what is the simplest mod to get more strength from a c3 IRS, other than u-joints?

 

If you have a stick shift and over 500 HP, the only way to have a strong enough rear setup would be to either install a Tom's 12-bolt setup or a solid axle. With stick and less than 500 HP or less than 700 HP with automatic (without transbrake), you can do the following:

1) stronger half shafts (preferably 3 inch thick wall half shafts)
2) solid Spicer U-joints
3) have your posi case polished and hardened spider gears installed
4) install a 400+ lbs/inch rear spring and QA1 shocks to prevent excessive squatting. Squatting is one of the major issues.
5) lower the strut rod mount (special bracket available) in order to prevent camber change when rear is compressing.

those mods shift the weakest link to the axles. However, without going to Tom's setup, I don't see how you can upgrade to larger axles. Those 17 spline stock axles just won't do the job with high HP applications.

 

GrandSportC3's recommendations are sound, as he's proven that a properly modded C3 IRS is capable of taking some serious pounding. However, not all of them are absolutely required if you aren't going drag racing. In particular, that stiff of a rear spring wouldn't be necessary (but do keep an eye on squat if you're running low). Also, eliminating camber gain is very important for drag race launches, but a no-no for cornering, so do take your own purposes into account before making significant geometry/alignment changes.

If you decide to stick with the 10-bolt and want someone else to massage it for you, I'd suggest getting in touch with GTR1999, as Gary definitely knows what he's doing. Myself, I'm a firm believer in the value of a 12-bolt IRS diff behind a stout mill, even on street tires. If Tom doesn't offer the ratio you want, try Henry's Machine Works (they heat treated my 3.55:1 gearset in house). Pepe Estrada designed the 12-bolt conversion for both companies, so either ought to do fine.

Additionally, install a solid diff locating kit and, whether you swap the stub axles out for HD's or not, I'd suggest switching to 1/2" x 3" wheel studs. FWIW, Guldstrand sells HD modified stubs, slip fitted for easy assembly and maintenance. "Blueprint" end play at .001" when you set them back up.

 

You guys are starting to worry me. I am am just finishing-off the engine transplant in to my '68 manual 427. It has 3.08 rear-end, M21 c/r, F40 suspension, 285/40/18's. New engine is 489 dyno'd at 528 bhp @ 5800 rpm, 563 ft/lbs @ 3800 rpm. I was prepared to 'be kind' to the transmission but I am now getting really concerned!

 

If you don't hook up, the tires will spin which reduces shock to the components.. On street tires, you should be ok but with over 500 HP, stick shift and traction, you will not get a lot of life out of a stock differential. (unless you launch and shift like a 95 year old grandmother). With automatic it's another story. I've seen high HP cars getting quite a life expectency out of their stock diff but all the stick cars with over 500 HP will need Tom's setup or a solid axle (in case that the car hooks up).

 

Oh well, seems like I will have to act like a geriatric till I get the measure of things!

 

With 3.08's and a close ratio...you're not going to be hitting it all that hard and those 18's aren't going to hook...so you'll be OK if everything is in decent shape.

The good solid u-joints and a Smart Struts kit to keep everything straight will go a long way.


JIM

 

Thanks Jim for input - do you think that 18"s (even with Toyo's) won't hook-up? I must admit because of the 3.08 ratio I don't drop the clutch when pulling away (if looking for fast times) - just engage it 'normally', accelerate, load-up rear-end and let it 'dig-in', and then put the hammer down relying on torque doing the job.

 

With enough torque the 18s won't hook up because they are a street tire. My 18s are not bad but I can burn them at will very easily.
BTW the 3.08 with a M21 will give you a terrible off the line gear. Even with a lot of torque the 6.77 1st gear ratio is a dog off the line. You need a M20 or a deeper set of rear gears.

 

The 18's are not going to hook really, but the poor gearing will help....UNTIL they start spinning. When they do...it's just going to require some judicious footwork. Little short sidewall tires are not made to accelerate forward that well. They do OK if there is enough footprint..but you'll notice everyone puts something with some sidewall on it for really hooking up.

528HP/563 ft lbs will be a handful...but will be fun from a 40 mph roll!

Your car is begging for a TKO or Richmond 5 speed!


JIM

 

Interestingly - despite the 'shocking' rear end & 1st gear ratio with the stock engine this worked extremely well even shifting at 3800/4000 rpm as I could guarantee very little wheel spin and when it 'came on gear' I would still be in 1st when all the others were about to change into 2 nd or 3rd. Quite how that will be with my extra 40-50% additional bhp/torque is another matter.

 

GrandSport C3 indicated to have you posi case polished. This is a great tip frequently left out of rebuilds. The posi case cracks before the gear set gives out. Once the case cracks the spiders gears and ring and pinion take a beating while you are trying to figure out what the noise "in the rear" is.

I also had a few cases now shot peened by my local engine machine shop. This seems to extend the life of the GM case.

Also stop worrying - when the rear end gives up you can address it then!

 

I just replaced stuff as it broke.

 

With a 3.08:1 differential, the Richmond 5 speed will be the transmission to get. The transmission is 1:1 in 5th gear. The TKO's 5th gear is an overdrive. The Richmond 5 speed, driving a 3.08:1 differential, will perform in first gear like a Muncie driving a 4.11:1. The Richmond will give a lot of pull in first gear.

Also, someone commented about using upgraded strut rods in the back. Tom's Differential sells their own brand of strut rods; they have heim joints.

 

I am still not totally convinced that a high numerical 1st gear ratio is neccesary with engines developing high torque at low rpm. If the car is going to let go in the form of uncontrolled wheelspin where is the advantage?
This may not have 'real world relevance' but some time ago using a 'desk top dyno' I did some 1/4 mile comparisons with differing auto and manual transmissions and rear end combinations using a HO small block engine. This did not take in to account reaction times or (apparently) allow for loss of traction etc. The interesting fact was that the best 1/4 mile times were achieved with a manual c/r 4 speed with a low numerical axle ratio!

 

You're right,it all depends on traction to a degree. You have an engine that develops peak TQ at pretty low RPM so I'm sure it's driveability is pretty decent with the low numerical gears. With peak TQ at only 3800 rpm or so...the cam has to be pretty mild.

But once the tires break loose...it's much harder to hook them up with low numerical gearing. You just have to work the pedals a lot to keep wheelspin under control.

But of course there are different characteristics that everyone likes. I've had a 2.20 low with 3.36's and it ran 55 mph in 1st gear easily. And with the 27" tires it was a slug at lower speeds. When it hooked the clutch slipped a lot, so that probably saved parts.

I later had my 427 in the Vette using 3.36's and the Doug Nash/Richmond combo with a 3.27 first gear and a 1.23 4th. That was a sweet combo with a final drive of 4.13 in 4th. I revved it to the 6500+ range and it ran great.

If you're happy with the Long legs of a 2.20/3.08 combo..go for it. But you would really be impressed with some deeper gear. Gearing helps all the way through the range..not just off the line.


JIM

 

Good comments Jim. I just won't know untill I try it out (which should be within next two weeks with luck)
The five speed with higher numerical 1st and overdrive 5th to me seems the best of both worlds, i.e more sensible for getting away and the overdrive 5th for some relaxed cruising. A few months ago I would have said the overdrive would be good for economy too - but now there is no such thing with our cost of gas.
At the end of the day the $/£/Euro cost of fuel (and the actual mpg figures) are only a number

 

Ask me how I know about cracked cases

 

That is an interesting failure! Emphasizes how they fail by cracking first for sure. Then the other parts get unhappy quick. I have had them crack from the window to the spider gear pinion pin several times. The polishing and shot peening seems to have cured that problem.

As an aside - I use NAPA u-joints with a grease fitting now. The u-joint should fail under duress before the stub axles (inner or outer) breaks.

 

This might be a dumb question or i may be missing something, but how does polishing help prolong the life of the diff, what does it do to it to the metal to prevent this from happening. does it harden the caseing.