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Old 02-16-2009, 04:06 PM   #1
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Default Spark Tuning on a Dyno

I'm trying to tune microsquirt on a four-stroke snowmobile, and we're to the point of tuning spark but I don't know what I'm doing.

We have a steady-state track dyno and exhaust-gas analyzer (the sled is for a clean-snowmobile competition), but no knock sensor at the moment. I think we're going to get a knock sensor soon but I would like to start tuning the spark table tomorrow.

So, does anybody have any tips for tuning spark? Around here everybody just says it's impossible to get it right without a dyno... but I have a dyno so I would like to get it right.

Priorities are Emissions, Fuel Economy, Power (in that order, unfortunately)

Relevant specs on the sled: 500cc, 2cyl, with catalytic converter, IRTBs, and microsquirt.
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Old 02-16-2009, 04:34 PM   #2
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get a rubber hose until you get a knock sensor. make it long enough so you can hold one end on the engine and one in your ear.

depending on the type of dyno, it may be slower or faster.

first you want to richen up your AFR across the board so it's "safe" at various spark settings. that's like 13:1-12.5:1 NA or 11:1 moderately boosted.

then do a baseline run and save the curve.

add some timing in small increments (2-4 degrees) in the cells the dyno plot takes you through (roughly a diagonal line from low rpm low MAP to high rpm high MAP) as well as a swath of surrounding cells.

re-do the power run and see where you've gained or lost power. also look at your AFR and see where the car is *LEANER* (therefore operating more efficiently and burning more fuel).

richen the "leaner" cells and evaluate the spark map by adding 2-4 degrees in areas where you gained power and subtracting 2 degrees from where you *LOST* power. make DETAILED NOTES of these areas.

re-run the power pull.

if you see increases in power after each run, continue to add timing. (and fuel as needed)
if you see decreases from a previous high power value, remove timing in half the increment you last changed it. (keep an eye on AFR)

example:

baseline at 90 kpa and 3000 rpm shows 50 hp, timing is at 20 BTDC
add 4 deg to that cell and new baseline is 55 hp (24 btdc)
add 4 deg to that cell and new baseline is 57 hp (28 btdc)
add 4 deg to that cell and new baseline is 54 hp (32 btdc)
remove 2 deg from that cell and new baseline is 58 hp (30 btdc)
add 1 deg to that cell and new baseline is 58 hp (31 btdc)
final: remove that last degree since it was beyond the MBT (minimum best timing) point and showed no improvement over the next lower value.

IMPORTANT!!!
If at any time you hear knock or detonation or whatever, first double check your AFR and make sure it's richer than you want it. if it is, remove timing until it first goes away, then pull another 1 to 3 degrees depending on margin of safety you want. do this even if you're not sure but think you may have heard it. safe. safe means not broke.

after you're all done with timing, re-tune your AFR for emissions or power or whatever you want. if it's N/A, you'll want to run 14.7 most places and 13.5 above 80 kPa. you can even run a little leaner 15-15.5:1 below 40 kPa if you want.

that should get you a solid tune.

oh and idle set to 10-15 BTDC (idle speed +20% and up to 20% over your idle kPa)
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Old 02-16-2009, 05:08 PM   #3
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Thanks! Is any of this going to be difficult because of the CVT? I know we don't generally get to run up typical diagonal path through the table, and there is essentially no overrun because the engine returns to idle as soon as you get off the throttle but I think I can work through those issues.

This engine is NA but has a pretty high CR in the area of 12:1 so I don't expect a lot of timing to be necessary.

We have REALLY high EGTs right now, so I'm hoping that tuning the timing will fix that. (the exhaust manifold glows bright red after only a few seconds at WOT) The VE table is pretty well tuned at the moment, so I know it's not because it's running lean.
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Old 02-16-2009, 05:13 PM   #4
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Our understanding of this Dyno is limited, but I think we will be able to hold various combinations of load and track speed, if we can figure out which buttons to press.
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Old 02-16-2009, 05:23 PM   #5
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CVT? errr can you lock it at one ratio?

I guess you could dyno several ratios... but they may be hard to compare.

glowing red means either too lean or too far retarded or both.

btw, one caveat with loading dynos is overheating. you wont get nearly enough airflow and the way i mentioned may be easier. but you can try it... it's just hard to do and hard on teh vehicle.
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Old 02-16-2009, 06:14 PM   #6
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the CVT is not lockable, it's just a regular snowmobile clutch.

We've got the overheating thing covered by putting an extra radiator in a tank of ice water, but I think I can adapt the general procedure to our situation. Thanks y8s!
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Old 02-16-2009, 07:40 PM   #7
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the cells in the map which are below the cvt locking rpm are not critical at all since the engine isn't under load at those operating points. just get a fair baseline on those and worry about the cells which see the engine under load.


An EGT gauge is also good to have, to see changes smaller than a dyno can tell. Typically a change of 1 deg in timing produces a change of 20-80 deg in peak EGT depending on the engine. Find out what that resolution is on your setup, and fine tune from there. What dyno is it?
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Old 02-16-2009, 07:56 PM   #8
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Crank the timing till she blows a hole in the piston or blows the head off the block, then back 'er off a little bit.

y8s pretty much nailed it, so do that. Look up "det cans" and make a pair. I so gotta do this one day.
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Old 02-16-2009, 10:34 PM   #9
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oh right, forgot about the EGT gauge.

incidentally, it pays to have an extra set of eyeballs around while you're tuning. bring a couple sixpacks and ask friends to come along and assign them to critical areas. one guy to watch EGT, one guy to watch AFR, one guy to watch the motor for fountains of lifeblood...
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