Have stock driveshaft balanced, or P71, or.. ? Wagon with 4.10s. - Chevy Impala SS Forum
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post #1 of 46 (permalink) Old 04-30-2017, 06:13 PM Thread Starter
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Have stock driveshaft balanced, or P71, or.. ? Wagon with 4.10s.

My wagon has 4.10s and its LT1 has bolt-ons good for 425hp or so at the crank. It develops a thrumming noise starting at around 80mph. It persists or worsens as speeds increase, up to at least 100mph. I'd like to be able to cruise the interstate at 95mph, vibration free, and be able to burst up to 130+ without driveshaft-related issues.

From what I've read, the vibrations are because the driveshaft is being spun much faster with the 4.10 gears that it was with the stock 3.08s (or whatever they were).

The nearby Napa Auto in Cape Girardeau will balance driveshafts for $75, and I was thinking about going that route. From what I've read, a driveshaft that is balanced up to 8000rpm should be good for up to 150mph with 4.10s. Dynotech Aluminum Driveshaft Balancing Speed I don't know up to what speed Napa will balance the the shaft to. I suppose I should call and ask!

Measuring from the center of the pivot point on the middle of the front u-joint to the pivot point on the center of the rear u-joint is just under 55". It's probably right at 55.5" between the edges of the pivot points.

Or, should I look into a P71 driveshaft? Would it be likely to meet my qualifications without needing anything besides possibly new u-joints (IE, not needing balancing)? I like the idea of just bolting in my original driveshaft after it's been balanced, rather than trying to find a P71, changing u-joints, etc. On the other hand, not having my car down for a few days waiting for the driveshaft to be balanced is a pro, too.

What do you guys think?

--'94 Caprice "SS" wagon, NO LONGER FOR SALE!
--'96 Roadmaster wagon, SOLD!
--'04 Mustang Cobra, '08 BMW M5
--1125R, 1050, VMax, VFR800, DRZ-400E
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post #2 of 46 (permalink) Old 04-30-2017, 07:18 PM
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I wouldn't get the stock shaft rebalanced. Most local places can't spin them fast enough. FWIW, I still run a stock shaft on my T56/4.10 car. If I did do a new shaft, it would be a 3.5 or 4" from Denny's. Or, one of the fat aluminum ones from a late Silverado pickup, cut down with the correct yokes. Cheap from any local yard, you just need a driveline shop that can weld aluminum (and my local one can).

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post #3 of 46 (permalink) Old 04-30-2017, 07:28 PM Thread Starter
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Cost is definitely a limiting factor--I've read good things about the Denny's shafts but, while I haven't seen anyone quote an exact price, I got the impression that they were north of $500. If Napa can balance the driveshaft to 8000rpm, is there any reason I shouldn't go that route?

--'94 Caprice "SS" wagon, NO LONGER FOR SALE!
--'96 Roadmaster wagon, SOLD!
--'04 Mustang Cobra, '08 BMW M5
--1125R, 1050, VMax, VFR800, DRZ-400E
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post #4 of 46 (permalink) Old 04-30-2017, 08:32 PM
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Yeah,the anemic diameter of the stock shaft for starters. If you were pushing less power with a shallower gear. You might get away with using the stock shaft. As it is? You're asking for trouble in my opinion...

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post #5 of 46 (permalink) Old 04-30-2017, 09:30 PM Thread Starter
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Quote:
Originally Posted by 1slow96 View Post
Yeah,the anemic diameter of the stock shaft for starters. If you were pushing less power with a shallower gear. You might get away with using the stock shaft. As it is? You're asking for trouble in my opinion...
What do you mean by "asking for trouble"?

Also, how is the stock shaft's diameter an issue?

The car is a semi-daily-driver that does get driven aggressively, but never truly drag-raced, and while I'd like to have the capability to take up to 130-140mph, I probably won't ever get it going that fast. As of right now, my primary goal is to be able to cruise at 90-100mph on the interstate without any driveshaft-related vibration.

--'94 Caprice "SS" wagon, NO LONGER FOR SALE!
--'96 Roadmaster wagon, SOLD!
--'04 Mustang Cobra, '08 BMW M5
--1125R, 1050, VMax, VFR800, DRZ-400E
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post #6 of 46 (permalink) Old 04-30-2017, 10:20 PM
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Flesh....

balancing a DS the length of ours at 8k rpm is serious driveline shop gear. I would be surprised a NAPA would have that. Dennys is one of the few shops that does

The "width" of the driveshaft is a critical part of the equation. Rotational mass

The stock DS is not going to be balanced for the RPM speeds it sees with 4:10's. It might get slightly better....but you need a wider DS that is well balanced. Dennys is what I have for my T56/4:10 SS with 1350 yokes. Tried getting the stock one balanced when I had 3:73...it didn't resolve the 80+mph vibe.

Is it north of $500...yes.

Inland Empire or DynoTech are two other options
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\'96 BBB 383/T-56

Last edited by BALLSS; 04-30-2017 at 10:33 PM.
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post #7 of 46 (permalink) Old 04-30-2017, 10:49 PM Thread Starter
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Quote:
Originally Posted by BALLSS View Post
The stock DS is not going to be balanced for the RPM speeds it sees with 4:10's. It might get slightly better....but you need a wider DS that is well balanced. Dennys is what I have for my T56/4:10 SS with 1350 yokes. Tried getting the stock one balanced when I had 3:73...it didn't resolve the 80+mph vibe.
Well that's.. disappointing.

Any input on the P71 driveshaft? A couple hundred bucks would probably fly under the spousal radar. $500+ definitely won't. "Just don't drive so fast, jackass..."

--'94 Caprice "SS" wagon, NO LONGER FOR SALE!
--'96 Roadmaster wagon, SOLD!
--'04 Mustang Cobra, '08 BMW M5
--1125R, 1050, VMax, VFR800, DRZ-400E
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post #8 of 46 (permalink) Old 04-30-2017, 11:11 PM
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Quote:
Originally Posted by TheFleshRocket View Post
Well that's.. disappointing.

Any input on the P71 driveshaft? ..."
No personal experience. Only read a few posts about them. IDK what results or if they are a "plug & play" swap

My ford truck has a big Spicer AL DS and it has 4:11's from factory so if what the P71 is like that it should be much better.

On another note, if you did the T56 swap, did you measure your DL angle before swap and = that on the T56?

There were 2 shims between my 4L60E and tranny mount. I needed them on the T56 (pic)

I mention this because DL angle off will cause vibes.....although your 8) mph screams DS.
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\'96 BBB 383/T-56
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post #9 of 46 (permalink) Old 05-01-2017, 09:56 AM Thread Starter
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Quote:
Originally Posted by BALLSS View Post
No personal experience. Only read a few posts about them. IDK what results or if they are a "plug & play" swap
My ford truck has a big Spicer AL DS and it has 4:11's from factory so if what the P71 is like that it should be much better.
On another note, if you did the T56 swap, did you measure your DL angle before swap and = that on the T56?
T56 swap? Hah, I wish! (I really do.)

I called the Napa in Cape Girardeau, and the tech said that their balancer spins the driveshaft at just 1140 RPM. He said that after they balance the driveshaft, they spin it up "faster" to check for vibrations, but that the machine doesn't register how fast it's spinning. The tech estimated it was probably somewhere around 1800 RPM. So, yeah, nowhere near 8000 RPM. Guess there's no point in bothering with that.

I'll continue looking into the P71 driveshaft, I guess.

--'94 Caprice "SS" wagon, NO LONGER FOR SALE!
--'96 Roadmaster wagon, SOLD!
--'04 Mustang Cobra, '08 BMW M5
--1125R, 1050, VMax, VFR800, DRZ-400E
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post #10 of 46 (permalink) Old 05-01-2017, 10:04 AM
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Originally Posted by TheFleshRocket View Post
T56 swap? Hah, I wish! (I really do.)

.
My bad, thought you had a T56

anyway with 4:10's you are going to need a wider, better balanced DS

\'96 BBB 383/T-56
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