Mathematical equation for HP VS. RWHP?

[jibbby]

My guess was, weight of drivetrain and the actual distance the power has to travel to reach the wheel makes all the difference...

This is why people seek out light weight rims and aluminum drive shafts and what not. Simply to lessen the weight of moving parts which inturn creates more power at the wheels...

I've also read something about reduced weight from moving parts is 100% more effective then reducing weight from non moving parts on your ride...is there any truth to that?

 

[markstoys]

I've also read something about reduced weight from moving parts is 100% more effective then reducing weight from non moving parts on your ride...is there any truth to that?

Removing unsrung weight (suspension, wheels, tires, etc.) will improve handling and ride quality, but it's still weight that has to accelerate from 0 to 60 MPH.

If by moving, you mean spinning, then yes. It's a HUGE difference, although it gets complicated because the farther mass is from the center of rotation the more power it takes to spin it. Also the faster the part rotates, the more it takes. Taking a pound off a flywheel makes way more difference than a pound off a driveshaft--not only because the flwheel is usually spinning ways faster than the drive shaft, but because most of the mass is so far from the center of rotation.

Mark

 

[stevechumo]

I think the SC400 1UZ is usually dynoed at 180 rwhp. That's around 28% loss. So if I make 350 rwhp, then how many bhp do I have?

 

[Tacoma_Kyle]

Thats a good point. Heavier tranny's will generally carry more friction and drag due to mass. More oil to pump as well. Err move.

But yeah good point, cowboy bebop.

 

[XR8tt]

By simply going from Tremec T5 to TKO I lost 6 rwkw...
On same dyno... Yes they can change from day to day...

It was re tuned for closed loop to save on fuel on long trips...
It was more economical at 80mph than 50 mph...
The lambda sensers where bought in at lower rpm...

 

[jibbby]

Thats a good point. Heavier tranny's will generally carry more friction and drag due to mass. More oil to pump as well. Err move.

But yeah good point, cowboy bebop.

Yep, I was thinking exactly what cowboy bebop said too...Is it more a flat figure then a percentage when calculating drive train loss with measured horsepower on a Dyno?.... That kinda makes sense because the drag really shouldn't be any different with a 500whp or 1000whp on the same car, right or wrong? The moving parts weigh the same and are identical on the same car.... Then again, if you have more horsepower the rotating mass should be greater as the parts would move faster and with that should create more drag when comparing 500whp to 1000whp on the same car..... Maybe it is a percentage??????????

Man I am stupped on this one......

 

[cjsupra90]

It increasingly requiers more power to overcome the mass and friction as the input power increases and thus the reason for it being a precentage.

As the input power increase so does the friction that is generated and thus more power used up to overcome this added friction. The faster you try to accelerate something, the more friction is generated.

Think of it as in brakes, (obviously this is the reverse of accerleration, but still the same pricipal is applied) the the faster you try to decelerate a car, the more heat that is generated because the friction was increased. Another way to look at it and can be tested if you do not have ABS is to try to lock up the wheels at 60mph, then again at 120mph (not that I recomend doing this as it is dangerous) but if you did, you would see that it take a noticable more amount of effort to lock up the wheels at 120 then it does at 60. This is because the wheels are inputing more power into the rotors and thus requiers more force or more friction to overcome this power.

Basically it comes down to this, friction changes as the rate of movement changes. The faster you try to move something, the more friction is increase and thus the more power needed or used to overcome this friction.

One other thing to remember or think of is, you can not create or destroy energy, you can only change its form. From this we can understand that friction is the process of changing kenitic energy to heat energy and with this, we can see that heavier power loads (kenitic energy) causes more heat (obviously heat energy) in gearbox's and diffs and thus more friction has been generated. We know that this is true or we wouldn't see the increase in oil temps that we do and race cars wouldn't have a need to run gearbox and diff coolers.

Hope this all helps everyone understand it better.

 

[jibbby]

cjsupra90....That was a very good breakdown..Percentage it is....

 

[XR8tt]

Yes but I wouldn't say its lineal maybe 18% for first 300 h.p .. But I doubt it would be twice as much at 600 h.p ???
Its the way dyno's measure ??
Drop a clutch on dyno ?? Or some torque converters can give strange readings.. A dyno operator would pick easy...
The weight also acts as inertia, once up and going its not holding it back too much..

 

[cjsupra90]

Yes but I wouldn't say its lineal maybe 18% for first 300 h.p .. But I doubt it would be twice as much at 600 h.p ???
Its the way dyno's measure ??
Drop a clutch on dyno ?? Or some torque converters can give strange readings.. A dyno operator would pick easy...
The weight also acts as inertia, once up and going its not holding it back too much..

Yes and this is the whole reason as to why chassis dyno's were not invented for rate power and also why manufactures always rate a cars power at the flywheel and not the drive wheels.......

Chassis Dyno's were invented for aiding in tuning and diagnosing drivability issues and thats it. A chassis dyno at best besides tuning and diagnostics is only good for seeing if there is a change from one setup to another.

 

[XR8tt]

Yes and this is the whole reason as to why chassis dyno's were not invented for rate power and also why manufactures always rate a cars power at the flywheel and not the drive wheels.......

Chassis Dyno's were invented for aiding in tuning and diagnosing driveability issues and that's it. A chassis dyno at best besides tuning and diagnostics is only good for seeing if there is a change from one setup to another.

YEP !! Even with engine dyno. Over the years there has been different ways of doing it .. With all accessories, sump full of oil etc...
If all cars are measured on same dyno, same day its close enough...
There are diffrences between brands of dyno also..
To tune properly with forced induction, the higher load are better...
Naturally an engine dyno is alittle out of the question once engine is fitted...Can't beat road / track tune to get top drivability also...

 

[Toysrme]

Until you're talking raw power outputs that can shove hundreds of pounds of weight to the rear of a car, FWD > RWD in a straight line provided the playing field of suspension geometry and differential type is level.

Its one thing to say RWD is just ungodly better down a strip when you're comparing a 4-link rear setup with a nice diff VS some unequal axle leingth open differential 3 link piece of crap.

You even the fieldwith a 4-link, LSD and axles in theball park of equal and suddenly RWD is out gunned in a line. You've got some 30-40lbs of recipricating weight atleast that much saved in how you package the car.
And the fact that weight transfer isn't quite as much as most people initially think. Most fwd cars have over 60% of their weight over the front so until you're talking huge horsepower and sticky launches guess what? There's still more wight over the front of a FWD car keeping those tires on the ground than your typical near 50% front / rear vehicle can transfer to the back to help hold the back end down.
Eventually you do get into weight transfer being a problem. My point is only that it's hundreds of horsepower more than most stock cars mak when you want to talk going in a straight line race.




Stock FWD is shitty because:
1) 3 link suspensions allow the suspenion to rock slightly backwards, letting pressure off the tire
2) General lack of a descent differential
3) Axle length axle length axle length

You guys wouldn't know, but what's the differance in a stock 250bhp'ish 3.5L v6 Nissan Maxima torque steer that will litterallyjerk the car over half a lane if you let it, and a 450bhp v6 Toyota camry's torque steer that will bearly budge the steering wheel???
Nissan uses un-equal axle length simply to give you that "sporty' feeling where Toyota's axle leingths are as good as you can ask for an OEM.

If you want to talk transvers powerloss in %. A transverse toyota v6 A/T (a540 / a541) will loose 22%. The matching manual transmission (with the open differential) looses 14%.

















On dynos. I always had a soft spot for my gtech pro/comp. Aslong as you do the orientation they're consistant for reasonable horsepower levels, and tell you what's left after everything has been said and done. Plus you only have yourself to blame if something isn't right.