Any ideas on how to accurately measure mpg’s on an older car with a carburetor?

[user removed]

Originally Posted by ncwa

So I couldn’t find any recent posts on this topic so I thought I would ask – does anyone know of an accurate means to measure fuel flow? I can’t use any of the modern Scangauges as I have a ’66 with a carburetor and an analog tach. I have been making a lot of mods and I just don’t think the pump gauge is cutting it unless I take a real long drive and get the same pump. The less expensive boat fuel meters seem to be discontinued and I can’t justify buying on of the $400 versions. Thanks for your help!

1966 model would have virtually no inspection restrictions, at least here in Virginia.

You might consider using a burette and a t connection into you existing fuel line at the point where the rubber hose connects the metal supply line to the fuel pump (assuming it is mechanical). You would also need a means to disconnect the main fuel supply and switch to the burette (mechanical cable to a switching valve).

This would give you very precise measurement of fuel delivered to the engine, but in a limited quantity depending on the volume of the burette. You would cap the burette with a vent fitting that was directed to outside air, or just place the burette outside of the passenger compartment in one of several places. The burette should be of a plastic type, not glass, and be impervious to your fuel, with no open top that could spill the fuel.

regards
Mech

[MetroMPG]

A compromise between finding & installing a fuel flow based meter (instant feedback) vs. tank-to-tank MPG calculations (feedback may be too slow for those trying to evaluate mods) is to calibrate your fuel tank sending unit and effectively create a "trip gauge".

- With your tank nearly empty, connect a digital multimeter to the fuel tank sending unit (measuring resistance)

- Add a small measured quantity of fuel (say .25 gal)

- Note the change in the multimeter reading

- Repeat until tank is full (multimeter reading stops changing with added fuel)

Now you have a graph with decent resolution of fuel level vs. sending unit resistance, which you can use to figure out fuel consumption. (Assumes you park in the same spot as when you calibrated the sending unit... and the sending unit isn't sticky etc. etc.)

[user removed]

Metro, I did just that with my brothers 91 Civic hatch to do some testing on a practically deserted road near his house. Although the OP would not have that problem the loop return fuel system added heat to the tank as the return fuel was warmed by engine heat. On one trip I went 9 miles and when I got back the reading had increased instead of decreased!

Once I figured it out and learned to compensate I was getting close to 90 MPG doing 15 to 35 P&G with engine off glides. No lights or traffic made it easy.

regards
Mech

[Superturnier]

Those old Zemco units (I have one) can be inaccurate in certain situations.

The fuel flow sensor has an optical sensor that counts the circulation of a small ball in fuel flow. It also counts the bubbles in fuel and if you have a mechanical fuel pump it typically creates bubbles when hot.
Especially when the engine is hot and you're idling at traffic lights the mpg reading goes nuts. But during cruising it can be quite accurate.

Sometimes I have played with a thought that how the MPGuino could be used in carbureted vehicles. At least it would be nice to see the differences in A-B-A testings when you have done some mods.

So, my idea is not to measure the fuel flow, but instead we measure the load of the engine. That could be done with a Manifold Absolute Pressure sensor, which typically give 0-5 volts according to the pressure.

If we can tell the MPGuino that "Manifold vacuum x rpm = engine load" then I think we should be able to see if certain mods will have less load on the engine. But of course we cannot see the real fuel consumption, since the carburettor's fuel delivery is not absolutely linear with engine load.

And to make MPGuino understand the vacuum signal, it would require changing the code or changing the 0-5Volts to PWM (pulse width modulation) and also we should input also the rpm info.

One other option would be using a MAF sensor, which measures the air entering to the engine. Then the rpm information would not be needed, I guess.

[ncwa]

Thanks for all great feedback. I am going to try to find a burette for my idle / regen / vacuum adjustments that I can do while parked in my driveway. Depending on the final setup, I may or may not use it for actual trips. I’ll post a pic if I get it working.
I also like the ideal of calibrating my fuel gauge by adding small increments of gas and measuring the sending unit’s resistance.

Has anyone tried mounting only the bottom of a burette to the fuel line to use it as a visual fuel gauge (like you might see on older water tanks)? Although MetroMPG’s method sure seems safer – and thank you Old Mechanic for the tip to this method.

Slowmover – it is a 66 Mustang with a straight 6 (200ci) engine and a brushless DC electric motor.

Thanks again everyone!

NCWA

Depending on the car there are (or were) spark timing adjustment boxes (MSD, and others) where one can advance or retard ignition timing. With the above gauges (and maybe a knock sensor) can can "lean out" the engine for best cruise parameters.

[ncwa]

Hey guys - after playing around with a few options I found an interesting data point. I burn 1 oz. of fuel each minute at idle! That is 0.47 gallons per hour. One way to look at it is that I need to be rolling along at ~14 mph while idling if my target goal is 30 mpg, which is a challenge in my 1966! I have no idea how some of you are getting close to 60 mpg’s!

[TheTestPilot]

Greetings Gentlemen
I have seen this & other posts about flow sensors for carbs with the Mpguino ,I have been tesing various devices over the last few months(talking cheap here),the current device I am testing is showing some promise.This sensor (2-30 litres/hour flow rate) is available from Futurlec.com at US$13.00 but needs modding to make it's signal compatable .
So far the results are looking ok , but more mods are needed to the sensor to at least stabilise the readings
I will try to add some photos & better explanation tommorrow.

Regards
Tp

[slowmover]

Originally Posted by TheTestPilot

Greetings Gentlemen
I have seen this & other posts about flow sensors for carbs with the Mpguino ,I have been tesing various devices over the last few months(talking cheap here),the current device I am testing is showing some promise.This sensor (2-30 litres/hour flow rate) is available from Futurlec.com at US$13.00 but needs modding to make it's signal compatable .
So far the results are looking ok , but more mods are needed to the sensor to at least stabilise the readings
I will try to add some photos & better explanation tommorrow.

Regards
Tp

Welcome, and thank you, hope to see more. The "trend" of agricultural or other small engines into vehicles would benefit by this also. Was discussing this kind of problem with a power equipment repairman the other day.

.

[TheTestPilot]

Thanks slowmover
here are some photos of the flow sensor ,it uses a single magnet (black dot) on the internal paddle wheel ,the other black dot is steel counter weight.
Externally the magnet sensor is hall effect device,in in original form the gph reading is extremely erratic , so far I have added another hall effect device externally & this improves the gph stability quite a lot ,I want to add a third device on top & see what we get.
From my bench testing of the mpguino it nees at least three magnet sensors to show smooth gph readings.

Tp