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Old 11-03-18, 01:50PM   #1
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Join Date: May 2016
Posts: 5
Default 12 bolt street/strip with tru trac rebuild

This was originally posted in GTOforum.com

Hi Everyone. In the fall of 2016 I shelled my 12 bolt, holing the cover from the inside with some sheared-off ring teeth. I never knew it ‘til I got home, then the oil on the garage floor was a dead giveaway. Full disclosure? It was an abusive relationship. The rear end treated me really nice. It was always there when I needed it. And in return I beat on it over and over and over. And for that I feel bad. But not enough to change my ways. Time for another rebuild. Counting rebuilds this will be the 7th rear end I’ve had under the car.

I rebuilt the 12 bolt as follows. I went with a Detroit Tru Trac differential. I like the fact they don’t require rebuilding like clutch style differentials, and they seem to do what I need—street durability and usefulness and straight line traction at the track. I kept my ratio 3.73. I bought the gear set, the rebuild kit, the bearings, the axle ends that positively retain the axles, and 31 spline custom length axles from Mark Williams Enterprises. They are right up the road from me in Louisville, Co. My builder was Pat at Pat Condon Race Cars. He did a good job. Both Pat and I know some of the folks at Mark Williams and they are knowledgable and friendly. The left main cap was replaced with a steel unit. The ????? were welded to the housing. The positive retainers were used at the housing ends, new 7/16 in studs were installed and the rear end was shortened by ¾” on each side. The yoke was replaced with a heavier unit as was the driveshaft yoke and the drive shaft was shortened accordingly.

More on narrowing the rear end. As you folks know, there is precious little room for tire under the rear of early A-bodies. Mine is a ’65 gto. I run 15 X 26 X 11.50 LT Hoosier DOTs on 8 inch Weld aluminum rims with centered offset. They are 26.5 inches tall. The 12 bolt before the shortening put the edge of the tire barely inside the fender lip even with the inside fender edge cut. For years I used airbags to gain clearance but they adversely affect the launch. Without the bags the tires rub when cornering. With the shortened rear end the tires are nicely centered in the wheel well with about ¾ in. clearance on both sides. The Hoosiers put about 10 inches of tread on the ground. I’ve thought about trying for 11 inches on the ground, splitting that inch and a half clearance I have, but it would require custom rims and new tires and the $1000 plus price tag doesn’t seem worth it. I was able to bend the brake lines to accommodate the shorter backing plate to backing plate distance, no cutting required.

I was surprised to learn the Tru Trac requires conventional lube. I used synthetic in the old clutch pack differential because of less friction loss when the rear end cools at the track. The synthetic is too slick for proper performance with the Tru Track.

My impressions after running the new rear end for 2 shortened seasons: I think the new package is slightly heavier and there may be a small increase of thru loss due to friction and heavier rotating weight. However my 60 ft times have stabilized. The car seems to repeat the 60 ft easily. Provided of course the tires don’t spin at the launch. My best previously with the old 12 bolt was a 1.68 60 ft. But that was once. Usually it was a 1.72--1.75 and maddeningly inconsistent. The 60 foots now are all around 1.70 and repeat over and over. The time honored laying 2 black stripes down the street seems to confirm that both sides are working equally. The price for the rebuild was well north of 2 grand. But the package will be bullet proof at my power level for some time and the consistency is welcomed.

One more thought. My car should 60 ft better than it does given its e.t. I have tried various shock packages, without much success. It does not seem to transfer weight on the launch. I might install a rear sway bar one of these days, though I hate to add weight to the car. Deeper gears than the 3.73 would also help the 60 ft, but that ratio is a good compromise with the short DOTs on my street/strip car. I don’t own a trailer and those same DOTs are on the car all season long. The street manners of the TruTrac are impeccable. I am pleased with its performance, both street and strip. Thank you all.
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Old 12-01-18, 09:23PM   #2
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Join Date: Mar 2011
Location: KANSAS
Posts: 169

Iv'e had a lot of cars, usually with pretty good power. iv'e never broke a good ol clutch type gm 12 bolt.

had one catch fire once though when the axle seal had come out, gear oil went into the brake drum on a long drive and whats the odds, a brake spring had broke rubbing on the drum sparked up and had a fire. pretty good flames to, backed into a mud puddle real quick and was able to douse it out. sheesh. original 69 GTO judge to!

had 2 people yelling "your cars on fire! " or i'd never known. saved the car and maybe me!
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Old 12-05-18, 02:15AM   #3
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Join Date: Jul 2002
Location: La Jolla , California
Posts: 3,801

Just remember street/strip machines are always going to be a compromise of sorts because your trying to make the car do 2 different things.

Is it a manual( 4spd or 5 speed w/ OD5th) or automatic trans ( 3spd or 3+OD4th)?
You could play around with gearbox ratios in the transmission without changing the rear gear so you still have your acceptable cruising speed when the trans is in high gear 1:1 if you are running 4spd manual or 3spd auto(1:1 final).
You just have to figure what gear the trans ratio change is going to put you in at the finish line. You want it still pulling to peak RPM just as it gets to the stripe.
You don't want a shift point happening before you get there or the engine will not recover in time from the shift before it makes the finish (slow down). depends on what distance you usually race to ,1/8th mi or 1/4 mi.
I had lower 1/2 planetary set put in my TH-400 because I'm running a 3.08 rear gear on the street and the car is doing double duty & not making huge power. At an 1/8 mi track I shift into second at about the 330 and just stand on it in 2nd until it crosses the line just before the shift light comes on again. Not revving to red line either.

Just a caution if you go with the lower 1-2 planetary set on a street driven 3 speed auto trans car.
Do not use the automatic kick down/"passing gear"feature or manually down shift the trans above 45/50 mph or you will risk over revving the engine instantly.
The difference between 2nd & 3rd for the correct downshift rpm at regular highway speeds WILL BE WRONG.
Gotta stay on your toes.
With your current rear gear ratio the lower 1-2 deal might not be the way to go.

If you went to an OD style trans the the rear end ratio could changed to a lower gear and the OD would compensate for it when in street mode.
Lotta people switching their old style 4 speed manuals out for 5 & 6 speed boxes now.
Automatic w/OD? The 4L80 trans is as good as the tough as nails TH400 ( gotta be able to make it to the BOP bell housing pattern though).
A GV external OD behind the trans will add some weight towards the back.

Other ride I drive every day is a small 4cyl truck that has a close ratio 5 speed OD manual trans in it and the rear end gear is 4.37:1 . In 5th gear on the highway at 65 mph it is doing about 3300 to 3400 rpm. Acceptable and not getting passed by everything.
Stand on it and it revs up to 6500 quick .
It used to have a 4.88 in it but that was putting it a 55mph in 5th gear OD out on the highway .LOL

Step Away From The KOOL AID !
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