Engine Swap Q's

If I switch my 4.3L 265 to a 5.7L 350..

1. Do I have to change transmissions? I've heard mixed answers, some people say they are the same trannys, some people say they arent. So does anyone know if they are the same, and if I have to switch trannys too?

2. Do I have to do anything with the ECU?

3. Are there any other parts in the car I would have to change?

I'm not sure on the transmission issue, Blue Bowtie, 96capricemgr or MT2500 can answer that. As for the engine, if you're doing the full LT1 swap and are using LT1 injectors, I've heard that it's pretty much drop-in aside from having to replace the MEMCAL or have the PCM reprogrammed for the LT1.

L99 to LT1 swap (http://tinyurl.com/7qmlf)
Near my bedtime and I am tired so I am taking the easy way out.

Alright..so from reading the link you gave me..it pretty much seems like I just need to switch to the 24 lb injectors that the LT1 uses, and possibly a new starter and fuel pump which I am going to need new ones soon anyways.

And someone in that topic said the tranny in the L99 wasnt built as well as the tranny in a LT1. So Im assuming he is saying they are different transmissions.

If anyone can give me a definate yes or no, I'd appreciate it.

They're the same type of transmission, a 4L60E, but the L99 might not have as many plates in the clutch stacks. Also, the 2-4 servo isn't the larger 'Corvette' size, so the 2-4 band won't engage as hard (leading to more slippage with the more powerful engine).

Oh ok. So basically if Im going to drop in a 5.7, Im going to need to get a new tranny..which I wouldnt mind a new one anyways.

I'll probably start off by switching the transmissions first, and then save up until I get more money for the engine. Does anyone know what a L99 would run like with a LT1's tranny? Im assuming there shouldnt be any problems with using the LT1's 4L60E tranny until I can get a 350 engine.

The LT1 trans will work fine on the L99, it shouldn't even be noticably different.

If you're going to have one rebuilt, make sure the shop installs a Beast sun shell.

I would say just put a Transgo kit in your tranny if it is still working fine. The tranny in my car now is the original case and such, was rebuilt badly once in preparation for the LT1 swap when that blew I used ProBuilt Automatics which is where I would recommend getting the shift kit from. The tranny now has 3 years with a lot more power through it and a recent filter change showed no clutch deposits in the pan which is a good sign.

I am still running the L99 starter and with the current modification my LT1 is 11.5:1 compression and the car has over 170K and it is the original starter to the car.

Far as the tranny goes my car today makes well over twice what the L99 did for HP like near 480hp where the L99 was just 200hp.

A "civilian" L99 car is likely to have the 7.5" rear axle which the LT1 will break if it gets traction but being an open differential that traction is unlikely, if you do have the 7.5" set a goal of eventually getting a 8.5" axle out of an LT1 b-body sedan. I used one out of a Roadmaster sedan larger drums and a much better price than the disc Impala or 9C1 rears.

Or you can do what I did last week and hit Ecology Auto Wrecking and get a complete, disc-to-disc 8.5" disc LT1 rear for less than $150. Mine was out of a '94.