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
6,460
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.
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.
That's why I've pierced the back side. I've tested the output on my scope by blowing air into the opening I created. It works.
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.
Actually, a MAF could work as well.
"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."
This I know. However, there's a fundamental difference between a MAP and a MAF.
The Aerodynamic-Drag on the Body of the Truck can fluctuate wildly,
depending upon the constantly changing environment, and Speed.
Again, this is something I'm well aware of. Which is why I want to know in real time what the effects of another vehicle are. As some are suggesting, using the fuel gauge doesn't provide the real time feedback I'm looking for. Yes, hills, gusts of atmosphere (wind), tire air pressure, all conspire to create an overall picture of performance. It's not the overall I'm looking for - it's the moment by moment indication of how another vehicle is affecting my vehicle.
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.
Doesn't have to be a computer fan. I have a bunch of DC motors scrapped out of old tape decks and other odd sources of motorification.
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.
I thought of this but building the Venturi means having an exit point for the airflow, which can change how things look.
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.
The available data via the mileage indicator is not sensitive enough to give me the real time indication of what's going on.

On the same road day after day mileage can vary from many factors. Traffic, speed, headwind versus tail wind or cross wind etc. all can change the reading. So I get an average readout by the end of the tank, which can indicate that during the last few weeks I encountered several differing factors that came into play for fuel economy.

I'm not looking for a way to increase gas mileage. It is what it is. I'm most curious about the effects of another vehicle directly in front of me. Not looking for "Meaningful" data, just a fun way of indicating what happens when you ride behind a big rig. Yes, I know I will get better mileage. But often times there's a cross wind which can drift the vacuum behind the rig to the side. If you're not in the vacuum you're not benefiting from it. Sometimes just sitting off to the side can be of benefit. Knowing where the air is going likewise helps to "Know" this. Benefiting from the knowledge is a different thing.

Again, accuracy be damned. This isn't a serious project, just an elementary school science project. Like what happens when you mix red and yellow food coloring in a cup of clear water. Just something for me to do besides going in the woodshop and sanding yet more drywall mud, then having a ton of baby powder all over everything in the shop. Yesterday I cut some wood for the railings around my deck. Today I'm going to be nailing those pieces so I can pass inspection (whenever that happens before mid October).
 

strantor

Joined Oct 3, 2010
5,695
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.
I think the point of the OP wasn't anything to do with measurable data. Just a real time indicator of relative wind speed.

In my case, I would have a readout of wind speed and direction.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
the whole anemometer + wind vane assembly sitting on my hood would have looked ridiculous
What I have in mind is an inlet port behind the grill. Nothing on top of the hood. However, if you want to make such a device then I'd recommend a magnetic mount using a tripod arrangement. Just be sure to NOT place magnets directly on the body of the vehicle. A thick piece of card stock will protect the paint from scratches. Then once the data has been gleaned removal is simple as pulling it off the hood. Or roof, or trunk, wherever you decide to place it.

Personally I think it's a fun project. More of an "In The Know" type thing than anything of practicality.
 

Juhahoo

Joined Jun 3, 2019
250
On the same road day after day mileage can vary from many factors. Traffic, speed, headwind versus tail wind or cross wind etc. all can change the reading. So I get an average readout by the end of the tank, which can indicate that during the last few weeks I encountered several differing factors that came into play for fuel economy.
Okay, I thought this was serious study of the benefits, not just for "fun".
I have built once such system that outputs fuel consumption in real time and that is much more fun to study what actually effects the fuel consumption and economy, in various situations, gears, wind, tailing, driving habits in generally. Accuracy of the measurement was around 0.5%
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
Okay, I thought this was serious study of the benefits, not just for "fun".
Just to be clear, I don't want to have to look at a gauge while driving. An audible tone and its change can be as effective in giving a real time indication of conditions. But then again, an audio tone buzzing constantly can become irritating. So having a switch to turn it off would be also in the consideration of things.
 

camerart

Joined Feb 25, 2013
2,762
Hi T,
If you watch smoke in a wind tunnel, there are spots where the pressure is greater, or even perhpas a vacuum, so placement of your intake to average this out at different speeds may be critical.

I would adapt the #24 idea, to give you an all round wind effect, and perhaps add a sound.
Cheers, Camerart
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
Normally the MAP sensor has just one air port - for the vacuum in the manifold. I've pierced the back side of it and with a power source hooked up, I've scoped the output with no pressure or vacuum applied. Then blowing on the hole I made the signal deflects from 0.5V to 4.5V, depending on how hard I blow on it. There's a raised nub on the back side about 3/8 inch diameter sitting about 1/8 inch proud of the rest of the body. I suspect I could probably poke a 3/8 inch hose onto that and then channel that hose to the grill. Yes, I know air pressure (or vacuum) will be different at differing locations. That's OK.

Let's assume a scenario where I'm on the highway and I'm hearing a tone roughly 1kHz. As I approach another vehicle, the tone increases (the 555 circuit I am considering changes its frequency to the higher when I apply a voltage to pin 5). Suppose it increases to 2kHz. A definite change that can be heard. Now suppose I approach a big rig. Pressure drops, voltage drops and the frequency goes even higher (or lower, haven't figured that part out just yet. Testing will prove which way the signal goes. Choice of the RC circuit will make a difference in what audio tone I'm listening to.)

From the change in tone and the degree to which it changes I can determine where the best place to ride is. Smaller vehicles pull less vacuum behind them, larger ones pull more. Big rigs will pull a lot of vacuum. I once heard a story - don't know if it's true - of a bicyclist who rode behind a big rig at highway speeds because of no wind restriction. Highway speeds could have been as low as 55 MPH - and the story could be complete BS. Nevertheless, the point is that behind the rig there's less wind resistance (vacuum relative to ambient static pressures). Discovering those changes will prove to be an interesting project when I get it built. And changing the orifice size will also be a matter of experimentation. Too large an orifice and I could be at full deflection by 25 MPH. Then again, they say that wind speeds produced at a human's lips can be over 200 MPH when blowing as hard as one can. I've never measured such things, only heard these to be somewhat true. Regardless, it's an experiment only. Just to see if what I suspect is true - that not always riding behind a big rig will be beneficial with a cross wind. I've watched rain kicked up by trucks on the highway and noticed how quickly it can be blown to one side, depending on cross winds. So somewhere to the side is more likely the best spot during such winds. In calm weather, directly behind would probably be the best. This will prove all that out. I may even make a video and post it on YouTube. But don't expect this to happen soon, I still have an addition that needs to be completed before mid October. I AM busy with other things. And the wife wants to take a vacation too.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
Found this video on YouTube.
Suggests what we all expect, better mileage behind a big rig. However, again, in cross wind conditions - that sweet spot may be not directly behind the truck.
 

BobTPH

Joined Jun 5, 2013
4,051
Brings back memories of my trips between home and grad school. This was during the oil "crisis" in the early '70s. I saved gas by drafting behind the big rigs on the PA turnpike and I70. It did up my mileage significantly, but I abandoned it because the added stress of paying much closer attention to my driving that was very tiring over the 600 mile trip.

Bob
 

LowQCab

Joined Nov 6, 2012
1,343
Notice that they are using the calculated Grams of Fuel per Minute vs Speed.
They are measuring the average "Power" required to
overcome the Drag generated by moving through Air.
They are not trying to measure "Air-pressure" at a specific point.
.
.
.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
abandoned it because the added stress
I remember those 70's. Used to live just down the street from a gas station. Selling lemonade to those in line for gas. So. Cal. Odd days gas for license plates ending with an odd number; even for even plates. Those with odd plates had the advantage. Every month started with an odd day (1) and at least seven months ended with an odd day. Leap year added a eighth month ending with an odd number.
 
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Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
Notice that they are using the calculated Grams of Fuel per Minute vs Speed.
They are measuring the average "Power" required to
overcome the Drag generated by moving through Air.
They are not trying to measure "Air-pressure" at a specific point.
Yes - I understand that. But in each case she and her father drove for 3 minutes behind the rig. It looked like the roadway was flat, but there could have been slight up-hills and down. An air pressure change, regardless of the hills will indicate a change in milliseconds. Having the sensor determine wind resistance will respond immediately. And likely I'll also see at what point behind the rig I hit turbulent air versus a cleaner vacuum. I've driven a van behind rigs and notices significant buffeting from side to side. Felt like a flag waving in the breeze. Fuel mileage will not respond quick enough.

Remember, this is a curiosity project, not something that's going to result in better fuel mileage. Like Bob said, tailgating a truck can be quite stressful. I know this to be true too. I've been there. Done that. Had low fuel and had to get to the next gas station. That can add stress as well. Having just two gallons of gasoline in the tank on a vehicle that gets 18 MPG and the next gas station is nearly 40 miles away - - - you WILL tailgate anything you can. Or slow down significantly.
 

LowQCab

Joined Nov 6, 2012
1,343
The Drag generated from pushing the body through Air,
with no Wind, and no other vehicles, creates a totally predictable, smooth curve.
Add cross-winds, and the tremendous turbulence behind a Tractor-Trailer and
You wind up with wildly varying pressures and Air-Speeds, and Air-directions
that cover an area up to ~15-feet square, or more,
all of those forces must be averaged.
You might be able to do that with 15 X 15 = 225 individual Pressure-Sensors,
but using the data from only one of those sensors will just result in
random crazy numbers that are not useful for determining anything except what
might be happening in one particular small area near the Sensor.
2-feet away from the original Sensor location could result in a
totally different, or even opposite measurement.

Any of the Crew-Chiefs of the various NASCAR Race-Teams
would "sell their Mother" to get the "Real-Time-Data"
that You imagine that You will have at your disposal.
They've got Millions to spend,
and an Army of enthusiastic genius-minds working for them.
They have Sensors on every part of the Car continuously relaying
Data back to the Pits via high-speed radio transmitters.

They don't bother with measuring the "Air-Pressure" in front of the Car.
.
.
.
 

shortbus

Joined Sep 30, 2009
8,966
They don't bother with measuring the "Air-Pressure" in front of the Car.
They also don't use a "mileage minder" or what ever you called it earlier in the post, like the old time vacuum gauge. Those had nothing to do with mileage other than telling you when you had the throttle open too far. They were though good for monitoring the health of the engine its self, but really nothing to tell you the load on said engine.
 

LowQCab

Joined Nov 6, 2012
1,343
They didn't directly tell You how much Fuel was being burned,
but they are a direct indicator of Engine-Load, by definition.
.
.
.
 

strantor

Joined Oct 3, 2010
5,695
I remember those 70's. Used to live just down the street from a gas station. Selling lemonade to those in line for gas. So. Cal. Odd days gas for license plates ending with an odd number; even for even plates. Those with odd plates had the advantage. Every month started with an odd day (1) and at least seven months ended with an odd day. Leap year added a eighth month ending with an odd number.
This reminds me of when I lived in Manila. They have a "coding" law where depending on your license plate # you can only have your car on the road even or odd days.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,460
using the data from only one of those sensors will just result in
random crazy numbers
Not looking to gather data. Just want to hear an audio tone fluctuate with the varying conditions I'm passing through at that moment.
an Army of enthusiastic genius-minds working for them.
I've seen genius' miss the obvious stuff because they tend to think big. Really big. So big that they overlook the simplest solutions sometimes. A truck got stuck under a bridge. Engineers and other equipment was there trying to figure out how to get it out. Some KID on a SKATEBOARD said "Let the air out of the tires." Nobody thought of that before the kid said anything. They were too busy calculating forces and angular momentum to figure out how to get the truck out.

NASCAR - ever see debris get stuck on the grill of the lead car? They tailgate someone to suck that debris off. I'm sure if they had a single air pressure sensor on the nose at that moment they would know just how things change when following someone that close. It's also the main reason why engines overheat when bump-drafting; no air flow. And when three, four, five or more get together nose to tail EVERYBODY picks up speed. The practical translation of that would be "everybody gets better gas mileage". But on a race track they're not interested in going slower to get better fuel economy, they want to be the fastest on the track.

At an IROC race Baby Al Uncer developed quite the lead. The rest of the pack formed a freight train and ran him down like nothing. He threw the block and he got knocked off the track. Overall, there's lower pressure on the nose of a car when it's that close. Whether using one sensor or 225 sensors mounted all over the grill, the consensus is going to be lower air pressure on the nose results in greater speed - or better economy.

I'm not looking for data. I just want to see how things change with vehicles pulling me along and when I'm the one pulling everyone. On the track, the freight train scenario - the lead car uses the most fuel. The tailing car uses the second most fuel while everyone in the pack is getting roughly the same fuel mileage. I don't want "Data". Just curios to know how the pressure on the nose changes with varying driving conditions. Even varying my own speed will cause variation in the pressure. But I am not looking for data. If I wanted data I'd book time in a wind tunnel with colored streams of smoke passing under and over the car. Sensors galore. Wind sock tape on the deck of the engine hood (bonnet) to show which way the wind is traveling under varying conditions. But like - was it Strantor who said he thought about an anemometer and weather vane on the hood? Wind sock tape would look somewhat silly too. But a pitot tube in the grill, mounted most centrally should give a decent indication of how things are changing.
 

strantor

Joined Oct 3, 2010
5,695
Not looking to gather data. Just want to hear an audio tone fluctuate with the varying conditions I'm passing through at that moment. [...] I'm not looking for data. I just want to see how things change with vehicles pulling me along and when I'm the one pulling everyone [...] I don't want "Data". Just curios to know how the pressure on the nose changes with varying driving conditions. [...] But I am not looking for data. If I wanted data I'd book time in a wind tunnel
Well, sounds like you're after some serious data! I think the only solution to this problem is a pressure sensor off a Mars Rover. Their output seems to lend itself well to audio conversion too (once the numbers have been crunched)

"Sounds from InSight's Pressure Sensor on Mars – NASA's InSight Mars Lander" https://mars.nasa.gov/resources/22204/sounds-from-insights-pressure-sensor-on-mars/?site=insight

On a slightly more (but not totally) serious note, could a simple microphone be used? Rectify its output, filter, amplify, turn into a tone. It would measure turbulence rather than pressure but I assume turbulence will coincide with with pressure? Unless there is some laminar air flow thing I don't know about.

When I was interested in this before, I did crude tests by simply rolling down my window and driving closer and closer until I could hear that I was in the low pressure pocket.

I stopped this because I ran over a tire that a big rig passed right over and didn't have time to react. No serious damage, could have been much worse. Lesson learned.
 
kudos lowqcab and a few others for your attempts at clarity, it seems to fall on deaf ears. there are a lot of ideas here but none are accurate and precise-well this is something the OP cares nothing about. it seems what is desired is a project to make a toy that makes noise relative to quickly changing air speeds somewhere at the front of a tailgating vehicle. a three wire computer fan offers two variable signals easily fed to an audio op-amp. Puncturing the rear of the map sensor effectively kills any accuracy it ever had. map sensor response time is slow which is why most efi systems went with maf. NAPA is wrong, a map sensor does NOT measure the airflow into an engine, it measures pressure on the absolute scale and does so slowly. the best option for reliable data is a scan tool that can display instantaneous fuel consumption. there are bluetooth and wifi obd2 modules that can connect to smart phones and laptops without a cable dangling near your feet for under $50. most are packaged with software that allows customized displays easily viewed at a glance such as round gauges or vertical and horizontal bar graphs.
the entire experiment is flawed, however, since the required distance to the vehicle in front necessitates tailgating which is dangerous, illegal and stupid.
 
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