Generating a BSFC graph using OBD-II data

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May 7, 2004
Nokesville, VA
I was just playing with the enhanced Ford data items that Alex Peper's OBD2 Vehicle Explorer supports and two of the enhanced data items that my 1996 Ford Contour supports are: fuel injector pulsewidth in milliseconds engine torque into torque convertor in ft/lbs (it has a manual transmission but the data is still there and the numbers went up as I loaded the engine by letting the clutch out while applying the brake so I think it's correct). and, of course, it supports engine RPM. It seems to me that I have all of the data right here that I need to make a BSFC graph for the engine in this car. Fuel consumption, and with a little math involving the torque and engine speed, horsepower. Now I just need a dyno so I can apply a constant load to the engine... [Frown] But maybe I can get something valid just driving it around and having it log those three data items and then load it into Excel? I'm not sure. And, of course, it all depends on how accurate that engine torque PID is. I'll start by getting that reading at highway speed and converting it into horsepower and see if it seems reasonable. What is it, 20HP to move a vehicle at highway speed? Could also answer the question of how much HP an a/c compressor takes...
Do you have a vacuum compensated fuel pressure regulator? That will need to be accounted for. If you look up your drag coefficient you can probably google the math you need to calculate the HP to push your car through the air at speed. Drag from the tires and bearings will be secondary.
I don't think the fuel pressure regulator will be a problem. In fact it'll keep things simple: My understanding of the purpose of the vacuum fuel pressure regulator is that it is there to keep the amount of fuel injected per unit of time constant no matter the intake manifold vacuum. At higher intake manifold vacuum, the fuel pressure regulator drops the fuel pressure because the high intake manifold vacuum means the injectors would flow more (the vacuum "sucks" the fuel out of the injectors more). At zero intake manifold vacuum, that is, wide-open-throttle, the fuel pressure regulator should increase the fuel pressure more. And, I suppose, for forced-induction applications where the intake manifold can actually become pressurized, it should increase the fuel pressure even more (since the intake manifold pressure restricts the flow of fuel out of the injectors). Of course I could be mistaken about all of this, but if that's not the purpose of the fuel pressure regulator, then why have it? Good idea about the drag coefficient. I think the old CarTest program has one listed although who knows if it's accurate..
A BSFC map would certainly be interesting to have. Is the torque actually measured by some force transducer or is it derived from other information like fuel flow, rpm and some finagle factor equation or lookup table? How do you like Pepper's system? Now that I have been given my wife's old laptop computer, I may need to get an OBDII reader. [Big Grin] If you don't mind, how about doing a brief writeup on the sytem and what you think of it. It's price and bazillion parameters it will read sure make it look good.
There's no sensor to directly read the torque, so it's calculating the value somehow. Pepper's system works well, the software has a lot of functionality though, and as a result sometimes it's not obvious how to do something. The help it comes with is ok. I really wish there were a support forum for Pepper's software but if there is, I haven't found it. I'll play with it some more (so if I do a writeup it'll be more complete) later on..right now I'm having a power window issue that just started today, the motor stripped it's gears and so I can't put the window up. Fortunately a rebuilt Motorcraft motor is only $75 and I can leave it parked in my garage until I get the new motor.
Originally posted by brianl703: I'll play with it some more (so if I do a writeup it'll be more complete) later on..
Please keep us posted as you play with it. I supect I'm not the only one that would be interested in it.
To calculate drag, you need the cross sectional area of the car as well as the Cd. You should be able to calculate the hp to keep a car at at certain speed by doing a coast down test. You would drive at 65mph on a flat road with no wind. Then put the car in neutral and time how long it takes to go from 65 to 55mph. As far as the math to compute that into horsepower, I will need to think about it a while.
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