Driveshaft critical speed
 

I'm putting a mild 4.8L/4L60E into my '89 Cheyenne. I'll be taking it to be dyno tuned, probably this summer. My concern is, "how fast do the tuners usually take a vehicle on the dyno drum?" I want to be sure that the stock driveshaft won't turn into a pretzel at some unfathomable speed that the truck will never see otherwise. I'm considering having a 3.5" shaft made, either steel or aluminum, but if the stock shaft will handle a few pulls, then maybe I'll keep it. If anyone kinda knows the answer to this, please let me know. Thanks

BTW, the truck has a 3.08 gear, and the little 4.8 likes to make some high rev's. The cam I'm running is a TS&P stg. 1 truck cam (208/214 @ .550 lift).

Re: Driveshaft critical speed
 

On my LS swaps I have the tuner set the rev limiter at 6400-6500.
I like to upgrade to 1350 u-joints while I'm at it.
You didn't say if you have a long or short box. That makes a difference.
I would bite the bullet and get a new one. I've only used steel and never had an issue.

Re: Driveshaft critical speed
 

They ran my truck in 4th gear, 1:1 ratio, with 4.10s and 30in tires.. I saw it hit over 130mph on the dyno screen...sounded like it was gonna twist outta the frame..I'll look tomorrow but I'm sure its was in the 6k rpm range
But if your having a driveshaft made, a good driveshaft shop takes all that into consideration
A aluminum shaft will have more dia than a steel shaft to do the same job..
Personally, I'd go steel

Re: Driveshaft critical speed
 

Thanks Gents. My truck is a short box with a 1 piece shaft. It also has the 3R type u-joints. Depending on where the engine/trans finally lands, a new shaft will be about 55-56". Going to a stronger joint at the front will not be a problem, as I can just get a slip yoke for the corresponding joint. The rear joint may have to be a hybrid, as the rear axle needs to see a GM 3R joint. Being an Ol'Geezer at 76, my stoplight commando days are behind me. The truck isn't being built for brute power, just good reliability with a little bit of snap. I still like to play, but I also know my limitations. Here's a pic of my truck, so you can see what I'm playing with

Re: Driveshaft critical speed
 

I'd be interested in what the stock shaft is like? How big? Was it a V6 truck before?

4.8L with cam, probably isn't going to pretzel the stock shaft.

I'm leaning towards, if it's not broke don't fix it.

However, I had ActionMachine.com make my 56.75 long 3.5" diameter aluminum driveshaft for my 6.0 4L80 swapped 68 shortbed. 1350 front and rear and it's a work of art. Forged Chromoly yoke's on both end. It's probably massive overkill...

Re: Driveshaft critical speed
 

They typically run the Dyno test is ran in direct drive, which ever gear that is in the transmission.

It is drive/3rd in the non-overdrive models.
Third in the 700R4/4L60 and variants.
Forth in the 6L80's as both 5 and 6 gear are over drives.

On my tuck the tuner used 3rd gear (4L60E) and the speed topped out at 130mph at 6000rpm with 3:73 gears. I had no driveline issues. I have a one piece driveshaft.
You can use this calculator to figure it out for your truck.
https://purperformance.com/p-29669-rpm-calculator.html

With your combination the drive shaft speed will be the same as the engine. Lets say 6000 rpm like mine. The rear wheel speed will be much higher because of your higher differential ratio (3:08) Tire speed will be close to 150mph.
You will want good tires. Any decent tuner will limit the engine/tire speed to your specification.
Good choice on the cam, perfect for the 4.8L. I always pick out the camshaft I want and purchase the next one down. It seems to work out better that way.
Tuners will reset the full throttle transmission shift points. An important feature for drive-ability. That way you can use the power where its made.
Cheers

Re: Driveshaft critical speed
 

150 mph, eh?! Talk about pucker inducement...almost like sucking on a lemon. There's nowhere in CT with roads long or straight enough to pull that off. I may be gettin old, but I aint lost my mind completely just yet. Anyways, this has been a good talk. Thanks. I guess I'll have a conversation with the dyno operator beforehand, just to see where their head's at. I'd sure hate to drive the truck there, then bring it home on a stretcher

Re: Driveshaft critical speed
 

I used to own a 2006 Corvette, it was all stock with 400hp.
It would go from a standing start to 150 in under 30 seconds.
Mind you this is all stock and they all did it with the base motor and both automatic and manual transmissions.
I never had the balls. However my point is modern vehicle are fast.

2022 Chevrolet Camaro is a 2-liter Turbocharged Gas I4 that makes 275 hp @ 5600 rpm and 295 lb.-ft. @ 3000 rpm. It is mated to a 6-speed manual. The 2022 four cylinder Chevrolet Camaro takes 5.35 seconds to get to 60 mph from a standstill and 14.7 seconds for the quarter-mile. The 1LS 4 cylinder does 0-60 in 4.1 seconds.
It has the V6. The 3.6 liter LGX V6 engine, having a power output of 335 hp (250 kW; 340 PS) at 6,800 rpm and 284 lb⋅ft (385 N⋅m; 39 kg⋅m) of torque at 5,300 rpm.

The 4 cylinder is only .2 seconds slower than a 1969 Z28 Camaro. Motor is less than half the size and get over twice the mileage.

I for one do not want to go back to the "glory days". But I would if I could pick up some bargains! Love the all stock and the Retro Rods. They, Retro, are gaining in popularity as they have a lot of the features of modern cars and the looks from the past. And the pollute less.

With your camshaft top end may be closer 5500rpm not the 6K I used as an example. That drops your tire speed to 140mph. No worries then!
You should know the safest place to stand is 45 degrees from the wheel.
Cheers.

Re: Driveshaft critical speed
 

I hear ya. I grew up with the "muscle cars" of the late '50's and '60's. Had a sail back '61 Biscayne w/tripower 348 and 4speed. Thought it was fast. Compared to what's available now, it's the proverbial slug! I still like the older cars, but that's a generational thing. That's why I want to bring this truck into the "present day" powerwise. It won't be near as quick as what can be had off the showroom floor, but then again, neither am I. As the adage says, "The older I get, the faster I was !"

Re: Driveshaft critical speed
 

I am an avid DIY person. Do everything I can myself as can be seen in my build thread. The drive shafts were outsourced to an experienced shop. Personally, I do not have the knowledge or experience to prevent a critical failure in this most important device. Do not have the shafts in hand yet but recall builder saying a 3.5" diameter steel shaft would be used in the rear of my SWB K10 with an LSA. About 650 hp and torque. Mind that drivetrain is 3/4-1 ton now and the ujoints are upgraded to 1410. Do not have specs in front of me but rear shaft is right around 46". Redline is about 5800 on the motor IIRC.

Re: Driveshaft critical speed
 

The modern z28's (LS cars) have aluminum drive shafts.
They also have what looks like an harmonic balancer on the transmission yolk.