J-Rho's '67 Camaro Z28 STX build Jason Rhoades builds a 1967 Camaro Z28 clone for SCCA STX autocross

24Oct/100

Agricultural 12-bolt and down the rabbit hole

Some of the parts needed for this build are getting harder to find, this is one of them.

The GM 12-bolt is an extremely robust rear-end with an  8.875" ring gear.  It is often compared to the ubiquitous 9" Ford rearend, the GM piece being a tiny bit less stout, but more efficient (less driveline hp loss).

I've been looking for one for a while, and one turned up on Craigslist.  The price was good, picked it up Friday.

It is a correct '67-built Camaro piece.  According to the owner, it had been sitting in storage for about 20 years, and then in his yard for the past several months.  Based on the dirt removed, by several months, he might have meant 120 months!

Inside the brake drums, an entire ecosystem was flourishing, all sort of spiders and insect carcasses abounded.  In this pic you can see the huge black spider that "owned" the passenger side drum-

I don't think I should count all that dirt as weight reduction, pretty sure it wasn't there from the factory 🙂

Getting this thing in shape is going to be a bit of work.

My plan was/is to get the car minimally prepped, so I could take it to a welder and have all the big on-car welding projects done.  I have taken a welding class, and have access to a good MIG, but don't trust myself to weld anything critical, like a live axle lateral suspension locator.

The lateral locating device will be attached to the rear axle, so the axle needs to be clean and ready for welding.  I also wish to package the device towards the end(s) of the axle.  The limit of how far towards the end the device can be located will be defined by the brakes.  Hmmmm...down the rabbit hole we go...........

So it looks like maybe I should get the rear brakes put together to ensure I don't get welded into a packaging dead-end.  Now, STX does have a "big brake kit" allowance.  Unfortunately, like many of the ST category's allowances, it is a rather Fisher-Price allowance ("My First BBK").  I can say that, having spent a few years on the STAC - aka the Street Touring Advisory Committee, the "subject matter expert" rules making/advising group for the ST category.  I get what they had in mind - the blingy cross-drilled stuff you see in the tuner magazines that are easy bolt-ons for popular cars.  Nothing exotic, no messing with the hydraulic system, nothing requiring any engineering on the part of the ST car builder, just bolt-on kit/kid stuff.

Now, there are plenty of "bolt on" kits for the first-gen Camaro.  However, none of them are really designed to work with the factory master cylinder, which is rather large (1.125" bore).  Those kits all come with some kind of late-model Camaro or Corvette master cylinder, often with a proportioning valve.

Can't change the master cylinder, can't change the booster, can't add a proportioning valve - at least, not in the classic fashion.  Some first-gen Camaros came with a proportioning valve, but that was only the heavier cars with A/C, not the Z28, and even if it did, I couldn't change it.

The flexible brake lines I can change, the calipers and bracketry I can change, and I can use any rotor equal to or larger than the diameter of the factory rotor or drum.  Even the smallest aftermarket stuff is about 12" (stock drums are 9.5") so the diameter won't  be an issue.  Since the calipers themselves are free, I was thinking I could use one small proportioning valve right at the inlet to each rear caliper.  Then I could say it is part of the caliper and thus legal.  I expect to receive emails from people saying "hey, that's not what they had in mind" shortly 🙂

But I'm not totally sure I even want to have a proportioning valve.  The Viper had one when I bought it.  I ran it that way for a couple events (nothing National, folks).  The car from the factory had shorter front tires than rears, and the previous owner raced it with equal sized tires all around.  Taking the car back to Stock, I removed the valve as it wasn't legal, or needed.  When the Viper was on the ideal f/r diameter tires, I thought its brakes were the best of any car I've ever driven, and a lot of that was the factory blessing the car with a healthy amount of rear bias.  When I ran a slightly (.6") taller front tire, it would try to kill the driver with too much rear bias.  So, suffice it to say, there isn't a ton of room for error in sizing the brake system, but the proportioning valve does give you a bit more leeway.

I won't go too far into designing a brake system here, but there are several key variables to consider.  In my case, some are fixed, some I know or can estimate, and some are free to change.

Fixed:

Master cylinder bore

Brake pedal ratio

Brake booster ratio

Proportioning

Known/estimated:

Static front weight

Static rear weight

Wheelbase

CG height

Maximum deceleration (use this to calc load on front and rear tires at max decel)

Front and rear tire diameter

Free:

Rotor diameter

Caliper piston area

...from all this there's a bit of thinking to do.  You can design a perfect system on paper, but if the parts don't exist out there in the world, it doesn't do much good.  Not going to have custom calipers made for this.  Have to see what is available from Wilwood, Baer, Brembo, Stoptech, etc., and piece together a working system from their available components.  Also have to consider - is the car going to get track time?  Autocross puts much less demand on brakes than does the track.  Most of my track experience is in the S2000, which had somewhat undersized and undercooled brakes, a constant problem.

If the car isn't going to see much hot lapping, then 12" rotors will probably be fine.  If it is going to be tracked hard, I'd probably want 14" rotors, as I plan to run only 18" wheels, and they'd provide the ultimate in thermal capacity.  The bigger rotors are heavier though, a disadvantage at autocross.  13" rotors might be a good compromise.  The larger diameter rotors also provide more brake torque with all else equal.

Speaking of weight, the stock rear drum brakes weighed 43 pounds between the two sides.  In my STX Z28 spreadsheet, was anticipating losing 7 pounds in rear brakes, would like to stick to that if possible.  Also have to retain some kind of parking brake, which adds weight.

Browsing Wilwood's brake kit chooser site:

http://www.wilwood.com/BrakeKits/BrakeKitApp.aspx

They ask about the axle offset.  Hmm.  Didn't have the axle apart enough last night to get this measurement, and even if I did, am I sure it would be right?  I'll be changing the limited slip, and didn't know if the axles in there were the stock size and length.  So one more trip down the rabbit hole, might as well get my limited slip now, with OEM replacement axles (no donuts here Civic guys! ;)), so I can put all that stuff together and measure my axle offset properly.  Will need the measurement to be final and exact regardless of whether or not Wilwood stuff is chosen.  They list three sizes, 2.75, 2.81, 2.91.  Again, not much room for error here!

Will go into a discussion about the limited slip choice later.  One somewhat unfortunate thing, is the GM 12-bolt isn't the hottest diff out there, so it doesn't have all the fancy choices some other cars get.  I've had excellent experiences with ATS/Carbonetic in the IS300 and 240SX, and the OS Giken has gained huge popularity in autocross circles as of late.  Still, there are some tried-and-true options for this car I think I can make work fine.

So I got things a bit further along today.  Didn't try measuring the axle offset, figured the bug guts might throw it off by .06" anyway.. 🙂

Whole thing covered in funk and some kind of white paint, got most of it off.  As bad as it looked on the outside, the insides look great.  Everything very straight and true, turning very smoothly.  It had a 3.07 open diff in it, so it's not likely to have seen much heavy use.  The gears all looked better than any of the Viper diffs I'd worked on!

Oh, and speaking of gears, one cool thing about old cars, is you had LOTS of choices-

http://camaros.org/drivetrain.shtml#AxleCodes

...and the Z28 could be ordered from the factory (unlike the dealer-installed cowl-induction or header options) with just about any of them.  3.73 standard.  These days, you have to order a whole big option package or something to get a different rear ratio, you never get to choose it individually.