Do the mods mentioned by the others sound good or any other things you think would help my goals with my self imposed limitations?

 

My car came factory with the Gymkhana suspension.
I have changed it up a little. Factory Bilstien's have been replaced with modern Bilstien's. I have cut and thus stiffened the front springs. I added a composite mono spring in the rear and a 3/4 inch rear sway bar. The factory gymkhana sway bar was a wimpy 7/16". Better than nothing. But not by much.
I really don't feel my car rides to stiff. Particularly at speed. At very low speeds perhaps a bit stiff.
On the track or twisty mountain roads, I wouldn't change a thing.
As per no decent tires in 15 inch sizes. These guys need to think outside the box. There are more tires than just Cooper, Mickey Thompson and BF Goodrich.
Which admittedly do suck in performance applications.
Tires are manufactured all over the world. Limiting ones self to just Mexican and American tires could be the issue.

 

The biggest limitation is sticking wirh 15" tires. The other mods, frame mods, coil over, etc. are not worth looking at because they will never produce their potential due to the tires.

 

There was an article on this car at the time it was written the suspension was pretty much stock. I mean no coil overs no tubular A arms. And it would hang with a C6. C4 would have no chance

 

I believe the most noticeable up grade is the wheel size along with good tires. At first for me, I went to 18" wheels with tire diameter that matched factory. (27") . I was pleasantly surprised. Later I've updated bushings, (rubber), Bilsteins, glass 330 rear spring, stock ' tiny ' sway bars, , , I can hang with any C4 that wants to have a go,,, and still ride smooth for a 8 hour cross-country drive. Don't over do it.

 

Go with Bilstein shocks. The handling will be much improved over stock units and the ride will still be "firm" but pleasant for normal driving. If you have stock rear spring, go with std. duty Bilsteins in rear and HD's in front. If you have a mono-spring in the rear, go with HD's all-around. The rest of the suspension upgrades depend on how you intend to use the vehicle.

 

Ok so you have committed to stick to stock wheel size and stock size tires.
This limits you to standard tire compounds (T-rated).
There are no high performance (summer only) tires or rubber compounds in those sizes.
The one choice some people use is the Avons. But they are really expensive like ~$600 ea. or so I hear.
Otherwise the good sticky compounds do not lend themselves to 60 series tires.
I ran BFG comp T/As in a 255-50-15 size years ago, but those sizes are long discontinued.

There are only a couple of brands on the market in those sizes. No Nittos etc.
So you will be limited to rather tall 60 series sidewall heights and sidewall roll-over, and probably .8-.9 Gs.
Yes 245's will work better, especially in the front.
With that in mind there are a lot of things you do not want to upgrade.
And maybe only a few that you might.

I would stick with stock mechanical suspension hard parts, a-arms, etc.
Stay with rubber bushings.
Just make sure all that stuff is in tip-top shape.
Fresh idler arm, tie-rods, rebuilt steer box, etc.

You will never get the car with these tires to handle exactly like your Z51, so concentrate on control and balance instead. That you can duplicate.
With the difference in tire tech you will never get the same cornering power tho, so don't sweat it, you can get sorta close. Most people appreciate the balance more the pure cornering power anyway.
Basically the stock car just rolls too much.
Stock front springs could get you there, if you have great shocks, but so will Moog 5758s, and easier, so do that.
You may want to drop the nose 1/2" - 1" .
I would probably cut 1/2 coil or 3/4 coil, to get around 1/2" or 3/4" drop.
Those 474# springs will stiffen up the front some.
I do not like what the 550s do to the ride & handling balance. We are aiming for neutral without going too stiff.

On the rear I would swap to a Fiberglass spring.
That greatly improves ride quality, and roll control at the same time.
A 360# from Van steel is a good complement to the 474# fronts. It keeps the ride frequency fr/rr bias similar to keep a smooth ride on the highway.

For sway bars a 1-1/8" Gymkhana or Van Steel front sway bar works well. It really helps chop the roll rate. Along with a 3/4" V-S rear bar.
All of this doubles your roll rate, from 462# stock, to like 982# now. That makes the car lean 1/2 as much in a turn.
In a 1G turn before it might move 2" now it is only 1"
The combination above also moves the understeer balance more neutral, just like the F41 racing setup.

The springs make the suspension ride frequencies go up significantly, from 1.2 to 1.9 cps. About like a F41 pkg, it will feel 60% stiffer than stock.
But it can ride better than an F41 due to the F/G rear spring.
There is really no need to go any stiffer, especially with 60 series tires.
The above spring/bar combo gives it less understeer, just like the F41, about 5% less than stock. (74% to 69% front roll).

Now for shocks you are going to need good shocks to make it feel tight like a Z51, Bilsteins are the only non-adjustables I would use.
Guys on here have a recipe for what works. Soft one on the front, firmer one on the rear due to the F/G spring.
FWIW, for even more ride / handling control, I would get single or double adjustable shocks. Vikings are good and more cost effective than QA-1s. It was started by old QA-1 engineers. That's what I would do. Warning once you try them, you may never be able to go back to singles!

Coil overs would do nothing for you except make it easy to change ride height. And spend your $. Once you set the ride height you aren't going to change it anyway.

The rear should be run at stock ride height for best handling. It has to do with the odd toe-in curve.

Use Van Steel's advanced street alignment settings, and you'll be good to go. 1/2* neg camber is enough. .6* or so max. on all four.

Options:
A Borgeson conversion PS box has a quicker ratio, and helps tremendously with tire feel. But only if yours is problematic already, then it is a very cost effective upgrade. But your gearbox has probably never been serviced, and at 50 years old, it will have a lot of slop. If your steering wheel feels like it has over 1" of slop, investigate. I think you would appreciate the faster gearbox and a smaller diameter wheel tho.

I don't like the negative rear camber that results from the rear rubber strut rods, resulting a twitchy rear end steering effect under load. If you find you are sensitive to that, run the metal heim joint strut rods. They stop a lot of the squirelliness, do not affect the ride. They make "click" and you need to make sure they stay tight. Makes rear alignments a breeze too. Poly does not work at that spot, it will not hold up.

Be careful with the spring & bar rates, you kind of need to do all 4 at once to keep the handling balance right.

Trust me it will respond in half the time and feel like a different car!
It should feel more like the Z51, it will just be a little short on cornering power, unless you change the tires.

 

Did anyone see the red C2 with the fat tires?

 

I might have missed, when "skimming" all of the responses, but I don't think I saw anybody mention a composite rear spring. Back when VB&P was in business, and my wife still had her C-3, I installed one of their composite springs in the car. It was a vast improvement in ride quality, without any "downside" in the handling department. As many know VB&P is long out of business, but the word on the street, is that VanSteel now makes that spring.

 

I think I have compiled a list of all the changes people have suggested and what I think I'm going to go with. Idk if I mentioned or not but I have a SB 350. I'm not sure if that changes the recommended spring rates or sway bar dimensions, so please chime in if that does change things.

Moog 5758 474# Front Spring
Vansteel 360# rear spring (the options are low arch or shortened - what do those mean?)
1 1/8" Front sway bar
9/16" Rear sway bar (this seems closer to the OEM F41 suspesnions for a SBC)
Bilstein HD front and Bilstein sport Rear (got this from another forum)
Front spreader bar
Frame stiffening based on the Chevy Power Book Borgeson PS (the 12.7:1 is really close to the 13:1 of the Z51 that I'm used to)
245/60/15 fronts
255/60/15 rears

Possible future if I want to go all out:
3/4" rear sway bar
Heim Rear struts
18" torque thrust rims or equivalent

I really appreciate everyone who has contributed something to this post. It means a lot to be surrounded by such an active and helpful community!

 

Del A Lum bushings will help

And this you won't find it in the power book


And these

 

I agree with your choices.
And I also agree with Reno's suggestions, if you go to the 18" wheels and modern H.P. rubber.
But if you stay with 15" rubber, you will get very little benefit from all of that, but the 18's would definately benefit.
Then you could pass your Z51!! It would be that good, as long as you have better rubber on the C3. We can cheat a little can't we?

I would stick with the 3/4" rear bar first time around tho.
You want to keep it pretty neutral.
It is almost exactly the combo that Van Steel recommends, but backing off to the Moog springs, from the 550s, makes it just a touch more neutral.
The 9/16" would give it more understeer. Your choice tho.

Another way to do some fine-tuning on the sway bars, and front/rear understeer balance, is to add some springs to the sway bar links, vs the solid stock sleeves. It softens the bar up.

 

I run the stock gymkhana front bar that came on my car, 1 1/8th inch. Stock front gymkhana springs with a full coil cut. Not really certain where that puts the front springs, a bit stiffer than stock I'm told.yet it still rides nice up front. A glass rear spring, 330Lb. Bilstien's all around B6' and 8's. And a 3/4 inch rear sway bar that I put on back when I did the rear spring, mid 90's. So, almost 30 years ago. I run the stickiest 15's I can find. Galaxy's. Poly A arm, sway bar and strut rod bushings, Poly links on the sway bars. Rubber on the trailing arm bushings.
A few other minor tweeks.
Gotta say I'm very happy with the handling of the car.
maybe it doesn't stick as good as 18 inch wheels and tires. But it gets around the track really well.
Driving the same car all these years probably helps.
I'm real used to it. Know just how far I can push her before she slides.
Back on track however, I started saying all of this because I personally have found a 3/4 inch rear bar to work really well. I put it on all those years ago. Never once felt I should change it.