Hello, been a member here for a long time but only recently decided to do an engine swap and get the car back on the road after 10 years lol. Anyways, I picked up a crate SP350/357 from my local dealer and plan to replace the tired 37 year old 305 LG4. The car has a 700R4 and as far as I know the car has a factory rear end (1985 TA). I called my local transmission expert who everyone recommended in my local "muscle car" Facebook group and to be honest he has me very nervous. I told him the engine I had and that I wanted the transmission beefed up/shift kit to handle it. It's rated 357hp and 407lb-ft torque, I also told him that the car is to only be used as a city driver with some highway use every now and then with the only serious work the car will have to do is if I put the pedal down for 10 or 15 seconds to pass a truck on the highway. He immediately told me that these transmission aren't very good and I'll be better going with a TH350, to which I asked "where would I find one?" and he says "good luck, hard to come by now". I'm wondering if he's thinking a little more race oriented then what I'm actually going to be using the car for. He did say he will take the transmission and rebuild/upgrade it for 1,700 CAD, but I just want to know should I still be worried about the tranny if I want to just go for a casual drive every now and again on a nice day? Genuinely concerned about this lol.

EDIT: He also said I'll need a new torque converter and said I would want one with 2800-3000 stall non lockup, this is jargon to me, I trust this guy because he's the recommended local tranny expert, but I don't trust him because I've never met him before just spoke over the phone. And if he is being more race oriented like I mentioned earlier I'm wondering if he is recommending the correct torque converter for me. Also anything special to look for when buying one? size? brand?

Hopefully this is the last thing, the new engine comes with a Flexplate part# 14088765, it's a 153 tooth and says torque converter pattern is 10.75", will this work with a 700R4?

 

I'd use what you have until it gives you problems. Sure not a 3 speed 350 trans though. The stock converter is probably a bit tight for the new motor but, again, I'd try it as is and seen how it all works. Not sure if that flexplate will work with the old TC though. Seems like yours would be a 10 1/2 bolt circle? There are flex plates with a dual bolt pattern and it's not a big expense.

 

Thanks for the reply, I was planning on replacing the torque converter anyways, but just wondering if the flex plate size matters to the transmission. Not sure on the flexplate that the car has now. And when you say use everything as is do you mean just throw the transmission back in without being built up any?

 

Those early 700r4s are not noted for durability, later versions were improved some. But if it is working fine now I'd run it. If/when it starts going I'd look for a performance built identical transmission and TC for replacement. Your transmission is basically a turbo 350 with an overdrive anyway - from what I understand?

 

#1: Absolutely keep using a LOCKUP Torque Converter!

The Transmission will operate significantly cooler, and last much longer, in regard to Expressway Driving.

The "High Stall Speed" Lock-Up Torque Converter is the absolute greatest Automatic Transmission Device of the 20th Century!
If needed; an approximate Stall Speed of 5,500 RPM could be Ordered (Engine Revs 9,000 RPM) with a Lock-Up Clutch.
Go for an Expressway drive, and Lock the Torque Converter while at a steady 2,000 RPM.

No High RPM drone Noise.
No super HOT Transmission Temperatures because the Engine has to stay at 6,000 RPM while driving on the Expressway.
Actually have some kind of Fuel Economy.

The days of using a NonLock-Up Torque Converter are LOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOONG GONE!


A 1985 Year Core THM700-R4 is not a Bad Core.
Better than the even Earlier Cores... but not quite as good as the Mid-Year 1987 and Up Cores.
A Rebuilt 1985 Core can be a great THM700-R4, especially with any of the GM Updates, and/ or any of the Aftermarket Upgrades
(All dependent upon Engine Power-Output, and Drive-line needs).

 

May seem odd, but the only 700r4 or 4L60E I had that lasted any length of time was built 10/82 and had nearly 300K on it before it failed.

Personally a 3spd TH350 or TH400 with tall axle gearing works extremely well. The cam in that engine is going to want to be up a little more in the rpm range than what a 700r4/2.73 gear lugs it to anyway.

 

Sounds like great advise. I'm subscribing!

Also, what a Blessing to have a former, GM Engineer on this site! Thanks.

 

LOL!
You might think so...
But all I usually do is bash on GM for their terrible products (Mostly circa 2005 to current Vehicles... they are HORRIBLE!)

 

That is because of the low Engine power output.

They are durable Transmissions... but did not handle much HP or Ft/Lbs stock.


Also, there is no reason that a THM700-R4 or 4L60E could not be built for 10,000+ RPM...
I have built many for small V8s that go to 11,000 RPM to 12,000 RPM.

 

It lived over 100K of that behind a 350 making over 300 hp and 380 tq in a 5,300 lbs vehicle with a 3.08 rear gear. At 150K the 305s upper end was rebuilt with ported 601s and a fairly hot cam. The 305 was making ~300 hp. It lived for 50K more before the 350 went in.

 

Like I said, low Power output.

 

380 TQ is more than a 4L65E/4L70E is rated to hold. Not exactly low output. Its more torque than a 5.3L makes.