No changes are needed to put one into a lighter vehicle, been happening with street rods and street/strip cars for decades.

If you are seriously racing it where you need every last bit of power to the tires, there are some places you could lighten components up/leave a clutch plate or two out to reduce rotating mass....but you don't need to do anything for your average trans swap.

The part most effected by the lighter vehicle weight will be the torque converter. Stall speed will get lower since the lighter vehicle is easier to get moving, the converter won't slip/stall speed as high as it would in a heavy vehicle.

 

Also take a look at the 200-4R overdrive transmission, which is sized closer in length to a TH350, so it’s usually an easier upgrade. It was also used in several GM performance vehicles, so it can be very durable when put together right. It’s just not as well known as the 700R4/4L60.

I’ve always read that the TH400 is stronger than the TH350, but less “efficient”. However, I don’t have a clear explanation.

 

Yep. The TH400 is hands down a stronger unit than the TH350. Out of the many reasons the 2 that stand out the most to me are the differences in the center support/its fit into the case and the planetary assemblies. The latter being responsible for most of the extra rotational weight of the TH400. More rotational mass is less efficient. I know folks debate the impact that has on things, but in my years of drag racing I never saw anyone really gain or lose anything switching between the 2. We were never chasing every hundredth of a second though.

Ericnova72 nailed it, as usual. No need to reinvent the wheel there.

I think the idea of making changes gets talked about a lot from the standpoint of how the transmission behaves. It's not a requirement, but more of a preference of how and when you want it to shift. Tire chirping shifts in a hotrod and driving Grandma to bingo in smooth shifting Cadillac both sound correct. A smooth shifting hotrod and chirping the tires on Grandma's Cadillac...both of those can and should get you slapped.

 

if you take a TH400 or 4L80 from a 6000LBS truck and use it for a smaller engine in a 2400LBS car, I have to assume there would be something that needs to change, but what? Would it just be easier on the transmission with no changes needed?

For one thing, if you're replacing a TH350 with a TH400 or a 4L60E with a 4L80E, your 2400 lb car will become a 2500 lb car. The bigger truck transmissions are heavier than the smaller transmissions by 90 to 100 pounds. In a light weight car, that's usually fairly significant.

The bigger transmissions also don't like high RPMs. The added rotating mass is an issue when you start spinning the input shaft over 5500 RPMs. If you're making power with higher RPM ranges and raising the red line on your engine, the 4L80E (and I believe the TH400) need some modifications to lighten and strengthen the rotating parts. The smaller transmissions might need modifications to handle the increased torque that comes with more power if you've raised the torque curve. The cost of the modifications required will be close to a wash, and the weight savings from the smaller transmissions usually pushes me in the direction of the beefed up 4L60E over a heavier beefed up 4L80E.

YMMV.

 

The TH400 and the 4L80E take more power to operate so they add parasitic power loss to the drive line assemblage. For a light weight, smaller engine vehicle this is an unnecessary overhead of power loss in addition the these transmissions greater weight than a TH350, 700R4/4L60 or 4L60E.

For a lightweight, small engine vehicle the TH250 or 200R4 is better suited in terms of weight and parasitic power loss. Many racers use a beefed up 200R4 just for these reasons.

Not that you couldn’t attach a GM Vee or inline 6 or inline 4 or small V8 if a TH400 happens to just be laying around for cheap, but it would exact a power loss and weight increase penalty to what you’re doing.

Bogie

 

This is for your proposed BMW motor swap?

Not sure the 4L80E or TH400 will fit the trans tunnel with surgery/ modifications

Then there is the parasitic loss as mentioned

 

Thanks for the replies.

This will not be for the BMW swap/“repair” because I do not have the BMW. Where I rent to live we can’t have non-running vehicles… it is sold. But I was happy to at least learn the value of how those cars are for handling and swap compatibility.

So, it sounds that the consensus is that any of the GM automatic transmissions will work in any weight of car. Just the main considerations being the rotating mass being perhaps problematic at different or higher RPMs like a lathe or axles spinning progressively faster. As compared to the mentioned 200-4R or others and the idea that adding weight and rotating mass to the transmission is simply best avoided if it’s unnecessary. I’m guessing a TH400 makes more sense for a truck and a TH350 for a Camaro simply as a factor of one being more suited and why use the heavier one for a car.

What y’all said makes sense. Sorry it takes me forever to respond. Interesting idea. I was worried that pulling a junkyard TH400 to use in some lightweight RWD like a Miata or UK cars would have problems, but it’s all good knowledge to learn and it sounds like a GM automatic works no matter what weight.

 

Thanks for the replies.

I’m guessing a TH400 makes more sense for a truck and a TH350 for a Camaro simply as a factor of one being more suited and why use the heavier one for a car.

The information in this discussion regarding the "build what you need" is spot on. Modest power in a lightweight (or really any reasonable car/truck weight) vehicle and you can use whatever fits the dimensions of the available space.

I can't speak to the weight difference in the overdrive automatics, but on a number of occasions I've weighed an empty (no fluid or converter) TH350 and TH400. The TH400 is heavier by about 13-15 lbs. Whatever is added by fluid volume and converter size will change that a bit. As I mentioned in my previous post, the extra weight of the TH400 is largely "got it where it counts" from a power handling standpoint.

If you start making good power things start to add up differently. The Camaro in your example would be a common vehicle to find a turbo LS or a big block with nitrous. The tipping point seems to be around 500-600 hp and with what's available getting into the 800-1000 hp range isn't far fetched. For me, it's a "dollar for dollar" thing. The budget and parts list to build an 800-1000 hp TH350 gets pretty involved. An 800-1000 hp TH400 doesn't need a whole lot. Apply the 800-1000 hp TH350 budget to a TH400 and the power limit will be much higher, for the same money.

The Camaro can be a mild to wild build and you're not going to gain or lose much between those 2 transmissions. If you see a difference of 2 tenths of a second in the 1/4 mile, something else is better or worse. It won't be the extra weight or the difference in efficiency.

I realize this doesn't apply to your inquiry on this aborted project idea. It's just some food for thought on future considerations on the next project idea. I'm a hobbyist and not a pro, but that information will hold up to scrutiny.

 

Not sure a TH400 would fit in a Miata without serious fab work on the body/trans tunnel