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  #1  
Old 05-07-2010, 12:57 AM
rider rider is offline
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Default DIY fuel-economy-o-meter

I recently drove a Leagacy Turbo-4 6 speed manual. I noticed that like my buddies M3 it had one of those fuel-economy gauges on the panel.

I kinda want one, just for funzies. I was also thinking that it wouldn't be too hard to make one. An electronic gauge that displays by voltage connected to some piece of the fuel system. Even a pyrometer with a custom gauge face could work.

My question is, if possible, what piece of the fuel system could I go off of?
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Old 05-07-2010, 01:04 AM
ratfrink ratfrink is offline
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Default Re: DIY fuel-economy-o-meter

Air flow meter? Dunno. I'm not too hot on vehicle electrics tbh.

The old ones work with just a vacuum switch on the intake manifold. My 'Yota had one that said 'ECONOMY' and had a green light when cruising and a red light when you put the hammer down. Real useful Toyota, I'd never have figured that out on my own! We used to joke that it knew the state of the world's economy. Back in the day you could get ones with a dial on them, but they were never particularly accurate.
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Old 05-07-2010, 01:06 AM
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Default Re: DIY fuel-economy-o-meter

Quote:
Originally Posted by rider View Post
I recently drove a Leagacy Turbo-4 6 speed manual. I noticed that like my buddies M3 it had one of those fuel-economy gauges on the panel.

I kinda want one, just for funzies. I was also thinking that it wouldn't be too hard to make one. An electronic gauge that displays by voltage connected to some piece of the fuel system. Even a pyrometer with a custom gauge face could work.

My question is, if possible, what piece of the fuel system could I go off of?
To clarify, something that states howe many mpg you're getting?
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  #4  
Old 05-07-2010, 01:10 AM
FireStarter FireStarter is offline
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Default Re: DIY fuel-economy-o-meter

If the car has an OBD-II port, there are units that plug in and give you MPG for some models of cars such as this: http://www.thinkgeek.com/gadgets/car/8426/
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Old 05-07-2010, 01:12 AM
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Default Re: DIY fuel-economy-o-meter

Quote:
Originally Posted by Virus View Post
To clarify, something that states howe many mpg you're getting?
Yeah, they have them in a lot of newer cars, the mpg changes according to how fast you accelerate, cruise control, etc etc. Like an engine temp gauge but with mpg.
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  #6  
Old 05-07-2010, 01:14 AM
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Default Re: DIY fuel-economy-o-meter

Quote:
Originally Posted by Virus View Post
To clarify, something that states howe many mpg you're getting?
Not nessisarily exact, I was hoping for a mechanical gauge, not a digital readout, that would kind of ruin the spirit of the Miata gauges.
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Old 05-07-2010, 01:15 AM
Dr rocker Dr rocker is offline
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Default Re: DIY fuel-economy-o-meter

I remember a thing you could get in the mid '80's that would do this - it had a digital display and could be plumbed into most cars. It was asked about in a classic car mag but they had no answer on it. I will have a look and see if I can find it.
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  #8  
Old 05-07-2010, 01:20 AM
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Default Re: DIY fuel-economy-o-meter

I'm also not sure if that thinkgeek thing will work because I'm running a megasquirt, not the stock computer.

I asked on a MS specific forum.
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  #9  
Old 05-07-2010, 01:20 AM
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Post Re: DIY fuel-economy-o-meter

Get a scan gauge it plugs into your OBD2 connector.
http://www.scangauge.com/products/
I see these things all the time,fucking car nerds who "THINK" they know about cars
Just the other day I had a guy come in from an other dealer telling me he had a sticking right rear caliper on a 2010 MZ3 speed.
I'm like MK what makes you think that?He then proceeds to tell me he is a graduated from MIT and is a engineer,I'm like---- so?
Then he produces from his glove box a infrared digital thermometer,The meter reads a whole FIVE degrees warmer on the right rear rotor than the other three!!
I had a hard time being polite and showed him the door. Then I knew why the selling dealer told him too GTFO.

Last edited by AngryOnion; 05-07-2010 at 01:33 AM. Reason: added a story no one cares about
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Old 05-07-2010, 04:10 AM
mrparks mrparks is offline
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Default

^Should have sold him a 5mm shorter pad.

I can empathize with him though. Sometimes the most random things drive you up the wall. Plus, it would have been a great story finding out how he thought to take the rotor's temperature.
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  #11  
Old 05-07-2010, 11:23 AM
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Default Re: DIY fuel-economy-o-meter

Looks like I can't use a scangauge, since I have a megasquirt, and it doesn't output OBDII codes.

So I guess it's back to my origional idea, some controller that interprets electrical signals and changes a gauge. Any thoughts on this? I was thinking of using a cheap chinese water temp gauge and replacing the face, but I still have yet to think of somewhere I can pull a signal from. Something on the fuel system, because of how it works, but I don't know where.
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  #12  
Old 05-07-2010, 04:25 PM
ratfrink ratfrink is offline
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Default Re: DIY fuel-economy-o-meter

Does the fuel pump draw more voltage when you press the accelerator, or does it run at a constant voltage? I think it's the latter unfortunately.
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  #13  
Old 05-07-2010, 06:43 PM
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Default Re: DIY fuel-economy-o-meter

Since you have megasquirt, there might be some way to take the injector pulsewidth and factor it by the vehicle speed sensor data.. You could probably get a PIC microcontroller to do the job.


Quote:
Originally Posted by ratfrink View Post
Does the fuel pump draw more voltage when you press the accelerator, or does it run at a constant voltage? I think it's the latter unfortunately.
Most of the time it is the latter. There are some 2 speed (and thus 2 voltage) pumps, and a few completely variable voltage pumps around.. but they're on newer vehicles than his Miata.
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Old 05-07-2010, 08:08 PM
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Default Re: DIY fuel-economy-o-meter

In most older cars the economy gauge is just a mechanical pressure gauge connected to the intake manifold. The principle behind this is that with rising manifold pressure, your fuel consumption also gets higher.
I think in your case you could also use a boost gauge for this purpose (one that also indicates manifold vacuum). They are available from many car tuning shops, in a lot of different styles, so it would match your car's style.
To connect it you probably just have to connect a little vacuuum tube from the gauge to you intake manifold, but make sure you connect it somewhere between the trottle body and the engine, not before the throttle body.
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Old 05-07-2010, 11:10 PM
MasterYoda210 MasterYoda210 is offline
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Default Re: DIY fuel-economy-o-meter

I'm no car electrics expert by far, but to get a rough reading, couldn't you piggyback a sensor onto the fuel pump (assuming it's electrical)?

Like if you floor the car, obviously the fuel pump will need to deliver more fuel, and thus some electrical current tells it to pump harder?

I know my dad's Integrale has a fuel pressure gauge, which goes up when he boots it, so maybe convert the reading of the fuel pump pressure into rough MPG?

It will probably be extremely rough like.

Just an idea.

EDIT: Just noticed Frink suggested that. Awwww.....

Last edited by MasterYoda210; 05-07-2010 at 11:13 PM.
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  #16  
Old 05-08-2010, 01:03 AM
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Exclamation Re: DIY fuel-economy-o-meter

YodaMaster, most fuel systems run the pump at high warp at all times.
The pressure regulator on the fuel rail regulates the volume of fuel too each injector.
Instead of Piggybacking the pump I recommend piggybacking each fuel injector.
Injectors run on a duty cycle that can be easily read-a small inductive probe on each injector harness.
Fuel pressure is known-size on orifice on each injector is known--duty cycle is known-- The next step is to verify the volume of fuel moved through the system.
Combine all this info with an odometer reading then you can figure out Miles Per Gallon.
Or you could just keep track of the amount of fuel you use and how far you have traveled.
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Old 05-08-2010, 01:23 AM
rider rider is offline
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Default Re: DIY fuel-economy-o-meter

Quote:
Originally Posted by angryonion View Post
Or you could just keep track of the amount of fuel you use and how far you have traveled.
I knew that was coming .

According to specs websites, the Miata tank is 13.1 gallons, or something like that. When mine is past empty, I've put 9.98 gallons in it. At most. My gauge is completely incorrect, and I don't feel like sitting on the side of the highway because I've run out of fuel .
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  #18  
Old 05-08-2010, 04:12 AM
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Default Re: DIY fuel-economy-o-meter

Since you're running a Megasquirt, can you not just monitor injector pulse width information off the Megasquirt (say, over the RS232 interface). If you know this, the lbs/hr the injectors flow, and the fuel pressure, you should be able to calculate how much fuel you're using at any given time. Do this on something small and cheap (like a beagleboard) and then start working on how to drive an analogue gauge from it.

Of course, that would just give you fuel usage over time. You need speed/location data to actually get MPG. You could find some way to capture speedo data from your computer. I think it would be easier to just get a GPS receiver. The GPS will give you location and velocity data once every second. Use the velocity data over the last second to give you instantaneous MPG or use the location data to calculate distance traveled over time and divide by fuel usage over time to get an average.
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Old 05-08-2010, 11:53 PM
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Default Re: DIY fuel-economy-o-meter

Can you people see my posts? I suggested monitoring the injector pulsewidth to get fuel usage data, and I suggested the VSS output, which is usually going to be 10000 pulses per mile, a relatively easy and free data source for your homebrew gauge controller. I also said that the majority of fuel pumps are single speed, and that only a few newer vehicles that have variable voltage pumps.


Last edited by MediumD; 05-09-2010 at 02:15 AM.
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Old 05-09-2010, 01:42 AM
Animal Farm Pig Animal Farm Pig is offline
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Default Re: DIY fuel-economy-o-meter

Quote:
Originally Posted by MediumD View Post
Can you people see my posts? I suggested monitoring the injector pulsewidth to get fuel usage data, and I suggested the VSS output, which is going to be 10000 pulses per mile, a relatively easy and free data source for your homebrew gauge controller. I also said that the majority of fuel pumps are single speed, and that only a few newer vehicles that have variable voltage pumps.

Doh! Sorry. After a bunch of discussion about fuel pumps, I just kept skimming through.

I agree with the solution you suggested before me.
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