# Help with Trans choice for OHC6



## azmusclecar (Sep 26, 2007)

I have a 66 Tempest with the OHC 6 (not the Sprint) and its mated to a 3 speed Saginaw. I'm not sure what gear ratio.

The rear in the car is a 3.08 open.

I'd like to go to a 4 speed BUT.....I have had so many numbers from Muncie Owners to Saginaw Owners to BW T-10s........

It all comes down to choosing the right 4 speed.

It seems to come down to math calculations as far as choosing the right tranny using the factor of 9 as GM did or does to choose a rear end ratio combined with the FIRST gear ratio in order to get the vehicle moving with minimum clutch wear and best off idle characteristics from the torque of the engine.

Here is how the math works:

Multiply the first gear ratio by the rear end ratio:

Using the LOWEST 1st gear ratio of a 3 Speed Saginaw= 2.54
Using the GEAR RATIO of my Tempest= 3.08

2.54 X 3.08 = 7.82

GM's magic number from what I have read in the Archives is 9
So you can see how you would need a lot of clutch slippage OR higher revs OR a torquier engine (power band) to get the mass of a 3200 lb gvw car.

Use the Saginaw 3 speed first gear ratio of 3.11
and the rear ratio of 3.08 and you get this:
3.11 X 3.08 = 9.57

This tranny and rear combo will move the 3200 car with much more efficiency, less wear and tear, and easier than the other Saginaw 3 speed.

*Have I lost the readers yet???????*

My 3 Speed Saginaw has a first gear ratio of either:

*SAGINAW 3 SPEED*
# OF SHAFT GROOVES 1St Gear 2nd Gear 3rd Gear
0 2.85 1.68 1.0
1 2.54 1.50 1.0
2 3.11 1.84 1.0
3 3.50 1.89 1.0

I have NOT pulled the tranny to count the grooves.

The Saginaw 4 speed has these gear ratios

*SAGINAW 4 SPEED*
# OF SHAFT GROOVES 1St Gear 2nd Gear 3rd Gear 4th Gear
0 2.84 2.01 1.35 1.0
1 2.54 1.80 1.44 1.0
2 3.11 2.20 1.14 1.0
3 3.50 2.47 1.65 1.0

*Muncie 4 Speeds*
GM Muncie	M21 CLOSE RATIO 2.20	1.64	1.28	1.00
GM Muncie	M20 WIDE RATIO 2.52	1.88	1.46	1.00

As you can see the Muncie Close ratio's 1st gear ratio of 2.20 & 3.08 = 6.77
Far below the targeted GM magic number of 9

The Wide Ratios 2.52 & 3.08 = 7.76

It looks like I would have to change the rear axle ratio even choosing the 4 speed Muncie Wide Ratio to use it while a Saginaw 4 speed with a first gear ratio of 3.11 and the 3.08 takes us back to the 3 speed first gear ratio.
3.11 X 3.08 = 9.57 
*
My whole point of this maybe pointless post was to try to ascertain the BEST choice for my OHC 6.*:leaving:

Math is one thing and numbers don't lie but the huge drop from 1st to 2nd gear in Saginaws seems to be their worst characteristic while the Muncies seem to be rev friendly through all the gears.

They all end up with a one to one ratio so ONLY a choice of an overdrive throws all this out the window but you can even do the math using the O/Ds.


Okay everyone at one time give a big GASP and go, good grief Charlie Brown.:smile2:

FLAME SUIT ON.......BRING IT:boxing_smiley:


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## ALKYGTO (Mar 29, 2010)

I agree with the fact that the Saginaw 4 speed first to second rpm drop is too excessive. I mean the fun ends when you shift because the bottom drops out and the guy next to you just pulled another fender.....

With that said looks like the bigger question is why stick with the 3:08 gear? A 3:23 or 3:55 would be the ticket and then just run a tall rear tire.


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## azmusclecar (Sep 26, 2007)

ALKYGTO said:


> I agree with the fact that the Saginaw 4 speed first to second rpm drop is too excessive. I mean the fun ends when you shift because the bottom drops out and the guy next to you just pulled another fender.....
> 
> With that said looks like the bigger question is why stick with the 3:08 gear? A 3:23 or 3:55 would be the ticket and then just run a tall rear tire.


I understand your point........I was hoping to find a user friendly tranny keeping the 3.08. This Tempest is bare bones, bench seat, rubber floor mat, I was trying to keep it all stock except for the 4 speed change out. I can do a tranny swap back to original myself but when it comes to changing out rear gears, my talent runs out and this gets farmed out. 

Thanks for your suggestion.....:thumbsup:....I'm trying to stick to just one part swap.


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## PontiacJim (Dec 29, 2012)

Well, here's my take. The 6cyl doesn't have the torque to use the first gears found in the Muncie 4-speeds coupled with the 3.08's, and you will have to slip the clutch a bit. So at this point, you have to use one of the Saginaw 4-speeds having the 3.11 or 3.50 first gear just as your 3 speed does.


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## azmusclecar (Sep 26, 2007)

PontiacJim said:


> Well, here's my take. The 6cyl doesn't have the torque to use the first gears found in the Muncie 4-speeds coupled with the 3.08's, and you will have to slip the clutch a bit. So at this point, you have to use one of the Saginaw 4-speeds having the 3.11 or 3.50 first gear just as your 3 speed does.


Thanks Jim............that was my thought but I think I just needed the push to a Saginaw when I can buy Muncies easier. I was trying to avoid the big rev drop as mentioned earlier. The Tempest is just a fun car to drive, not a race car by any means. It's a fun cruiser.......guess that 3.11 will be what I need.......
I appreciate your input.........:thumbsup:


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