Stock 340 - Swap 3.23 for 3.91 gears - is Suregrip overkill?

[Titan1969]

Thanks in advance. I have a stock '73 340 with open diff 3.23. Im going to 3.91's to give it more bottom end. I wont mind the high revs, its a street car very little highway time. Will a suregrip be needed with 3.91's or with a stock 340, is a sure grip really overkill and I should just swap the gears and call it good. No intentions to go to the strip with it.

 

[Chryco Psycho]

I would use a sure grip & some sticky tires

 

[heminut]

I would use a sure grip & some sticky tires

I second this! With an open diff you will pretty much only spin one tire, the right side one usually. It tries to lift up from the driveshaft torque and unloads sooner.

 

[Heywodja]

These things are already light in the rear end, a suregrip is a great upgrade, and as stated above it will improve traction.

 

[cuda joe]

like everyone says you are not going to like a open end rear with that ratio it will spin one tire real easy

 

[rklein71]

You will hate an open diff with 3.91s.

 

[mrmopar340]

I run a 17 inch tire and with a 3.91 gear and an 833 my rpms at 70 mph were right a 4,000. With a 3.23 about 2,700. Sure grip definitely needed but if you cruise the highways be ready for some high rpms

 

[540HemiCuda]

If you do go with a Sure-Grip, just start with a different housing. Used ones are usually available for $100 or so. That way you can go right back to your 3.23s if you want and could sell your 3.91 set-up if it comes to that. You will really like the difference the lower gears make on the small block. But finding a tranny drive gear for the 3.91s can be tough. A&A Transmissions used to have them, but I don't know about now.
Dr Diff does a lot of Mopar stuff and will tell you straight about what gears will be more noisy and which will be less so. But be prepared for gear whine with the modern lower gears. They are just not as precise as the originals. And depending on which housing you use, get away from the crush sleeve and go with the spacer and shims. It's a little more involved, but if your diff guy is competent, he should not have a problem. www.doctordiff.com/drivetrain/mopar-8-3-4

 

[rklein71]

I agree with 540HemiCuda, having two pumpkins that you can swap is a big bonus. I dropped in a 2.76 gear set in my 70 440+6 Challenger for a cross country trip when I changed duty stations in the Navy. Florida to Idaho, man, those high gears were sweet for the road trip. Once I got to Idaho, back went the 3.54 sure grip.

 

[Titan1969]

ok thx. Ill pickup the Yukon complete diff with SG and 3.91, then its an easy swap.

 

[heminut]

ok thx. Ill pickup the Yukon complete diff with SG and 3.91, then its an easy swap.

Great choice! With my first Cuda, a 340 4 speed, I had two pumpkins. One had 4.56 gears and the other had 3.54 gears, both were limited slip. I also have two pumpkins, both limited slip for the Cuda I have now with a Gen lll 5.7 hemi and a 727 torqueflite. One set is 3.23 and the other is 4.10. Swapping pumkins is a fairly easy job and have both highway gears and 'fun gears' is great.

 

[Ricks72Chlgr440]

Great choice! With my first Cuda, a 340 4 speed, I had two pumpkins. One had 4.56 gears and the other had 3.54 gears, both were limited slip. I also have two pumpkins, both limited slip for the Cuda I have now with a Gen lll 5.7 hemi and a 727 torqueflite. One set is 3.23 and the other is 4.10. Swapping pumkins is a fairly easy job and have both highway gears and 'fun gears' is great.

Wow, 4.56! Bet you could climb a telephone pole with that! Swapping 3rd members is not difficult when you're 25 but as we get older not so much! Ugh!

 

[Xcudame]

All good advice regarding the two different center sections. It's not a bad swap and easily done in a few hours, but Rick is right, the older I get, everything seems to weigh more than it did 25 to 30 years ago and I notice more aches and pains that take longer to heal. I have an old floor jack (steel and weighs over 100 pounds) that has a huge cup saddle on it that works great for swapping pumpkins.

Be sure to protect the seals when you pull the axles out! Actually leaving them part way in when swapping the gears prevents the splines from accidently cutting them!

 

[heminut]

Wow, 4.56! Bet you could climb a telephone pole with that! Swapping 3rd members is not difficult when you're 25 but as we get older not so much! Ugh!

You're absolutely right! I'm 70 now and I use a floor jack.

 

[Ricks72Chlgr440]

You're absolutely right! I'm 70 now and I use a floor jack.

I've done that too, but even getting that doggone chunk up on the jack is a struggle!!!

 

[Xcudame]

Like lifting something the size of a ham that weighs 100#! But better lifting it a few inches than a foot and a half, right?

 

[Ricks72Chlgr440]

Yeah. Doing it on your back is real fun too. Love all that stinky gear oil running down your arm. Last time I did that my son helped me. Ah the memories!

 

[Xcudame]

Skunk juice!

 

[Avalanche]

Dr. Diff also sells complete center sections

 

[Spyder]

I wouldn't necessarily upgrade to a suregrip as a standalone thing in a cruiser, but if the third member is coming out, I'm not putting a single tire fryer back in.