Rolling rpm issue
08-09-2003, 12:52 PM
#1
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Rolling rpm issue I'm running into a problem programming a Cammed 99 Corvette
M6.
I've got the idle(in park) working good for AC and non-AC.
The issue comes while rolling on the road.
With the AC on, the rpm jumps up to 2000rpm until I bring
the car to a stop, then it catches a good idle.
With the AC off, the rpm starts to hunt around until I come
to a stop, then it catches a good idle.
2 problem symptoms only while the car is rolling.
Idle is set to 950 with AC on and 925 with AC off.
The IEngine Cal-Idle and limiters have tables mod'ed.
Idle speed, IAC park position have been bumpbed some.
I figure that the issue has a mph relationship, and I am
targeting the Throttle follower tables(already some mod's),
since they reference the mph.
BUT why would the rpm have a different symptom for AC on and
then AC off when there's no reference to AC status?
Any help is appreciated,
Jay Johnson
jayjohnson@rocketmail.com
08-09-2003, 01:31 PM
#2
Re: Rolling rpm issue 1) You may have drilled the TB (if you did) too much
2) You need to play with the P/N airflow decay tables. Notice the MPH that you are having the RPM issue at and adjust the table at those points....SLOWLY. You can make the car barely drivable if you make large changes to these tables.
Good Luck
2) You need to play with the P/N airflow decay tables. Notice the MPH that you are having the RPM issue at and adjust the table at those points....SLOWLY. You can make the car barely drivable if you make large changes to these tables.
Good Luck
08-09-2003, 05:41 PM
#6
Re: Rolling rpm issue Ok I'm Really confused on this one. Rate of Decay - if you increase the rate of decay it should decay faster and then go to idle quicker...wrong?
Should we Decrease Rate of Decay?? How can we go in opposite values and have same net result - is it due to binary values? Do I need to get out an old Math text. I'm too old (48) for this ****!
joel (Bink)
Should we Decrease Rate of Decay?? How can we go in opposite values and have same net result - is it due to binary values? Do I need to get out an old Math text. I'm too old (48) for this ****!
joel (Bink)
08-09-2003, 09:18 PM
#7
Re: Rolling rpm issue This is a long post, but I think it is worth the read for most. Sorry, I tried to shorten it.
The P/N Decay tables are a little confusing because they are control modifiers. We are starting to get into the math (which sucks) of how devices in the car are controlled.
On every control system you have three primary components: Target Value, Gain (K), and Damping.
The target value is the value that we are trying to reach. For the IAC (inlet air controller) this value is a calculated value that will keep the car running. This is reffered to as the Target IAC and can be monitored with any scantool.
Gain is the rate of change at which the IAC servomotor will attempt to get to the Target IAC value. Very fast Control Rates, or high gains, will cause the engine RPM to get to its target faster, but it will often overshoot (we refer to this as surging). Very slow rates will cause the engine to lumber to the target RPM and often hang up (we call this the Cruise Control Effect). For our purposes the Gain of the IAC motor is fixed (we cannot change it).
Damping is a functional adjustment to the Rate of Change, or gain. Damping's primary function is to properly modify the Rate of Change of the system so that the IAC motor can move as quickly as possible towards the target (as high a gain as possible) yet still stop accuratly on the Target IAC position.
A good example of a control system is your suspension. The Target Value is the normal loaded position that the vehicle sits at when motionless. The Gain, is the spring that is used in the suspension system. The damper is the shock that is used in the suspension system.
How does Decay come in to play?
Decay is a Damping modifier. It is like an adjustment on a shock. If you turn the adjustment too far down, no damping, the car will bounce like an old cadillac down the road. If you turn the adjustment too far up, lots of damping, the car will jar and the suspension will not properly protect the car from imperfections in the road.
The IAC Decay tables are the same. They are a modifier to the Damping of the IAC control system. Notice that all of the Decay values are not greater than 1. This is because a value of 1 is Full Damping. Any value less than one reduces the damping of the system.
If you look at the P/N Airflow decay table you will notice that all values above 18 mph are a 1. There is a very good reason behind this. How would you like to be traveling down the road, put the car in neutral, and have the car die. No power steering or power brakes at 80 MPH! Because of this GM has made the IAC system as sluggish as possible once you get above 20 mph. The "1" on the table means FULL DAMPING, or MOVE THE SYSTEM AS SLOW AS POSSIBLE. This way the IAC motor get the car down to its target idle VERY slowly. There is little chance that the car will die as the RPM very slowly moves to its target.
Now look at the P/N table for the points less than 20 mph. This is the around town, stoplight to stoplight, area of the table. When driving around town and you throw the car in neutral, you don't want the RPM's to hang out at where you put the clutch in. As a result, GM has curtailed this portion of the table to make the IAC controller very aggressive. The IAC motor will move to its target position very quickly allowing the RPM of the motor to drop quickly.
Anybody with a car can prove this point. While moving below 20 mph, put the car in neutral. Watch how fast the RPMs drop. Now, go out on the freeway. Put the car in neutral. Watch how the RPM will hang for a second then VERY slowly make their way down to just above Idle RPM (it will never make it to idle because the RAM air in motion jams extra air in).
How does this apply to a cammed car?
On cammed cars we often get ourselves into trouble because we alter the TARGET value of the IAC motor. When you adjust the IAC in park and gear tables you are messing with the Target value of the IAC motor. On a cammed car the overlap and reversion of the gasses cause less air to be contributed to combustion. As result we have to let more air into the motor to keep the car running. This is why we increase the Idle Air table for cammed cars.
Unfortunatly, altering the target IAC position messes up the decay tables. Because we have additional air being let in now, the IAC motor appears to be sluggish.
Example: (the values are just for this example) A stock car will demand 20 grams per second of air to idle at 800 rpm. When on the throttle the IAC opens up and takes its running position (a factor of the idle position). Suddenly you get off the throttle. The IAC is now responsible for keeping the car running. Open, the IAC motor admits 100 grams per second. The IAC motor must now get the air being admitted into the engine at as quick a rate as possible without stalling the motor. The IAC motor, looks up on the decay tables for what speed it is going and sees that it can remove 10 g/sec of air at a time without killing the motor. It does this, and the car gets down to the desired idle quickly and stable.
Now imagine that you have adjusted your Idle Air table to accomodate for a cam. The car now demands 50 grams per second of air to stay running. Additionally, the IAC now admits 200 g/sec of air with the IAC open because the running position of the IAC motor (when you are on the throttle) is a function of the idle position of the IAC motor. Now, when you get of the throttle, the IAC motor is still responsible for getting the RPM of the car down to a reasonable value, but because the Decay tables are the same, the amount of air that the IAC motor moves at a time is still 10 g/sec. This is a drop in the bucket when the IAC has to go all the way back down to 50 g/sec. As a result the RPM moves down VERY slowly.
The fix for sluggish drop in RPM is to increase the rate at which the IAC motor is allowed to return to its target IAC. Being the Gain of the IAC system is fixed, we are left with controlling the damping of the system. And the only value that we have available to control the damping is the Decay of the system.
Reduce the Decay of the system to accelerate the speed at which the RPM of the car changes. Be careful though, because if the RPM changes too fast the RPM will overshoot below the idle RPM and the car will die due to lack of RPM.
Anyhoo, I hope this gives a full explanation as to why we want to decrease the decay in order to get rid of 'auto-rolling'.
Cal: I have no idea why increaseing the table to its max has worked. Naturally, you can see that multiplying a damper by something greater than 1 is just completely goofing up the controls equation. But if it works...... why not.
Good Luck,
Kevin
The P/N Decay tables are a little confusing because they are control modifiers. We are starting to get into the math (which sucks) of how devices in the car are controlled.
On every control system you have three primary components: Target Value, Gain (K), and Damping.
The target value is the value that we are trying to reach. For the IAC (inlet air controller) this value is a calculated value that will keep the car running. This is reffered to as the Target IAC and can be monitored with any scantool.
Gain is the rate of change at which the IAC servomotor will attempt to get to the Target IAC value. Very fast Control Rates, or high gains, will cause the engine RPM to get to its target faster, but it will often overshoot (we refer to this as surging). Very slow rates will cause the engine to lumber to the target RPM and often hang up (we call this the Cruise Control Effect). For our purposes the Gain of the IAC motor is fixed (we cannot change it).
Damping is a functional adjustment to the Rate of Change, or gain. Damping's primary function is to properly modify the Rate of Change of the system so that the IAC motor can move as quickly as possible towards the target (as high a gain as possible) yet still stop accuratly on the Target IAC position.
A good example of a control system is your suspension. The Target Value is the normal loaded position that the vehicle sits at when motionless. The Gain, is the spring that is used in the suspension system. The damper is the shock that is used in the suspension system.
How does Decay come in to play?
Decay is a Damping modifier. It is like an adjustment on a shock. If you turn the adjustment too far down, no damping, the car will bounce like an old cadillac down the road. If you turn the adjustment too far up, lots of damping, the car will jar and the suspension will not properly protect the car from imperfections in the road.
The IAC Decay tables are the same. They are a modifier to the Damping of the IAC control system. Notice that all of the Decay values are not greater than 1. This is because a value of 1 is Full Damping. Any value less than one reduces the damping of the system.
If you look at the P/N Airflow decay table you will notice that all values above 18 mph are a 1. There is a very good reason behind this. How would you like to be traveling down the road, put the car in neutral, and have the car die. No power steering or power brakes at 80 MPH! Because of this GM has made the IAC system as sluggish as possible once you get above 20 mph. The "1" on the table means FULL DAMPING, or MOVE THE SYSTEM AS SLOW AS POSSIBLE. This way the IAC motor get the car down to its target idle VERY slowly. There is little chance that the car will die as the RPM very slowly moves to its target.
Now look at the P/N table for the points less than 20 mph. This is the around town, stoplight to stoplight, area of the table. When driving around town and you throw the car in neutral, you don't want the RPM's to hang out at where you put the clutch in. As a result, GM has curtailed this portion of the table to make the IAC controller very aggressive. The IAC motor will move to its target position very quickly allowing the RPM of the motor to drop quickly.
Anybody with a car can prove this point. While moving below 20 mph, put the car in neutral. Watch how fast the RPMs drop. Now, go out on the freeway. Put the car in neutral. Watch how the RPM will hang for a second then VERY slowly make their way down to just above Idle RPM (it will never make it to idle because the RAM air in motion jams extra air in).
How does this apply to a cammed car?
On cammed cars we often get ourselves into trouble because we alter the TARGET value of the IAC motor. When you adjust the IAC in park and gear tables you are messing with the Target value of the IAC motor. On a cammed car the overlap and reversion of the gasses cause less air to be contributed to combustion. As result we have to let more air into the motor to keep the car running. This is why we increase the Idle Air table for cammed cars.
Unfortunatly, altering the target IAC position messes up the decay tables. Because we have additional air being let in now, the IAC motor appears to be sluggish.
Example: (the values are just for this example) A stock car will demand 20 grams per second of air to idle at 800 rpm. When on the throttle the IAC opens up and takes its running position (a factor of the idle position). Suddenly you get off the throttle. The IAC is now responsible for keeping the car running. Open, the IAC motor admits 100 grams per second. The IAC motor must now get the air being admitted into the engine at as quick a rate as possible without stalling the motor. The IAC motor, looks up on the decay tables for what speed it is going and sees that it can remove 10 g/sec of air at a time without killing the motor. It does this, and the car gets down to the desired idle quickly and stable.
Now imagine that you have adjusted your Idle Air table to accomodate for a cam. The car now demands 50 grams per second of air to stay running. Additionally, the IAC now admits 200 g/sec of air with the IAC open because the running position of the IAC motor (when you are on the throttle) is a function of the idle position of the IAC motor. Now, when you get of the throttle, the IAC motor is still responsible for getting the RPM of the car down to a reasonable value, but because the Decay tables are the same, the amount of air that the IAC motor moves at a time is still 10 g/sec. This is a drop in the bucket when the IAC has to go all the way back down to 50 g/sec. As a result the RPM moves down VERY slowly.
The fix for sluggish drop in RPM is to increase the rate at which the IAC motor is allowed to return to its target IAC. Being the Gain of the IAC system is fixed, we are left with controlling the damping of the system. And the only value that we have available to control the damping is the Decay of the system.
Reduce the Decay of the system to accelerate the speed at which the RPM of the car changes. Be careful though, because if the RPM changes too fast the RPM will overshoot below the idle RPM and the car will die due to lack of RPM.
Anyhoo, I hope this gives a full explanation as to why we want to decrease the decay in order to get rid of 'auto-rolling'.
Cal: I have no idea why increaseing the table to its max has worked. Naturally, you can see that multiplying a damper by something greater than 1 is just completely goofing up the controls equation. But if it works...... why not.
Good Luck,
Kevin
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08-09-2003, 11:08 PM
#8
Re: Rolling rpm issue Kevin, thanks for putting the time and effort into that detailed response. I'm an Electrical Engineer, and I've studied feedback control systems, so it all makes sense, except that decreasing the decay would seen to imply an increase of dampening to me. But it may be just a mater of semantics, and if decreasing it makes the system respond faster, that's great because that means I can improve my present tune. Who knows, maybe the learn cycle is actually what finally started making the idle come down for me. I'll try reducing my decays back to stock for a week and see what happens.
08-10-2003, 12:14 AM
#9
Re: Rolling rpm issue Cal, I'm not saying the way you did it is wrong. Hell, for all I know my method could be the absolute worst way to do it. If it works for you (and some other people on this board) why not stick with it.
I'm am going to try your method when I get back to the shop on Wed. If it works better, I'll change my ways.
Its refreshing to see a different approach to the same problem.
I'm am going to try your method when I get back to the shop on Wed. If it works better, I'll change my ways.
Its refreshing to see a different approach to the same problem.
08-10-2003, 12:50 AM
#10
Re: Rolling rpm issue OK ...I get it. I thought of nuclear decay, half lives(t1/2)and biosystem decay rates. I thought we were adjusting "rate of change" -which I now know is "Gain".
It now makes sense! Thanks.
joel(Bink)
It now makes sense! Thanks.
joel(Bink)
08-10-2003, 01:13 AM
#11
Re: Rolling rpm issue I guess we are kind of adjusting the 'rate of change' just going about it in a f-upped way.
There's got to be a better way to play with the IAC motor in edit (gain directly etc...) but I don't think Ken has dug them out yet. JMO
There's got to be a better way to play with the IAC motor in edit (gain directly etc...) but I don't think Ken has dug them out yet. JMO
08-11-2003, 11:54 PM
#13
Re: Rolling rpm issue
I'll try reducing my decays back to stock for a week and see what happens.
08-12-2003, 06:18 PM
#14
Re: Rolling rpm issue Thanks for the update. It usually takes me about a day to figure out the combination of IAC and decay numbers that I want to use for a specific cam (the big ones)
I have to try alot of different combinations before I find one that I am happy with. And even then I have to tailor that combination further for the different cars that get the cams.
Good Luck
I have to try alot of different combinations before I find one that I am happy with. And even then I have to tailor that combination further for the different cars that get the cams.
Good Luck
