But the 15/16 nor 7/8 are really good permanent solutions for that kind of power, period- especially not long term.

You have to be the only LS4 owner I've ever heard say that, lol. IMO, the transmission was a terrible choice for the LS4 application- the car is too heavy, and the engine makes too much torque. Modded L67's had been blowing them for years, I don't know how they thought it would be fine with a few minor changes. Plus- who designs a performance oriented car, with little-no room to expand in power or beat on the car? People are going to mod the cars, and race them- why not design in a little wiggle room, give them a different trans option, or put in a trans that could handle the power (ie 4t80 like the bonneville gxp)

The other option of running it underneath is definitely a worse option though since heat rises. You can always header wrap it and hope it doesn't crack- but realistically, it has a built in cooler- and you can do an external cooler if needed. As long as those temps are in check, and nothing is excessively hot laying directly on the trans (ie turbo hot side sitting on the aluminum trans bellhousing)- then there isn't much issue with heat. The breakages on the turbo LS4's are the same problems as the big build 3800's- there are just multiple parts in the engine that are way under spec for 500+ lb-ft of torque, especially launched hard on slicks.

The cartuning kits come at ~500whp AFAIK (so not far from that 640 figure at the crank), there really isn't a mild 'option' (although I suppose you could turn the boost down).

 

In my car I don't think that some of these issues apply. Murci rebody has all kinds of ventilation and the engine compartment doesn't get hot even with the turbo. Also I have a separate transcooler with it's own grill and I am putting a larger one in it even though I don't think it's a problem, but I have room for it. I'm going to install the 300m input shaft with the 7/8" chain. The input shaft sheared right at the o-ring by the torque convertor spooned. The thinnest part of the shaft. Thanks for all the info. I like to share my experiences too.

 

Sorry, double post

 

Of course the 4T80e would have been a better choice..But at the same time would it have fit? Also you got the weight difference, if I remember right the 4t80 weights a good 50% more than the 4T65E-HD, and also costs quite a bit more...Also read that the 4t80 wont bolt up to any LS style engine..Maybe I should rephrase what I said lol...for 320 hp(which is what im at, don't plan on going any higher) they could have made it plenty reliable but didn't. I mean we all wish they came with 6T70s but that didnt happen

 

IMO, they were making a lot of stuff from scratch specifically for FWD (ie block, crank, accessory drive, etc)- I'm sure something could have been done.

IMO actual static weight isn't that big of a deal in these cars anyways- its not like they're some lightweight autoX cars, another 50, 75 pounds on the front end- I just don't see it mattering that much. Extra rotational weight sucks due to less whp, but when its that, or have a trans that can't hold the power- I'd rather take a little less power to the wheels.

I don't see this being a valid justification. Sticking in something which clearly isn't sufficient doesn't help anybody. They end up spending more in warranty repairs, and people end up spending more in post-warranty trans repairs. Plus, ultra-low trans reliability ended up hurting resale value down the road for those who bought. I've seen countless times on various forums where people are asking about an LS4 car, only to get turned off by multiple horror story posts of transmissions blowing with "little-no abuse" (lol, yeah right huh?). They wouldn't pull something like that on the new corvette.

Neither did the 4t65 though, hence the special block. I remember reading stuff about other interference issues, but for a company that big, with that much auto-engineering staff, it couldn't have been impossible.

No doubt- I'm just saying they should've done something from the start. Either put in a trans that can actually handle the power if you don't want to re-engineer the trans; or put the time into re-engineering the trans so you have some margin of over-engineering into it. It would be one thing in a normal commuter car- but doing something like that in a performance vehicle that will get beat on, and will get modded, seems kinda dumb.

6T70 would've been cool for sure.

 

Maybe I've had misinformation but here are the reasons I rejected the 4t80. 100 lbs heavier and no upgrades available. The 4t65 heavily modified by GM in their drag car stood up to over 1000hp. Granted it's no good for the street and in fact I just passed up a chance to buy one on eBay brand new for a really good price, but I'm taking some lessons from them. I'm replacing the input shaft with a 300m and using a 7/8" chain. Kevin builds a 4t65 with a machined case so that he can install extra 3rd gear clutch discs and a few other things. Also tep builds a strong 4t65. There are a few guys with fieros running built 4t65s with 600+hp and the transmissions are holding up. they're breaking axles and stuff. I'm only planning on a run or two at the track to get an idea of what times it's capable of and then a few short wot runs on the highway just for the fun of it so I hope that I won't have to go all the way on the tranny. Dave at tep says to treat the 7/8" chain as a maintenance item.