has no built-in pressure sensor to provide feedback to the PCM, so all it can do is gauge shift quality based on time it takes to complete a shift. Ebay seller cmbclalb sells a good one that I've used for a long time. A healthy 4L60E will have around 70psi at idle in Park, Neutral or Drive, and climb to around 150psi in reverse. You should see over 200psi at WOT at some point in 1st gear.
5. The 4L60E is a bit of a hybrid transmission in that the shift solenoids command the START of the shifts, but the actual shifting is still completed by hydraulic valves, unlike something like the Chrysler 41TE where the solenoids directly fill the hydraulic circuits with no additional valves. Sometimes these shifts take a long time to complete, so if you're hitting the rev limiter and you have sufficient line pressure during the shifts, you need to reduce the shift MPH to get the shift to complete on time. A looser converter, numerically lower rear axle, more engine torque, and more vehicle weight will all exacerbate this late shift behavior.
6. Shifting harder IS shifting faster, and vice versa. It's really a balance point. For modded vehicles, the only way to get your 4L60E shifting perfect assuming it's in good mechanical health is to dial it in over the course of a datalogging/tuning session, or in my case, a road trip. Avoid hard shifts, period. It is needlessly shock-loading the transmission and driveline. The clutches are there for a reason, and smooth quick shifting will not wear the clutches at any appreciable rate.
7. The 4L60E is now a mature transmission since it's been out for so long. Sonnax and others make many upgraded parts for it. There is no longer any excuse for an unreliable 4L60E. If your builder can't keep the 3-4 clutchpack together and you have tuned it for good 2-3 shifts, it is HIS fault, period. If your builder thinks Sonnax is crap, then you need to find another builder as I have found no one else who has explored the real reason for 3-4 clutchpack failures (input housing distortion and valvebody hydraulic leaks) more thoroughly than Sonnax. I do not work for Sonnax and never have, but I have spoken at length with their lead 4L60E parts developer and he knows his stuff.
8. There have been various electronic shift improvers marketed throughout the years. These are basically resistors spliced in series with the EPC solenoid, reducing current and thus shifting line pressure up across the board. It "feels" firmer, but you've also needlessly raised line pressure at idle and part throttle while doing nothing for WOT. Avoid these components.
9. A couple of quick notes on valvebodies, accumulators, bushings, and hole drilling.
A. Valvebodies, especially ones with higher mileage, are very likely to have more leaks around the valves. Pay attention to this, and a good rule of thumb is that if Sonnax makes an aftermarket valve for a particular spot in the valvebody, they did so for a reason.
B. Accumulators are basically fluid shock absorbers that work only at part throttle (at WOT they are slammed against the end of the bore and they don't have much effect). It was fashionable for a time to block accumulators in place in high-power applications. I would not recommend this for any street car as there is no way to dial out enough pressure to soften the shifts enough to compensate. This is especially true for the 2nd accumulator (1-2 shift). If you have taken out all the line pressure from the 1-2 shift, I would check the accumulator first; then check the 1-2 feedhole to see if it got drilled real big, then the servo to see if someone put in a bigger servo, and finally the 2-4 band to see if someone installed a wider band.
C. Anytime you have a 4L60E rebuilt, have all the bushings replaced, and install any upgraded Sonnax bushing Sonnax makes. This is the number one way you can help avoid internal leaks and maximize fluid control.
D. Be real cautious about big hole drilling. A little goes a long way. Also, don't let anyone convince you to enlarge the shift solenoid feedhole. If you do, it'll actually take MORE TIME to complete the shifts because those circuits are a feed-bleed setup that requires the feed and bleed orifices to have certain relative sizes.
10. ALL of the 4l60E electronics can be accessed and replaced without dropping the transmission.
Tuning suggestions:
1. Just like engine tuning, the transmission MUST be in good health before you tune it. If it's got low line rise, fix that first before you tune. Commanding max pressure is a fool's errand if the trans is only generating 150psi at WOT and you really need 200+ to get good shifts in the vehicle/powertrain setup you have.
2. Line pressure is calculated by adding up the various line pressure tables and then dividing that by the "max line pressure" value to get a % duty cycle for the EPC solenoid. DO NOT RAISE THE MAX LINE PRESSURE value. You would effectively be lowering pressure across the board by doing so. BTW the units are wrong - it's not psi. It's just counts.
3. Think of line pressure tuning as two separate regions; during shifts, and between shifts. If the trans is not slipping in gear, then leave the line pressure tables alone and ONLY reduce the "Shift time (sec) vs. TPS vs. shift." In the last LT1/4L60E I tuned, the WOT shifts were perfect at 0.600 sec for 1-2 and 0.875 sec for 2-3. Leave the 3-4 shift at 0.00 seconds because the 4th gear apply piston is small and needs no softening. That 1-2 shift value gave me just a slight chirp from the rear tires. I tried 0.400 sec but it was too harsh and barked the tires. 0.800 sec wasn't firm enough and was actually cutting pressure during the shift according to the logger.
3. I would only up line pressure if you think it's slipping, OR if you installed a larger throttle body which is causing more engine torque to be applied for a given TPS. If anything, lower line pressure during cruise will give you better fuel economy because the pump isn't working as hard.
4. TCC tuning is a bit of an art. Lots of useful flags in the 8051 binfiles for keeping the TCC locked during various TPS% events, downshifts, etc. I've tried a lot and personally I like the way GM setup the stock TCC behavior. The TC serves as a rotational shock absorber; shifts without the TCC unlocked are jerky and unpleasant.
5. The '93-94 4L60Es did not have a PCM-controlled TCC apply rate. That came in 1995, with the PWM-controlled apply solenoid & valve. The valve tended to wear the valvebody and cause leaks (the P1870 code in '96-up). You can disable this in the PCM by setting all the TCC apply/release rates to 100% and 0%. If you have a '95 4L60E, strongly consider repairing the valve correctly via a Sonnax kit (either find a shop that has the $$$ reamer and take the valvebody to them to do it, or just buy the reamer and rEbay it when you're done). Don't just block the TCC regulator valve in place at full open, because this will put max line pressure into the torque converter, ballooning it during WOT and possibly damaging the crankshaft thrust bearing in the process.
I would strongly suggest going for a drive with a real line pressure gauge hooked up to the trans and a datalogger going, monitoring the actual line pressure versus commanded line pressure, in gear, at idle, during shifts, and at WOT, BEFORE doing any tweaks to the tune. You will learn volumes about what the PCM is doing.
