BAWC Forums

[quote=""Wolf_Zero""]I'm sure I'm missing something which is kinda why I made this thread. Honestly a simple log of Stock vs. CFDS where it has RPM and Time would quickly solve this. But my car isn't running right now and I no longer have a stock driveshaft.[/quote]

The gains from a CFDS are very minimal. If your old DS is so worn, it's giving false knock readings to the ECU, you'll definately noticed an improvement in the launch, put in terms of HP to the ground, the gains are very minimal.

People on 3si like to brag about them just so they can justify spending the grand on it to themselves.

[quote=""TajMan""]As has been said, its not just "freeing up power".
This is rotational weight.
Less weight rotating = it takes less to change that current rotating state.
Engine revs quicker, FEELS more responsive in certain scenarios.
Goes hand in hand and more noticeable with a lightweight flywheel, lightweight pulley, etc. Nice light aftermarket pistons/rods will also make a similar difference in this rotational weight reduction.[/quote]

Already covered that bit. The majority of the mass is close to the point of rotation so doesn't have as much of an effect on the amount of energy required to get it to rotate (ie. it has low rotational inertia). Yes it does take a little bit more energy to start rotating but not that much more. I feel the same way about lightweight, stock sized pulleys. With something like a flywheel or a wheel it's more noticeable because a good portion of the mass is further away from the point of rotation which creates more inertia.

As for lightweight pistons/rods, that's linear motion being converted into rotational motion. So you're not actually reducing the rotational weight, what you're reducing is the amount of inertia the pistons/rods have. By doing that you're reducing the forces being exerted on the rotational mass itself, but your not actually reducing the weight. To do that you would have to get a lighter crankshaft.

Like I said, I'm doing quite a bit of physics because of school. I'm no physicist, but it is my minor.

[quote=""Monochrome""]The gains from a CFDS are very minimal. If your old DS is so worn, it's giving false knock readings to the ECU, you'll definately noticed an improvement in the launch, put in terms of HP to the ground, the gains are very minimal.

People on 3si like to brag about them just so they can justify spending the grand on it to themselves.[/quote]

Didn't think about false knock readings from a worn out driveshaft, that could be a possibility. I kinda figured people wanted to feel good about spending a grand for a bit of carbon shaft.




I've also realized that I've been using incorrect terms. Anytime you see me say something about unsprung mass ignore it (to lazy to go back and edit posts right now). Don't know where the hell I got that from. The term I was really looking for was Rotational Inertia. I'll go back and edit them tomorrow when I'm at work.

[quote=""Wolf_Zero""]By doing that you're reducing the forces being exerted on the rotational mass itself, but your not actually reducing the weight. To do that you would have to get a lighter crankshaft.
[/quote]

not quite. the crankthrow-conrod-piston is still a basic crank-slider mechanism. the only difference here is that instead of purely rotational inertia, there is the component of reciprocating inertia as well. classically, conrods were modelled as having two components. one linear mass lumped onto the piston, and one rotating mass lumped onto the crankpin (rod journal). this split was determined from the conrod CG (from the rod journal axis) as well as the moment of inertia about this CG. in any case, the jist of it is this: since one end of conrod works in a constrained fashion, its free body motion can always be broken down into these two components when examining it's effects on the crank.

you can see that increasing a conrod mass contributes to increasing the total system MOI directly as the rod's rotating mass. however, you are correct that a cyclic acceleration-deceleration (as the rod's linear mass plus piston) only applies retarding forces.

[quote=""Monochrome""]The gains from a CFDS are very minimal. If your old DS is so worn, it's giving false knock readings to the ECU, you'll definately noticed an improvement in the launch, put in terms of HP to the ground, the gains are very minimal.[/quote]

[quote=""Wolf_Zero""]
Didn't think about false knock readings from a worn out driveshaft, that could be a possibility. I kinda figured people wanted to feel good about spending a grand for a bit of carbon shaft.
[/quote]
I'd say the false knock scenario is very rare. You're talking about 5 counts of false knock before the ECM starts to pull timing. 5 counts is pretty noticeable on a logger, especially at low RPMs and normal acceleration.

I think most people with a CFDS would agree that the biggest benefit is seen at low RPMs, off-boost acceleration, and pulling away from a dead stop. For a long time, I kept my logger on any time I drove my car (as more of a knock gauge). I never saw knock off of a start-up or at low speed acceleration, and yet my car feels better with a CFDS.

[quote=""Monochrome""]
People on 3si like to brag about them just so they can justify spending the grand on it to themselves.[/quote]

What's there to justify??

Not spending a grand and a half on a new OEM one? Eliminating 3 non-greasable/replaceable U-joints, 2 lobros joints and 2 carrier bearing, for 2 greasable/replaceable U-joints? Spending a couple hundred dollars to replace the carrier bearing and having a driveshaft shop replace the U-joints...?

[quote=""thestealth""]What's there to justify??

Not spending a grand and a half on a new OEM one? Eliminating 3 non-greasable/replaceable U-joints, 2 lobros joints and 2 carrier bearing, for 2 greasable/replaceable U-joints? Spending a couple hundred dollars to replace the carrier bearing and having a driveshaft shop replace the U-joints...?[/quote]

I can rebuild a stock driveshaft for half the price of a CF one, but I was leaning more towards spending so much money on so little gain in performance.

You don't have to believe it, but the car feels a lot better with a carbon fiber drive shaft. If I had to build a car again, that would be one of the first mods I get. Money well spent, in my opinion. The car has completely different characteristics for stop-and-go driving.

In all honesty, I'd say I'm usually pretty critical of parts that I install on to my car. When I bought my wheels, I was able to shave nearly another 30lbs off of the drivetrain. Its pretty much universally accepted that saving a significant amount of weight in the wheels does help performance. As much as I had hoped and expected to feel a difference, I couldn't feel a damn thing when I replaced the wheels.

I put on a carbon fiber hood and lightweight seats, and didn't notice a difference with either. At best, the weight savings is 30 lbs (static) apiece, and it was unnoticeable. The CFDS, on the other hand, saves between 35-40 lbs, yet it is very noticeable.

Like I said, you don't have to believe it if you don't want to, but pretty much everybody who puts a carbon fiber driveshaft onto their car says that their is a noticeable difference in the feel of the car.

[quote=""Hannibalzero""] The car has completely different characteristics for stop-and-go driving.

[/quote]

Especially stop and go driving, like idling around parking lots looking for a spot, or those times when you're stuck in rush hour traffic...even though, that is really not a fair comparison as my stock drive shaft was pretty wore out...

[quote=""Hannibalzero""]Like I said, you don't have to believe it if you don't want to, but pretty much everybody who puts a carbon fiber driveshaft onto their car says that their is a noticeable difference in the feel of the car.[/quote]

Oh I'm not saying it doesn't make a difference. Just that the bang for the buck isn't that great.

If you want great bang for your buck get a thai hooker
sorry couldn't avoid the joke