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David Todd

Intake popping while on Cyclical Idle.

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I've got a 1991 Subaru Legacy with an EJ-22 Turbo swap (motor is the entire closed deck variant) no mods, stock injectors.. I'm running the throttle body at 18% open with cyclical idle keeping the revs around 1800; I've tried playing around with the settings some, I've got it setup to where it's pretty rough. Every now and then I'll get a muffled pop sound back through the intake, it's loudest at the filter. I'm not exactly sure what's going on, but it sounds like a lean idle pop. I know that at that point on the fuel map (around where the engine is idling on cyclical idle) I'm seeing AFR's of 13.5. Unfortunately I can't check the AFRs while on cyclical idle since the sensor just reads far lean. I've never seen any flames, nor is it a loud bang; it sounds like it's happening in the runners. At cyclical idle I'm seeing -10kPa to 2kPa manifold gauge pressures, and the entire car smells rich; which isn't really new to my car. I just want to make sure what's going on isn't a serious problem? Maybe it's too much ignition advance? (about 23 degrees) or I need to change the injector timing? (375 degrees) Also, on cyclical idle, is it a fuel cut? ignition cut? both?

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hi there, i'm having same experience too, to reduce it, i smoothed the cyclic idle by adding the limit cyclic idle, up to 800 rpm. my customer car is evo4 grup n rally car with plug-in ecu. i really like to konw too, what cause the poping. hope got any explaination soon. thx.

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Hi David.

Are you using the advanced mode?  If not, may be turn that on, and manipulate the start and end cut patterns.  You can soften your cyclic idle up like this and will find most likely the 'pop' will disappear.  It's not a firing event you are hearing, as technically there should be no fuel in there to pop :)

Visit the Anti-Lag Settings portion of the help file for a full explanation on these advcanced settings.

Let me know how you get on.

Jurgen

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well, it definitely is a firing event. my IAT spikes and I see a jump in intake pressures when I hear the pop; so it's definitely a fire event. I changed my plugs right before the race that weekend and drove it, softening up the cyclic idle so that it came out of idle easier and into anti-lag. At the transition point I got lots of pops back through the intake. Car is running pig rich, and I'm getting ignition events so I don't think it's the engine missing and firing the other plug during an intake event. I might try swapping to a coil on plug setup and see if it disappears.

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Can you guys please confirm that you are running the latest ECU firmware.  There has been significant changes to anti-lag and cyclic idle in recent firmware versions.  There are now two different types of cut mode for cyclic idle.  Constant and adaptive.

As you guys have described, the popping is fuel lighting in the intake manifold.  Remember that in a wasted spark application, a spark is fired in the cylinder opposite in the firing order at the same time.  So, with a firing order of 1, 3, 4, 2, a spark is fired in the no 4 cylinder at the same time as no 1.  If no 1 is firing, the valves in the no 4 cylinder will be 'rocking', but as the spark is advanced, it is more likely to occur while the no 4 intake valve is much further open than the exhaust valve.  There is plenty of fuel in the intake manifold and on the port walls to light.  Add to this the higher intake pressures and temperatures present during cyclic idle, the fuel is much easier to light.

So, I can not guarantee this, but I would be surprised if changing to direct spark did not fix this problem.  Possibly retarding off the ignition timing during cyclic idle would help, but that may lead to higher exhaust temps etc...  

The cyclic idle system uses a fuel only cut, but even if fuel is cut for a cylinder there is still fuel on the port walls from previous injection events, so a leaner charge can enter the cylinder and possibly be lit in a wasted spark arrangement as the spark fires while the intake valve is open.

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Well, I hope you have a surprise outfit. I already had two twin-channel ignitors; so I just wired them up to run four brand new ignition coils I bought so I could swap over to direct spark and absolve myself of blowing flames out the wrong end of the motor. Well, it didn't work. I left the throttle stop set at about 5% and played with the settings/throttle inside the car; after I couldn't get it to clear up any I just took a 50Hz log for the better part of ten minutes. When you look at/analyze this log, just look for the IAT spikes, it's unmistakable. Over the duration of the log the TP% will vary as I'm keeping it far open by foot, instead of constantly adjusting the stop screw. I should note that it seemed that the further the throttle plate was open, in comparison to a low idle speed (~1500rpms) the more often an intake backfire event occurs. I could be wrong, or backwards, but it just seemed that way when I was messing with it. So I'm looking for answers; I don't think I'm running too much ignition timing... I've tried adaptive vs constant.. the only thing that seemed to help was a throttle opening around 10-12 percent and an idle limit of 1200 rpms. Hoping you can help, I go racing sunday and as it is I'm leaving the antilag off, afraid I might do damage on cyclical idle. David

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Yup, there is definitely evidence of popping in the intake.  At this stage I have no definite answer for you.  There is no evidence of anything going wrong in the log file.  There is nothing to indicate the cause of the popping.  It could be a mechanical problem possibly to do with valves being pushed open but that is not common at low RPM.  The log shows the result of the pop, not the cause.  Is it possible that it is a lean misfire?  Does the problem occur under the same conditions without cyclic idle turned on? (eg same RPM, MAP, TPS etc).  I would suggest now that you take your car to a Link dealer and have them investigate the cause of this problem further.  

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hello, just noticed this older post and i think i can help: i workend on many gr.N cars that use jacked up throttle solution for Als, and this is more fequent on wasted spark (evo) but can also be seen on direct spark it is mostlly related to cam overlap because on engines with vvt (mivec and avcs) you can retard intake while on cyclic and cure this (on ceratin ecus which have douan vvt maps -one for als ,one when you are on off als- wold be a great feature if added to Link). comon solution is : 1. use a less aggesive cyclic-meaning engine should be alowed to have bigger rpm limit range(engine sound like haunting for idle-rather than constatnt rumble), 2. use more cut (fuel is best if single possibilty, fuel and if ign best if supported)on wasted spark and non sequential injection is quite difficult 3. use less neg timing-this causes less pulses in exhaust which bounces back froum manifold through cam overlap and into the cylinder that is on intake stroke- from which this can be transsmited to inlet manifold -hence the poping sound and pressure spikes 3.if you have ecu that suports doual VVT tables- use max retard on intake (0 if possible)and max advance on exhaust - to obtain minimum overlap 4. less oppening on throttle but this limits the als - this is why dbw is great -you can have diffrent positions while on or off als this whill cure the popping but will also eliminate the nice exhaust note- which turns into a hounting sound

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