Transmission Limitations
 

I'm sure you've seen my thread(s) about planning a turbo build. I wanted to know in depth the limitations of the transmission in specific circumstances. The turbo kit I'm putting together could possibly make 300-400 whp. Would the trans be able to hold anything near that? I will have low spring rates in it, and have it tuned reasonably, and I definitely plan on babying the trans until I can get it built. I definitely won't be doing any drag strip launches, just street driving for now. Thoughts?

I feel like I remember reading that the stock hd transmission will handle up to like, 300-320 hp. Not completely sure about that though. I'm actually going to follow this thread and see if any of the more knowledgeable guys can shed some light on this.

These things are a crapshoot. I would just add the costs of a fully built transmission to your overall build cost. Run what you have until you ultimately blow it, and then do the rebuild / swap.

My first trans went stock, my 2nd one has lasted even though boosted. It is another stock trans.

-Just a couple things ive picked up over the years...

Stock 4th gear cannot hold any power whatsoever..

The difference between the heavy duty and "non" is a stronger differential which doesnt really matter unless youre spinning the wheel(s) or making power while turning..

Its hit or miss with these garbage transmissions.

Personally I have decided to scrap the idea of ever turboing (or doing any significant power mods) and instead I have been looking for a manual RWD V8 powered vehicle to use as a toy and dump my money into..

I mean, dont take this as discouragement. As long as you know the risks and hassle that can come with this I say, go for it. I cant wait to start seeing your progress with the project. We need more people to mod these cars.. The few that are left anyway..

Its impossible for anyone to say with any level of accuracy for the following reasons:
-We have no idea what kind of mileage / condition the current trans is in. To be fair, you really dont know 100% either unless you've recently had it apart. It could literally die tomorrow without a single mod added. Plenty of stock transmissions have died with totally stock engines.

-We have no idea what the tune will look like. The tune is a major factor in life span. Certainly at the upper end of that range, no amount of tuning is going to save it, but it can play a big factor in life depending how it's set up.

-The large range given. 300 to 400 whp is a giant range as some of the major failure points pop up in there. 300 is doable on a fresh rebuild with some good clutches and a few other minor tweaks. People have run 300 whp on good condition stock transmissions for reasonably decent mileage. But in that span as you go up, both the input shaft and drive chain will become issues. I mean we are talking double (or more) stock power here.

-Usage. There's a big difference between someone with 400 whp that actually uses it and a car with "400 whp" that spends almost its whole life at 10% throttle because the owner is terrified of grenading it. Keep in mind, even if you plan to putt around at 10% throttle forever, its going to need to go WOT & near WOT multiple times to get the tune dialed in. Worst case is you pay a few hundred for a tune and end up paying a few hundred more to trailer the thing home to sit because it barfed trans parts all over its first time at WOT.


One thing to keep in mind, the big 4t65 shop requires their top of the line $4000+ 'level 4' build to actually handle the top half of your suggested power range; that should give you a good idea of how far you'll be stretching the trans.


I know I've said it in a few threads, but I feel it's relevant here again. If you're serious about getting into this, forget buying any more performance parts and do the trans first for whatever power level you really think you want. There is nothing worse than dumping a bunch of money into performance parts on this platform and then never really using it because you're terrified of the transmission blowing. If it does blow, it may also be at a really bad / unsafe time.

Would it be worth picking up some replacements for the normal wear items in the trans, plus the chain and input shaft, then replace them myself? I don't want to max out the turbo kit just yet. Just looking to get more enjoyment out of the car, and the experience of building the kit.

Not to be rude, but have you ever rebuilt a trans before? That's a pretty giant undertaking if you've never done one before. The other hard part about DIY'ing it just going by the manual is that you're not going to do many of the minor tweaks and mods that a real 4t65 shop would know that help from experience beyond just replacing hard parts.

The big risk with transmissions is that all of the stuff is so close together and running off the same fluid. So you screw up one thing and it comes apart, next thing you know, the debris has circulated around and destroyed everything.

This is a pretty good car to start learning turbo kit fab work on. It's literally just capping off the downpipe connection, adding a Y with a flange to the crossover and running a simple dump pipe to get a functional hot side. Cold side can start as simple as a single looped pipe to the TB especially if you're not really planning on cranking it up from the start anyways.

I haven't rebuilt a trans. I always research, and learn along the way. And yes, I understand theres a risk in screwing up the rebuild. Just like theres a risk of one of my welds being crap, and sending bits of metal into the turbo. But i'm going to practice, and learn. Hopefully I wont screw it up, but if I do, I'll fix it. For me, the thing with putting together a turbo kit, vs buying a built trans, is the money. Yeah, it'll cost about the same in the end. But with the turbo kit, I can buy pieces along the way, and have something to work on which motivates me to keep going. Whereas the built trans requires months of savings on my current salary, during which I don't have a project to work on.

Main reason I ask is the extreme level of difficulty with transmission work compared to just about any other system on the car as you've got complicated mechanical, electrical, and hydraulic systems all working together.

With welding, assuming you've got a quality welder setup, you could teach the average Joe how to make functional exhaust welds in maybe a half hour or less. Wont be pretty, may not last forever, but itll be enough to function and not leak for awhile. It's also easy and cheap to practice. You can just buy a length of pipe and just keep cutting it up and welding it back together.

With transmissions , tech colleges have entire course lines dedicated to learning how to do this properly as it's such a complicated system. After the 6 month trans course in my high school auto mech, probably 15-20% of the students still weren't able to do it properly and the class population was mostly made up of people heading into the auto repair field (ie familiar with basic mechanics/diagnostics).

My point is more that it's just not typically a field where people just jump in and try to DIY without some sort of professional instruction. Not to say its impossible, but its just a really steep learning curve, especially starting with a 'performance' build thatll be pushing the limits of the components. Beyond the learning curve, the cost can be prohibitive too. There are some specialty tools required, and like engine rebuilding, all it takes is one small goof up and you could be throwing thousands of dollars worth of parts in the trash can.

Like with head porting, if you're serious about wanting to learn, I'd suggest 2 things-

1. Go buy the ATSG 4t65 manual. Itll give you the bare minimum instruction needed to successfully rebuild this trans. Only downside here is that it assumes you already understand transmissions so there is no explanation of how the parts work together. It would probably be worthwhile to supplement that with a tech college course book on transmissions as well. Doesnt have to be the latest and greatest, these transmissions are 25 years old anyways.

2. Go buy the cheapest local 4t65 you can find and walk through a full rebuild. I wouldn't spend a dime on new parts, I'd just take it completely apart and then pretend the originals are your new replacements and put it back together. You likely wont be able to test for functionality (unless you got lucky and found the same year range / code transmission as you've already got in your car), but itll give you a lot of understanding about the process and if you think you'll actually be able to pull it off on a real trans where theres money on the line.

I totally see where WovenScout is going. Get your hands dirty and actually dabble with this stuff. I also understand what Bumpin is saying. To Bumpin's point, this is is why you will hear about some home mechanics or people restoring a car rebuilding a motor from a bare block, but not a trans (and they will send the trans out).
But I have a big appreciation for what WovenScout wants to do. Bumpin actually suggested a thought I had after reading Woven's last statement. I completely agree, get a hold of a salvage 4t65e (of possible, a match for the one in your Monte now). If it does have an actual problem, could be a great exercise in finding the problem. If you are handy at a junk yard that let's you pull parts, you can probably pick up a transmission cheap (near me it can be less than $100). And as for the rebuild manuals, if you have a good library around, they might actually have the manuals (odds are they won't but worth checking). I have also seen a variety of YouTube videos about the 4t65e. Some are with the trans out of the car, looks like a professional shop working on it. Completely worth leveraging some of that information too.

If you are lucky to find a match for your transmission, you gain two items from this:
1. - Ability to practice on a 100% matching transmission
2. - If comfortable, could potentially do a full rebuild, maybe some upgrades on this donor transmission (minimizing down time with your car if you need it).

Also, you may want to consider not using this car as a daily driver once you are ready to begin doing the turbo upgrade. The reason I say this is that you may find yourself working out a lot of different items that may take some time before it is 100% reliable again.

I really feel the practice transmission is an awesome idea. In expensive, if something goes wrong you are no worse than you were. Admittedly, because these cars and parts are so common, I have gotten donor parts to do almost all my mods with.