HP vs lap times

[Protein Filled]

One more thing to note on your remark there Nate. I am not sure exactly why, but you can have a 95 hp SV and a 95 hp F3 and the F3 will be faster on the straights. I think it has to do with how fast a 4 cylinder bike spools up compared to a twin. I know this from our team running SV's at Brainerd. The inline 4's would come flying by you once the drive out of the corner was nullified.

[Nate R]

Now that's interesting. TZracer, or anyone else have any input as to why? Torque curve? Regardless, the I4 can do the same amount of work as the Twin if they're both 95 HP. Is there a reason for this other than gearing?

[tzracer]

Inertia (rotational) - crank, transmission, clutch, sprockets, rear wheel/tire. Less energy tied up in rotation, more energy for translation.

There is a trade off (as with most everything) having less inertia in the system. When the tire breaks loose, a bike with less inertia will spin up faster making it more difficult to control - even worse in the rain. Eddie Lawson had Cagiva add mass to the crank of its GP bikes to make them easier to ride.

[Nate R]

But if more energy is tied up in the twin, then how can they do the same amount of work? How do you measure this difference? Don't they have the same amount of HP available at the crank?

Let me put it this way: If you remove some weight from the crank, doesn't the bike then make more power on the dyno?  

[tzracer]

But if more energy is tied up in the twin, then how can they do the same amount of work? How do you measure this difference? Don't they have the same amount of HP available at the crank?

Power is the rate at which work is done. 2 engines, same power, same rate of work. Work is converted to energy. For a given amount of time, both engines will do the same work, and hence the same TOTAL energy. The energy (kinetic) is divided up between rotating parts and moving the entire motorcycle from a to b. If you have less rotational inertia, less energy is tied up in rotating parts, which leaves more for translational kinetic energy, then the motorcycle will accelerate at a higher rate.

Let me put it this way: If you remove some weight from the crank, doesn't the bike then make more power on the dyno?  

Lighter crank should make no more power, but the engine will spin up (accelerate) faster. If your dyno is using the rate at which the engine accelerates (inertial dyno) to calculate power, the dyno can read an increase. But is it really an increase in power or a poor way of measuring power?

If you lighten the drum of an inertial dyno, will your power reading change? Did your power really change?

[Nate R]

But we're talking about power available at the crank. (as in, measured on a dyno) Sure the power didn't change at the  piston head, but the available power is AFTER those rotating parts. The power made at the piston head doesn't mean much if the losses between the piston and crank output are different between 2 engines.

I guess it must come down more to the area under the torque curve. Sure they both make 95 HP at X rpm, but the 600s torque curve may have a lot more area under it, meaning you spend more time closer to that peak while accelerating than the SV would.

[tzracer]

But we're talking about power available at the crank. (as in, measured on a dyno) Sure the power didn't change at the  piston head, but the available power is AFTER those rotating parts. The power made at the piston head doesn't mean much if the losses between the piston and crank output are different between 2 engines.

It depends upon the dyno. An inertial dyno will show more power from a lighter crank, but it will also show more power from lighter wheels.

A brake dyno (one type is an eddie current dyno such as a Factory Dyno) will show no power increase due to a lighter crank.

Power is measured differently on the 2 types of dynos.

Neither type will show a power increase for making your bike lighter (non rotating parts).

So the question is what is the best definition of power? A lighter crank will increase acceleration, but not top speed - no increase in peak power. So does a lighter crank really increase power?

The way I view it, if it does not show up on a steady state dyno run (brake dyno), but shows a change on a inertial dyno, your power was not increased, but your mod will help acceleration. Neither dyno replicates the load placed on the bike while actually accelerating. I do not consider one type better than the other, they (can) tell you different information - you just have to know how to listen.

Perhaps for racing an inertial dyno is better. You can indirectly regulate modifications to the driveline besides power modifications. A smart tuner would know that some mods will not increase the dyno output on a brake dyno, but would on an inetial dyno. One could make these mods without going over the hp limit, yet have a faster bike.

I look at engine/driveline mods in 2 categories, ones that increase power (brake dyno) and ones that increase acceleration (inertial dyno).

[HAWK]

 ???

tzracer you're hurting my head again.

The way I view it, if it does not show up on a steady state dyno run (brake dyno), but shows a change on a inertial dyno, your power was not increased, but your mod will help acceleration.

This statement reads that increase in available torque will increase acceleration but not power. One of these days I will find a satisfactory connection between HP, torque and acceleration.

Now where did I put that tylenol........

[clarkie]

think of it this way,

on an inertia dyno before the lighter wheels/tires/flywheel/crank etc it took 5 seconds to accelerate from 3,000rpm to 13,00rpm, with the lighter rotating parts the same run takes 4 seconds.

now if you think about the inertia dyno measuring distance over time you can also compare that to coming out of a corner and accelerating down a straight.  before if it took you 10 seconds to accelerate down the straight (changing gears as you go) now with the lighter components it may only take you 8-9 seconds.

but like it was mentioned there is no direct comparison between the dyno and on the track.  

there are other things that come into play as well, like the bigger 190/55 (or 190/60) tires are a lot heavier than the 180 tires and on the dyno the 190 tires show a hp loss, but if you can get on the gas a lot quicker and have more grip on the 190 tire the acceleration you lost with the heavier tire, is offset but the fact that you accelerated earlier in the corner

[H-man]

???  tzracer you're hurting my head again.

Not me!!  PLEASE keep it up!

I'm just lovin' and learning from threads like this and the "Pounds and Horsepower" a while ago. 8)

I never learned physics in HS nor college.  Looking back, I've regreted not having had the practical, intellectual exercise.

Though I wonder whether I would have felt that way back then when a grade would have been on the line.  ;D

[tzracer]

???

tzracer you're hurting my head again.

That is my job, according to my students I am pretty good at it.

[HAWK]

That is my job, according to my students I am pretty good at it.

I'll say, Point tzracer.... ;D