25% drivetrain loss? WHAT?!?!

[sparq]

I was over surfing JacksonRacing.com for the hell of it when I came across the Prelude dyno charts... I noticed they DYNOED the car at 154hp... Ummm, Honda says 200hp ~ what gives for the 25% drivetrain loss, anything a lightened flywheel could help ~ also what other things could help to get more of that power to the ground??? I thought the "average" was 15%... damn it thats a whole 10% im missing out on

[fritz_269]

Most 5G manual Preludes dyno at about 165-170 peak (15-18% loss). 155 is a bit low, but it shows off their system well, so I'm not too surpised.

And a lightened flywheel will not change your horsepower.
:smoker2:

[Riceburner91]

Originally posted by fritz_269
Most 5G manual Preludes dyno at about 165-170 peak (15-18% loss). 155 is a bit low, but it shows off their system well, so I'm not too surpised.

And a lightened flywheel will not change your horsepower.
:smoker2:

Yeup...fritz is right.....of course! Whats up fritz?? How you been doing?? Dood the Thermal is better than I imagined....such a pretty low tone...ahhh....Done anything new to the 'Lude?
ttyl

[sparq]

Originally posted by fritz_269
Most 5G manual Preludes dyno at about 165-170 peak (15-18% loss). 155 is a bit low, but it shows off their system well, so I'm not too surpised.

And a lightened flywheel will not change your horsepower.
:smoker2:

I didnt say it would ADD horsepower, but from what I understood the main loss in power is from the engines mass as it rotates... and a lightened flywheel helps shaves some pounds, thus letting power get to the ground faster? I dunno, im an idiot... performance stuff is still sorta new to me ~ im an audio guy :smoka:

[fritz_269]

Your drivetrain mass has absolutely nothing to do with the amount of horsepower produced.

But to get down the strip, the mass of the drivetrain must be accelerated, just like the mass of the car itself. The difference is that the drivetrain turns in circles, so it has rotational inertia rather than translational inertia like the car. Translational inertia is just the mass of the car; rotational inertia is the mass times the radius of rotation.

Every pound taken from drivetrain components plays double duty. First, it lightens the overall mass of the car - always good. Second, it lessens the rotational inertia of the drivetrain. The flywheel is particularly good for reducing rotational inertia for a couple of reasons: One, it has a large radius, and thus a large rotational inertia, and thus will have a greater effect on the rotational inertia than any other single part. And two, it's before the gearbox, which means that in a 1/4 mile run it has to be accelerated from low to high RPM after every gear change (3-4 times each run).

None of these things change the power output - they're quite similar to changing the overall mass of the car (just a little more effective, pound for pound ). Since acceleration ~= power / mass, you end up with greater acceleration.


Note: The above is true for "brake horsepower" (BHP) - the true definition of horsepower. Some methods of dyno measuring (particularly inertial dynos aka DynoJet) will give different results with different drivetrain inertias, this is a known measurement error. And it's why a DynoJet will give different hp ratings for a 2nd gear pull and a 4th gear pull on the same car.

[fritz_269]

One more quick thing - as said before, you don't lose power when accelerating things. The only way take power from the wheels is to transform it into another kind of energy - in this case, heat from friction. It is solely the friction within the drivetrain that is responsible for the difference in the measured wheel hp and flywheel hp.

Physics homework - How does the principle of conservation of energy apply to the power produced by an internal combustion engine? Trace the energy path from the gasoline to the end of the 1/4 mile strip.
:smoker2:

[Rgacke]

I had heard that for every pound you take off the flywheel was like takeing 50lbs off the car, is this comparison true???

[fritz_269]

Originally posted by Rgacke
I had heard that for every pound you take off the flywheel was like takeing 50lbs off the car, is this comparison true???

This depends on a LOT of different things. Weight of the car, original weight of the flywheel, diameter of the flywheel, were the weight was removed from the fly, what gear you are in....

Reducing weight on the fly will have it's greatest effect in 1st gear, and it's least effect in top gear.

I suppose that some lightened flywheels could be equivalent to 50lbs off the weight of the car, but only in first gear. In top gear, it'd probably be more equivalent to 10 lbs off the weight.

But not all modded flywheels will be that good. Mass removed from the hub means almost nothing to the rotational inertia, but mass removed from the edge of the disc is worth many times it's own weight. Simply knowing how much mass was removed tells you nearly nothing about the reduction in rotational inertia, you must also know where on the disc the mass was removed from.

Although a ligtened flywheel will certainly help you incrementally with acceleration (particularly in the low gears), this is not the major reason to get one. IMHO, the major advantage to a light flywheel is the speed at which you can shift. With the clutch out, the flywheel is the only load on the engine; so with a lighter flywheel, rev-matching on either an up or down shift can be much faster.

[flylwsi]

back on the main question...
the basic drivetrain loss is about 20%...
this is because the power is lost through the trans and wheels before it is measured, as opposed to directly at the crank... it is also good to note that part of that loss comes from engine driven accessories, power steering, etc, which are not included when dynoed from the factory with just the motor...

[Someguy]

SAE net, which is the typical "book" horse power sited by most auto makers is the horse power messured at certain conditions (tempature, pressure) at the flywheel with the complete exhaust and engine driven accesseries in place.

[pvang31019]

let's put this in regular words....JR rated it that low to make their charger look good when the numbers turned out after the install....

[flylwsi]

i talked about the flywheel and pulley idea for a long time today... and came to the realization of a few things....

when you say a flywheel or pulleys increase power, it is only whp that is increased, as the overall power of the motor doesnt change, like if you added a header...

this is a common fallacy of many.

basically... for example.

you have 155whp, and say, 200 at the crank (the manuf rating) if you add a lightened flywheel, you might pick up a coupla whp, but not at the crank... so you are letting more get to the wheels but not changing the actual motor's hp... same with pulleys. you are freeing up hp instead of increasing it...

i just figured i would spill the obvious out, b/c it was a huge realization for me, and i figured this was a good spot to drop it...

sorry if i am stating the obvious, but alot of people dont realize the diff b/n freeing and adding hp, or just dont care...

[fritz_269]

Originally posted by flylwsi
when you say a flywheel or pulleys increase power, it is only whp that is increased, as the overall power of the motor doesnt change, like if you added a header...

this is a common fallacy of many.

basically... for example.

you have 155whp, and say, 200 at the crank (the manuf rating) if you add a lightened flywheel, you might pick up a coupla whp, but not at the crank... so you are letting more get to the wheels but not changing the actual motor's hp... same with pulleys. you are freeing up hp instead of increasing it...

Sorry, but you're still mistaken. Changing the rotational inertia of the drivetrain does not change the power, either at the flywheel or at the drive wheels. Read my 2nd post in this thread again. Brake Horsepower (BHP) is a measurement that is completely independent of any mass that the power is trying to accelerate.

Taking out rotational inertia (lightening the drivetrain) reduces the effective mass of the entire car, and thus will increase your acceleration (since acceleration ~= power / mass ). But the power doesn't change at the flywheel, drive wheels, or anywhere else.