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Dynamohum Dynamohum is offline
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Join Date: Jan 2008
Location: Midland Ontario Canada
Posts: 173
Originally Posted by JFairman View Post
"Then tune around this by increasing in spring tension at the AFM."

Thats the opposite of what I do when tuning L- jetronic for more power.
L-jetronic is similar to motronic, but it's analog, fires the injectors 3 at a time alternating on a 6 cylinder engine, is less accurate and only does the injection and has nothing to do with the ignition.

I loosen tension on the barn door spring in the AFM so it opens easier and farther so the motor breathes easier and injector dwell time is increased for any given rpm and then compensate for the barn door opening farther at idle and being too rich by opening up the CO air bypass screw in the AFM so the barn door closes up and the AFR is correct at idle.
Then install an adjustable rising rate wide band fuel pressure regulator on the end of the fuel rail so you can have higher fuel pressure under acceleration and zero manifold vacuum.
You can also take the stock fuel pressure regulator and take 2 sockets of the right size and squeeze the ends of the metal housing together a little bit in a vise. That increases the spring pressure inside the FPR and raises the fuel pressure, but an aftermarket adjustable vaccum controlled rising rate FPR is wayyyy better.

Along with that I install a 10,000 ohm linear taper potentiometer in series with the engine temp sending unit so you can add resistance to the sensors reading and make the ECU think it's colder out than it is up to 30 below zero so injector dwell time is increased for alot more fuel with the turn of a knob on the dashboard.
Of course there are alot of different EFI injectors to choose from but I never turbocharged a car with EFI so didn't need to.

I love the simplicity, accuracy, and adjustability of EFI and I wish CIS was never invented.
Great info,
As you mentioned above the L-jetronic is analog, only does the injection and has nothing to do with the ignition. So on this system its safe to say that the input from the temp sending unit only has the affect on injector dwell (which your clever potentiometer mod takes advantage of)

Since the Motronic ML3.1 has both an analog and digital side to it The Intel 8051 Microcontroller - The Land of NoVA/DC/MD area Porschephiles and 2 temp sensors, 1 in the AMF housing and 1 on the cylinder head and controls both fuel and ignition, I wonder if we can do the same potentiometer mod? Perhaps someone can enlighten us,

I also believe that ML3.1 can retard ignition using temp sensor input when confronted with very high ambient temps when there is a greater risk of potential detonation. But by how much?

I've read the plus side of the Motronic system over others is in its ability to compensate for mild build changes and during warm up. My understanding is that this system was tested to operate in a wide geographical heat range from extreme hot to extreme cold which is a plus and we might be able to take advantage of the built in temperature compensation to further adjust our ignition needs while on boost.
Thank You for your time, Paul. We do because we can.
87 911 3.2 (Turbo conversion, build in progress, Thermal Barrier Coatings, High Pressure Dry film coatings) Modified heads, boat-tailed case, ARP hardware, OBX Header, 930 clutch disk, G50 Trans
89 5.0 Mustang convertible (For Sale)

Last edited by Dynamohum; 01-27-2010 at 09:37 AM.. Reason: correction
Old 01-27-2010, 06:38 AM
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