Rear Gear reccommendations

[phat rat]

So you have a Munice 4 sp, but you don't know your rearends gear ratio but it go 65 mph at just off idle with the 396. Does this about sum it up?

Forget it, sorry i asked the question.

You're getting po'd because no one can answer your question? You asked a question without information on what rpm you want the motor to be turning. To be cruising at 1200-1400 at 65 mph that motor must have been lugging like crazy. Don't expect this new motor to do that. As far as trans gear ratio do some research, they are listed on the net

[chimp koose]

The engine will be most fuel efficient at about the torque peak for rpm.RPM/trans gear ratio/diff ratio x tire dia x 3.14/12/5280x60=mph. with that much torque and that big a carb ,if the engine doesnt see some rpm the carb may be running in the idle/off idle circuit and fuel economy will suffer. What type of dominator are you running? Some dominators have little in the way of off idle fuel metering causing problems with part throttle operation. Sounds like a fun car,should be a beast when you crack the throttle!

[enjenjo]

Looking at the ratios for the 6 speed, I find 5 gear sets, depending on the application. But, first gear is around 3.00 on all of them, and 6th gear is about 0.50.

From your description, I suspect the current rear gear is likely a 2.75 or 3.00. That would make a final drive ratio in  4th gear on the Muncie of 3.00 to one. The 6 speed would be the same in 4th gear, but in 6th gear the final ratio would be 1.50. With a 28" tire, that is 300 mph territory.

In 1st gear with a close ratio Muncie, a 3.00 gear gives a 6.6 starting ratio. That may explain why it feels doggy with the 396. But first gear with the 6 speed  gives a starting ratio of 9 to one, which is more in line with 600 hp.

So as I see it, a good all round ratio would be about a 3.50, or a 3.75. In both cases, with that much engine, first gear, and sixth gear are going to be pretty much useless on the street. but it gives you decent choices if you want to install a bigger tire and race it.

[oj]

Looking at the ratios for the 6 speed, I find 5 gear sets, depending on the application. But, first gear is around 3.00 on all of them, and 6th gear is about 0.50.

From your description, I suspect the current rear gear is likely a 2.75 or 3.00. That would make a final drive ratio in  4th gear on the Muncie of 3.00 to one. The 6 speed would be the same in 4th gear, but in 6th gear the final ratio would be 1.50. With a 28" tire, that is 300 mph territory.

In 1st gear with a close ratio Muncie, a 3.00 gear gives a 6.6 starting ratio. That may explain why it feels doggy with the 396. But first gear with the 6 speed  gives a starting ratio of 9 to one, which is more in line with 600 hp.

So as I see it, a good all round ratio would be about a 3.50, or a 3.75. In both cases, with that much engine, first gear, and sixth gear are going to be pretty much useless on the street. but it gives you decent choices if you want to install a bigger tire and race it.

Thanks, i believe the current rear gear to be about 3.77ish.  I suspect that you are right - 1st & 6th will be useless.    
I believe i'll leave the rear gears as is and when after the headers are made i can provide 'real' info and then you guys can give informed advice.  
We shall see what we see, thanks again oj

[Glen]

,if the engine doesnt see some rpm the carb may be running in the idle/off idle circuit and fuel economy will suffer.

Good advice there


Its a personal preference.  I would want to be at 72-75 MPH cruising at the perfect RPM in 5th gear.....Then use 6th gear for 80 MPH plus.  Thats my personal preference.  since its not EFI, the carb needs to see a certain RPM to work properly at a cruising speed.  I would also want my cruise RPM to be at a point the cam is happy and not choppy.

[oj]

A brief update to the Chevelle.  After stuffing the engine in i boxed it up and sent it down to Kooks' Headers in NC.  They did an incredible job, 2 1/4 primaries, 4" collectors, 3" thru the removable X member into the muffler and out to the rear bumper, all stainless.  I'll get some pics, the work is beautiful.
Anyway, i drove it yesterday for the first time and have some carb problems to deal with but the gearing isn't as i thought.  1st gear is managable - you just don't put your foot down unless you have lots of room - but i never got out of 4th gear.  It was a brief shake-down run and i didn't go onto 4 lane highways, i drove it a couple miles out of town, turned and came back and never took it above 60.  When i get it running better i'll see about getting it into 5th.