700R4 vs. 4L80e vs. TH400+GV

[MJMullinII]

Just a simple question (hoping to here some personal experience stories :)).

Which would you recommend to go behind a 502 Ramjet Big Block?

I've seen places that sell 700R4s they claim will handle the 500+ torque, but have heard others tell nothing but horror stories. I've always heard the 4L80e and the TH400 were basically rock-solid, but (again) others have said the 4L80e because it has an overdrive gear is automatically weaker.

So, for a n00b, I'm basically looking for first hand experience before making my choice.

Thanks.

[rustyk]

I got this second-hand, but I had the Peninsular Diesel engine spec'ed to 430 lb.-ft. torque, below the 450 lb.-ft. 4L80E limit.

However, I've contacted a friend with the 502 in a motorhome, and will let you know when he answers.

[rustyk]

Bill (a VERY experienced mechanic), replied,

"Short answer: All three can.


Long answer:....I have the TH475 with a few mods. At the time I did the conversion, putting in a 4L80E was not as easy as it is now, so I went with the 475 and a Gear Vendors. I know both of them pretty well, so that was a factor. If I were doing it today, I would probably do the 4L80E, owing to the lock up converter. I have that on my truck, and it is good. I have never rebuilt one myself, but I'd take a chance there. I do have an aversion to driving something I can't fix myself.


Ultimately, the TH400 could be considered the strongest, but I think that would be due to its simplicity. There are just fewer parts to fail than a 4L80E.


However, if you are talking about the ht502, which is a torque motor, the TH700R4 or 4L60E are not a good match, mostly because of their gear ratios. The lower 1st and 2nd gears are not needed for startup, owing to the torque of the ht502, and they leave too big a gap between 2nd and 3rd for RV use. The engine would buzz too much in 2nd and lugs too much in 3rd on some climbs. I went through this on an Olds tow car. A TH400 cured it. If hard high RPM, high load shifts are avoided, the 700 would be fine. Mine was just a little beefed, and showed no distress towing a heavy boat.


With the ht502, the issue is more heat than the strength of the tranny. Tranny damage is done by horsepower at high rpm hard shifts. We RVers don't do a lot of that. I don't even have a kickdown switch andy more. This avoids full throttle downshifting on a hard climb. I let up on the throttle a bit for a downshift, just to save the tranny. What the 502 does is make a whole lot of heat in the torque convertor. The only trans cooler that has done the job for me is the big Long one from Canada. I monitor the converter out temp and switch on the elec fan if any heating starts. Oh, yeah, synthetic fluid.

A locking converter would cut a lot of that heat, so the 4L80E would be good. Funny, Packard did that in the late 40s, but no one else followed."