Big Block into a 6 Cylinder Convertible - Rear Swap?

[red73mustang]

So I have this pipe dream of putting a rather large 385 series big block into my 73 rag.  I've already secured a 429 Block and a C6 Trans but my concern is will it be necessary to replace the rear diff from the current 8 inch 6 cylinder rear to a 9 inch?

What is the weak point on the smaller rear (axles, pinion gears), are there limitations with gear ratios? 

I would rather spend the money on the motor instead of the rear 

Chet

 

[Jeff73Mach1]

Power breaks shit. To run more power than a stock 302 through a 8" rear end is possible for a while on street tires, but if you ever want to be able to take advantage of the extra power, it has to get to the ground. If you can't afford the motor and the rear end, then you can't afford the motor.

When building more power you should have upgraded u joints, driveshaft and yokes. You need frame connectors and your rear suspension needs to be in good shape. A driveshaft safety loop is a necessity. Then you need a traction device of some sort and now that it will go faster, you need to be able to stop it faster.

When I built a hot engine, I spent more than the cost of the engine on upgrading my clutch and bellhousing and driveline and such. I don't mean to be a downer, but you would be surprised at how much engione damage can be caused by driveline failures.

 

[73pony]

+1 on what Jeff said. I built my rear end, trans, front suspension and rest of drivetrain prior to my engine.

 

[Don C]

What is the weak point on the smaller rear (axles, pinion gears), are there limitations with gear ratios? 

All of the above are too weak for serious power. I don't know of any limitations on axle ratios with the 8-inch rear end. You just need to keep in mind that the lower (numerically higher) the gears are, the smaller (and weaker) the pinion gear becomes.

 

[will e]

Unfortunatly I have to agree with the responses. The good news is you should be able to find a 9" rear pretty cheap.

With the additional power you simply must use subframe connectors in a convertible.

Without some traction control all you will be able to do is cool burnouts. So Limited Slip rear end and some kind of traction bars. I like the 'shelby style' myself. Click on 'my garage' and go to modifications tab to see what subframes and traction bars look like.

And you will need to replace the front and rear springs. The 6cyl front springs are not made for the weight of a Big Block. The rear springs won't handle the power, although the traction bars will help.

Don't get discouraged. You have a dream and a motor and tranny. Put together a budget. It might be a while before you get to swap the motor in but you want the car to handle whatever kind of power you end up with.

 

[secluff]

What is the weak point on the smaller rear (axles, pinion gears), are there limitations with gear ratios? 

As others with excellent info have already posted, ALL the above are weak points on the 8" differential. Was fine for stock small displacement engines but not up to the task for big horsepower and torque applications. I have personal experience with the 8" in a 67 289 4bl 4sp Mustang. With a mildly modified engine I was able to shear the pinion shaft twice, twist the rear yoke, u joint, u bolts and flange on rear of driveshaft several times. The last time it let go there was a imprint of the pinion gear teeth in the rear of the housing. The fix was a locking 9" out of a 67 390 GT Mustang and driveshaft from a 68 428 Mustang. So while my 289 was strong, it was no where the torque monster a 429 is! Save yourself some time, grief and money and go ahead and budget for differential upgrade. You'll enjoy your car a lot more if you "Nip it in the Bud" now.  

Nothing sours a car romance more than having something catastrophic happen right after you first put it on the road!

 

[turtle5353]

I am running an 8.8 rear end out of a mid 2000's explorer. My motor is close to 600 hp and it's still holding up. You can get 3.73 gears, posi, disc brakes, and 31 spline axles all in one shot and the bolt pattern matches our rims. These rear end are plentiful and cheap at salvage yards.