Gearing and Rear Discussion TKX

[rirealtor]

As the build continues along with my insomnia, my attention is now about the current rear end set up and whether it should change and even be upgraded.
Project Norma Jean is a 67 Charger with 383 stroked to 496 with TF240 heads and TF Roller cam(.600 lift). Tire size is 26", and have selected the TKX close ratio (2.87,1.89,1.28,1.00, .068)
Currently have 3.23 in the back and wonder if that may not be enough for this combo in the lower rpm and if may lug on highway. Perhaps a 3.73?
Street Driven vehicle; peak torque at 4200.
Second question is will the existing 8/34 handle the power? 580HP at 5200 and 646tq at 4200. I cant imagine big launches with the vehicle and the tire selection (Nitto) just wouldnt hook, but does make me wonder if an axle upgrade should be considered or is that overkill.
Appreciate your thoughts and experiences.

 

[ruffcut]

I'll insert my two cents here. I have a stroked 400 to 512 and using TF 240 heads and the same (.600 lift) hyd roller cam.
I'm using an 8.75 with cone type gear setup. and 3.55 gear and 29 inch tall tire.
The specs on that cam called for 3.73 gear
You are using a shorter tire, so I think 3.23 gear would be okay for street application.
If it were all out race with slicks, then maybe upgrading the axles and possibly the rearend itself may be in order.
In conclusion, I think you'll bee just fine. JMTC. ruffcut

 

[Bighouse]

3:55 is right in between.

 

[Cranky]

The 8.75 should survive on the street so long as you don't like side stepping the clutch pedal. Gots to be one of the quickest way to tear stuff up imo. And well, if you damage the rear, then upgrade it.

 

[33 IMP]

Your overdrive ratio is .68, not .068.
With your 3.23, final drive in fifth would be 2.19, quite a bit higher than I would run.
With 3.73s, final drive would be 2.53. I could live with that.
As for whether your 8 3/4 will survive.... that factory put dana 60s behind 440s with a stick, then 9 1/4s after no more Dana's. IF you NEVER put good traction tires on it, and if you NEVER do 5000rpm clutch drops, the 8 3/4 MIGHT be fine..... but you will have more torque, more first gear ratio, and more horsepower than the car that the builders decided needed a dana, even with garbage G70x15s.
(I would probably put in a dana with 3.73s, or a nine inch Ford, with 3.70s. 4.10s would also be considered.)

 

[gkent]

Interesting. My build is a 526 stroker, .600 roller, Indy EZ-1 MW heads. I only ran this through a very old "desktop dyno" and came up with very close numbers - 600 tq @ 4000 and 550 hp @ 5000. The gear I decided on is a 3:55 with roughly a 28" tall tire. So I tend to agree that with the shorter tire you should be ok with the 3:23.

When I was piecing this build together my plan was to make the same torque that it took to launch the car on slicks with 4.56 gears when I had my 383 in it - only do it with 3.54's. The above numbers are what the math called for. As yet to be confirmed !!

In many past posts though the opinions were that the 8-3/4 would not hold up. Dana S60 gets ordered next week !!

 

[pnora]

I went with the 3.73 Strange Dana 60 from Dr.Diff. Anything lower and you will find first gear like a crawler gear in a truck. My Hemi is detuned at 642HP and 650TQ and I am out of first gear by the time I cross the street.

 

[beanhead]

A general calculation for giddyups is "starting line ratio", which is transmission 1st gear x rear gearing. Most consider an SLR of right around 10 to be ideal....of course that's more for racing. Many will say the big strokers don't need a lot of gear..but on the street, when you're only using part throttle and the carb primaries to get going more gear makes you quicker (funfactor!)

The TKX has a 2.87 1st gear.
So, using your gear you have now to compare the 727's 2.45 1st----
727 & 3.23 = 7.91
TKX & 3.23 = 9.27

727 & the 3.73 = 9.14
TKX & 3.73 = 10.71
That's kinda like running a 727 with 4.30s, but having a 727 with 2.54s in 5th gear...a nice sweet spot for a street car IMO.
You can play around with the calculators and your tire size to get ballpark RPMs and MPH etc.

As for your other question, 8-3/4s are tough...mine's holding up with drag radials, clamped leafs and over 600 lb/ft BUT it's an auto car and does spin on hard launches. With a manual and sticky tires I'd be considering an upgrade back there as well.

 

[Mopar Hunter]

That's kinda like running a 727 with 4.30s, but having a 727 with 2.54s in 5th gear...a nice sweet spot for a street car IMO.

727 with 2.54’s in 5th gear?

 

[beanhead]

727 with 2.54’s in 5th gear?

Compared to 3.73s and a TKX, yes! I'd love to do one but I just can't stomach the 8K price tag to convert my auto car.

 

[Darter6]

I think you'll bee just fine.

BUT, years ago I ran a TKO in a 71 Duster, 372 small block with 3.55's.Had 27'' rear tires. I had always felt that the car would have been happier with 3.73 gears. It had a healthy cam,specs I do not remember from 21 years ago.
I did feel under 60 mph the car was lugging in overdrive.

 

[INTMD8]

I went with the 3.73 Strange Dana 60 from Dr.Diff. Anything lower and you will find first gear like a crawler gear in a truck. My Hemi is detuned at 642HP and 650TQ and I am out of first gear by the time I cross the street.

I love the 4.56 in mine. 13.08 starting line ratio.

 

[69L48Z27]

I put a 1966 big block Corvette together earlier this year with a TKX close ratio and the factory 4.11’s. Thing was great. If it was my Mopar, I would be in the 3.73 or even 3.91 camp. Why the short tire?

 

[Mackman]

Hey, gkent, I may have a new S-60 for you. Your inbox is full.

 

[gkent]

Hey, gkent, I may have a new S-60 for you. Your inbox is full.

Just cleaned it out ... that's 20 minutes of my life I'll never get back (this site is SO user friendly !!)

 

[weedburner]

Second question is will the existing 8/34 handle the power? 580HP at 5200 and 646tq at 4200. I cant imagine big launches with the vehicle and the tire selection (Nitto) just wouldnt hook, but does make me wonder if an axle upgrade should be considered or is that overkill.
Appreciate your thoughts and experiences.

Engine power/torque isn't much of a factor when it comes to breaking rearends. Even a 7-1/4 can hold up to a 646tq engine in 1st gear. The real factor is how much peak impact is sent to the rear while the clutch is engaging.

These cars rolled off the assy line with a B&B pressure plate. The B&B gets it's clamp pressure from fairly short coil springs, which lose clamp pressure pretty quick as the clutch disc wears. That means the pressure plate needs quite a bit more clamp pressure than required when the clutch disc is new, just to make sure it still has enough clamp pressure to hold when the clutch disc is near the end of it's life. Basically, these cars came from the factory with a clutch that is far more likely to break the rear when the clutch is new. They likely had to upgrade the rears just to get them past the warranty period.

Due to the design of a diaphragm spring PP, you don't need excess clamp pressure when the disc is new. That means overall pedal effort will be much less with an appropriate diaphragm vs the B&B. An appropriate diaphragm will also be far less likely to break your 8-3/4 rear.

Most aftermarket clutch manufacturers will steer you to more clutch than you need, just to make sure they don't get a reputation for selling weak clutches. The important thing to remember is to choose an appropriate clutch for the application, avoid something that is overkill. For a 650tq engine, IMO a single dual friction disc with a 2800lb PP would be about right.

Grant

 

[Kern Dog]

I pondered the axle ratio back when I did the conversion in my car. I've had 3.55s in the car for years.

Tremec 5 speed conversion in a 1970 Charger

I looked at what the new Challengers used for gearing. They have a 6 speed with something like a 2.9x 1st gear and a 3.92 axle.
Then I thought of how the 3G Hemi is basically a small block that needs some rpms to make power while a stroked big block makes far more torque.
With that in mind, I decided to keep the 3.55s in place. I had a 28" tire, now a 26.5. The shorter tires do help acceleration some.
Regarding the 8 3/4 strength....It is a matter of what the axle actually "sees".
If you're dumping the clutch with grippy slicks, you will destroy it eventually. If your tires break loose easily, the axle never sees the full extent of the power. My tires spin when the clutch engages under heavy throttle so I expect my 8 3/4" axle to last awhile.

 

[lewtot184]

In my opinion the .68 is too steep. I believe the .82 to be more versatile.

 

[Mopar Hunter]

Compared to 3.73s and a TKX, yes! I'd love to do one but I just can't stomach the 8K price tag to convert my auto car.

8K?, I thought @Kern Dog said it was around 6K

 

[pnora]

In my opinion the .68 is too steep. I believe the .82 to be more versatile.

Mine is .68 and is perfect with the 3.73 gears. At 65 mph it is right around 1,700 rpms.