Rear end advice for racing with a 4 speed

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340MPR

340MPR

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Hey guys,

I just got into drag racing my 69 dart last year broke my first parts, the rear end crapped out on me. My car has a 340 that makes 440HP at the crank, 4 speed, 741 case 8-3/4 rear end with 3.91’s, Mickey Thompson ET Street SS tires. After taking the rear end apart I found that the drivers side axle splines twisted and the inner side gears of my sure grip were wiped along with that portion of both axle splines, the outer side gears were fine. To get the car back in action and to make the last race of the season I rebuilt the sure grip with new, one piece side gears, new clutch plates, and found an old pair of factory axles. The ‘66 dated ring and pinion looked to be in good shape and I left them as is.

The last race of the the season was just this past weekend and I raced without issue and now I want to properly come up with a game plan. A lot of the local mopar guy’s think that the 8-3/4 will do just fine for my power level and that I should just get a new pair of axles and keep running it. I will say that they all run automatic’s and aren’t racing a 4-speed. I talked with Dr Diff and Cass figures if I’ve twisted axle splines than it will happen again, even with new axles so he’s leaning me towards a dana60.

I made around 50 passes last year and about the same this year and I’m hoping to keep that pace up. Down the road I’ll probably get slicks for the car and maybe coax some more power out of the car but nothing too drastic. It’s a true street/strip car.

I appreciate any advice from guys who race 4 speeds, thanks. Mike N

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While a Dana is going to hold up to slicks and a glued track, your going to notice the heavier rear end while on the street. It’s not going to handle as well. Usually the weak spot on an 8 3/4 is that the housing flexes and that can be solved to a point by bracing the back side, but that may not be your problem. Race duty axle’s may solve your problem. A Chrysler 9 1/4 rear axle while still is heavier than an 8 3/4 rear axle may be an option.

It’s always a compromise for a street/strip car. Either you give up street manners for racing durability or you live with parts fatigue while racing for street manners.
 
A lot depends on the type of clutch you are running, if you don't have an adjustable one than check out the clutch tamer @weedburner . I went with a dana 60 @Newbomb Turk s advice since he figured I would be stepping up the power levels later and it turned out to be sooner so he was right.
 
If you want to save yourself a lot of time and money put a Dana 60 or 9" Ford in there with 35-spline axles and you'll never have to think about issues again.

Tom
 
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I got about the same power, but I don't race. Well one day I did, back in 2004. I made 4 passes on an old airport runway, no prep. Car went 93 in the Eighth.

My axles are Resplined C-body axles, in a narrowed rear; I don't have your kind of problems.
I gotta get me a pair of those tires!
What else are you running back there?
 
I would suggest the Dana 60 for sure.

When I ran a 440 4 Sp 1970 Barracuda Gran Coupe back
when they were new, I broke several 8 3/4s
the first year until I sprung for a Dana.

I do not think you will notice the weight difference very
much.
 
GTX JOHN said:
I do not think you will notice the weight difference very much.
Click to expand...

People argue and fight about this all the time. The bottom line is a Dana 60 will weigh more, but that weight is also putting more weight on the tires.

So you actually get better traction because of it. If you do it right a Dana 60 can be done very reasonably.

Tom
 
ch1ll said:
While a Dana is going to hold up to slicks and a glued track, your going to notice the heavier rear end while on the street. It’s not going to handle as well. Usually the weak spot on an 8 3/4 is that the housing flexes and that can be solved to a point by bracing the back side, but that may not be your problem. Race duty axle’s may solve your problem. A Chrysler 9 1/4 rear axle while still is heavier than an 8 3/4 rear axle may be an option.

It’s always a compromise for a street/strip car. Either you give up street manners for racing durability or you live with parts fatigue while racing for street manners.
Click to expand...

Adding a Dana will not hurt handling or street manners.

These cars are all nose heavy in stock trim or anything close to it, and better balance improves handling far more than a small increase in overall weight will hurt it. If anything the Dana will improve F/R weight bias and may even lower the CG a smidge.

There’s a small increase in driveline losses, but that would hardly be noticeable, if noticeable at all, on the street.
 
By the time I narrowed a dana, bought gun drilled axles from Moser and installed rear disc brake setup I was within 10 lbs of the 8.75. I got tired of fixing the 8.75 and decided to go dana. Have not worked on the rear axle since except for lube change. The axles from Moser came with a 10 year warranty.
 
I run the shortened C-body axles and braced my housing in my 68 Dart RB 500 stroker. Only raced it twice though. I do agree with going with a Dana. I would have but I already had the 8 3/4 and a low budget.

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I 2nd (4th?) a D60. Found out that an 8 3/4 doesn't like 4 speeds once a 3200 lb car start's hitting low 11's back in the early 80's.....well, it probably didn't like the 6k rpm launches :D but once the 60 was under the car, the broken 8 3/4 problems went away. Also, it doesn't matter much if you back brace the 8 3/4 since that doesn't stop pinion gear deflection much if at all.....and Roy is right about the weight difference. Don't know if he used a spool or not but that reduces weight too. Years ago I weighed a stock Dana 60 with LSD against an 8 3/4 also with an LSD and it was just under 50lbs difference. The 8 3/4 came out of an E body and the 60 came out of a B.
 
Put a Dana 60 in it with a good Power Loc sure grip (not a trac Loc diff) and a good set of axles and never worry about it again.
I have a Dana out of a 3/4 ton Dodge truck narrowed to fit my Dart, it has the 1/4 inch thick axle tubes (a lot heavier than the car axles) and didn't lose any E.T. at all when I switched from the 8 3/4.
 
PROSTOCKTOM said:
People argue and fight about this all the time. The bottom line is a Dana 60 will weigh more, but that weight is also putting more weight on the tires.

So you actually get better traction because of it. If you do it right a Dana 60 can be done very reasonably.

Tom
Click to expand...
Takes less power to run a Dana vs 9inch also
 
Go Dana 60 with a spool, then you save LOTS of weight. The sooner you switch the sooner you will stop the bonfire of money spent repairing that 8.75

8.75 with a 4 speed and traction is going to break again, and again,
 
What clutch are you running? Just a wild guess, but a big diaphragm clutch with a huge amount of base load, with ceramic disc or discs?
 
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For a leaf spring car you cannot beat a S60 from Dr. Diff. The problem with a leaf spring Mopar having a back braced rear end is shock clearance. If you run the back braces all the way to the housing ends to be most effective you have to hack out big chunks of the bracing to make room for the shocks. I have been able to get away with my 8.75-742 clutch style Sure Grip because of my 235 60 14 tires. That said I have fragged a few steels in the Sure Grip on multiple occasions.
 
Your clutch is what twisted the axles. The more torque capacity a clutch has, the faster it pulls the engine down when you dump the clutch, which results in a sharper impact sent to the drivetrain. Most install an aftermarket clutch and think nothing of putting a 650tq capable clutch behind a 450tq engine, not realizing that excess clutch torque capacity is counter-productive and leads to bog/spin/broken parts problems. A proper clutch for the OP's 340 only needs about 2500lbs of clamp pressure on an organic disc to hold the engine after the shift into 4th gear. Aftermarket PP typically has quite a bit more, simple solution is to shim the aftermarket pressure plate until it barely holds after a WOT shift into high gear.

A pretty common misconception is that you only want the clutch to slip during launch, truth is there is more to clutch tuning than just dialing in the launch. For proof just look at the data logs from any fast adjustable clutch car, it's pretty common to see half a second to a full second of clutch slip after the shift into high gear. They could easily adjust their clutch for less slip after the shifts, but data tells them the car is quicker and more consistent with the slip.

Even after the PP clamp pressure is optimized for the application, you still won't have enough clutch slip for a proper radial launch. Simple way to get what you need during launch is to add a ClutchTamer. A 'tamer momentarily reduces clutch clamp pressure, which in-turn allows the clutch to slip longer than it otherwise would in 1st gear. That gives the car more time to gain ground speed before the clutch locks up, which helps keep the engine operating up where it makes more power.

Grant
 
450 hp with a 8 3/4 is do-able with an auto trans. With a four speed I would plan on a Dana 60 or Ford 9". A Ford rear might not help resale value (if that means anything) but a Dana certainly will.
You can spend money on the 8 3/4 to make it stronger, but it still might not be strong enough, and upgrades are expensive.
If it were me? Id get a strange 60, and sell the 8 3/4 before you break it again.
 
8 3/4 backbrace installed... $200?
Good 30 spline axles... $3-500.
Detroit locker (assuming you don't want a spool on the street, or having to repair suregrips repeatedly) $900.
Total $1500-1700.
Dana 60 from strange... $2500-$3000. Minus sale of 8 3/4 $500
With SLICKS and a four speed, a Dana or 9" swap is a no-brainer.
 
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j par said:
View attachment 1716310555View attachment 1716310558View attachment 1716310557View attachment 1716310556View attachment 1716310554
I blew the caps off the 8 3/4... built a d60 and done...
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In the photo of the housing getting narrowed up. That's a Dana 60 HD housing. Those are boat anchors with and not something I would ever use under a car.

Getting a housing from 60's or 70's Ford F250 is a great place to find one, but you have to be careful in the mid 70's up because you could end up with a Dana 61.

The later they get past the 70's the heavier the housings get and worse yet the axle tubes start getting really thick. Some are close to 3/8" wall.

Tom
 
413 said:
Go Dana 60 with a spool, then you save LOTS of weight. The sooner you switch the sooner you will stop the bonfire of money spent repairing that 8.75

8.75 with a 4 speed and traction is going to break again, and again,
Click to expand...
Also save a couple hundred dollars buying a spool vs a power lock carrier
 
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