Since we sell quite a few rears and suspension we run into this question everyday. A lot of old timers will tell you a 9" is stronger then a 12 bolt................... Welcome to 1987! Back in the day, if you was on the cheap, you could snag a 9" from a truck and it was strong as hell. There were not a real strong aftermarket for 12 bolts, only spools for a long time which are not real street friendly.
Now with the 33 and even 35 spline Tru Trac differentials, support covers, Ford 3.15 bearing ends, bolt in Ford style axle options. New 12 bolt housings that are stronger then the old original housings, the 33 spline 12 bolt is as strong or stronger then the 31 spline Ford 9". The difference in strength (if any) at our high horse street levels 500HP to 800HP, there is no real worries about either system holding up to the power
For me, the difference maker IF (Big I F) I can use a 12 bolt is keeping my car all GM and putting more power to the ground. Yes a 12 bolt has less power absorbtion and will put more power to the ground then a 9" Ford. By it's location, the pinion in the 9" Ford tries to climb the ring gear stealing power the 12 bolt does not give up. It is a small amount, maybe 10 to 15 horsepower, but at 300-500HP it could be 3 to 5 percent and we spend a lot for 10 or 15 horsepower at the flywheel only to give it up to the rear end.
Also from a cost perspective, the cost are almost identical for a double throw down, 33 spline, Performance Cover, Big Ford ended 12 bolt or a Nodular Center, 31 spline, Big Ford ended 9" Ford. So for me..........my choice is a 12 bolt....................IF I can use a 12 bolt!
Now, with all the said, I just got my new suspension yesterday for our next shop project...........and it is a Fabricated 9"! But it also runs a watts link and that is another subject entirely! (no watts link with a 12 bolt). And at our horsepower levels, we will be giving up less then 1% power (you do the math) because of the 9", so we have 10 to 15 horsepower to give up.
Now how was that for making a strong arguement for 12 bolts and then muddying things up at the last second!
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Frank Serafine
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