2:73 gears with Muncie 4 speed

[Todd Devine]

I just finished a year long 70 Z28 build and have questions about gear ratio.
The car was a father/son project that sat for about 10 years before I purchased it. I am not sure if it is a real Z28 as I don’t have a trim tag and it was a Los Angeles built car.

It has a CTB stamped block with 186 camel hump heads, and an original polished winters intake. The trans. Is a Muncie close ratio 4 speed. Also has a floor mounted gas pedal.
The rear end I am assuming is not original to the car and is a 10 bolt open rear end. I calculated the rear gear ratio at about a 2:73 as it is revving about 2500 rpm at about 65 mph.

My question/ issue is it is a complete slug off the line and needs a good amount of RPM to take off from a stop. Once moving it seems to go pretty good.
Is a 2:73 rear not compatible with the Muncie gearing?

I do plan on changing the rear to a 3:55, or 3:73 with posi. Would this make a huge difference on acceleration from a stop?

 

[1980RS]

Welcome. I would ditch the close ratio Muncie and try and find a 2.88 or 3.42 low gear Super T-10, that way you don't need to swap rear end gears.

 

[72SS9681]

If it's an original Z, I would find the correct 12 bolt and gear with the 3:55 or 3:73 gears you talked about and yes this will make a huge difference. If your not worried about the 12 bolt and the rear end in the car is a 8.5 inch 10 bolt (71 and later corporate rear end) re-gear as mentioned and enjoy the car. Sounds like someone either swapped a 4 speed into and automatic car or changed out the rear end. Have you checked the code on the rear axle to see what it is? Might help solve the mystery. I've never seen a car with a 4 speed higher than 3.08 and the one I drove was a dog.

 

[Todd Devine]

I don’t mind changing rear end ratio as I planned on adding a posi carrier anyway. After doing a little research on the net it looks like the m21 was never designed for freeway gears. My decision now I guess is 3:73 or 4:11 gears.

 

[Todd Devine]

It appears to be an 8.5 rear end. I am assuming if it was a real Z the original 12 bolt is long gone.

 

[Todd Devine]

I am not concerned with correctness as previous owner smoothed the firewall, changed to pro touring suspension and brakes and did some other mods. I have added vintage air and Dakota digital gauges, etc

 

[72SS9681]

Depends on what you want to do with the car. For street driving I would go with the 3:55 or 3:73. For me the 4.11 is too low for the street but that's personal preference. Before you buy parts check the code on the right axle tube and make sure its not a 70 8.2 inch 10 bolt otherwise you'll run into problems.

 

[72SS9681]

BTW nice job on the car, looks great!

 

[Zspoiler]

If its a real Z*28 that year should have a 12 bolt rear end with a 4 speed ,Usually with 3 :73 gears ,Not a 10 bolt .If its and 8.2 10 bolt you could find a 12 bolt and or an easier to find 8.5 rear end with and 3:42 and or 3:73 gears .

 

[tom3]

That transmission has a 2.20 : 1 first gear. Combined with a 2.73 rear ratio that's really making for lousy take off at red lights. Not a good trans for a daily driver for sure. Not that good with a 3.70 rear end either as I recall from many years ago. Getting pretty expensive but a Tremec five speed (3.27 : 1 first) would be ideal for that car although the cruising rpm might be too low for the camshaft design.