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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?
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.
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,
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.
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.
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.
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.
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.
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.
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.
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.
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