blackbetty89

I've got an 89 camaro with stock 2.73 gears. I"ve heard people refer to changing rear end gear ratios and say that some people dont have enough engine to turn that ratio of gears. Has anyone ever heard a statement like this? Is there any sense to it? For the record, I've got the TBI 305 L03, MSD 6A, MSD Blaster coil, ACCEL 8.8 wires, headers and y pipe, exhaust cut-out, k&n open element filter, BBK underdrive pulleys, a/c and smog delete, and when necessary a 120 shot of nitrous. Future mods include LCA's, panhard rod, and aluminum driveshaft which will all be added at the same time as the gears. I'm considering 3.73 gears but if i don't have "enough motor" to turn those, I'll opt for 3.42's. Can you guys give me some advice?

jimmy_mac

Actually it's harder to turn a higher gear like a 2.73 than a lower one like a 3.73. Many don't like the 3.73s with the TPI motor because the 3.73s get you to the upper rpms quicker and keep you up there between gears. So your not utilizing the lower part of the torque range of the TPI motor as much.

I would think anything to help the poor TBI'd 305 get through the gears quicker would be better though. And if you plan on modifying it for more HP in the upper rpms later, the 3.73s would be a good all around gear.

M1tch

what about 3.42's? I have a 305 tbi

sofakingdom

iTrader: (1)

Yes that statement has been heard.

It's a little bit over-simplified; but it's basically true.

Gear ratio determines the relationship between engine speed and road speed. Obviously both the transmission gears, and the rear gear, make up the total final ratio. In your automatic trans, 3rd gear is 1:1 in the transmission; 1st and 2nd are higher than 1:1, and 4th is lower (.73:1 IIRC), otherwise known as an "overdrive".

You have a 305 TBI motor. It makes peak torque at around 3200 RPM or something, and peak HP at around 4200. Torque (the amount that the engine is pushing on the car) falls off steadily between those 2 RPMs, and then falls off DRASTICALLY above 4200 RPM. In other words, there is no sense in EVER running your engine beyond 4200 RPMs or whatever it is, because it will PUSH ON THE CAR HARDER if you shift gears. Which is why you literally get stuck in traffic behind 2 r¡cers that are "racing", rather than them pulling away from you, like you'd expect "racing" to cause; that stupidity they call "racing", which is nothing more than annoying each other with their fart pipes while holding the car in too low of a gear, prevents their cars from achieving even the limited acceleration they might be capable of if properly operated. But I digress.

Consider what would happen if you put, let's say, 5.13 gears in your car. That's almost twice the ratio you have now. That would raise the engine RPM at any given road speed to near double what it is now.

Your car will do, what, 22 mph or something at 4200 RPM in 1st gear. Now imagine taking off from a stop, and instead of 22 mph, the engine reaches its peak output at 11 mph, and it's time to shift. Then, right now, in 2nd gear, the engine reaches peak power (4200 RPM) again, at 34 mph or whatever. OK, with your new gear, 2nd gear tops out at 17 mph. And so on. I don't know if the mph values I used are accurate (probably not), but it's the principle I'm trying to illustrate, not the actual numbers. You can plainly see, that it would be basically impossible to drive the car any faster than about 50 or 55 mph.

So, you can easily put so much gear in a car, that when the engine reaches the highest RPM it can produce useful output at, the car is still not going as fast as it needs to.

In the 1/4 mile, what you get is a car that launches like hell, and then for the whole last half of the track or whatever, does not accelerate any more at all. It might put down a good 60' and 330', maybe an OK 1/8, but a terrible 1/4 ET and an even more terrible 1/4 mph.

M1tch

i just took my car on the highway and did about 95.. It was pathetic. After about 65 mph the 3rd and 4th gear are so long it hardly moves out of 2500 rpm range..

So what could i do? Is it possible to shorten 3rd gear and overdive? What should i change my rear end gear to?

* The car will still shift at its normal rpm range correct? It will just be shorter? Or am i wrong?

sofakingdom

iTrader: (1)

For a stock TBI car, 3.23 or 3.42 work pretty good. 3.73 is too much, unless something has been done to uncork the motor. (exhaust, cam heads)

You can't really change the transmission gears. Changing the rear end gear "shortens" all of them.

The TBI auto cars came with a 2.73, and the 5-speeds had 3.08s. The auto cars REALLY suffer with that. I guess they were the CAFE sacrificial lambs.... such that it didn't matter how awful they made the car drive, they HAD TO do that, to keep from getting hit with the $1500 or whatever it was gas-guzzler tax.

blackbetty89

So with my set up and modifications, should I go with 3.73's or 3.42's? I will be swapping to a carbed 350 later on and plan to stay with the gears i choose. What do you recommend? Gas mileage and freeway driving are the least of my concerns, but I do want to take into consideration 1/4 mile times.

jimmy_mac

Like has been stated, the 3.73s may be to much for your stock TBI 305 for a good quarter mile pass. But having a stock 305 TBI is a temporary thing for you. And for the short term, it will hit 0-60 speeds pretty good with the 3.73s. I mean why take a stock 305 TBId car to the drags anyway? If you build a good carbed 350 with a decent cam that will make power to 6000 rpm, the 3.73s will be the way to go.

M1tch

I plan on getting headers/ exhaust and an open element air filter with the gear ratio changed.. So I should go with 3.42's?

91ChevyRS

Thats what I am gonna be getting most likely...see the problem is my camaro is my daily driver and I dont want anything too eccentric.

The 3.42s I think will be perfect, 3.73s sounds like a bit of a hassle to me....I dunno why.

The stuff you plan on doing should match well with the 3.42s

robertfrank

iTrader: (27)

3.42 `s dude make a big difference,mod`s in the sig