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Old 06-27-2009, 03:35 AM   #1
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Default Higher idle, higher minimum dc?

So I am running a higher idle (1100 for now, in part for alternator activation and to minimize stalling with AC), is there any harm in raising the minimum dc to match it (at stable idle I run around 28-30) is there any harm at setting the min to 27?

I'll attach a log so you can see what the idle looks like, if that helps.

Is there anything else that might go askew with the higher idle setting?
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Old 06-27-2009, 03:48 AM   #2
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And if anyone takes the time to look at my log, if you could tell me if the AFR noise is normal I would appreciate it. (the log is taken during a cruise, some 10psi points but mostly cruising)

{the car is a 91 turbo, LC-1, 550 injectors, MSPNP9093 - latest rev, 10Ghires}



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Originally Posted by aseer View Post
So I am running a higher idle (1100 for now, in part for alternator activation and to minimize stalling with AC), is there any harm in raising the minimum dc to match it (at stable idle I run around 28-30) is there any harm at setting the min to 27?

I'll attach a log so you can see what the idle looks like, if that helps.

Is there anything else that might go askew with the higher idle setting?
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Old 06-27-2009, 10:31 AM   #3
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Quote:
Originally Posted by aseer View Post
is there any harm in raising the minimum dc to match it (at stable idle I run around 28-30) is there any harm at setting the min to 27?
No harm. In fact, one should tune one's MinDC value to as high a number as possible (1 or 2 points below normal warm idle) to assist in the transition from cruise to normal idle.

Your AFR signal in cruise looks perfectly normal to me.
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Old 06-27-2009, 10:54 AM   #4
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My minDC and closedDC are around 7 or 8, why is everyone so much higher? My idle screw is all the way in...
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Old 06-27-2009, 11:15 AM   #5
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What we're doing is observing log data to find out what our actual DC is under normal warm idle, and then setting the MinDC parameter to be a point or two below this.

To understand the advantage of this, you have to think for a moment about how the idle program works. When you are at normal cruise (throttle and/or RPM above idle limits) the idle DC sits at the closed value. When you go into neutral and release the throttle, the RPMs begin to fall until the idle program activates. At that point, it starts out at the MinDC value and begins seeking a stable idle.

A lot of folks have problems with the engine wanting to stall during this time. This is caused by the fact that the idleDC is starting out at too small a value, and taking too long to work its way up to a value adequate to sustain the engine. By increasing the value of MinDC, it starts out at a value which is very close to normal, and thus prevents RPMs from dipping well below normal.

Realistically, there is no reason to have the MinDC set to a very small value. The ideal number for it is one which is as high as possible without being so high that it causes idle RPM to exceed target RPM when the engine is warm and running normally. The sweet spot is usually 1-2 points below what you observe to be the lower average value in a log of warm, closed-loop idle. IOW, if in closed-loop idle your valve typically oscillates between 26-28 (occasionally dipping to 25) then I'd use 24 or 25 as Min.
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Old 06-27-2009, 01:08 PM   #6
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@Joe: I understand this completely and that is exactly what I have: if I raise it above 8 or 9 my idle will rise to 1200rpm or more. Let alone setting it to 28. This was from the start, my PNP map had a 2.5krpm idle
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Old 06-27-2009, 01:44 PM   #7
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Thanks for taking a look (and the response).



Quote:
Originally Posted by Joe Perez View Post
No harm. In fact, one should tune one's MinDC value to as high a number as possible (1 or 2 points below normal warm idle) to assist in the transition from cruise to normal idle.

Your AFR signal in cruise looks perfectly normal to me.
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