The Stovebolt Site Forums The Shop Area Driveline Rear Axle Switch on '54 Suburban

I'm installing a '79 Firebird rear end in my '54 'Burb. Do I put the original springs above or below the new axle? And, if I reverse the springs, will that help me center the axle with the fender opening? This project will have a 300hp 350V8 tied to a 700R4 and the '79 Firebird rear end is a 2.73 posi; the front subframe is also '79 Firebird. It has a '69 Impala factory A/C system, with a 2" lowered dash to accommodate the vents and A/C controls. But I'm up in the air about axle over/under the springs. Anyone's help will be greatly appreciated.

Which ever way you install it, be sure the spring locating pin aligns with the hole in the spring mount. You will have to replace it if you mount it above the springs. Any trailer shop or spring shop will have new pins. Allen head bolts also work.

When above the springs, the axle tubes are likely to hit the frame rails.

To center the tires, you will have to drill a new locating hole in the spring pad or make a adapter with one pin and one hole offset. The u-bolts will still work as long as you don't move back to far. Most go about a 1" to 1.5" back, yours may vary.

Your choice of gear ratio is interesting, in overdrive, your final gear ratio will 1.91 :1 . A much better gear would be a 3.23, 3.55, or 3.73. Even a 3.08 will net a 2.16 to 1 ratio in overdrive. I really doubt your 300hp will be able to pull the overdrive with out eating a bunch of fuel. See here for a chart,
http://www.crawlpedia.com/rpm_gear_calculator.htm

example, 30" tall tire
2.73 gear ratio
60 mph
.70 over drive
1284 rpm, you better hope this is diesel with a ton of low rpm torque. Plus your burb is the size of a barn, so add in a bunch of wind resistance. Even my little 1/2 ton '37 truck eats fuel when the wind is strong, they are not aerodynamic in any way shape or form!

Joe

The original axles were mounted above the springs. If you mount the replacement axle under the springs, you will have a serious "down-hill machine."
I am really hoping your 2.73 is a typo for 3.73.
I don't think you need to reverse your springs.
I welded the axle perches sold by Classicparts to a 12 bolt GM axle (3" tubes). They have an extra hole way off-center for the thru-spring bolt head, which locates the axle in the original front-of-center position.
Unless you plan on using giant tires, this location will work for you, and look more original.
Carl

In your other Rear Axle Spring Fitting Thread.
You didn't mention that you were going to put in a 300 horse power engine.
With that kind of horse power you should not use the original leaf springs.
They are not wide/stiff enough for that kind of torque.
The original leaf springs were designed for a 130 hp engine.
If you use the original leaf springs, because of the axle whip,
you will wear out the pinion bearings prematurely.

Go to a spring shop and tell them what you want to do.

No matter what leaf springs you use.
The Leaf spring is under axle.
That is how to mount your '79 Firebird rear axle.

If you put the leaf springs on top of the axle, the rear of the Suburban will look jacked up.

This discussion should be in the Hi-Po shop

I concur.

Thank you, men, for your comments. I can always count on stovebolters! What if I used my rebuilt '79 Chevy truck 350, only 225hp . . . would that eliminate the concern about my original springs -- and the 'weak' 2.73 with the 700R4? Or, should I just use a 350 turbo transmission -- or my rebuilt and strengthened 2-speed Powerglide? Comments, please -- suggestions?

Since the thread hasn't moved yet I will jump in. The concern about spring size would bother me if I was going to do a lot of hot rodding. If you plan on daily driving with no burn outs or sticky tires, the stock springs would be OK. You can always add one more spring clip to the front half to help hold the pack together better. And as long as you are running a 2.73 axle in a heavy truck, spring wrap probably won't be a problem. Beings this is a suburban, I would bet the spring are fairly heavy already.

So save a bunch of setup time, a T350 trans will be a better choice.
( http://700r4.com/faq/ratiocmp.shtml ) It doesn't have the lowest first gear ratio, but you won't have the expense or weight of a 700r4. Stay away from the powerglide, they are outdated and like starting in second gear all the time.

Sweet is at our house, helping John with some equipment stuff. So ... I will move the thread to the HiPo forum.

Peg

Good information on the link Joe H! I am getting ready to put a 700r4 behind a 350 with a 3.08 rear axle, just like your link talked about.
I one time considered using a TH350 until I looked up the 1st and 2nd gear ratio differences between a TH350 and a 700r4 online.
Always nice to get confirmation from multiple sources.
Carl

I decided to use my 225hp '79 pickup 350 and now have4 to decide whether to go with a 700R4 or 350TH. Daily driving, dseveral 1500-2500 mile round trips to visit kids and grandkids. No hotrodder.