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

[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