C2 Rear Differential Change

[Ralph E.]

What would happen if I changed the stock differential, 3:70 gear ratio to 3:08.
I have a 327/300 4 Speed convertible? Does the small block have enough power to drive this rear end?

Ra;ph #37280

[Tom D.]

Sure - even 250 HP and 3:08/1 is fine *NM*

[Duke W.]

Re: C2 Rear Differential Change

It's the torque curve of an engine, not peak power, that dictacts suitable axle ratios. Both these engine had considerably more low end torque than the SHP/FI engines.

The 250 and 300 HP engines included a standard 3.36:1 axle with all three transmissions. With the (wide ratio) four-speed, a 3.08 axle was optional.

Duke

[Duke W.]

PS

Just to be clear, a 3.70 axle ratio was NOT available with the 250/300 HP engine and the only optional ratio was 3.08 with the four speed only - no optional ratios for the three-speed manual or Powerglide.

Duke

[Ralph E.]

Re: PS

Duke,
My NCRS Spec Guide list the 3:70 rear for a 1965-1967 with a 327 engine. Since '67 base engine was a 300HP, I assumed that included both small blocks.

[William C.]

Re: PS

Standard rear with 300 hp was 3.36, but 3.70 was optional in the mid-years.

[Joe L.]

Re: PS

Ralph------

No, a 3.70:1 ratio was not available with the 300 hp engine for ANY 63-70 Corvette regardless of transmission.

Also, the only 4 speed transmission originally available with the 300 hp engines was the wide ratio (2.52:1 1st gear for 1967). If you still have the wide ratio transmission installed in the car, the 3.08:1 will be ok. However, if someone has installed a close ratio four speed (2.20:1), then I would not recommend using any rear ratio numerically lower than 3.36:1. Even 3.36:1 will be marginal in that case.

[Ralph E.]

Re:C2 Differentials

Thanks for your help. Back to the original question. It sounds like changing to a 3:08 rear could work out just fine if I choose to go that way.

[Joe L.]

Re:C2 Differentials

Ralph-----

It should IF the car still has a wide ratio four speed. Otherwise, with a close ratio, I don't think that you'll like the "off-the-line" performance with the 3.08:1.

[Larry S.]

Re:C2 Differentials

I have a question if you dont mind.

Why are you going so deep on the gears? I would think thats quite a jump.Some place on line I had found a chart explain the difference in the gear changes in rpm's

I am going from a set of 4:11's to a set of 3:36's I was told for me it would be like staring out in third gear on my car. M21

I have a 427/425 and was able to start in third gear NO problem so I thought it would be great to have 3 more gears left to shift to.

[Terry D.]

Re: PS-Joe-I'm Confused

Joe
Are you saying the NCRS spec guide is wrong on rear end ratio's? It lists mid years all the way up to a 4:56. If that's the case what else is wrong in the spec guide and why are they selling them with bogus info? Just curious.

Terry

[Tom M.]

Re:C2 Differentials

Larry,

I think it was on '67heavern's site. Glad to read on the CAC you are going back to stock, you seem half way there already,

Tom

[Ralph E.]

Re:C2 Differentials

Larry,

I was just wondering about making this change because someone has offered me a 3:08 Rear. I thought for highway driving it would be an improvement. Less RPM on the little engine. I have always thought I had a 3:70 rear. I can't read the markings very well. It looks like "A-something" but it is worn and not very legible.

Ralph

[Wayne K.]

Re:C2 Differentials

Ralph,

A good way to know what your rear gearing is is to jack the rear of the car up ( both wheels off the ground ) and put a piece of tape on the drive shaft. Have someone turn a wheel once around and count the revolutions of the shaft. 3 1/2 turns would indicate 3:55, 3 3/4 turns 3:70, etc.

Wayne

[Bill Jackson]

Re: C2 Rear Differential Change

Drive the 3:70s around using only 2,3,and 4th gear. That is what it will feel like if you change to the 3:08s. If it feels ok around town yhen you will really like the 3:08s on the highway. I went from 4:11s to 3:08s with a close ratio 4spd many years ago without regret although if I could magically have any ratio I'd try 3:36 or so.