Rear End

[MacDee]

In one of the other threads, Jeff W queried about which rear end I might be using when I swap out my tired 170/slug-o-matic for a fire-breathing 200 with C4. I stated that I was intending to stay with the its wimpy 7-inch rear. There are actually some specific reasons I say this, so I would like to present them here and wait for a barrage of feedback....

It all starts with the Dodge Dart I had in my youth. It had the "Power Pack" 273 V8 with hot cam, 4-barrel and dual exhaust (after-market). It had the wimpy 6-cyl rear end (MoPar, of course) and I never had a lick of trouble with it. As I recall, if you had the V8 with a manual trans, you got the 8-inch rear, but the engineers at Chrysler apparently believed that the auto trans would make it unnecessary to use the heavy rear in that application. Also, as I remember very clearly, that motor had very weak low-end torque. It was a dog until the tach reached 4000 when it took off like a bat [from the depths of darkness]!!

With the cam I've selected for my 200 build, I'm expecting a performance curve very similar to that old Dart. It's going to be a little weak in bottom-end torque. Although I know I should not compare FoMoCo and MoPar components directly, I kinda am, and am thinking the little rear end should be adequate assuming I don't get too rambunctious with it (Neutral/Drop burnouts?). Further, with the weak low-end torque, I'll need a fairly short gear. The rear I've got is 3.20:1, which is a pretty good ratio for this cam. If I were to locate an 8-inch with 4-lug axles, it would probably be a 2.79 or something and I would feel a need to have it re-geared to be compatible with my motor.

Next, the wimpy little rear end is, of course, lighter than the 8-inch. Lighter is Better (...if it'll stay Together!).

Finally, I also got the rear end and drive shaft from the '67 Mustang that supplied the engine and C4. It had the wimpy rear end also, and, as far as driveshaft modifications are concerned, I think I may only need a length adjustment of the 'Stang's driveshaft as I believe both ends of it are correct for my application.

Okay...

Let me have it!

[Jeff W]

Gary, I can think of another reason for you to keep the little guy...

When I install my eight inch, you can have my 3.50:1 for free as a spare.

[redfalken]

Gear ratio is the main reason I swapped. I now run about the same RPM at 70 than I used to run at 55. Not that I'm cruisin 70mph too often but it's a much better highway cruiser now. That may not be important to some but I think it's a big plus.

Here's a write-up I did with some thoughts on the swap.

http://redfalken.com/pages/rearend.html

[pbrown]

As Kenny stated, the gear ratios would be why I would change the rear end on a wimpy six engine car .

First gear with a Ford-o-Matic and a 3.5:1 rear end ratio gived you about the same starting gear as a C4 first gear and a 2.79:1 rear end. So by changing from a Ford-o-Matic with the stock rear to a C4 with a 2.79 rear is like adding an overdrive to the car. It's a great thing on the faster roads of today.

[falcon cobra]

Hi gary..you could keep the 6-cyl. rear end in your car, if it works good and does now howl or other bad sounds it's good to go..I used my little wimppy one with a 302 for 5 years before puting in a 8" and it was working fine when it came out, lots of burnouts and a few races, ECT.ECT. trust me it will work with you hot 200....john h

[MacDee]

Gear ratio is the main reason I swapped. I now run about the same RPM at 70 than I used to run at 55. Not that I'm cruisin 70mph too often but it's a much better highway cruiser now. That may not be important to some but I think it's a big plus.

I'm betting the balanced 7-main 200 will LOVE IT cruising 70 at 3100 rpm!

First gear with a Ford-o-Matic and a 3.5:1 rear end ratio gived you about the same starting gear as a C4 first gear and a 2.79:1 rear end. So by changing from a Ford-o-Matic with the stock rear to a C4 with a 2.79 rear is like adding an overdrive to the car.

I'd rather have the shorter First than the longer High. Better hole shot.

When I install my eight inch, you can have my 3.50:1 for free as a spare.

WooHoo!! Even BETTER hole shot!

Hi gary..you could keep the 6-cyl. rear end in your car, if it works good and does now howl or other bad sounds it's good to go..I used my little wimppy one with a 302 for 5 years before puting in a 8" and it was working fine when it came out, lots of burnouts and a few races, ECT.ECT. trust me it will work with you hot 200.

I rest my case...

[Luva65wagon]

If you can find a 4-lug 8" I'd go that way, but they are not so easy to find (at least not that I've seen). If it's sound now, I'd just use the 7" until it goes, which as often as it's probably driven may be a long, long, time.

[Luva65wagon]

Hi gary..you could keep the 6-cyl. rear end in your car, if it works good and does now howl or other bad sounds it's good to go..I used my little wimppy one with a 302 for 5 years before puting in a 8" and it was working fine when it came out, lots of burnouts and a few races, ECT.ECT. trust me it will work with you hot 200....john h

And for what it's worth I just rode in John's car last weekend and if it (a wimpy six rear end) would last 5 years with what John runs, I think you'll be OK. John's car is scary quick (or maybe that was John's driving). Dang'd if I didn't start wishing I had one like that again. Of course I'd probably start getting tickets again too.

[redfalken]

And clear a spot next to the garage to start collecting 7.25" rearends that people are dumping. Since the carrier is part of the housing, it's easier and cheaper to just swap them if they start to go. I've never looked but I've heard several people say that parts are getting really tough to find as are people who work on them.

I agree with Roger. Since you only drive for recreation, what you have should last a long time with normal wear and tear. I don't think anyone would argue that a 7.25" couldn't handle a mildly built 200. It's plenty sturdy.

But...grab an 8" 4-lug if you get the chance.