AboveTheLogic

I just did something stupid and bought a reman 4.11 rear diff to swap into my '79 which has a T56.

I'm running the factory driveshaft which has been shortened to 23" and has solid 1330 joints at both ends. Slip yoke into the trans is full loop (no straps/bolts), yoke in diff is factory.

Tires are 255/45/ZR18 and I don't intend to go larger. I might buy a set of 275s on 15" wheels someday for the drag strip but that will only be after I'm confident I won't explode everything.

Pondering driveshaft/half shaft loops.

Driveshaft shop said 1330 joints are rated at max 5,000rpm which will be just under 100mph. I see others on this thread have 1330 joints on chromemoly driveshafts.... does that help? What is your top speed?

Fastest I've ever taken this car is 125mph but I think it will do 150+. I can see sometime in the distant future doing a mile run up to 150+ at an event or something. I do take it up to 90-110mph periodically on empty desert highways here in NV.

Can you guys who have done this before give me some brutal advice on what I need to do here? I'm running a mostly stock drivetrain behind the trans yoke.

Engine is around 380hp/420ft lbs and I don't intend to do any kind of power adders.

427Hotrod

All I can say is they've been run a LOT faster than that over the years. Would be the least of my worries as far as speed. Might bust one if you hook up hard.....but that's at low speeds.


JIM

AboveTheLogic

Yeah I'm thinking I hope if there is going to be a failure that I'd hear/feel some warning signs of a loose joint. I've experienced that before -- vibration when letting off the gas that goes away when you give it some is a clear sign of a failing joint.

4-vettes

I'm thinking ask another drive shaft shop. I've never heard such a thing. Ever.
How much torque your putting through them would be a much greater concern in my humble opinion.

Rescue Rogers

Driveshaft speed ???? Really????
If they are good, run them. I ran 1310s drag racing my 327 to 7000 rpm...no driveshaft issues. Put in a 5 speed with 1310 1330 conversion joints and 4.11s with a big block....no issues.

Get the loops for piece of mind but something else will usually break first. If its a worry, never run slicks

gkull

Over drive and drive shaft rpm is very simple math. My intended top speed scarred me into building a drive shaft safely loop. I also did the math and determined that 4.11 or 4.30 gearing was going to have me at peak hp in OD at 175 mph. Which was the maximum speed in my class at the Nevada Silver State 92 mile open road race. My motor at the time peaked about 6300 rpm. Just perfect for 4.11 and 28 inch nascar road racing slicks. Very easy to cruise at just below 175 mph on those highways.

Ive had drive shaft rpm at over 9000 rpm. My engine, tranny, and rearend are solidly mounted

gkull

T56 with. 50 OD makes for effective 4.11 gears to be 2.055. My 92 c4 grand sport could go faster in 5th gear because of only having 400 some Hp. 4.10 gearing.

You need to put some thought into aerodynamics and limit the amount of air into the engine area and under the car. Stability and safety are essential

gkull

Have you improved your brakes? Even with the best pads and fluid stock c-3 brakes are totally inadequate for over 3200 pounds at high speeds. I had to stomp on my brakes on a Nevada freeway many years ago and it gets you pumped up when pushing as hard as you can and you are not slowing down. With max performance tires c-3 brakes can not lock the tires at anything over about 90 mph. Over 130 in a panic stop it's a pucker factor!

From summit racing you buy like a nova drive shaft safety loop and instead of bolting to the tunnel sheet metal you weld it on the tranny cross member.

AboveTheLogic

Thanks everyone this all gives me more confidence.

I found a few references to a 5,000-6,000 RPM max for 1310/1330/etc units and that got me concerned. Guess I won't worry so much about it. Driveline angle is relatively flat too so that helps.

Critical speed on my driveshaft with its length of approx 23" is very high. It was just the joints.

The shop I consulted is the same shop that shortened and balanced my driveshaft many years ago. The diff is an eBay unit that claims to have been rebuilt. Will inspect it when it arrives.

Good reminder about controlling air under the car. I've taken it up to 125 and the front got light and I've kept it under 105 since. I have dreams of setting it up for silver state someday.

Thanks again.

wwiiavfan

I would think a real good balancing job would be prudent as well.

AboveTheLogic

Brakes are stock and fresh - except for the rubber lines I have a few old lines I want to address. Will keep the upgrade in mind before taking high speed anything seriously beyond cruising the open highway or drag racing.

I was looking at those safety loops. Was thinking the same as what you said - instead of bolding to the tunnel, mount to the cross member. The loops aren't expensive. I will probably do that.

I think they did a good job balancing it in 2016 but their machine only goes up to just above 3,000 rpm and I guess that's normal and there is some other shop in the country that can spin them faster.

gkull

My advice is to get into a local SCCA road racing out at the strip. Or some other club. I m from Vegas and started out racing motorcycles and moved up. I did the drag racing in my vette and cycles.

Before you can do something like the open road racing you have to submit your racing resume and be excepted or else you have to attend their road racing school at some additional cost and still be in the lowest category for average speed.

Rescue Rogers

If you are just at the dragway, 385 hp isnt going to get you above 110 mph so you dont have to worry about that right away. If you post a dyno curve you will see that you may be spending all your time between 4500/5000 rpm and 6500 rpm depending on where you torque curve meat is.....IF you get your gearing right so you are at max rpm at the trap then you would be in forth at 6500 rpm @ 1:1....so all that talk about driveshaft yolks and 5000 rpm is baloney, you are going to above that all night. No one at your HP has spent a ton of money on their drivelline unless they are running slicks and dropping it into gear at 3500/4000 rpm which I dont suggest doing that because you will not only snap u joints, you will blow up your tranny, blow up your rear, and possibly twist your half shafts, tear your snubber apart which will put the differential into your storage area....

OldCarBum

I bought these half shaft safety loops from Tom’s Differential
They can be bolted on or welded onto the differential crossmember.
I won’t be using them on my build.
If you are interested, send me a PM and I can give you a good deal on them.
Greg

Rescue Rogers

Ive got something like that...those are nice

leigh1322

I had the much longer driveshaft in my Camaro up to 6-7000rpm, quite a few times. 1310 u-joints too IIRC. 150+mph .On the track tho.
No problems to report.
The L88s at Lemans went over 200mph. I wonder what their driveshaft speed was?

gkull

Really low rpm. They ran non OD transmissions with numerically low ratio rearends. On some tracks we installed 2.73 gear sets. Capable of 200 mph at only something like 7000 rpm So the drive shaft speed was just engine rpm in a 1:1 top gear. Bonneville type Vettes run 2.56 and get going in the 230 - 240's

leigh1322

427Hotrod has cracked 200mph in his 67, so even with tall gears he must be in that 7000 rpm range.
With 411s you should get 136mph out of it at the same driveshaft speed.
Drag cars do that every day with much longer driveshafts.
The vette driveshaft is so short I just don't see it as an issue, as long as you have it well balanced.

AboveTheLogic

Really looking forward to experiencing this...

Rescue Rogers

its going to be a fun ride