69DartDave
Well-Known Member
Retrofitting EFI into a slant6-powered '69 Dart has been mostly fun, but also a little frustrating. Frustrating due to some unintended consequences, but I hope that others can use this information to learn from and maybe apply to their own projects.
The biggest change (other than the 2" hole you gotta drill in the firewall to route the new wiring harness through) is the fuel system, and my son and I knew that before making the decision:
The car came with a 5/16" diameter fuel feed line. Which is great, since you can re-use that as the return line.
The rub is: the fuel tank sending unit. If you want to re-use it, you technically could for a slant-six powered Mopar. Just put an 5/16-to-3/8 adaptor just outside of the tank to neck upsize the original line before the fuel filter. The fuel flow rates for a slant six are low enough to be fed by a 5/16 line, so long as the entire run isn't 5/16. Then, add a new return line that terminates at, say, the fuel tank fill tube (there are aftermarket fittings to help do just that).
But that's not what we did. We bought the only off-the-shelf sending unit I could find that has both a 3/8 feed and 5/16 return line built-in to the bulkhead. Which I thought was great. Maybe I should have looked around more? It was, after all, listed in RockAuto's web site as fitting a 1969 Dodge Dart 3.7L car. But the Ohm range is advertised as being different than stock, which I didn't know enough at the time to pay attention to. Who knows what the stock range is? (yes, I should have researched it more)
Bottom line: The stock fuel gauge is now going to be WAY off.
How much? Well, I did some instrument testing of the stock sending unit and the new sending unit, as well as estimated the height/volume relationship of the fuel tank itself (not linear due to not being a perfectly rectangular box). When the gauge first reads empty, there's still going to be about 7 gallons present. :???:
Here are my findings. Skip the first two plots, unless you're a nerd like me.
The conclusion is that I need to replace the stock fuel gauge with an aftermarket one that is properly calibrated to the new sending unit. That is do-able, but something I'd rather not have to mess with. Live and learn (and share the story).
The bottom-most plot is pretty cool; the original calibration has been reverse-engineered here to prove that Dodge did a good job with the stock unit providing a nice, linear full-to-empty gauge reading.
Anybody know who makes a fuel gauge that's calibrated to 10 ohms full and 80 ohms empty?
The biggest change (other than the 2" hole you gotta drill in the firewall to route the new wiring harness through) is the fuel system, and my son and I knew that before making the decision:
- You need a bigger feed line diameter, a 3/8".
- And you need to add a new fuel return line.
The car came with a 5/16" diameter fuel feed line. Which is great, since you can re-use that as the return line.
The rub is: the fuel tank sending unit. If you want to re-use it, you technically could for a slant-six powered Mopar. Just put an 5/16-to-3/8 adaptor just outside of the tank to neck upsize the original line before the fuel filter. The fuel flow rates for a slant six are low enough to be fed by a 5/16 line, so long as the entire run isn't 5/16. Then, add a new return line that terminates at, say, the fuel tank fill tube (there are aftermarket fittings to help do just that).
But that's not what we did. We bought the only off-the-shelf sending unit I could find that has both a 3/8 feed and 5/16 return line built-in to the bulkhead. Which I thought was great. Maybe I should have looked around more? It was, after all, listed in RockAuto's web site as fitting a 1969 Dodge Dart 3.7L car. But the Ohm range is advertised as being different than stock, which I didn't know enough at the time to pay attention to. Who knows what the stock range is? (yes, I should have researched it more)
Bottom line: The stock fuel gauge is now going to be WAY off.
How much? Well, I did some instrument testing of the stock sending unit and the new sending unit, as well as estimated the height/volume relationship of the fuel tank itself (not linear due to not being a perfectly rectangular box). When the gauge first reads empty, there's still going to be about 7 gallons present. :???:
Here are my findings. Skip the first two plots, unless you're a nerd like me.
The conclusion is that I need to replace the stock fuel gauge with an aftermarket one that is properly calibrated to the new sending unit. That is do-able, but something I'd rather not have to mess with. Live and learn (and share the story).
The bottom-most plot is pretty cool; the original calibration has been reverse-engineered here to prove that Dodge did a good job with the stock unit providing a nice, linear full-to-empty gauge reading.
Anybody know who makes a fuel gauge that's calibrated to 10 ohms full and 80 ohms empty?
