Re: Identify 3707308 differential???

[Wayne C.]

Thanks Joe, That is helpful, you're right on the money.... as it turns out, my friend wasn't sure if the last digit was an "8" or a "6", I misunderstood him because he wrote the number as "3707308?6"; in speaking to him, I told him the first 7 digits were a GM part number and I couldn't explain the extra digits... I'm sure he told me there appeared to be a pair of extra digits after the 7-digit part number, one of which he couldn't decipher... but apparently I confused him and he meant he couldn't tell if the last number (of 7 digits) was an "8" or a "6".

The current ratio seems to be 4.11:1 but he hasn't pulled the carrier and doesn't want to do that until he has the parts for a ratio change (axle is currently working fine); he says with rear off ground both wheels turn in the same direction when one wheel is turned, so he thinks the differential is a posi (I'm not sure if that's truly a valid test, but I admit I'd expect the wheels to turn in opposite directions on a non-posi unit).

So are you saying that the carrier (based on the casting part number) cannot be a posi or handle a numeric ratio lower than 3.36:1? He'd like to go to at least 3.08:1 to get the freeway cruising engine rpm down to a reasonable level. If his current carrier is not/cannot be posi nor be converted to 3.08:1, what are his options?

Unfortunately I'm away from home for an extended stay and don't have access to my collection of reference and vendor information. Can you recommend a source for advice and parts to convert to a numerically lower ratio (even if only to 3.36:1)?

Thanks,
Wayne

[Joe L.]

Re: Identify 3707308 differential???

Wayne-----

The carrier was not originally a posi. Whether someone previously modified it and installed a posi and/or a non-original gear ratio, I could not say. With these C1 units, I'm not sure if the non-posi carriers can be modified for posi installation. However, I THINK they can be so-modified or, possibly, a posi installed without modification.

Lower numerical gear ratios can definitely be installed in the carrier; I don't see any problem there, at all.

One thing to keep in mind, though: C1s generally used relatively high numerical gear ratios and there was a good reason for this. The 283 cid engines that most of these cars used really didn't have a lot of torque. So, the lower gear ratio made for better street performance, even if highway cruise was less than ideal.

So, if the engine involved is still a 283, I think I'd be reluctant to use a rear gear with as low as a 3.08:1 ratio. I'd say a 3.36:1 would be about as low as I'd go.

For advice and parts, I'd contact Tom's Differentials (www.tomsdifferentials.com) or Randy's Ring and Pinion. Either will be able to provide advice on this issue far better than I. Plus, they'll be able to provide just about any parts that are required and desired.

[Wayne C.]

Thanks, Joe! *NM*