Z28MJ

car is an a4, H/C/I with a 2.73 rearend. should put low 400s HP down to the wheels.


Went to a highly reputable tuner this morning, and they would not put the car on the dyno cause it had a steel drive shaft and 2.73 gears. He said that if I had 3.43 gears I would have been fine, but that I needed to go get an aluminum shaft out of an m6 car.

this did not make to much sense to me, but I am still learning so I figured I would come here for an explanation.

Thanks

Blackpanther99

iTrader: (8)

Did you ask why? Id be more scared of a aluminum driveshaft as far as breaking, then again a steel driveshaft is even more dangerous.

wrd1972

iTrader: (12)

That sounds strange to me. That gear ratio means the ds will be turning slower which I would think would be less likely to fail. Plus the steel is stronger. Maybe they are fretting over the possible imbalance it might have that has a history of plaguing the oe shafts.

alexus6798

iTrader: (24)

That's my first time hearing something like that. Subscribed to learn what you find out.

Z28MJ

when I asked they told me "the dyno gets up to 140 plus which is too much for a steel DS, cause steel DSs have a lower torque rating than aluminum DSs. So it would snap the tail shaft and send it throgh the floor boards."

jimmyblue

iTrader: (11)

No, that's *** backwards. Steel shaft and steeper gears
is how you overspin the driveshaft and crack trans tail
and/or pinion snout. There is no downside however to
getting an aluminum OE driveshaft in there (M6 or 3.23
A4 cars). They're not even expensive.

1 FMF

iTrader: (1)

I don't understand what affect the rear axle gear ratio has.
On a chassis dyno you're doing a 3rd or 4th gear pull which is a 1:1 ratio in the transmission and run it up to redline which is 5500-6000 rpm. so that is what the drive shaft will turn to regardless if you have a 2.73 axle ratio or a 4.10.
only thing the rear axle ratio will do is alter the wheel speed and how fast the dyno drum spins.

Exeodus

iTrader: (1)

I have a steel driveshaft with a Dana 60 with 3.54 gears and mine was run to 137 MPH on the dyno with no issues. Was this a Chicago based shop?

HCI2000SS

iTrader: (2)

Either there is a misunderstanding or these people are idiots lol

wrd1972

iTrader: (12)

These guys are morons and I would not let them near my car. They probably have a 4-banger ricer dyno anyway and they are worried that an American V8 might blow their delicate little flower dyno to the moon.

Z28MJ

yeah, since I found out the aluminum one is better, I have started looking.

Z28MJ

They tune TT 2000hp+ v8's......so I don't think that is the problem.

1FSTZ71

iTrader: (11)

Now I understand this theory on the trucks, the long steel driveshaft on a 3rd gear pull starts flopping around like a noodle and busts, saw one on an old truck the other day go bang on the dyno:



I think the theory is that since you have the "higher" gears, the shaft will be experiencing the "stress" for a longer time compared to a "lower" geared car. That truck took forever to build up RPM on the dyno and then about 4500 it popped. I know my stock steel shaft vibrates bad over 110mph compared to my aluminum one, both balanced. Would you probably be fine on the dyno, hell ya, but I think they are trying to cover their butts. The steel shaft has a tendency to flex under a load more than an aluminum one

1 FMF

iTrader: (1)

be careful how you explain it. it's not necessarily because it was a steel drive shaft that caused it to break.
Trucks are a different story because you can get a long bed truck the rear axle is further away so you need a longer drive shaft. This is some times a reason you have a 2-piece drive shaft. Other than simply over torquing the shaft, the issue is critical speed of the drive shaft. I assume this is what the dyno shop was worried about for the original poster but like i said rear axle ratio does not affect that on a dyno under most cases for cars.
A drive shaft can only spin so fast before it becomes unstable and bends and destroys itself. The critical speed is influenced mostly by the length of the shaft, the longer it is the lower it's critical speed. then you get into modulus of elasticity of the material (steel vs aluminum) and shaft diameter and wall thickness. So you have to factor those things in to know, it's not simply an aluminum shaft is better than a steel shaft. you can easily go with an aluminum shaft in an application (by getting one with too small a diameter) and exceed it's critical speed and destroy it. For the f-body the oem shaft i think is around 46" long, and the steel shafts were 2.5" diameter? would also need to know the wall thickness of the shaft, then you can look up the critical speed of it so you know.

1FSTZ71

iTrader: (11)

^ what he said lol. I think you know what I was getting at though, like you said it's not solely the material but a combination of things. So the guys at the shop aren't idiots maybe just overly cautious