manga_blue wrote:Nevertheless you'd think that at those speeds you'd be able to ram more air in. Really, how hard does a turbo push compared to a 180kmh wind?
Dann, where are you drawing your air from?
2 part question, Firstly I have a sealed pretty well designed cowl intake. And some recent datalogs suggest that im making a little under 0.5 PSI of 'boost' at 50km/h.
Compared to a turbo, well patricks 1.06 at wakefield 'Casper', runs 15psi. Stock se is about 7.
The way you test air intake effectiveness.
Plug laptop into aftermarket computer, that has a map sensor (map sensors accurately measure pressure in 1 kpa increments).
Stop the car and turn the engine off at the exact piece of road you will take the reading. (protip, use a hill)
Take a reading of ambient air pressure.
Drive the car down the hill at speed.
Turn the engine off, note new pressure.
Total improvement is new minus old pressure.
Ie, I tested ambient at 98kpa, this may or may not have been perfect(nominal pressure is said to be 101, but depends on height above sea level, air temp etc) but im using the same measuring device so it doesnt matter.
Drive down hill, turn off engine, brake so im doing exactly 50kph, read datalog.
I found 101kpa at 50kmh, or 3kpa improvement, or 0.44 psi.
Aerodynamics are a subject with dimishing returns. it takes the square of the speed worth of power.
Ie, Double the speed takes 4 times the power, triple takes 9 times.
Im not sure if this means Ill get 4 times the pressure at 100kph, or 2psi. I need to do more tests, but its hard to roll at 100 while looking at a laptop without getting booked.
Hellmun wrote:
Weight is definitely a factor, use skydiving for a simple example. My force acting downwards is my mass * gravity. Therefore a change in my weight will change my total force exerted downwards. Since drag is a force(mass of air multiplied by the acceleration your body is exerting on it) by changing weight you will change the amount of negating force required to reach the same velocity. With the constant acceleration of gravity this actually means the higher weight increases your top speed. With a car in the horizontal plane the increased weight has a similar effect, it pushes the car down harder increasing vertical force, thereby increasing friction on the road and reducing the force available to overcome the aerodynamic force resisting in the horizontal plane. Being that the force is acting perpendicular it's simply a smaller factor because it's calculated at a tangent. Plus as mentioned, racetracks aren't really that big so the effect it has on reducing acceleration through drag really limits practical top speed. So I think quite relevant to discussion.
I agree fully. Though only due to increased rolling resistance as youve said but I feel that needs to be very clear.
Weight does not limit top speed. Acceleration is a rate of change, it measures how fast we can change speed. more weight equals more mass to accelerate slowing acceleration. Speed is purely Force VS Efficiency.
Its how hard you push vs how hard friction (including air friction, or air resistance) pushes back, when your friction caused by the tyres and air resistance equals your power you stop accelerating and reach terminal velocity.
but it does take 4 times the power to go double the speed.
TL;DR You need power and low friction for top speed, not low weight. the low weight only helps shorten the distance before you reach top speed.
When I hit 225 I was on a very long straight. Mind you the car would have gone faster, it was still pulling.
Ill also add I also have a non standard undertray.
Dann
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