Octane for stock GT?

Discussion in '94-95 5.0 - Specific' started by SIIaCanuck, May 19, 2016.

  1. SIIaCanuck

    SIIaCanuck New Member

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    After chasing a bunch of little problems, I've finally got my car running well . . . on 91 octane premium.

    It's now got:

    170lph pump
    New fuel filter
    Adjustable fuel pressure regulator set 36psi (vacuum off)
    New cap/rotor & 8mm performance wires
    Timing at 10 degrees

    Was still pinging under light load (70mph up hill in cruise) and went lean over 4000rpm so I cleaned the MAF, cleared the codes and tried again.

    Bingo, problem solved. Smooth as silk and pulls very nicely to 5500+.

    So, as the manual says 87 octane regular, I put in a tank. I immediately got some low load pinging back (nothing in cruise at 70mph but as soon as I hit a steeper hill, it gives a light rattle).

    I've read several forum threads about the computer in '94-'95 5.0 Mustangs having too much part load advance for 87 regular but I want to get some experienced opinions. I've got some 5.0L experience with my cousin's cars but they're all Foxes, so don't have the 3-D programming that the SN95 has and they were running 12-16 degrees base timing with no problems that I know of.

    I'm going to dump in a tank of 89 next to see what happens but I commute 230 miles to/from work three days a week so burning fuel that's 20% more expensive adds up. It'd be nice not to have to run 91 all the time.

    I'm planning on getting into the computer but was going to wait until I put my heads, intake and cam in. Maybe it's worth doing now and backing off on the part-load advance.

    WOT and/or 3000rpm up doesn't seem to have a problem, it seems like it's 1/2 throttle and below and below 2800ish rpm, hence my thought that it's an ignition advance issue. The previous issue affected WOT and threw 181/189 codes, so it was clearly a lean issue.

    This is my first SN95, and it's teaching me a whole lot about the last 5.0L! It's kinda fun but, for a car I rely on every day, I'd rather not spend every morning listening to the engine for nearly 2 hrs listening for pre-ignition rattle.
     
  2. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    It's weird that you are pinging with 87 octane and 10 deg timing. I run 10 deg timing and 87, and I do not get any pinging...
     
  3. 96blak54

    96blak54 Legend

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    Definitely sounds like pre-ignition. The maf could still be a culprit given its age.

    To me it sounds like carboned cylinders. Carbon clinging onto the head and piston surfaces creating hot spots for fuel to combust easily. Even on the intake valves back, carbon collects.
     
  4. ranger56528

    ranger56528 Active Member

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    I run 87 in both without any issues but I did put a On3 turbo on the 95 and I'm going to try a tank of 91 to see if there is any noticeable difference,other then that I run 87 in everything I own.