Hp needed for % of Speed Increase

[SHughes]

Not to high-jack the thread, but this is a great topic and I LOVE the formulas.  IF they are accurate, I need far less power to meet my goals than I previously thought.

And power is certainly not the problem I have...it's money to make the truck safe to drive that's the problem.

[agrassfidf]

Is there a formula that involves weight, or does that not matter when you already have hp and top speed? Is there a reasonably accurate formula for motorcycle?

[LSR Mike]

Dr Mayfield,
Mayf, for all the Listers, has a site with many specifics including the Cd, frontal area etc... of many production cars and trucks,
along with simple explanations...

http://www.mayfco.com/dragcoef.htm

[1212FBGS]

theoretical vehicles set theoretical records every theoretical time.....
Kent

[Seldom Seen Slim]

Kent, you should add that you're speaking theoretically, I theorize.

[Peter Jack]

But it's so true!

Pete

[desotoman]

Dr Mayfield,
Mayf, for all the Listers, has a site with many specifics including the Cd, f :evil:rontal area etc... of many production cars and trucks,
along with simple explanations...

http://www.mayfco.com/dragcoef.htm

Mike,

He forgot Roadsters!!!!!!!!!!!!   

Tom G.

[Peter Jack]

And he runs one ------------ just a little newer than the rules allow!!!

Pete

[LSR Mike]

Interestingly enough Tom, I stole an old method from my high school Photography instructor to calculate the frontal area for #300. He was a Air Force Photo Intelligence Interpreter.

Shoot a pix of the of the intended vehicle with a known value object in the frame. From there you can calculate the frontal area of the vehicle. In Vietnam they knew what the bore of rifle the VC was carrying, therefore he could tell you what size shoe he (the VC) was wearing.

I have since lost the excel file I was working with to approximate the HP necessary to put that brick over the record, and we really never knew how much HP we were producing because the LSR tires would never hod traction on a chassis dyno. but alas, we know what happened to that engine...I still have some (very broken) parts of it in the "offering to the gods of speed" section of my LSR chapel.

[thecarfarmer]

DRAG goes up as Square of velocity. HORSEPOWER goes up as CUBE.

Really?

Maybe someone can explain how this works...

If most of the drag on a car at speed (like >100 MPH) is aero, and aero drag goes up as a second power function of the increased velocity, how come HP needed would go up as a third power function?

In other words, if air resistance is the largest single factor in drag, and if it goes up 4X when I double the speed... where's the other "horsepower vampire" hiding here? 

Just tryin' to understand...

-Bill

[hotrod]

In other words, if air resistance is the largest single factor in drag, and if it goes up 4X when I double the speed... where's the other "horsepower vampire" hiding here?

Just tryin' to understand...

The speed itself is the 3rd factor.

If you could drive in a vacuum the power required to go faster would go up directly in proportion to the speed. With air resistance you also increase drag at the square of the speed. speed squared x speed = speed cubed

Larry

[panic]

In Vietnam they knew what the bore of rifle the VC was carrying, therefore he could tell you what size shoe he (the VC) was wearing.

Plus or minus 100%...

[Jack Gifford]

Bill- what you were missing is that power is force times velocity.

[tallguy]

We're not talking about any specific car, here.  Nor any Cd.  There's a formula that sez that if you get the car (bike, whatever) to go, say 180 with, say 200 HP, how fast you'll go with 250 HP.  I think that Avanti comes closest, but I think it has to do with the square of the percentage of increase, not the cube.  I don't need anyone to figure it out for me -- I'm good with formulas -- I just don't have that one.

Stan

Aerodynamics is not the only thing that plays a role here.  There are other frictional things coming into play.  As it happens, some of them work in a similar fashion, mathematically (i.e., as the speed doubles, the required power increases by a factor of 8 -- that is, if other miscellaneous things such as change of viscosity due to temperature change don't come into play).
At fairly high speeds, aerodynamics is probably the major biggie, and for it, the horsepower required increases as the cube of the speed.  Of course, available power is not the only important thing.  Appropriate gearing, traction, and durability of the machinery is also critical to success.  So is a smooth-enough surface for the tires to push against. 

Regarding using specific "target" numbers for speed or horsepower, it's not likely that anyone would have a vehicle set up so precisely that its maximum horsepower output from the engine would happen at a steady, exact 180 mph consistently over the timed mile.  So we tend to approximate.  I believe that an approximation based on total drag increasing as the cube of the vehicle's velocity is a good approximation.  But the real "proof of the pudding" happens on the race track, not on someone's
computer.

[Stan Back]

I said that 14 years ago?  Yikes!!!  I musta stole that from someone else.