1st Gen, I dig your writing style, you are one funny guy!

I probably should stop posting so quickly. I ask a question and then after a few hours of playing with calculators, reading, digging up some specs… I would probably pose the questions differently.

My motor has more mild cam and makes good power between 2000-4800 RPM, and good torque from 2400-3200.

So this being said, I don’t want 5th gear lugging on the freeway at 65/70mph. I’d like to be cruising at 70/mph around 2200 RPM. In playing with Tremec’s calculator, it seems I can get there a few different ways, assuming a 27in tire. I could stick with .68 overdrive and get away with 3.73… puts me at 2200 RPM at 70mph. 1st gear ratio is 10.78, which seems like it should get up and going no problem.

If I go with the .72 overdrive, I get the 3.27 1st gear, so if I do 3.55 in the back I still get 2200 RPM for 70mph. 1st gear ratio increases to 11.6 though… too much for street driving?

 

Put in a set of 3:90's with a 12 bolt and Detroit locker.

 

Boy, wide tires! Mine were (and will be) 245/60R15, which is slightly less diameter, and already fill those wheel wells! Here is a look:

I know I've said for years "if some's good, more's better" but I try to only say that where it's likely to be true. My rims are Centerline rivet models, and once upon a time, I measured them, but can't seem to find it at the moment, and they're 190 miles away. Anyway, I haven't checked to see what losing 1/2" in diameter does to RPM's but just sayin'... I know in my 4-wheeling days, we used to find that there is such a thing as too wide, in relation to the weight of the vehicle, and at least in snow or clay, fat tires were bad news.

 

One thing to keep in mind about the 12 bolt. All SS cars have the 12 bolt that included the 290 HP 350. Automatic or Stick. We could say that was a marketing thing. GM also offered offered a 275HP 327. If you ordered that with a manual GM forced the 12 bolt on you. This was not marketing. If I remember right if you order your car this way GM gave you the 12 bolt. GM engineers though the failure rate in the first 12 months of 10 bolts would be high enough to justify just giving people the 12 bolt cause in the long run this would cost GM less. That does not mean every 10 bolt would fail. With the high cost of repairing a rear at the dealer a failure rate of 4-5% could be considered unacceptable. I am not saying you should or should not get the 12 bolt just passing on information. Course the 10 bolts not worth a lot of money so swapping after the 10 bolt blows is going to be the same price.

The other thing to keep in mind is that when these cars where built they had small fiberglass tires on them. A meaty set of radials will add to the stress on the rear.

The 275hp 327 4 speed became known as a poor mans SS. because you got the Muncie trans along with the 12 bolt. It was a nice bargain.

 

My suggestion is, and evermore shall be: 12-BOLT. There is a profound reason why Chevrolet elected to put 12-bolt rear ends into big block Camaros. They did the math. They realized that no typical Camaro owner can resist periodically turning his machine into the burner of two fuels, gas and rubber. When that happens, sometimes 10-bolt rear ends are less than appreciative of the physical forces involved. Sometimes, not always, but as Gorn says, they did the math. One time is one time too many.
Now, to add insult to injury even 12 bolts have differences. Beyond whether it's a 12-bolt truck vs auto, I am told there are two different possible axle sizes, 1.29" with a 30-spline count, and 1.41" with a 33 spline count. Mine turned out to be the wrong size, and my shop owner said, and I quote: "...fun with care would be the way to describe the best method to utilize your car." What he means is, be judicious. The caution Gorn added is correct: these modern tires have more bite, less tendency to spin, and therefore greater forces trying really hard to turn your axles into a pretzel. Or strip splines, or do really unhappy things to your center section. My choice of an Eaton 'Detroit Truetrac' differential makes that equation even worse, as Eaton states that the Truetrac is much better at transferring torque to the axle with the most bite.
Anyway, you might take the statements of the two different shops as perhaps biased at a 'capitalism' level. The profit from immediate work vs the delay inherent in you deciding to take the time to do it better might factor into their advice. Their bias might well be weighted toward immediate income. Also, it is possible that they mostly see modern cars with drivers who don't often seek to define strength of materials experientially. I say, appreciate your Big Block carefully. Spending more up front is often cheaper.

 

That was a great little motor,I had one in a '68 Impala 2dr fastback with a T-400 and 10 bolt posi.
It was a low mile one owner car when I got it back in '76,wish I had it back it was one of the best cars I ever owned.

 

Alright, as I am a glutton for punishment, I opted to put the Posi and 3.73s in the current 10 bolt. Time will tell if I come to regret that decision, but hey… Live and learn.

Transmission & hydraulic clutch kit arrive on Friday. Tires should arrive around then too. Expect to have that all done by mid-Feb.

In the meantime, buying a bunch of little stuff to clean up the car…. new weatherstrip and felt kit, dome light, center dash panel, missing rear window crank… just things to keep me busy.

Also, found a local shop with a dyno and carb guy. Would love to strap her on and get a proper tune, but I assume I’ll want to wait 500 miles or so to give the rear, clutch, and trans a chance to break in.

 

Glad to hear you are moving in a forwardly direction! I was worried that we 12-bolt enthusiasts might have scared you off the site. The failure rate of 10-bolts was very low in reality, just high enough to fail Chevy's "cost to repair vs cost to equip" analysis for big-block animals. I suspect their metrics included a factor for the likely maturity (or lack thereof) of the intended Camaro ownership...

Regarding your clutch, not sure what brand you are getting, but the story is probably consistent across brands. We chose a McLeod Super Street Pro, which is rated to 550 HP. In reviewing their FAQ's, they recommend 500-750 miles of stop adn go city driving, with no drag racing or dyno tuning. They are looking for 1200-1500 clutch actuations to properly seat the clutch disc with pressure plate and flywheel, before exploring full throttle power. OUCH.

In their break-in instructions, they then add, in bold type, and I quote: "Do not run the vehicle on a chassis dyno prior to full Break-In procedure (Will VOID Warranty)"

The shop that is restoring my Camaro wants me to put 500 miles of any sort of driving on him, then return for total vehicle inspection, tightening bolts, etc and at that time they will flush and replace ALL fluids stem to stern. Well before they turn the car over to me, they will have broken in the motor, tranny and differential in the air, and drive tested it 100 miles to gain confidence that they are providing a safe and reliable vehicle. Plus that 100 miles of driving around with the civilians staring gives them a feeling of pride and satisfaction, I suspect. I remember how I felt so many years ago when driving Ricky Camaro, when I would notice people noticing him. I did an install for three months once in Vegas, and would drive him on the strip and it was always cool how much attention he got, even though I was surrounded by ungodly expensive exotic cars. People just love these American hot rods from that point in time. Steel tube headers and nearly non-mufflers just rock.

 

Forgot to mention, it was 3.08 in the rear before. Hoping the move fo 3.73 is significant enough. Time will tell.

Looking forward to getting the transmission in there. Will report back in a couple weeks when she’s back on the road. Will be a while before I get to really judge how much of a difference, but I’m optimistic.

 

blacknwhite....add me to the list for adding a 12 bolt. Think of it this way, It's just a little extra insurance.
I had one in my 68, and that had a built 327 and turbo 400. Never had to worry about the rear or the tranny, with any motor I dropped into it.
I had the original 327 which ran pretty healthy, then added a blower, then dropped in a 302/ 4 speed and then dropped in a 396.
Good luck bring this car back to life and have fun with it.