Diff ratio to suit

[Tczz]

Hello ... just trying to work out how the car would likely perform with a ratio change. At the moment the 8 3/4 has a 456-1 on the posi, I am want to change it out to what i have on the shelf which is 3.23-1 just trying to get the car to be abit user friendly for driving highway speeds without the motor getting screaming or looking for another gear to drop into .
Question... is.... The torque-converter has a stall rating of 3500 so how would that ratio effect the type of performance that the 456 is producing ? just trying to get an idea of that before it goes in or set up a new crown wheel that would work well enough in both worlds street / strip
thanks for the advice out there

 

[Outsider]

Well it won't be as quick, but if you make a lot of torque and your cam is not too big it will run. Seems like one of the major changes you could do in an afternoon and check it out for yourself. Personally I like something like 3.9 gears in a performance car that I don't intend to daily drive on the highway, but lot's of people like the 3.23s for the gas mileage.

 

[RemCharger]

I'll put it to you this way, my 67 Charger healthy 383 stock weight went 13.69( very best) to a 13.19(only time out) from 3.23 vs 4.56.
I still run the 3.23s.
It'll still peel the tires, but you have to wind it up to get some action. A 440 wouldn't care as much.

 

[Detective D]

Seat of the pants feel most of what you will miss out on is how first gear runs. Once you get moving, the gears might be a bit taller but the car will feel familiar driving it, just might have to get used to when it will kick down to second vs how it is now, but it will move right out almost as good once you get past the slower start.
You can't really have it both ways unless you figure out how to add an overdrive to the system.

To put it in perspective, Police cars have 3.23, they kept that right to the end in 78. Police cars also had a 140mph speedometer, so you know....
When it comes to your converter, weight of car and how big the engine is will matter more then the ratio IMO.

 

[1967coronet]

3.23 is a good all around ratio.
Tire dia. will matter with that converter.
26" rear tire those 3.23 will give you 2200 to 2300 at 60mph , speedometer and tac differences not withstanding.
That 3,500 converter better be pretty tight. Plus extra cooling cap.
Myself I would swap it for a 2,500 stall.
All depends on what your plans are for the car.
The 4.56 has to be fun as hell.

 

[#41]

Changing out the 3rd member is an easy project, so just do it and see how it works. THEN maybe decide about the converter, or put the 456's back in.

 

[Geoff 2]

- diff ratio is always a compromise
- comparing to another car is reasonably useless, unless every component is the same.
- & then there is the unsuspected. A Pontiac I tune for drag racing had the 3.31 12 bolt diff changed to a 3.73 Ferd 9". No other changes made. Ran slower with the 3.73.
- as a general guide: if you want better economy, less noise, use a numerically low gear ratio. If you want performance, use a higher ratio

 

[Tczz]

3.23 is a good all around ratio.
Tire dia. will matter with that converter.
26" rear tire those 3.23 will give you 2200 to 2300 at 60mph , speedometer and tac differences not withstanding.
That 3,500 converter better be pretty tight. Plus extra cooling cap.
Myself I would swap it for a 2,500 stall.
All depends on what your plans are for the car.
The 4.56 has to be fun as hell.

appreciate what your saying, the tire diameter is 28" less a little sitting on the car ... does have a bigger tranz sump , could do with a better cooler perhaps not to sure how tight the converter is .. what are the physic's of this converter business from one to another
The engine is a 63 dated MW rb block with SR440 Indy heads which were rated with alot of work to them as a bolt on 100 hp. Reputed hp for the engine was about 700hp but I don't have the paperwork to say..
MSD Dizz without the 6al box do have a one to go in for another day of fettling around .
one other question thats been wearing out the brain is the plug gap for these plugs NGK 5671-10 factory set is .021 has been suggested to bump them out to .040 ... compression is 225-230 psi cylinder pressure ...
cheers

 

[1967coronet]

appreciate what your saying, the tire diameter is 28" less a little sitting on the car ... does have a bigger tranz sump , could do with a better cooler perhaps not to sure how tight the converter is .. what are the physic's of this converter business from one to another
The engine is a 63 dated MW rb block with SR440 Indy heads which were rated with alot of work to them as a bolt on 100 hp. Reputed hp for the engine was about 700hp but I don't have the paperwork to say..
MSD Dizz without the 6al box do have a one to go in for another day of fettling around .
one other question thats been wearing out the brain is the plug gap for these plugs NGK 5671-10 factory set is .021 has been suggested to bump them out to .040 ... compression is 225-230 psi cylinder pressure ...
cheers

Sounds like a fun set up . On the converter, you can tell pretty easy just by feel.
Loading on trailer , will it go on with light throttle ?
Driving down the street , RPM below stall limit if you change gears will the speed follow the RPM ?
If it does its fairly (( tight )).
Have fun just keep a eye on the heat / fluid if you swap to the 3.23.