HP @ crankshaft vs HP @ rear wheels?

[Black2003Cobra]

Light weight drive shafts, wheels, etc won’t increase HP at the flywheel (well…except for the flywheel itself) but it will absolutely increase power at the rear wheels. There is an inertial power loss that results from spinning up (i.e., rotational acceleration) those rotating masses. You save the “static” weight, too, of course. But don’t forget about the frictional losses in the drive train. They’re significant.

 

[PARALYZER]

Lightweight wheels, driveshaft and flywheel will not increase hp. It will only reduce the mass the engine needs to accelerate.

So if I double the weight of mt wheels, driveshaft, and flywheel, my rwhp will not go down?? I find that hard to believe. I know torque would be greatly reduced.

 

[Kovachian]

So if I double the weight of mt wheels, driveshaft, and flywheel, my rwhp will not go down?? I find that hard to believe. I know torque would be greatly reduced.

Wrong, torque will increase. Your engine will rev slower, but once up to speed it will have more inertia (am I using the right word? heh) You increase rotating mass, you increase torque but since your engine won't produce work as fast you lose horsepower.

 

[Steeda30]

I love being able to say that I have 535 bhp and 590 lb-ft. These cars are awesome! :rockon: :thumbsup:

 

[SlowSVT]

Wrong, torque will increase. Your engine will rev slower, but once up to speed it will have use more inertia (am I using the right word? heh) You increase rotating mass, you increase torque but since your engine won't produce work as fast you lose horsepower.

Yea, he'll have a very big flywheel.

Actually any torque increase will be from the heavier flywheel that will give up it's energy when he releases the clutch. The heavy wheel will be an extra drag on his engine when he's excellerating though. The heavy wheels will pose an extra burden on his brakes when he wants to stop.

WS-6, all that lightweight stuff will allow your engine to wind up faster and excellerate quicker (lightweight wheels will also help you stop fater) due to the decreased mass. But they will not increase the overall amount of hp the engine is able to produce.

 

[Kovachian]

Yea, he'll have a very big flywheel.

.....
WS-6, all that lightweight stuff will allow your engine to wind up faster and excellerate quicker (lightweight wheels will also help you stop fater) due to the decreased mass. But they will not increase the overall amount of hp the engine is able to produce.

Are you sure about that? B/c that goes against everything I've learned from Nascar engine builders. Hypothetically if my engine could rev to the moon in the blink of an eye, it's producing work much much faster than it does now, so horsepower increases. The engine isn't necessarily any more powerful, it just produces a given amount of power in a much shorter period of time. Horsepower is the ability to produce work over time.

Wait what's the subject of this thread again? lol sorry for the hijack.

 

[SlowSVT]

Are you sure about that? B/c that goes against everything I've learned from Nascar engine builders. Hypothetically if my engine could rev to the moon in the blink of an eye, it's producing work much much faster than it does now, so horsepower increases. The engine isn't necessarily any more powerful, it just produces a given amount of power in a much shorter period of time. Horsepower is the ability to produce work over time.

Wait what's the subject of this thread again? lol sorry for the hijack.

Man I was not suppose to be online this long.

He was talking about mass external to the engine (flywheel, driveshaft, wheels).

Yes, allowing the engine to rev quicker will help it get to the rpm where the engine makes more hp but once it's there the added weight will do nothing to detract from the overall amount of power the engine will make. It will just take longer to get there.

What your thinking about is the reciprocating mass inside the engine. Then the answer is "yes". Reducing the weight of the pistons, wrist pins and connecting rods will have a direct bearing on the HP output.

The problem is the crank shaft has to constantly accelerate and decellerate the pistons and a portion of the rod mass. Imagine the power consumed by this process. Maybe some physic geek can do a calculation and tell us how much power is consumed by accelerating and decelerating 8, one pound pistons, add a pound or two for each connecting rod moving at 3.500" displacement @ 6500 rpm. OUCH!

 

[Black2003Cobra]

Let me try this again. Accelerating a rotating mass takes energy, just as is the case when accelerating a mass in a straight line. There’s no disputing it. I’m sure most of you are all familiar with Newton’s second law of motion, i.e., F = m*a. And perhaps some of you will also recognize the expression for power as P = m*v*a. Well…the equivalent expressions for rotational motion are, TQ = I*alpha and P = I*omega*alpha, where I is the mass moment of inertia, and alpha and omega are the angular acceleration and velocity, respectively. If you will accept this, then it’s not a big stretch to see that power is also P = TQ*omega for rotational motion. (If you don’t, then I would suggest you go get any of a number of good basic physics books. You’re sure to find all this discussed there in great detail.) Anyway, the upshot of all this is, to accelerate a mass in a circle results in both a power and torque loss. That’s why you hear so often that rotating masses are “bad”. It’s because they take energy for both translational (along a straight line) and rotational (spinning) acceleration. However, although this results in a loss when you’re accelerating down the drag strip, a rotating mass stores the energy it took to accelerate it in the form of kinetic energy. This is given by 0.5*I*omega^2. This is why some guys argue that a steel flywheel helps them launch better. It’s because this stored energy increases if you increase the moment of inertia (rotational “equiv” of mass) at a given rpm (omega = pi*rpm/30 radians/sec). Then, when you dump the clutch (an inelastic collision) this energy is transferred to the wheels which helps get you out of the hole. (On the other hand, it “hurts” you as you try to accelerate the heavier flywheel down the track. Hence, several racers will tell you they rather raise their rpm to help w/ the launch and keep the aluminum flywheel to minimize the energy it takes to accelerate it as they move down the track.) When these rotating masses aren’t accelerating (alpha = 0), there is NO power or torque loss as you can see from those equations I gave.

 

[Kovachian]

That actually clears it up alot, thanx. I'm still on a steep learning curve with physics and engine theory.

 

[PARALYZER]

Thanx, I understand clearer now. I was getting confused for a sec.