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

[sherlock9c1]

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?

[EagleMark]

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...

[RobertISaar]

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.

[sherlock9c1]

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?

[RobertISaar]

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.

[EagleMark]

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.

[RobertISaar]

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....

[sherlock9c1]

Good points guys - thank you for that. Mark, did you see MAP start to go up once you passed MBT in spark advance?

[EagleMark]

It was awhile ago but IIRC analyzing data from known stretches of road seems MAP went up and TPS% was up, exactly opposite of what you want.

My bottm line was I could find no improvement in stock LT1 table, stock motor RoadMaster Wagon. Maybe if it were a couple 1000 pounds lighter or had higher gear ratio? Also was the first GM OBDI I could find no improvments in, this must have been a changing moment in GM tuning as it has followed through to later OBDII years. Just look at the tables and you can see they actually did something right.

[sherlock9c1]

Strange. Adding a little timing to my '95 LT1 made a noticeable difference in the amount of TPS required for light acceleration. I will keep that in mind as I tinker. If nothing else, it's worthwhile to understand all this stuff, as I am planning to try a different set of injectors and perhaps a supercharger at some point soon.

[EagleMark]

Les TPS would be good.

I corrected my staement above:

It was awhile ago but IIRC analyzing data from known stretches of road seems MAP went up and TPS% was up, exactly opposite of what you want.

[Fast355]

Les TPS would be good.

I corrected my staement above:

I would agree with you if you are holding air/fuel ratio as a constant, however you want a balancing act. You want to create as much torque as possible from a specific air/fuel charge, but you also want to minimize pumping losses at the same time. Over the years of tuning I have found that slightly retarding the timing as the MAP decreases and rpm rises helps keep you in a MPG sweet spot. For example in most of my tunes, my 50-60 KPA @ 2,000 rpm timing will be higher than my 20-40 KPA at the same RPM.

[Fast355]

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....

I have never seen anyone hammer bearings, unless they just go crazy with spark advance.

[1project2many]

Over the years of tuning I have found that slightly retarding the timing as the MAP decreases and rpm rises helps keep you in a MPG sweet spot.
Halleluja! Another believer in less timing can equal better mileage. Setting the timing to occur later can generate more pressure when the piston has greater mechanical advantage over the crank.


I have never seen anyone hammer bearings, unless they just go crazy with spark advance.
It only takes a few degrees too many and and it only took a couple of days of light driving to do it. The heater core was getting plugged so I wasn't getting much heat. We were having a cold spell and I was buried with outside projects. So I added a couple of degrees more advance to the spark timing in the 30-50 kPa range at cruise rpm on the highway to try and generate a little more heat in the head. Second day after the changes, I stop at the end of the exit ramp, and the low oil pressure light turns on. The car had run for a year with no trouble and now, with a couple degrees more advance, I had to replace the rod bearings. I might still have them... the upper halves have nice, shiny spots where they should be dull gray. Pulled spark back out, installed new bearings (and larger oil pump since I was in there) and ran for several years before pulling the engine and dumping the body when it rusted out.

[RobertISaar]

^ one example that came to mind.

the difference in power between optimal advance and any advance used beyond that is getting used to push the rod bearing into the crank pin... that's never a good thing.



the peak pressure point is something like ~21* ATDC? if you had an ideal combustion chamber, you could have the spark event take place then and instantly the cylinder pressure would go from what it rises to from the effects of the static compression to the pressure caused by turning a ~300*F air/fuel charge into a ~1500*F fireball.

unfortunately, a chamber that can do this is either impossible or impractical. so in the real world of street-driven vehicles, we end up with a situation where whenever you can run less advance and make the same amount of(or more) power, it's a good thing for power and economy. emissions.... IDK, that's never been my target for improvement.

[bobdec]

sherlock, not to regress but here's a post where you add a resistor on a switch in place of the IAT. It sends a constant temp voltage level to the PCM that is warm enough to maintain closed loop and other fueling tables something like 60* C would work. You then change one row of coolant temp spark correction table for that specific temperature range. It's been used to retard spark when switching on nitrous. http://www.hptuners.com/forum/showthread.php?t=41946