Vacuum Guage and Digital Fuel Gauge - Fuelly Forums

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Old 02-05-2006, 03:29 PM   #1
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Vacuum Guage and Digital Fuel Gauge

In my intro, SVOBoy asked me to post more info on the vacuum gauge and digital fuel gauge install.







The 2 5/8" Autometer Vacuum gauge is installed in a pod on the A pillar.







I tapped into the vacuum line that runs between the throttle body and the valve on the fuel rail.

When fully warmed up, the car idles at about 650 rpm, 22-23 in/hg. When driving, I try not to drop below 10 in/hg during acceleration and I upshift at 15 in/hg. When going downhill with foot off of the accelerator, the gauge shows 25 in/hg. According to the shop manual, the ecu shuts off the fuel injectors when the throttle is fully closed and the engine rpm is above 870. I try to take advantage of that as much as possible.

Again, during accel I shoot for 10 and upshift at 15 in/hg. In Pittsburgh, it is quite hilly so I find myself approaching uphill climbs in 4th gear at 10 in/hg, then downshifting to third and holding 10 in/hg. Some hills require 5 in/hg to hold speed but I have never had to drop to 2nd gear.

On long downhill stretches I typically shut off the engine completely and coast for 1 to 2 miles. On my drive to work, I have three occasions when I can coast for about 2 miles so this contributes to the 50 MPG tanks. The trip back only has one 2 mile coast. I have spent lots of time optimizing my commute to maximize fuel efficiency. My route is slightly different for the del Sol than for the Prius because, well it would take lots of typing to explain. Suffice it to say that much of my fuel efficiency can be attributed to staying off of the interstate and using the service roads that run along side it. The speed limits on these side roads range from 25 to 45 mph. I do spend about 6 miles of the 25 mile commute on the interstate where the speed limit is 55. This alternate route takes an additional 5-10 each way, but the distance is actually shorter by a fraction of a mile.



Here is a picture of my digital fuel gauge. It is a DMM set to ohms which lives in the center console. In del Sols, you have to remove the rear storage compartments to get access to the fuel pump/sensor. In civics, just remove the back seat. There are two sets of wires, one to the fuel pump and one set to the fuel sensor. I just cut the fuel sensor wires and extended (by splicing more wire) them into the center console, basically adding 2 ft of length. In the middle of the wire extension I cut them and reconnected them using a connector. I also installed a matching connector on the DMM. So I can use the analog gauge in the dash by connecting the two connectors on the extended wires. Or I can use the digital gauge by connecting the DMM to the connector that goes to the fuel sensor.

As you can see from the picture, the resistance is at 2.7 ohms, which is nearly full. At 2.0 ohms, the sensor is maxxed out. I always fill to 2.5 ohms to get the most accurate tank MPG calculation. This is much more accurate than guessing or topping off.



Here is the fuel sensor map that I have developed by filling my tank in 1/4 gallon increments on three separate occasions. I take a reading before leaving work every morning to get an idea of how my tank mileage is progressing. My garage floor is perfectly level so I get a good reading every time. I also use the same pump to fill the tank and the ground is level there also.

So far I am able to use this method to determin e tank MPG and can get within 1 MPG. It ain't perfect, but it helps.

Does anyone have an OZTrip Computer? That would make the Vacuum Gauge and Digital Fuel Gauge unnecessary.

Dan
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Old 02-05-2006, 03:51 PM   #2
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VERY COOL! How cheaply do

VERY COOL!

How cheaply do you think I can get a DMM that will serve this purpose? Also, how small do they come? I'm wondering now if someone more electronically inclined could turn that display into something a bit more self contained (could be mounted on the a pillar or something)...

Very very cool. The line you tapped for the vacuum gauge actually goes from the fuel rail to intake manifold though, if I'm looking correctly.

One of the things that can be done with chipping is to use your CEL light as a shift indicator so that instead of reading the gauge you will see the flash at like 2500 and know to shift.
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Old 02-05-2006, 04:35 PM   #3
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Re: VERY COOL!

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Originally Posted by SVOboy
How cheaply do you think I can get a DMM that will serve this purpose?
if you watch for sales at your favourite parts store, you can get them cheap. i've bought a couple at $9 each over the past few years. the ones i got normally retail for $30-$40 (all figures $CDN @ .85 $US). door crasher type deals.

you could probably eBay yourself a good deal also.

ps - nice work, dan.
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Old 02-05-2006, 04:41 PM   #4
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There are mini ones on ebay

There are mini ones on ebay for 9 shipped, , I'll get one soon once I finish closing down my ecu business.
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Old 02-05-2006, 04:52 PM   #5
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DMM

Harbor Freight they go on sale once in a while and they also give them away for free on $40 orders sometimes. Usually you can get them for $2.99 - I have purchased a lot of them over the years for my scooter projects and once calibrated they hold up pretty well and do not drift much at all. Once I figure out the resistor network inside that is needed to make it function as a 20 volt meter I will be able to reduce the unit down to the display and battery size.
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Old 02-06-2006, 02:25 AM   #6
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Re: VERY COOL!How cheaply do

Quote:
Originally Posted by SVOboy
VERY COOL!

How cheaply do you think I can get a DMM that will serve this purpose? Also, how small do they come? I'm wondering now if someone more electronically inclined could turn that display into something a bit more self contained (could be mounted on the a pillar or something)...

Very very cool. The line you tapped for the vacuum gauge actually goes from the fuel rail to intake manifold though, if I'm looking correctly.

One of the things that can be done with chipping is to use your CEL light as a shift indicator so that instead of reading the gauge you will see the flash at like 2500 and know to shift.
I have 2 more just like it I bought on ebay. I would give you one but it needs a mini 12V battery, which costs more than the DMM costed, about $5 at radio shack. You are probably better off just getting the one on ebay for $9 shipped. Just a few more notes about the digital fuel gauge, you can't use the 12V from the car to power the DMM, you get bad readings. Also, you get bad readings when the ignition or accessories are on. You need to turn the ignition off completely before a repeatable reading is obtainiable. I just take a look every morning before work, because the car is usually cooled to the same temp. As you know, temp also effects resistance readings.
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Old 02-07-2006, 05:20 AM   #7
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Update on Digital Fuel gauge

I just thought I would track this tank on this thread to give an idea what the data looks like and how the accuracy turns out. On Monday morning, 2/6 I was at 2.8 Ohms, 6 miles. This is slightly less than a full tank as i had to drive bach home from the gas station. This morning I was at 10.9 Ohms. Now the first gallon or two is not that linear so the data is probalby off a little. Here is the MPG calculation.

10.9 is 1/3 of the way between 10.5 (1 gallon used) and 11.7 (1.25 gallons used). So I calculate 1 + (.25/3)= 1.08 gallons used. The trip odometer shows 56 miles but I need to add 3.5% for the larger tires which I calibrated over a 50 mile stretch of I -79. So 1.08 gallons used for 56*1.035=58 miles equals 53.7 MPG.

I will be back tomorrow with another update.
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Old 02-08-2006, 04:46 AM   #8
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Re: Update on Digital Fuel gauge

Quote:
Originally Posted by krousdb
I just thought I would track this tank on this thread to give an idea what the data looks like and how the accuracy turns out. On Monday morning, 2/6 I was at 2.8 Ohms, 6 miles. This is slightly less than a full tank as i had to drive bach home from the gas station. This morning I was at 10.9 Ohms. Now the first gallon or two is not that linear so the data is probalby off a little. Here is the MPG calculation.

10.9 is 1/3 of the way between 10.5 (1 gallon used) and 11.7 (1.25 gallons used). So I calculate 1 + (.25/3)= 1.08 gallons used. The trip odometer shows 56 miles but I need to add 3.5% for the larger tires which I calibrated over a 50 mile stretch of I -79. So 1.08 gallons used for 56*1.035=58 miles equals 53.7 MPG.

I will be back tomorrow with another update.
End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG

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Old 02-11-2006, 11:09 AM   #9
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Re: Update on Digital Fuel gauge

End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG

Two days sick, no driving.

End of day #3, 163 actual miles, 20.6 Ohm, 3.125 gallons, 52.3 MPG
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Old 02-11-2006, 11:17 AM   #10
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Looks like a good tank so

Looks like a good tank so far.

I just found a DMM in my basement, think it's my teachers but I wanna hook it up before I return it to make sure I can get the hook up working well for when I buy my own DMM (it's only 9 bucks after all).

Do you have any pictures of how it's wired into the fuel sender thing?
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