I have the opportunity to purchase a rebuilt Differential C7AW-E-71 with 3.25 gears for $350. Is this a smart move? I have a 69 Coupe with 351W M code FMX trans. Currently it has the stock 3.00 gears. The car is stock and I don’t go racing but will the 3.25 add enough performance increase to make this all worth the while? Thoughts. Also is that a good price for the differential? Thank you.
If the new differential has limited slip and you currently have an open diff, then yes, go for it. If not, you’ll have a hard time noticing a difference other than your speedometer will be off. I recently switched from 3.00 open to 3.55 LS and was disappointed in the in the change it made.
Keep in mind, if you do much highway driving, your engine rpms are already pretty high at 70+mphs. In some areas in Texas the posted limit is 80. I ended up with an overdrive transmission with my 3.00 gears.
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I’d suggest you find a gear ratio spreadsheet calculator that allows you to input your specific car data including tire OD, final drive ratio, trans gear ratios and plug and chug to see the impact in RPM at various speeds. The Tremec one is my favorite.
My belief is that the change from 3.00:1 to 3.25:1 final drive will not make much difference in seat of the pants feel.
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I was thinking of 3.50 or 3.70 for me. Good for a romp but not too tall for highways.
I do not know much about fmx trans but several companys offer low gear set for c6.10 percent lower in low and 5 percent lower in second. With a 2.75 rear gear you get the acceleration of a 3.00 but a high gear of a 2.75. Not an od but makes them more drivable
I’ve put just under 3000 miles/ year on my ‘68 with ‘69 351W with C4 and 3.00 gears the last couple years. When I get on Interstates with a long run and 70 MPH speed limit ( traffic running 75 to 80), I start wishing for an AOD. So to answer the OP, it really boils down to where you are going to drive and how much. What’s fun on two lanes and what works in modern day interstate travel can be different.
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For the price I think you should do it. Personally I think 3.00 with a poisi is ideal - but you won’t know until you actually try it.
You’re cheating Royce. Either of your FEs will make you smile with 3.00s. But noise does make a difference and we’ve all become used to civilized quiet cars on the freeways. As a reference, on E70-15s, with 3.50 gears, 3000 rpm gets you 70 mph, 4000 rpm gets you 95 mph. That is kind of noisy. I didn’t used to think it was an issue, but as I’ve gotten older I like to be able to talk to my grandson as we go to a car show. Although Royce didn’t say it exactly this way, his point is spot on. Decide what you want your car to be. If you want nasty, go with 4.11s. If you want fun and refined, your 3.00s are just fine. You probably wont be able to feel much difference going to 3.25s.
Wait, I just went back and read the OP. Isn’t the C7AW-E-71 a nodular case? Go buy that thing for $350 if you still can.
It’s not a nodular case. No “N” on the case. Would be nice though
Some early N cases do not have the N on the outside. But do have the extra rib. See picture for rib location.
Yes this is a case with the N.
The big N on the outside of the case did not appear until mid 1969 model year. So all earlier cases did not have the large N in the casting. The cast number is C4AW - so the C7 case is not likely the right one but the low performance 28 spline “single rib” version. Still it is easily worth $350 for the case and the Locker unit.
Here’s a case with the C4AW and no N for reference.
