Hey guys I am building a mild 355 and I'm shooting for 380whp/400+wftlbs. I currently have a slightly modded LB9 with an 89 700R4 with 131K miles on it that shifts real good, it never slips and has been maintained well. I am wondering if it is worth it to build it up to handle the 355, or build a WC T-5 we have laying around to handle it. Obviously the T-5 would be more fun, but I would be doing an auto-manual swap at the same time. I guess I'm asking which would be more cost effective/reliable? Thanks. James

some fundamental things on the 700r4 are inherently weak. Like the reaction shell. Behind a hi po motor the "beast" is recommended because it prevents stripping out or breaking altogether, but the sad part is a teardown is required. a servo for the band is a good idea to upgrade and doesn't require a full trans teardown. The corvette servo should suffice. If you do decide to tear the trans down and replace the shell-the pump could bennefit from better boost valve and int/reverse valve to firm up the shifts more and give more holding power. .5 boost and .296 int/ rev. work good to make a really firm shift. The transgo 2-3 kit addresses the servo and valves but doesn't include them but is a good kit for performance use as it allows automatic shifts but can also be held in any gear manually. Doing these small changes goes a long ways in the 700r4.

I've done all the basic upgrades to a 700R4 as ed o has suggested however the weak link appears to be the 3-4 clutch pack.
My latest trans lasted 4 quarter mile passes before the 3-4 gears went away and it has the .5" boost valve, corvetter servo, etc. etc. That was a low mileage unit from a 90 IROC that I modified.
I've managed a 12.9 sec pass with another trans before this one and I smoked that one too although it had a questionable history. ( It had all the basic upgrades )
Once I got past the 375-400 engine horsepower mark (based on my trap speeds) it seems these transmissions show themselves for what they are. I feel that unless you get inside one and make some major improvements, it won't last for long.
Keep in mind that until I made the latest round of engine upgrades, I beat on the 700 all the time however that set up was good for only low 13's and it too eventually gave up the same way.

Yeah, I hear what you guys are saying. I'll probably do what I can to the 700R4 for now and just go easy on her with the 355 until I get the money to have one built right. Thanks for all the input, it always helps.

I see all these people on here talk about how weak the 700R4 is. I have two 700s, 1 in a 85 corvette and the one in my 84 Z/28. Both are stock. The vette with NOS is putting down close to 500hp and the Z/28 is making mid 400s on motor. I have never had an issue with either of them. Now I am sure that they will eventually give out but the vette makes around 350HP on motor without the NOS and its been fine for over 3 years now. The Z/28s is been going strong for over a year.

What I'm saying it that if mine are struggling on this long then I think yours should last at least as long if not longer.

neither tranny will survive very long based on traction and how much you abuse them...

my 700R4 started slipping really badly when i built my mild 357cid motor and my T-5 is just about to blow its self up... the case flex is what you need to watch with the t-5

if you wanna have fun and reliablilty go for a t56

Yeah I really wanted to go the T-56 route, but it just seems like such a pita swap. Plus T-56's arent cheap.

trust me its always cheapest to do it right the first time.... i had a t-5 laying around thats why i went this route... but i know its only a matter of time

so do as you say not as you've done you're saying.

well my motor doesnt put down much power maybe 300hp 350-400tq and i toasted a 700r4 in less than a month and the t5 has shifting issues after about 3 weeks... this is after i spent like $600 on a flywheel and clutch set up for the t5.

I'm basically reiterating a lot of what's already been said in this post, but yeah, I wouldn't waste any time or money on the T5. They're just too weak to last in the long run behind a healthy motor. For a good manual trans you'd have to lay out the cash for either the T56 or a Tremec TKO. 700r4's can be built up to be stout as hell, but you do have to tear it down completely and upgrade all the things everyone else has mentioned. If you talk to Dana at ProBuilt Automatics, he can set you up with everything needed to make your 700r4 virtually bulletproof. His website is www.700r4l60e.com. It'd cost you roughly $1200 to beef up the 700r4. Probably $2k or more to set up a strong manual trans. For now, if you put the vette servo, transgo shift kit, and a stall converter in your 700r4, it'd probably hold up for a while. As stated above, the 3-4 clutch pack is usually the first thing to go south on them.

Yeah thanks for all the replies, I've spoke with Dana a couple times, he's always very helpful, since the car has a 700R4 as it as I will just have it built, if it was a manual car I'd dish it out for the T-56. Thanks for all the help guys.