New project - your opinion - "Old MAP sensor to be used to measure wind pressure on front of car"

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,461
I have an old MAP sensor (Manifold Absolute Pressure) sensor that has three pins "+5V…Vout…Gnd" I've pierced the back side of the MAP sensor to measure air pressure. Accuracy doesn't matter. What I want to do is mount this somewhere on the front of the vehicle and see how much wind resistance I'm getting. Remember - accuracy be damned, I don't care. I want to be able to compare the difference between open air driving versus pulling behind another vehicle. No - I don't want to tailgate. I want to see how much the pressure changes (actually hear, as I want to build an oscillator that is voltage dependent - easy enough). I'm more interested in your opinions on the mechanical aspect of it.

Like jet aircraft with their "Pitot Tube" probes on the front of the jet, air rams into the Pitot and the computer senses the wind pressure and calculates the air speed. Again, to hell with accuracy, I don't care about what my air speed is, I just want to be able to determine when I'm behind a vehicle how much difference it makes. And that "Difference" is relative. Again, doesn't matter. More wind pressure the higher the pitch. Less wind pressure the lower the pitch. I have ears that are calibrated enough for the project. I've always wondered about when there's a cross breeze and you're in the next lane to a big rig, where's the sweet spot for cruising while reducing the load on the engine.

No, I'm not trying to make a device to improve gas mileage, I just want to know when I'm pressing harder on the pedal to maintain speed versus the subtle difference it might make when following another vehicle. Another curiosity has been wind resistance on the nose of the vehicle when facing a headwind, versus a tail wind.

I've been working on building my woodshop and am tired of drywall and mudding. My shoulders ache from all the over-head sanding and mudding. So I want to take a break from that and put together an oscillator circuit that is voltage dependent. The measured range of the sensor on a 5VDC supply ranges from 0.5V to 4.5V. So somewhere in between that I need a mechanical way of sampling the air pressure on the nose of the car. Tacoma to be exact. I'm guessing that the size of the orifice will make a difference too. So your opinions please.
 

zophas

Joined Jul 16, 2021
156
The size of the orifice always makes a difference....but that is another matter.
I suggest you take a look at the Pitot Tube design on aircraft and see if you can model something up for your car.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,461
Someone's comment must have been deleted. Tried quoting their post. In short, they suggested a computer fan spun by the air. Well, that's a fair assumption, measuring the change in fan speed via measuring the generated DC from the small motor. However, I'm figuring this to be working at highway speeds, so likely the air speed would definitely overpower the motor and spin the heck out of it. It's dubious I'd get any detectable changes in nose air pressure. But it's a good thought. Thanks to whomever was making that contribution.
 

LowQCab

Joined Nov 6, 2012
1,345
This plan will not create any useful feedback.
Simply monitor the Voltage of the stock MAP Sensor on your Engine,
it's a far more meaningful number.
The Old-School name for this was a "Mileage-Meter" or simply, a Vacuum-Gauge.
It's a direct indicator of Engine-Load.
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.
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Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,461
The size of the orifice always makes a difference....but that is another matter.
I suggest you take a look at the Pitot Tube design on aircraft and see if you can model something up for your car.
Had already thought of that. Smaller orifice means less pressure entering the sensor. Perhaps I'll start with something very small. If I don't like the results I can drill the hole a little larger. If I use the end of a 3/8" hose I suspect I'll get full deflection at very low speeds. That may be useful for driving around town, but at those low speeds wind restriction doesn't come into play very much. It's those highway speeds my truck doesn't like.

If I push the truck along at 80MPH I get between 13 and 16MPG. Headwinds can REALLY suck down the gasoline. I rarely exceed 75MPH. After owning the truck for a while it appears the best speed for the highest MPG is around 60MPH. That's a little slow, so I don't mind using more gasoline to maintain a reasonable highway speed. It's just the curiosity of how much benefit I get from riding behind a big rig. Again, not tailgating. And since those BR's are slower, I often end up passing them and ending up in clear air conditions. So it's hard to really tell how much difference riding behind a truck makes.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,461
This plan will not create any useful feedback.
Simply monitor the Voltage of the stock MAP Sensor on your Engine,
it's a far more meaningful number.
The Old-School name for this was a "Mileage-Meter" or simply, a Vacuum-Gauge.
.
.
.
Dashboard has a mileage meter. But with fluctuations in pedal position the information changes fairly rapidly, however, that means focusing my eye on the dash. I had an older Tacoma with cruise control but it was a 5 speed manual transmission. That meant that when I was going up a long hill the engine could be working harder and harder and I didn't know it until cruise control shut down. So I put a vacuum gauge on the A-Pillar so I could see when the engine was really working hard. Under such conditions I could down shift until over the hill.

I want an audio tone that can tell me how much affect the air on the nose of the truck has. Whether I end up pressing harder on the pedal or not - the changes will be made more obviously. And as I mentioned earlier, being behind a truck with a cross wind negates any reduction in air on the nose of the vehicle. So maybe riding in the next lane over, behind the bumper of the trailer might be the sweet spot. Or like in NASCAR side drafting. Maybe. These are the things I hope to learn.
 

LowQCab

Joined Nov 6, 2012
1,345
The MAP Sensor is designed to measure between
ambient Atmospheric-Pressure, and MINUS (roughly) 12 PSI, ( close to a Vacuum).

It is not designed to measure Pressures in excess of Atmospheric-Pressure.

Map Sensors are available for Turbo, or Super-Charged-Engines that will
measure "positive" pressures,
but Tie-Rapping one to your front Bumper will provide ZERO useful information.
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ElectricSpidey

Joined Dec 2, 2017
1,972
"Side Drafting" in NASCAR is when you place your car in such a position as to force a great deal of air across the other car's spoiler in order to slow it down.

"Side Drafting" is sort of a misnomer.
 

ericgibbs

Joined Jan 29, 2010
14,203
hi Tony,
As LowQCab points out , its really a MAF , Mass Air Flow sensor, normally mounted pre carb' input to measure air intake vacuum level, for fuel control.
E
Added;
 

scorbin1

Joined Dec 24, 2019
96
Someone's comment must have been deleted. Tried quoting their post. In short, they suggested a computer fan spun by the air. Well, that's a fair assumption, measuring the change in fan speed via measuring the generated DC from the small motor. However, I'm figuring this to be working at highway speeds, so likely the air speed would definitely overpower the motor and spin the heck out of it. It's dubious I'd get any detectable changes in nose air pressure. But it's a good thought. Thanks to whomever was making that contribution.
A computer fan might not be a bad solution actually. Similar to the pitot, you could build a bell housing around the fan and limit the size of the inlet orifice to reduce the amount of air pushing through it. However I wouldnt look for the dc voltage produced by the fan, especially since some are brushless and wouldn't produce DC anyways, but get a 3 wire fan and hook up to the speed sense wire(should be a HAL effect sensor). That should be pretty reliable setup.
 

ElectricSpidey

Joined Dec 2, 2017
1,972
From a reliable source (NAPA)

"In fuel-injected automotive engines, a manifold absolute pressure (MAP) sensor is used to continuously monitor the amount of air flowing into the engine, so the computer can calculate air density, adjust the amount of fuel to spray into the combustion chamber and adjust the ignition timing. In some vehicles, a mass air flow (MAF) sensor is used. While the two are interchangeable, a MAF sensor measures flow rather than density."
 

LowQCab

Joined Nov 6, 2012
1,345
There are 2 standard ways to measure the Air-Flow into the Engine.
Most modern Cars use both.
They are .......
MAP-Sensor, (Manifold-Absolute-Pressure) plus RPM,
which then runs through a Computer to "calculate"
the "estimated" Mass-Air-Flow into the Engine,
and,
a MAF, (Mass-Air-Flow) Sensor,
which creates a direct measurement of the Air passing through it by weight.
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.
.
 

LowQCab

Joined Nov 6, 2012
1,345
All of this discussion is getting rather off-point.

The Total Aerodynamic-Drag CAN NOT be measured by way of Air-Speed or Pressure.

The only reliable indicator is Engine-Load, which just happens to be
exactly what determines the Fuel-Mileage of the Truck,
the determination of which is the desired outcome.

The Aerodynamic-Drag on the Body of the Truck can fluctuate wildly,
depending upon the constantly changing environment, and Speed.

Completely Isolating the Aerodynamic-Drag,
from the Mechanical-Drag,
and the Load variations caused by Inclines and Declines of the Road,
is virtually impossible,
and can only be reliably achieved in a purpose-built "Wind-Tunnel".

Please take note that the Wind-Tunnel operator
does not care what the "Pressure" is anywhere in his facility.

The "Pressure" at any chosen point on the Body of the Truck may change radically,
or oscillate for no readily apparent reason.

The "average" Drag on the Body of the Truck is measured at the Tires.
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.
.
 

zophas

Joined Jul 16, 2021
156
I initially suggested the computer fan but then deleted my post thinking that you already had a MAP sensor. I was not sure if the fan would give you any voltage if spun by air.
 

Juhahoo

Joined Jun 3, 2019
250
I want to be able to compare the difference between open air driving versus pulling behind another vehicle. No - I don't want to tailgate. I want to see how much the pressure changes
Build a device that measures accurately your fuel consumption, this gives you some data how much it benefits to drive behind another vehicle.
 

LowQCab

Joined Nov 6, 2012
1,345
If you put your MAP sensor inlet in the side of a venturi, you will get a vacuum reading as air goes thru the venturi.
This will result in an extremely low resolution, noisy signal,
that will only roughly approximate Air-Speed, in a very small, localized area.
It will not even remotely indicate the Total-Aerodynamic-Drag on the Body of the Truck,
which is what would theoretically be needed.

There is NO external Sensor that will provide the requested data.
.
.
.
 

strantor

Joined Oct 3, 2010
5,695
I've gone down this path of thoughts before and I don't see why you couldn't just mount an anemometer on your hood to measure relative windspeed. In my case I wanted relative direction, so the whole anemometer + wind vane assembly sitting on my hood would have looked ridiculous and that's why it never passed the idea phase. But what you want is much simpler. You could probably just run the pulse output of the anemometer into an audio amplifier to a speaker and get audio feedback of your relative wind speed easily. And maybe instead of an anemometer sitting on your hood it could be a front-facing propeller on the grill which would look less silly.
 

Juhahoo

Joined Jun 3, 2019
250
I've gone down this path of thoughts before and I don't see why you couldn't just mount an anemometer on your hood to measure relative windspeed. In my case I wanted relative direction, so the whole anemometer + wind vane assembly sitting on my hood would have looked ridiculous and that's why it never passed the idea phase. But what you want is much simpler. You could probably just run the pulse output of the anemometer into an audio amplifier to a speaker and get audio feedback of your relative wind speed easily. And maybe instead of an anemometer sitting on your hood it could be a front-facing propeller on the grill which would look less silly.
What information this would produce? Audible tone that goes up and down.. It is obvious that tailing will benefit, but method described here doesn't provide any measurable data for such purpose.
The most simple way without any additional sensors is to measure your real time fuel consumption to tell how much less it takes to go the same speed behind another vehicle.
 
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