i've got a 1991 camaro rs 305 v8 auto, i have recently bought a set of richmond 3.73 rear gears and an auborn posi both for $200, both new, what all is involved in putting in the gears and posi. i was planning on taking it to a shop to do for me, but otherwise, WOULD I HAVE TO REPROGRAM THE ECM SO THAT IT KNOWS THAT I HAVE DONE THE SWAP, AND HOW WOULD DOING THIS AFFECT MY TOP SPEED, my speedometer goes up to 110mph and the car cuts out at 115. i have the original aluminum rims on it. A little more about my car, its got edelbrock headers, flowmaster, K&N extreme kit(the lid is also a filter), trick flow phenolic tbi spacer, a B&M ratchet shifter, 4 inch cowl hood. i dont plan on making it a drag car the posi being the last mod. i got the posi/gears because i thought it was a very good deal. any and all comments, ideas, advice, is welcome, CHEVY POWER

 

the ecm won't care what gears you have. not sure if you have eletric speedo or not. the speedo will be off.

 

does the ecm take speed of the vehicle in to account at all to figure shift points, is it preset to shift at certain rpm's or at mph's? is there any where i can find out how much the top speed will drop

 

The ECM doesn't shift the transmission. The only thing it does to the trans is operate the converter lockup clutch.

Get the right speedo gears and it will all work correctly.

There's no way to figure out how much "top speed" you'll "lose". I'd guess it will still just barely make it to the speed limiter, much like it does now. It might get there a little faster though.

 

Well first off i'll start by saying my friend has the exact same car as you, and he did the exact same thing your doing. You dont have to do anything with the ECM. That doesnt control anything, its the PCM that controls the tranny. Regardless Anyways all that will be off will be your speedometer. However once the swap is done i strongly suggest u get the speedo correct immediatly. My friend didnt do this, and b/c the speedo thinks your going faster it puts a LOT more miles on the odometer than your really driving, and then u wont know you real mileage. Anyways who cares how much it will lower your top speed, youll be amazed at the dramatic improvement in acceleration. Youll be changing your gears a whole series(2.73 to 3.73). It will bring your quarter mile times down at least a whole second. heres a pic of my buddies car who did the same thing.

 

The transmission is not controlled by any kind of "whateverCM". It is entirely hydraulic, except for the lockup clutch.

The facts remain, as I stated them. Get the right speedo gears, and the ECM (which controls the lockup clutch) and the speedo won't even know you changed the rear gears. They'll work exactly like they're supposed to.

It's impossible to guess how much changing the gears will affect your ET. I can't feature it taking a second off of a car like you describe, especially with the stock cam and stock swirl-port (anti-RPM) heads. I'd expect more in the neighborhood of .3 - .4 seconds in the 1320. If the engine doesn't have the capability of producing meaningful power at the higher RPMs that the new gears force it to run at, it won't go any faster. It's actually possible to put too much gear in any given car, and slow it down; although that probably won't happen here.

 

pcm what the hell is that and what does it do since it doesn't control the trans

 

Agreed.

I did it.

I installed a 3:73 into a stock LG-4 T/A. The motor ran out of power way too early in the rpm band that the gear (and my foot) was forcing on the engine.

 

maybe next time, do the gear swap to compliment your mods....i.e. if you did what i did, ported ald polished your heads, manifold, oppened up the exhaust...carbed it....basicly put all your power in your mid range rpm area....then go 3.73.....

it sounds more like your stock engine just doesnt breathe past 4k rpm....not that the gears took your power away....open up your air intake --whatever it may be-- and open up your exhaust....then tell me what happens

other than that..lets not focus on what has to be done to the ecm --nothign at al-- and lets focus on the PITA a gear swap is if you have never done it....im half way into my first swap and its well..less than pleasent.....if you arent good at mods and taking precise measurements, i highly reccomend you have a pro do it. im pretty good with tools , and have moded the hell out of my engine and i still find this extremly annoying to do...