Hello All, I am now at the end of my drivetrain upgrade. I’m sporting a 327-cid, 3.58 stroke Y-block with around 400hp and C4 w/2000 stall converter. I am now considering a ’57 Thunderbird rear axle with 28-spline 3.1 gears vs Currie 9” with all the bells and whistles and 3.25 gears.

The 3.25 ratio and True-Trac is desirable, but is it worth $3k more.

All opinions wanted…

Thanks!




.150 Stroked Y-Block:327.25 ci @ >1hp per ci

I don't think you want a locker if it will be for street use.  There is the Detroit Locker which automatically disengages when turning and is better suited for street use because of that.  There are also really good geared differentials like Detroit Tru-Trac which function more or less like a clutch type differential but have a higher bias which can apply more torque to the wheel with grip than a clutch type is able to.

I got a powder coated center section and axles from Moser a few years ago and that was a little over $2k so $3k for a complete new unit doesn't sound out of line.  A lot more than a questionable used unit, of course but that doesn't really compare.

You may need shorter gears than 3.25 depending on how the low end output of the engine is.


Lawrenceville, GA

28 spines are good and not the weak link some say. We ran one in a #3000 dirt 1/2 car and never hurt an axle or carrier. I'm not sure on a locker or other device to limit slippage on the street. Today's modern cars do it electronically for a reason. With no where near your hp I'm running a 2.74 gear on the street and very happy with it.. I'm not a light to light any more. Good Luck

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Something is definitely needed to limit slippage on the street.  The number one reason is right hand turns, and particularly right hand turns going up hill and even more so, right hand turns going up hill when it is raining.

Too many times with my truck, I would start to pull out and the right rear would just spin.  If the spin did not occur, there was plenty of time to pull out but because of that, time is running out.  If far enough out, the guy behind has pulled up so I can't back up and if there's a car coming, I have to go.  I've been in some pretty close calls because of that.  Sometimes it isn't very predictable so while I'll concede that some instances could be bad judgment on my part, there certainly are times that it is not.

So, now an argument may be "well, trucks are light in the rear and isn't your engine modified?"  Yes.  I also have an '88 Firebird that is stock, 170hp and has a far superior suspension setup and weight distribution than the truck.  Exact same problem.

Since installing the Tru Trac in my truck, the problem no longer exists.  I have to make it spin if the road is wet.  It doesn't just happen in most cases, anymore and no negatives I've come across, yet.  The car will need a rear end rebuild soon and it's getting upgraded, too.



Lawrenceville, GA

PM sent Brent.

Durham Missouri

I’m going new rear axle with Truelok.
3.25 gear ratio, 27” tall tires, 65mph, 1:1 trans = 2700 rpm
I’m hoping the 2000 stall converter will get me off the line, if not, going to try higher stall and maybe a taller tire.

As Always: Thank You for sharing experience!




.150 Stroked Y-Block:327.25 ci @ >1hp per ci

My past experience tells me that you should go with a 31 spline since you are going to race some. The price difference is not that much difference.

  Vic Correnti

I've been running the 28 spline in my '57 Bird since I bought it in '65. Its possible to break things in it if you realy try but if your rear suspension is set up to eleminate wheel hopp it will take a lot. I'm running a 3.89 Traction lock in mine now. So for so good. Whats the difference between a Traction lock and a Truelock?

I believe the Traction Lock “Track-Lok” is Ford’s name for a limited slip which uses clutch plates. Check out this article which discusses Locking vs. Limited slip

This article How to determine the right gear ratio for your car really digs in.

Vic the awesome picture of your front wheels off of the ground is not displaying, I hope you will get that fixed. What an awesome display of torque from the mighty Thunderbird. That’s representing!





.150 Stroked Y-Block:327.25 ci @ >1hp per ci

FORD used two clutch systems- EQUA-LOCK and TRAC-LOC. TRAC-LOC replaced the earlier style in 1968 as it was newly designed and stronger.

DETROIT LOCKER was a mechanical ratcheting assy. FORD offered those also.

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