GEAR CHANGE

[RIDGERUNNER]

OK,,, HERE GOES.. I BOUGHT MY SON A 1969 F-100 2 WHEEL DRIVE... IT HAS A BUILT 390 OUT OF A 65 GALAXIE... C-6 TRANNY.REBUILT, MODIFIED.... THE VIN TELLS ME IT HAS 3:25 GEARS..... WE WANT TO GO WITH 4:88 GEARS WITH POSI, SINCE ITS FOR 1/4 MILE..... WOULD FINDING A DANA WITH 4:11 GEARS BE CHEAPER THAN REBUILDING THE 9"?? OR DO YA'LL THINK IT WOULD BE WORTH WHILE REBUILDING THE STOCK 9"??? NOT COMPLETELY SET ON GEAR RATIO BUT DO WANT IT LOWER THAN 3:25... ANY IDEAS????????

[theskytoucher]

You would probably be better just rebuilding the 9"! Parts are readily available and you dont have to find a new axle!!!!

[Thunderfoot]

There are lots of factors to both.... I believe that the 9" in there now is only 28 spline and is not very strong for Drag racing, you would want the 31 spline axles for doing that. So a rear end change is probably going to be needed... or at least after market axles...

The 9" rear ends are nice for changing gears as you can remove the center section and swap it with another or get the gears changed without having to take the whole truck to get it done... The Dana will be faster even with the same gear ratio as it has a lot less parasitic drag in the gears.

The 4.88 would be way to much for that 390 unless your going to put a real TALL tire on it and still wind it to the moon... These are more Small Block high RPM gears. The 390 has lots of torque so it doesn't need that much gear, I wouldn't go over 3.90-4.11 for this... (just to give you an idea, I have a 302 with 4.56 gears, 4,500 stall converter and 26" tall tire, motor turns 6,700-7,000 in the 1/4 mile... in a 3,000lb car...)

[big green]

I am a 9" fan my self. But like thunderfoot said most stock 9" had 28 sline axels, which are nowhere near good enough for the 1/4 mile. If you decide to rebuild the 9" upgradeing to 31 spline axels must be done. Because of the 390's tourque I personally would run nothing to much bigger then a 4.11.

DL

[FORDification]

The '73-'79 pickups have 31-spline axles in their 9" rearends...AND the rearend is several inches wider, to widen the track width and bring the rear tires back directly behind the fronts.

The only problem is that the rear spring perches on these housings are each several inches farther outboard. However, if you've got a welder, relocating a couple spring perches to enable you to bolt the dentside rearend into your bumpside is childs play, and worth the effort, especially in your case.

[willowbilly3]

You might also want to go with a better center housing. The stock ones are weak arounf the pinion pilot bearing and will break there. You can at least inspect it to make sure you have one with some meat in that area but I have seen them that had core shift and were very thin casting where it is machined out for the bearing.
Also I read some dyno data on the parasitic drag of a 9 inch. The very thing that makes them tough, the low pinion placement, also makes them rob more power. It is probably a moot point but worth mentioning.

[RIDGERUNNER]

I WANT TO THANK YA'LL FOR YOUR INPUT AND ADVICE,,,, I HAVE DONE SOME PRICING ON REPLACING THE GEARS AND SPLINE IN THE 9" I HAVE NOW..... PRETTY SPENDY.... I BELIEVE THAT FINDING A REAREND ALREADY BUILT WOULD BE THE LESS EXPENSIVE WAY TO GO... I LIKE THE IDEA OF THE 73-79's WITH THE 31" SPLINE.... LITTLE WELDING IS NO BIG DEAL,, I BELEIVE WE WILL NOT GO HIGHER THAN THE 4:11.... THANKS AGAIN FELLAS..........