Real HP loss numbers

[Daniel Ciobota]

Shannen Durphey wrote:

> Ok, I'm working to understand this.  The chassis dyno has no brake, so
>
> they measure the change in acceleration of the drums to compute
> power.  This means that anything that changes the rate of acceleration
>
> of the wheels changes the rate of acceleration of the drums, and the
> calculated horsepower.  This also means that without acceleration, the
>
> chassis dyno cannot measure horsepower, hence no steady rpm
> measurements can be taken and no tuning done at any given rpm, yes?
>

Correct.

> This would also mean that claims like "lighter pulley adds 5 hp" are
> mrginally correct based on chassis dyno measurements, but technically
> should say something like "lighter pulley increases acceleration
> equivalent to adding 5 more hp", since at any given rpm the hp level
> is ~constant, regardless of the mass of the rotating components.
> Yes?

Two for two... :-)

> I only ask because I can see no other way that claims for replacing
> rotating components with lighter ones can add power.  But I sure can
> see that they'll change the rate of acceleration.

Advance to the head of the class, you get an A+. Seriously, in the real
world, you want to consider anything that would make a difference in
your et numbers (hopefully, a positive one).  Even though pullies don't
add actual hp, they do help your car accelerate faster; that's what
counts, right.  Brake type dynos eliminate all inertia effects and give
you actual engine hp (minus frictional losses); however, they totally
ignore effects of lighter flywheels, different trannies, etc.  There's
no perfect dyno, but as tuning tools both accelerometer and brake type
dynos are invaluable.

Daniel