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8" rear upgrade

Badass70

Badass70

Member
I've been reading a lot of posts about what differential to stick in my 8" rear and can't decide. First a little back round car is a 70 coupe 302, 40 over roller motor with a 550 lift cam, Rpm intake , 670 carb, afr heads 1650 I think they are, and bottom end fully built.
About 5 years ago I changed the ring and pinion from stock to 3.25, which felt better but honestly just doesn't feel like it gets up and goes. I have a c4 with a bm shift kit and a 2200-2600 stall. My plan is to put a true trac in my 8" but I'm undecided on gears, do I go for the 3.80s or do I just go to 3.55, I'm not a track guy, rarely go but want to once in a while just when I get the itch, do some highway driving , and some city. Do I go 3.80 and upgrade the trans to an overdrive trans? Or just stick with 3.55?
-Joe
 
I'd stick to the 3.55 unless you are the kind of guy who likes to step on it ALL the time from a standstill. Even at that ratio highway rpms will be up there with the C4. The cost to go to an overdrive auto is steep and quite frankly if you are serious about a big gear you should be thinking about a manual trans too just to make the driving experience that much more fun.
 
Yeah that's what I'm thinking, my engine builder took the car for a ride after the Motor was done and was like yeah this car should have more off the line just feels sluggish, so recently decided the car needs some love, I've been on the fence with gear change. At the very least I'm upgrading it to a true trac, with the 3.55. Just hope the investment makes it faster off the line. Oh and not really a lead foot type of guy but when i get the look at the red light, when that light turns green I want to be out!
 
The other thing not talked about is suspension work to get everything available out of the set-up you have. If your tires hook, that 3.55 gear, modified trans and your engine will be more than enough to make you smile.
 
Have the Shelby drop springs springs up front, and drop leaf springs in rear just can't remember for the life of me what they are.
 
Those rear springs are likely not very stiff being more about lowering than anything else. Nothing wrong with that just not really meant to plant the tires on launch. Everything is a trade-off with cars. Putting a bit of "pre-load" on them with something like a set of Cal-Tracs would make a difference just not sure if it is really worth it for your goal. The gear change may be all you need to be happy. Start there and see how you like it. You can always add a set of bars later.
 
The Ford 8-inch carrier is good for up to 400 horses at the axle, or about 450 at the crank. If you have an automatic and street tires you might get away with a few more ponies. The weak point will be the axles themselves or the differential unit.... check the specs of the manufacturer. IIRC, the TrueTrac is rated for up to 400 hp or so. Also, much of the durability factor is in the assembly. A rear that's assembled out of spec isn't going to last at lower horsepower levels.
 
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