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Old 07-26-2003, 02:15 PM   #1
QA Guy
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Join Date: May 2001
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2002 WRX
Silver

Default Why is timing not RPM-derived?

Looking at the timing maps for the UTEC on TurboXS' website, I see that timing is highest at low RPM, declines to its lowest advance in the midrange, and goes up again toward redline. It does not advance as much as it did at low RPM, though.

In addition, the timing does not seem to be proportionally related to RPM. Two or three sites in a row might have the same timing advance, with the next site retarded 5 degrees.

Questions:
1) why doesn't the timing start low and advance all the way through to redline, where it would be at its highest point?
2) why is the timing steady for two sites, for example, then takes a big step down, instead of going down half as much twice as often?

Here are my thoughts on why timing should be RPM determined:

Constants:
1) The piston travels a fixed path.
2) The flame front propagation time is constant or fairly so within one engine.
3) There is a fixed point where the flame front's push on the piston will have its most beneficial effect (14 deg ATDC, IIRC).

Variable:
This seems to leave only RPM, which is directly proportional to the speed the piston makes its trip.


So if, in the flame front propagation time, the piston travels 12 degrees at 3600 RPM, it would travel 24 degrees at 7200 RPM. Which brings me back to my questions.

Can anyone answer my above two questions?
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Last edited by QA Guy; 07-26-2003 at 02:37 PM.
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Old 07-26-2003, 03:00 PM   #2
cRayZee
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2007 Legacy Spec B

Default

There are other variables:

Air Fuel mixture
Air temperature
Air Density/Boost/Vacuum/How much air
Fuel Octane


These variables define how fast the mixture combusts, so you'd have to set the timing according to these variables as well.
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Old 07-26-2003, 03:51 PM   #3
Concillian
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Default Re: Why is timing not RPM-derived?

Quote:
Originally posted by QA Guy

2) The flame front propagation time is constant or fairly so within one engine.
3) There is a fixed point where the flame front's push on the piston will have its most beneficial effect (14 deg ATDC, IIRC).
2) is not true when you talk about drastically varying boost pressure and AFR. Low RPM is high advance but the car never EVER sees 2000 RPM and 100% boost pressure.

3) Maybe, but if you tune for this on pump gas, YOU WILL DESTROY YOUR ENGINE. You are not tuning for MBT, you're tuning for not getting knock on pump gas.

The sites are that way because the engine is more susceptible to knock at particular load points. The design of the engine makes this so. I don't necessarily know why it's so, but it definitely is so.
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Old 07-26-2003, 05:36 PM   #4
SlideWRX
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Default

Looking at the TXS maps, the timing drops significantly at about 3000 rpm because that is when boost comes in. One of the variables is load, partially determined by manifold pressure. manifold pressure will change the speed of the flame front, and the predisposition of knock. With higher boost, the flame front travels faster, so the ignition is retarded. The odds to knock is much higher, and that retards timing. After 3000 rpm the curve roughly follows rpm, but there is enough happening that adjustments are needed in spots.

Tom
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