Hello all,

I wanted to see if I could get some advice on what ratio to go with on my rear end rebuild that I'm planning on for next year. Some basic information on my car is as follows:

71 mostly stock cutlass s
Th350 trans
350 rocket
Aftermarket cam from previous owner
4 barrel 750 Holley
Edelbrock intake manifold
Headers and dual exhaust
MSD ignition system
Stock 10 bolt rear, guessing it's something in the mid 2's. Open diff.
24.5" tall rear tires

The car is setup for highway cruising and can do over 100 mph without a problem, I don't have a tach but I cruises at a low rpm at 65-75. I think I may add a tach before I finalize my rear diff ratio decision. Anyway the car is peppy once it gets going or if I keep it in 1st or 2nd gear but from a stop it's not real quick and won't spin tire even with the open diff.

I've been talking to a couple of specialty shops to get pricing on a rebuild of my diff and axel for an upgrade to limited slip and lower gearing. I've done a good amount of research and found that people are going with ratios from low 3's to mid 3's. I'm not looking for something crazy just a ratio that will have a big difference on my off the line power and still be able to drive on the highway at 65.

Recent pics for reference.






Thanks in advance,

Pete

 

Pete your working on the wrong end of the car.

i went thru this in the past couple years i swapped my stock 2.7x open rear to a 3.42 posi and found i could still just barely spin the tires w my stockish 140k mile 350.

undeterred i found a 455 had it rebuilt and put that in now that 3:42 posi works likemad to get some traction! downside is at 70mph im a tad over 3200rpms w 255-60-15s...i kinda wish i still had the 2.7x rear but w a posi in it.

no matter the car is a lot funner to drive now.

my dads old 71 350 supreme wouldeasily spin the stock open diff (prolly also a 2.xx) i suspect its the dizzy curve that gives it the lethargic low end, my 350 had a later model HEI in it.

BTW for reference i paid $1200 for a complete rebuilt and refinished 3:42 posi axle set-up that i swapped into my car.

 

Thank you Retro, my first quote was for $1,300. What did the whole 455 replacement cost you and what else did you upgrade for it?

 

hard to put a number on the 455 as i added a bunch of stuff like frame stiffners, sway bars, new mini starter, mtr mounts, new qjet, w z manifolds...the list goes on, 455 was $500 rebuild was $ 4k ish.

theres a running rebuilt 455 on worc or bost cl $1500

i have a 455 core for sale $455 in mass, but if i was you i would chk out that cl 455

 

I had that size tire for a long time. With a 2.56 rear under the car the engine was around 2000 RPM at 60-65 MPH. With the 3.23 rear it was around 3000 RPM. Acceleration with the 2.56 was unimpressive.

Seems as if your engine should be able to spin a single tire, so perhaps it needs some tuning?
Do you know what the initial advance and total advance are?
Has the Holley 750 (which seems kinda large) been jetted / tuned for your application?
Assuming it's a vacuum secondary carb, has the secondary vacuum opening been adjusted? I had a Holley 600 back in the 80s and it took a lot of tuning to get it to launch as well as the factory QJet.

 

Are you SURE about that? They look taller than that in the photos. 24.5" is a VERY small tire. Even a 225/60-14 is taller than that. The photo looks like 245/60-14s, which would be about 25.5".

 

Joe,

I found that the BFG 245-60-14s are shorter than what's stated in the specs. Here's a pic I just went and snapped of one of my old ones - I know the tire is nearly bald but it never measured 25.5" even when new.

 

I have 245 60 14 and they measure exactly 24.5", I know that dimension because the rear end shop asked for it.

I had the timing done when the MSD was put in this summer, I have not had it tuned or dino'd or done any mods to the carb setup.

 

Your timing curve sucks for power. Its way to slow and your not getting max advance until 4k rpm. Your old stock distributor will work better with the stock setup.

 

So I should adjust back to 35 and 52 degrees?

 

If it were me I would adjust the curve (or have it done). There isn't enough advance built into it IMHO.

 

The vacuum looks good, it's the mechanical that needs work.

 

I heard back from the rear end shop and they stated the following:

"If you have 8.2" rear, 3.36, 3.55, 3.73, or 3.90 gears are available. If you have the 8.5" rear, 2.73, 3.08, 3.23, 3.42, 3.73, 3.90, or 4.11 gears are available."

How do I know if I have a 8.2" or an 8.5" rear? My diff cover has 10 bolts and no indents on the sides.

Thanks,

Pete

 

All 71 and up cars used the corporate 8.5 inch 10 bolt so that's what you should have. I went through the same process with my car and decided to go with a new posi unit and 3.42. I wanted good low end acceleration and still be able to cruise at 70 and 3.42s are a good compromise. If your really worried about running too high of rpms at higher speeds a 3.23 would also be a good upgrade option.

 

Thank you everyone for all the help

 

But seriously, work on tuning your existing combination before dropping even one dime on a 455 or new rear gears. Your right foot should be able to spin a tire with ease.

 

I would run 38 total and 50+ with vacuum advance. You want max mechanical advance in before 3000 rpm. I found 38 total felt stronger on the street even though 36 was faster at the track. My 245/60R14's BFG's measured around 25" when bald. With those short tires, I would run 3.08 gears.

 

That picture tells me what the 245/60R14's will look like on the back of my car, looks good. What size do you have on the front?

 

235/60/14 up front

 

Anyone know a good shop in Mass to get my car dyno tuned?

 

Thank you, I'm going to try to find a good shop to get it dyno tuned.

 

I'm having the same issues and I'm going to take mine to a tune shop first

 

Looks good. I plan on the same with my current 88 Cutlass factory aluminum rims on my 70 S.