I recently posted about how "low to go" for a 1970 454 LS5. I currently have 4.10s. The general concensus was 3.08s with the exception of two replies which suggested 2.73s. The concern being that. off the line performance would be compromised by using anything below 3.08s The torque for the 454 LS5 is 500 foot pounds at 3200.

I want to keep the car stock so overdrives ther transmissions can't be considered.

I found a tire size/rear end gear =RPM at 60 mph calculator at (http://www.drivetrain.com/rearend.html ). The current combo is 3200 RPM at 60 MPH. With 3.08s it would drop to around 2250 RPM @ 60 MPH. With 2.73s would be 2000 RPM @ 60 MPH. It seems to me that if torque peaks at 3200 at 500 foot pounds, there should be enough torque to still create great acceleration with 2.73s?

Please share your thoughts and especially actual experience with changing your own gears

I have been unable to find a dyno curve read out for the 454 LS5 any sources ?

Thanks for all your help

Ruti[B]

Be carful with that much torque and a 2.73 gear because you could tear up rear ends and your drive shaft I would opt for the at least a 3.08.

A call to drivetrain after posting this afternoon was revealing. Evidently the "carriers" for 12 bolt mains are by numbered series. A gears with numerical numbers that start with "4" as in 4.08 and above have a four series carrier, and gears in the threes require a 3 series carrier. Carries are about $425 for an aftermarket brand. But the real news is that 2 series gears and carriers are no longer available new. So 3.08s are the gears for me along with 1000 bucks in parts for all the everything.

I will update afterwards to report on the results

Ruti

in the q&a part of the june issue of chevy high perfornce magazine a guy asked a question about rear end gears and tire size. if you go with a shorter tire you gain a higher gear. the question is on page 131 and is entitled short and wide.

A call to drivetrain after posting this afternoon was revealing. Evidently the "carriers" for 12 bolt mains are by numbered series. A gears with numerical numbers that start with "4" as in 4.08 and above have a four series carrier, and gears in the threes require a 3 series carrier. Carries are about $425 for an aftermarket brand. But the real news is that 2 series gears and carriers are no longer available new. So 3.08s are the gears for me along with 1000 bucks in parts for all the everything.

I will update afterwards to report on the results

Ruti

I have a 2.73 gears and carrier for a GM 12 out of a 1972 El Camino Custom 400. I'd be willing to swap. E-mail me if your intrested.

There are two issues I see with the 2.73 gears.

The first has been noted that you will have a tendency to break rear end and drive shaft components easier as the stresses on parts from a hard launch are greater for the 2.73's than 3.08 (not much, but some).

The second is tuning of your carb to compensate for the gears you are discussing. To compensate for the 2.73's, you will most likely have to lean out the carb for the hard launch (low RPM) so it does not flood and "bog down" which will in turn hurt the high end RPM's by limiting the amount of fuel needed for 3700 RPM's on a 454. Unlike modern fuel injection which can be tuned for the entire RPM range, a carb has a much more limited adjustment range.

If you are talking about every day use, you can undoubtedly find a medium ground tuning range for the carb. For racing, I believe you will have a hard time tuning for both ends of the carb range, thus hurting potential 1/4 mi. times.

I have a 60 Elky, a big heavy boat. I am setting mine up with a '71 454, roughly 450 HP. I have a Ford 9" rear with 3:31 gears which I believe will be the best overall gears for street / strip use. I am still looking for a stick trans that can handle the torque and give me an overdrive gear to save some gas-mileage. Depending upon first gear ratio, I may wind up going down to 3:08 since the overdrive will handle top end RPM's.

Let me know how happy you are with the gears you choose - I am interested to see how they work out on the strip . . .

Dan

Useful calculator for determining rpm's at various speeds

mph X (times) gearing (3.08) / (divided by) wheel height (diameter) (24 inches) X (times) 336 = rpm

Today I took the car out and tried driving at differant rpm's to observe several things. Noise, engine power, vibration, transmission performance (auto trans). The 454 has a lot of torrque from about 16-1700 rpm and builds fast. When driving at 40 mph with the 4.10 gears the motor is turning around 2200 rpm even at this rpm the noise is not insignificant, but tolerable, the power is great, the vibration is not a problem, the transmission is really in top gear (third) in fact the tranny shifts into third at about 24 mph under light throttle. By droping to 3.08s the rpm would drop to about 1700 rpm at 40 at the bottom of the power curve but livable at 60 the rpm would drop from 3200 with 4.10s to about 2600 with the 3.08 and the 2.73s drop another 200 or so. So I am leaning towards 3.08s althought the differnace between the 3.08s and 2.73 is rather minimal.

In terms of tearing up the drive train my mechanic said the following

You'll burn up your tires and put a little more stress on the torque converter but said that the same amount of power goes through the drive drain regardless of the gearing?

I would like to hear some more about this issue of higher gearing creating more drive train stress

Ruti

Hey Ruti,

Your mechanic is correct in that the same amount of power is going through the drivetrain regardless of the gearing. However, most drivetrain parts are broken or blown at the bottom end or the top end of your RPM's. The top end is easy - something was worn out or overwound and in general exploded. That statement is very generalized, but in most cases, it holds true.

Bottom end breakage comes from that hard torque hit when you mash down on the pedal. You are asking stationary parts to go from a standing stop to high RPM's in a short amount of time. Your current 4.10 gears allow rotational movement in the axle housing with more ease than 2.73's or 3.08's will. It takes more torque to get that initial movement going, thus increasing the possibility of breakage - and at 500 ft/lbs of torque, there is a lot of potential to twist up some stuff under your car.

Also keep in mind that the better you can plant your tires (reduce or eliminate tire spin), the more torque is transferred through te driveline. Breakage can be overcome with beefier parts like your 12 bolt axle housing, boxed control arms, a cage in the car and so on - so don't sweat it too much.

They say "if you're not breaking parts, you are not driving your car hard enough . . ."

Dan

I have a pretty stout small block in my car making almost the same numbers as your engine with a 3000 stall and 2.73's and off the line punch is not a problem, neither is acceleration on the highway, just grab second and go. Thats the beauty of low gears, you can accelerate much longer before you run out of gear. I can pull 1st to 45 and bury the speedo with second, and I still have another gear to grab. If you have the power, which you do, and want a car that will pull for a long time and keep you pinned in your seat go with low gears. Also about breaking stuff, you'll be fine as long as you don't dead hook the car, just keep your tires as the fuseable link.