GM 4T40-E Transmission Torque Limit

[burly]

I have a 2003 Malibu which uses the GM 4T40-E tranmission. According to GM's site on the transmission, http://www.gm.com/automotive/gmpower.../apps/4t40.htm

the maximum engine torque that the transmission is rated for is 190lb-ft. Now, the largest engine that the 4T40-E is mated to is the 3.1L, which makes 190lb-ft of torque, and is the engine that is in my Malibu. These are my questions: Is the tranmission truly only reliable up to 190lb-ft or is GM just stating that number since that is the torque rating of the largest engine they will mate it to in production? If the tranny is reliable (as in, at least 100K) at more than that, how much more? Has anyone had any experience with this transmission?

The reason I ask is, I would like to do a few performance modifications to the engine but A). the car is still under warranty B). even after the warranty is up, I want to maintain the car's reliability. Ideally, I would like to have the engine making ~200-215HP and ~210-225lb-ft of torque at the flywheel. It currently is stock, rated at 170HP/190lb-ft at the flywheel. What are your thoughts on my plans?

[SaabJohan]

Gearboxes aren't usually fully reliable to that much more than what the engine with most torque fitted with it can deliver. Perhaps it's safe to add about 20% or so before the transmission will start to become unreliable, but car manufacturers are usually quite hard on this, they will probably class many "chipped" cars as unreliable.

[burly]

While nothing is guaranteed, my goals seem reasonable then? If I'm looking at 210-225lb-ft, thats less than 120% of the factory rating, which yields 228lb-ft. Like I said, I know that it is not an exact science, but I was thinking along similar lines when I came up with the numbers I stated early, which are an 11-18% increase in torque and an 18-25% increase in HP. I am not drag racing the vehicle, nor am I street racing it. I'm looking at the power increase for two main areas: Easier drivability and more highway passing power. I do 90% of my driving on the interstate, about half of which is with luggage through the mountains. To me it seems like these increases, along with some changes to the suspension and tires, would put the vehicle in its "sweet spot" for highway driving.

Do these goals make sense and sound feasible? Also, given the nature of the driving, are these goals likely to be reliable with the current tranmission and drivetrain, or am I looking at a trannyswap? Would switching to synthetic transmission fluid be beneficial and are there any other modifications that can be done to protect the transmission that are cheaper than a full replacement?

[Holyterror]

Okay, let's keep things simple. You want better driveability and more passing power. Here are my suggestions, with brief explanations:

1. Synthetic transmission fluid
   • Smoother shifts
   • Lower transmission temperatures
   • Reduced wear (as a result of reduced friction)
   • Will not affect warranty
   
   This is an easy one. Try Amsoil ATF. I am big on synthetic lubricants, and this is definitely worth the extra cash considering how often most people change their fluid (almost never). There will even be a marginal improvement in power, since less of it will be wasted by friction.
   
   
2. Transmission line pressure modulation
   • Faster, more solid shifts
   • Reduced wear
   • May affect warranty (see below)
   
   At high speeds, changing in and out of your overdrive gear, your transmission requires more line pressure than at any other time. Increasing line pressure will help the transmission spend less time slipping (wasting energy and causing wear) and more time propelling you forward. This is easy to do with an electronically-controlled transmission like yours. The AutoTrans Interceptor is an example of a cheap device that will do this. And while it might affect your warranty, you can easily remove it, so nobody will know it was ever there. The firmer shifts are a little less luxurious, but they definitely make the car feel more powerful. Most transmissions made today are made to shift slow and soft, supposedly more "female-friendly." That's why we need little gizmos like the Interceptor.
   
   
3. Transmission cooler
   • Lower transmission temperatures
   • Requires some plumbing
   • Will probably affect warranty
   
   If you've already gone through the first two steps, then this one is a little less important. However, if you were towing anything, I would definitely recommend getting a transmission cooler. Heat is the number one enemy of your transmission. This is something that will be a bit harder to remove than the above, but will give you a much wider margin of safety when running more power or heavier loads (or both).

Although heat is the transmission's number one enemy, poor engine performance is a close second. Keep that engine in tune! As for your power increase, keep it modest, and I don't imagine you'll have any problems. I can't see them actually making a transmission that will give out 1 ft/lb above the motor's stock peak. When you get the ATF, go ahead and switch to a synthetic motor oil too. Amsoil ATF and 0w-30, perhaps? That might give you all the extra performance you're wanting. Remember, start small and go from there.

[quaddriver]

The 4T40 is simply a 3T40 with ability to overdrive the 3rd gear.

If you recall, the 3T40 was the stock auto tranny behind the buick 3.3 which made 205? ft so we know the rest of the unit is stout, the only consideration is the od clutch. However, at WOT conditions the tranny will never hit 4th gear so thats not a consideration.

Some malibus and cutli were equipped with the 4T60E, if you are REALLY going to increase the gen 3 a large amount of hp, then finding a core and making the proper PCM change is an exercise of how easily you can find your metric sockets.

btw - on GM emodel trannies, do not use any devices that alter signals to the solenoids, the PCM is 2 way, it instructs the tranny to behave and then times the answers, if it gets wrong answers it will first try to correct the condition with altered shift timing, and failing that, will retard engine power and enter limp mode. the emodels raise line pressures with engine load, there is no such thing as anything like shift delay at anything other than light throttle (and even then you know from experience that the PCM retards engine output at the moment of shift)