Aerocivic - how to drop your Cd from 0.31 to 0.17

[ebacherville]

Hmm, here is a link to one and it come up page load error???
http://img.photobucket.com/albums/v4...vic/Above5.jpg

maybe you have them cashed in your browser??

[hypertex]

Originally Posted by Coyote X

The other thing would be using some relays to switch them out you can add several silicon diodes in line. You get a .7V drop across a diode so add 5 of them in series will drop you 3.5V. Just make sure to pick big enough diodes to handle the load.

Originally Posted by basjoos

Coyote X, good idea about using series connecting the headlights and also using doides to customize the voltage drop.

Your alternator/battery will still have to provide the standard voltage. Yes, headlights are dimmer but there is still 12+volts feeding the diodes. Why not just use a smaller fuse or feed wire to limit the draw?

[Xringer]

Using diodes to drop voltage is smart, since there is hardly any power loss to heat.

Using a "smaller fuse" won't work, since a fuse is a safety device, not a voltage dropper. The fuse element melts in half to protect from over-current. That causes an open circuit. Lights out..

A long thin "feed wire" would work. If it were long and thin enough to drop 3.5 volts,
the remainder of the voltage would be applied to the headlamps.

BUT, the wire would get warm. 3.5V x ~10 Amps =35 watts of heat. Wasted heat.

~10A is assuming 2 lamps at 5A each. 13V x 5=65W lamp..
I have no idea what wattage headlamps are used these days..

[Bicycle Bob]

Hi Basjoos, thanks for all the postings, etc. Have you ever done any tuft testing to detect turbulence on your car? I'm particularly curious about the area under the tail, since I have always drawn much shallower angles.

[fit7ad]

Originally Posted by Xringer

Using diodes to drop voltage is smart, since there is hardly any power loss to heat.

Using a "smaller fuse" won't work, since a fuse is a safety device, not a voltage dropper. The fuse element melts in half to protect from over-current. That causes an open circuit. Lights out..

A long thin "feed wire" would work. If it were long and thin enough to drop 3.5 volts,
the remainder of the voltage would be applied to the headlamps.

BUT, the wire would get warm. 3.5V x ~10 Amps =35 watts of heat. Wasted heat.

~10A is assuming 2 lamps at 5A each. 13V x 5=65W lamp..
I have no idea what wattage headlamps are used these days..

High school physics teaches us that P = V * I. This is true even if you drop the voltage across a diode instead of a resistor. You will need huge heat sinks to dissipate that kind of power.

A better way is to use a PWM circuit with a power MOSFET. You can look for DIY kits/circuits or DC motor speed controllers. You might still want a bypass switch for 100% brightness since the controller is not 100% efficient and it probably won't give you 100% duty.

[basjoos]

I've tuft tested the back window and it has smooth airflow as seen in the rear view mirror, but I don't have an easy way of viewing tufts on the underside of the boattail while I am underway. I suspect the are under the tail has a fairly smooth airflow since I live on a dirt road and any areas of the car subject to unattached and eddying airflows quickly get coated with a fine layer of dust. One of the unanticipated benefits of aero modding my car has been that it stays a lot cleaner now. No smashed bugs on the front and dust on the back.

[Xringer]

Originally Posted by fit7ad

High school physics teaches us that P = V * I. This is true even if you drop the voltage across a diode instead of a resistor. You will need huge heat sinks to dissipate that kind of power.

A better way is to use a PWM circuit with a power MOSFET. You can look for DIY kits/circuits or DC motor speed controllers. You might still want a bypass switch for 100% brightness since the controller is not 100% efficient and it probably won't give you 100% duty.

I just knew I shoulda went to high school!
I've used small diodes for years to drop voltages, but never really ran any at high currents. Down at a few MA, they down seem to make much heat at all.

Anyways, it's not that much per diode. 5A x 0.7V = 3.5 Watts each.
Depending on the size of the diode, you might not even need a heat sink..

I remember about 35 years ago making a 12V 30A supply for a ham radio project. I mounted the 4 diodes on a big heat sink that was bolted to the chassis. So, I never noticed it getting hot. But, one cold winters day,
I was out in a cold radio hut and noticed how warm it was by my old 12V PS.
Running about 20A, it must have been putting out a lot of heat from the whole chassis..


Anyways, I was wrong about the lamp current too. Since adding the diode drops in series with the lamps will increase the overall circuit resistance and therefore drop the current (saving some battery/Alt power)..
At the cost of less candle power..

[Bicycle Bob]

I wonder if dust is shaking off that surface.
I'd be happy to observe under-tail tufts for you if we were neighbors, but perhaps the easiest do-it yourself approach would be to tape a digital camera under the tail. Even a very basic one will usually make movies a few minutes long.
I've heard that another quick test for turbulent/laminar flow is the speed of evaporation. If you wet your car and drive a bit, the last wet, shiny areas have laminar flow.

[Cd]

We would all love to see some tuft testing videos like that !

( P.S. : Basjoos : I'm building a 3D model of our car for CFD testing . )

[MazdaMatt]

Basjoos, do your aero mods affect your highway acceleration rate? ie, is your 60-80 time faster than a stock civic hatch? Have you done motor or gearing mods that would affect this?