Driveshafts and such

[FTA1995]

OK,

Anything but the simple things with me... I know..

First off, Aluminum driveshafts; Does having less rotational mass cut the parasitic power loss down enough to where I would want less strength where it counts?

Second off, I'm a bit worried about mine, getting a nice unusually strong vibration, verified driveshaft vibrations, above about 90 mph, in neutral, engine dropped back down to a stepped idle. Yes, spider gear and whole rear end is in great condition, I'm sure its the driveshaft.

Third, Dexron Mercon III ATF in my T56, is there a better idea? What would some of you guys recommend? My guess is the ATF is used because of tight tolerances?

Thinking about a new DS after replacing the stupid forward pinion seal that goes bad on the stupid 10 bolt...

Thanks everyone!

[Injuneer]

Steel driveshaft vibrations at high RPM is a very common problem on the LT1-equipped 4th Gens. Its not a balance problem, as much as a dimensional problem causing a low RPM critical frequency, and an input yoke that was not always concentric with the tube. The problem was so bad that there was a "technical service bulletin" (TSB) allowing dealers to replace the steel driveshafts free while the car was still under warranty. The typical "cure" is simply finding a 1LE or an LS1 aluminum DS, fully interchangeable. On a stock engine, or even a beefy normally aspirated setup, the aluminum DS will generally hold up. With a large power adder, the aluminum shafts have been known to fail.

The switch from steel to aluminum might save about 4#. The HP savings at the rear wheels is very small. See this dyno test of the 1LE aluminum shaft vs. the stock steel shaft:

http://www.ws6.com/mod-1.htm

That shows a 1.1 HP increase in "peak HP", and a 1.8 average HP change across the dyno RPM test range. Torque increase about 5 lb-ft.

I went to the extreme of buying an ACPT carbon fiber 3.8" DS, and while there was a larger weight savings over the steel shaft, the CF shaft failed while the engine was still running a "get it running good enough to drive to the body shop" tune. I learned my lesson and wouldn't consider anything less than a 3" chrome moly DS with billet yokes and solid u-joints. But again, that for an application that generates 800 lb-ft.

[FTA1995]

the carbon fiber seemed like another good idea, but i didnt even mention it because I know its going to be expensive as hell.
Why's that fail, specifically... vibrations and jerking from a bad tune like you mentioned?

[Injuneer]

The input yoke assembly (metal) broke loose from the inside diameter of the CF tube, where it was bonded together. At first ACPT claimed it was my fault - they said the DS was too long and that the front slider had bottomed out in the tranny housing, putting a compressive load on the joint that it wasn't designed for. They managed to come to that conclusion without even seeing the failed parts.

Over a period of three months I went head-to-head with the ACPT engineer, and in the end he agreed it was likely it failed for some other reason. My feeling was that the bond had not been made under the required antispetic conditions that are required by th eprocess. I had to reach an agreement where the shop I bought it from (T. Byrne Motorsports) was the middleman. They had (correctly) ordered the length. Tom was able to show that the length he ordered, and what the shaft actually measured, was slightly shorter than the stock DS, and was the same length as the Mark Williams chrome moly shaft that I bought to replace the ACPT unit. ACPT wanted me to send them the CF shaft, so they could destructibely test it. I said I would agree, but if they determined it was not a material or bonding failure (their fault), I would get all the parts back, so I could have it tested. They wouldn't agree to returning the parts to me, even though at that point I had paid $780 for a piece of junk.

In the end they agreed to refund my money, excluding shipping and the labor charges of changing it out. I had to send it back. The shop that installed it (along with the engine) decided they would avoid CF in the future, but when they went to Mark Williams for a DS for their own 7.0-second Pro-5.0 Mustang, they were told that because of the length, and the extreme RPM capabilities of the engine (9,500) the only shaft they could supply that would provide the required critical frequency would be a CF shaft. Worked fine for them.

[Wild Willy]

Slightly off on a tangent- the aluminum driveshaft may not noticably increase actual RWHP, but the lower rotational mass may allow faster acceleration- a difference that may not be observable at a sustained output, but *may* allow the max output be reached slightly faster-

Also, from experience- don't assume that all LS1 driveshafts are aluminum- mine wasn't coming with the high-mileage stock 2.73 gears that GM thinks belong in a Z28- I swapped with a B4C driveshaft- not sure if that is the stock LS1, or a more high perf model-

[ConElite]

Drive the speed limit (Below 90mph) and you wont have to worry about the vibration!


I have the same issue. I have an Alum. DS. I get it around the same MPH and above. I have been meaning to change it out but Have not had the chance. Im sure a 1LE or LS1 Alum. unit is enough for your power output which is around 400ish HP im guessing. LS1 guys are pushing close to 500hp with the Stock Driveshaft with no issues. You dont notice the difference in power at all to be honest. My brother did the swap on his 96 and nope. It is a lighter unit so the rotation mass is less which is always a good thing.

I use the Mobile 1 ATF oil. The Dex. III is recommended, But when I asked my old co-worker (the Certified Tranny guy at Chevy where i used to work at) he said it should be fine. Kinda $$ but runs smooth even though my trans is going out!

[FTA1995]

It should be fine, I don't see any reason as to why it wouldn't. My last dyno numbers are a secret .

I was thinking about swapping the DS soon... but there are some other things I need to look for first.

I'm going to look for a well balanced steel DS and see what I can find.

A bit to busy right now with education and the wife to really dedicate much time to it right now

Thanks guys

[ConElite]

I

A bit to busy right now with education and the wife to really dedicate much time to it right now

Thanks guys

Thats Why I will never get married!.



JK

[FTA1995]

"Sweatheart, can you please grab me the slip jaw open head?" as I'm half pretzeled between the steering rack and a header.

*Hands me a flathead screwdriver*




I'm not married YET but I think its going to be a fun ride.

[ConElite]

LMAO

Thats why I try and find girls that actually like cars and wanna try working on them.

When do we get to know the dyno numbers on that girl?

[FTA1995]

In good time my friend

[Injuneer]

Slightly off on a tangent- the aluminum driveshaft may not noticably increase actual RWHP, but the lower rotational mass may allow faster acceleration- a difference that may not be observable at a sustained output, but *may* allow the max output be reached slightly faster-

That's exactly what the dyno test measured - the difference in the energy consumed in accelerating the rotational mass of the driveshaft. The dyno test is not made at "sustained output". Since the only thing that was changed between the two tests was the driveshaft, it demonstrates the difference in the energy consumed by the driveshaft over the time period of test. The important thing is the change in torque at the rear wheels, since that is the only thing that causes the car to accelerate.