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Thread: Quicker way to do Spark Hook test on the street for LT1s and others?

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  1. #1
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    Quicker way to do Spark Hook test on the street for LT1s and others?

    I'm reading several of Greg Banish's books and just finished reading the "spark hook" test again as a way to find Maximum Brake Torque (MBT) as a function of spark timing. I have a long straight flat hill near my house that seems to work well for slow sweeps under various loads. In the context of my OBD1 $EE 8051 LT1s, it takes time to upload new tunes to the LT1 to vary spark timing and I worry about weather conditions changing too much from run to run to really be comparable. As an alternative, I was pondering perhaps loading multiple PCMs ahead of time, each with the same spark tables but shifted a few degrees relative to each other and then immediately swapping them out between runs.

    BUT... As I'm nosing through the various tables in the '94-95 LT1 $EE 8051 setup, I see a "spark correction vs. MAP vs. Coolant Temp." Has anyone ever tried adding a switch into this circuit and then adding a potentiometer or resistor box to the circuit once you got to the test site to command a particular coolant temp and thereby vary commanded spark timing? One could then setup the spark correction table with a particular set of correction factors per temperature and very quickly change resistance values to sweep spark timing within the context of a single run. For example, if the driver held the vehicle speed constant by varying TPS and then varied the coolant temp which forced the spark advance to vary, then all you would have to do after the run is look at the data logs to find the minimum TPS/MAP needed to maintain a given speed. You could repeat this test multiple times at various operating points without the need for multiple tunes or musical PCMs.

    Is this feasible? Or quixotic?

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    You can try all you want, I did and all I got was a couple degrees here and there without knock retard starting... and that was with 92 octane, I'd get knock retard on stock table during shifts with 87.

    The main spark advance in these is already pretty maxed out, people use it for performance SB engines. Compare it to a Corvette and it has like 10 more in some spots, but it also has aluminum headsm more gear and weights like half of a B body.

    The spark advance coolant table is all 0 in warmed up areas, it adds a little when cold and if you get hot takes some away. Works quite well as is...

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    yeah, if you really wanted to do quick spark-hook type stuff, either need to mod the PCM and temporarily replace the PROM with an ostrich or another emulator.

    or, figure out the ALDL mode4 commands and command different amounts of spark advance in a steady state.
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    I should clarify - I was meaning only to setup the spark correction vs. coolant tables for testing, and once the optimal values were found, set the table back to original. So this way I completely bypass having to do more invasive stuff like what Robert mentioned, and instead simply setup the spark correction tables for temporary test values and add a switch and potentiometer to the ECT wiring, then return to stock when I'm done.

    Mark - do you mean WOT, or part throttle? I know some guys who have added a lot more timing here and there without knock, although it was all in part throttle areas and I don't know if the tables I saw were at or beyond MBT or not. How clean were your combustion chambers when you did that?

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    the PCM isn't going to like the coolant temp being misreported at all.... cylinder airmass corrections are highly dependant upon accurate coolant and IAT values. messing with them will cause both fueling and spark issues, ones that may not be immediately apparent.
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    Quote Originally Posted by sherlock9c1 View Post
    Mark - do you mean WOT, or part throttle? I know some guys who have added a lot more timing here and there without knock, although it was all in part throttle areas and I don't know if the tables I saw were at or beyond MBT or not. How clean were your combustion chambers when you did that?
    Part throttle is where I got added spark to stay wothout knock, I was specifically looking for more spark a highway cruise speed for more MPG. I got it in there and no knock... milage dropped like a rock! I think all I did was cause some pre detonation... there's a point where you loose HP, just before causing knock, it also creates a lot of piston heat.

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    once you pass the spark advance threshold where you produce best power, assuming you haven't run into knock yet, that's the area where you'll start hammering the bearings....

    that's the area where novice tuners almost always end up, since they believe the most advance they can run without knock is the best spot to be....
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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