Cam Overlap Surging, causes?
Pro
Joined: Dec 2001
Posts: 542
Likes: 0
From: sacramento
I understand the Air Fuel packing... and think we are saying the same thing as to combustion theory.
There is the gross packing thing and also how evenly the air fuel is mixed. Fuel atomization, injection timing relitive to intake valve timing, and air volicity also some of the factors. That is, our large intake ports, batch injection, and relitively low fuel pressure also has an effect on the quality of our idle.
I suspect this is a big reasion we need closer to 14/1 to maintain a quality idle but can run around 15/1AFR at cruse without issue.
Now the fun stuff.
As you know, there is a lag between throttle and the EFI's response the way motronics functions. It has to first sense a change in air flow or one has to nail the WOT switch.
Playing with the settings can make improvements in throttle response but the system design still has a major short comming.
This is why I am proposing that one way to achive faster response might be to run it ideal AFR and ignition settings at low load ahead of demand to put one in the sweet spot before the injection system would be able to adjust to acceleration values.
However, there is a problem with this. Iit might be ok on a race car to cruse at 13/1 at 29 deg of advance but this would be less than wonderful on a street car.
Fun stuff!
There is the gross packing thing and also how evenly the air fuel is mixed. Fuel atomization, injection timing relitive to intake valve timing, and air volicity also some of the factors. That is, our large intake ports, batch injection, and relitively low fuel pressure also has an effect on the quality of our idle.
I suspect this is a big reasion we need closer to 14/1 to maintain a quality idle but can run around 15/1AFR at cruse without issue.
Now the fun stuff.
As you know, there is a lag between throttle and the EFI's response the way motronics functions. It has to first sense a change in air flow or one has to nail the WOT switch.
Playing with the settings can make improvements in throttle response but the system design still has a major short comming.
This is why I am proposing that one way to achive faster response might be to run it ideal AFR and ignition settings at low load ahead of demand to put one in the sweet spot before the injection system would be able to adjust to acceleration values.
However, there is a problem with this. Iit might be ok on a race car to cruse at 13/1 at 29 deg of advance but this would be less than wonderful on a street car.
Fun stuff!
Thread Starter
Racer
Joined: Mar 2008
Posts: 317
Likes: 2
From: Rhode Island
Want to revive this old thread:
I converted my 3.2 setup and now I'm running a MAF instead of AFM. I re-wrote the entire Air Fuel code in the motronic to properly support MAF.
And the engine still exhibits the surging at lo-load lo-RPM just as it did with AFM. But I learned a few extra things: I can actually induce the surging real bad if I remove ignition in this problem area. And if I add ignition in the problem lo-load 2000-2500RPM area it does get better but I can't totally eliminate the surging. I do think that the SSIs with M&K 2in2out plus these Web Cams 20/21 are the issue. Next step to try would be to separate the left and right side exhaust but that won't be easy for me.
I also think I can get my hands on a basic 2in 1out Sport Muffler and may try this next.
I converted my 3.2 setup and now I'm running a MAF instead of AFM. I re-wrote the entire Air Fuel code in the motronic to properly support MAF.
And the engine still exhibits the surging at lo-load lo-RPM just as it did with AFM. But I learned a few extra things: I can actually induce the surging real bad if I remove ignition in this problem area. And if I add ignition in the problem lo-load 2000-2500RPM area it does get better but I can't totally eliminate the surging. I do think that the SSIs with M&K 2in2out plus these Web Cams 20/21 are the issue. Next step to try would be to separate the left and right side exhaust but that won't be easy for me.
I also think I can get my hands on a basic 2in 1out Sport Muffler and may try this next.
Last edited by scarceller; May 2, 2014 at 01:30 PM.
Pro
Joined: Dec 2001
Posts: 542
Likes: 0
From: sacramento
I am so rusty at this but I suspect it would help if you had a Throtle Position Sensor capability and ran it more as a reference system for acceleration response. Then use MAF more for cruise and part throttle.
Sensing acceleration demand from airflow change may be the bigger issue.
As a work around maybe you could replace the throttle switch with a vac loss switch and use it triger the acceleration map.
Sensing acceleration demand from airflow change may be the bigger issue.
As a work around maybe you could replace the throttle switch with a vac loss switch and use it triger the acceleration map.
Thread Starter
Racer
Joined: Mar 2008
Posts: 317
Likes: 2
From: Rhode Island
Keith, the MAF works flawless even accel enrichment and excellent throttle response. I have 2 cars running the system, one is a stock 3.2 with SSIs and 2in1out Sport Pipe. This car runs flawless.
The Other is my personal 3.2L Euro with Web Cams 20/21, port polished honed intake, SSIs with M&K 2in2out pipe. This car has surging/bucking at 2000-2500RPMs at very light loads like parking lot crawl. Other than this surging it runs fantastic and pulls about 230 RWH (SAE) on the dyno. It's this bucking surging I'm working on and I suspect it's Exhaust related.
The Other is my personal 3.2L Euro with Web Cams 20/21, port polished honed intake, SSIs with M&K 2in2out pipe. This car has surging/bucking at 2000-2500RPMs at very light loads like parking lot crawl. Other than this surging it runs fantastic and pulls about 230 RWH (SAE) on the dyno. It's this bucking surging I'm working on and I suspect it's Exhaust related.
I am so rusty at this but I suspect it would help if you had a Throtle Position Sensor capability and ran it more as a reference system for acceleration response. Then use MAF more for cruise and part throttle.
Sensing acceleration demand from airflow change may be the bigger issue.
As a work around maybe you could replace the throttle switch with a vac loss switch and use it triger the acceleration map.
Sensing acceleration demand from airflow change may be the bigger issue.
As a work around maybe you could replace the throttle switch with a vac loss switch and use it triger the acceleration map.
Pro
Joined: Dec 2001
Posts: 542
Likes: 0
From: sacramento
Got it.
I think you are on the right track for what it is worth. That is a classic flat 6 issue it seems.
Had it on some of my tuned mfi cars. T cam not as bad as the S or modified S cam.
My worse stumbling issue was from having my mfi fuel distributor miss timed so it was not injecting fuel right as the intake valve opened. Wow that was bad.
3 cylinders in one exhaust manifold can create solid salvaging opportunities. But probably not through out the full rpm range. I would bet on this.
Added overlap may just add to the effect.
Good luck. Cheers.
I think you are on the right track for what it is worth. That is a classic flat 6 issue it seems.
Had it on some of my tuned mfi cars. T cam not as bad as the S or modified S cam.
My worse stumbling issue was from having my mfi fuel distributor miss timed so it was not injecting fuel right as the intake valve opened. Wow that was bad.
3 cylinders in one exhaust manifold can create solid salvaging opportunities. But probably not through out the full rpm range. I would bet on this.
Added overlap may just add to the effect.
Good luck. Cheers.
Addict
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Joined: May 2001
Posts: 12,830
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I'm tuning a Porsche Carrera 3.2L with cams that have 7* overlap. Car surges 2nd gear lo RPM (2000-2500RPM) lo-load.
I'm sure it's the cam overlap but I want to understand what happens during overlap period. I think one of 2 conditions could be occurring:
1) At lo-RPM lo-Load we have throttle body very much closed so we have very high vacuum in the intake. One theory is that the minute the exhaust valve opens while the intake is still open (for 7* period) the high vacuum present in the intake will instantly suck exhaust back out of the exhaust stream back into the cylinder and then into the intake. If this is happening then the cyl has some exhaust in it prior to actually filling the cyl with intake mixture. Making matters worse is that if enough exhaust also flows into the intake then we have a polluted intake mixture as well, this means the intake mixture would not contain as much O2 and thus we will have a rich intake mixture. Also the extra exhaust sucked back in is now un-metered air that the MAF has not seen.
2) Theory #2: If the exhaust resonance happens to align with the RPM range that is surging then we would have exhaust pulling phenomenon going on. This is when the exhaust sound wave travels back up the exhaust to hit the back of the valve while it's still open. If this happens then we have negative pressure (vacuum) wave behind the exhaust valve. Then if the intake opens at this same time then the negative exhaust pressure would instantly pull the intake charge into the cyl and straight out the exhaust valve. This un-burned mixture in the exhaust will cause the WBO2 to read lean because un-burned fuel contains tons of un-used O2. If you are PT Dyno tuning in this area under these conditions you CAN NOT trust the AFR reading from the WBO2 because it has extra O2 and will cause you to read lean. The issue here is the tuner may add more fuel to fix the artificial lean condition only to make matters worse.
So which theory makes sense?
Please advice.
I'm sure it's the cam overlap but I want to understand what happens during overlap period. I think one of 2 conditions could be occurring:
1) At lo-RPM lo-Load we have throttle body very much closed so we have very high vacuum in the intake. One theory is that the minute the exhaust valve opens while the intake is still open (for 7* period) the high vacuum present in the intake will instantly suck exhaust back out of the exhaust stream back into the cylinder and then into the intake. If this is happening then the cyl has some exhaust in it prior to actually filling the cyl with intake mixture. Making matters worse is that if enough exhaust also flows into the intake then we have a polluted intake mixture as well, this means the intake mixture would not contain as much O2 and thus we will have a rich intake mixture. Also the extra exhaust sucked back in is now un-metered air that the MAF has not seen.
2) Theory #2: If the exhaust resonance happens to align with the RPM range that is surging then we would have exhaust pulling phenomenon going on. This is when the exhaust sound wave travels back up the exhaust to hit the back of the valve while it's still open. If this happens then we have negative pressure (vacuum) wave behind the exhaust valve. Then if the intake opens at this same time then the negative exhaust pressure would instantly pull the intake charge into the cyl and straight out the exhaust valve. This un-burned mixture in the exhaust will cause the WBO2 to read lean because un-burned fuel contains tons of un-used O2. If you are PT Dyno tuning in this area under these conditions you CAN NOT trust the AFR reading from the WBO2 because it has extra O2 and will cause you to read lean. The issue here is the tuner may add more fuel to fix the artificial lean condition only to make matters worse.
So which theory makes sense?
Please advice.
One theory is that the minute the exhaust valve opens while the intake is still open (for 7* period) the high vacuum present in the intake will instantly suck exhaust back out of the exhaust stream back into the cylinder and then into the intake
Typically the sweet spot is in the 5500-6500 range and the sour in the 2-3k range
W/a muffled exhaust these effects are drastically reduced and the best to hope for is sufficient flow
There is a condition called lean surge where at light throttle and if the mixture is too lean surging will occur, so the first thing I'd check is to see if the A/F mix is where you want it at the trouble point
Thread Starter
Racer
Joined: Mar 2008
Posts: 317
Likes: 2
From: Rhode Island
I have a WB02 setup in the car with a chip emulator to tune the PT IGN and Fuel Maps. This weekend I spent a bit of time tweaking this problem area 2000-3000 RPM at very light loads where the surging is occurring. What you describe as Lean surge seems to be the issue but the mixture is not lean. What seems to really help is to add ignition advance, so I set advance at 40-58* (2000-3000RPMs) and left mixture at Lambda 1.0 and this certainly helped. But then I decided to throw more fuel at it and altered mixture to 0.9 and the surging was eliminated! Then I tried to find the sweet spot for mixture and it's right around 0.94 Lambda.
So your theory on lean surge is correct! But to fix the issue I can't run closed-loop because this brings the mixture back to 1.0 and the surging returns.
One other solution is to try more timing but I need to be careful here because pushing timing more than 50* gets the distributor rotor way out of phase from the post and I think it could result in cross firing the previous post? To really see this I'd need to drill a hole in an old distributor cap to see where the rotor phasing is. I know the 84-89 cars have centrifugal advance for rotor phasing but I'm not sure how kicked in this is at 2000-3000RPMs.
I think that this engine setup with hi-compression does not fill cyl much at these light loads so the actual amount of ingested air is minimal. This means that the amount of fuel/air in the cyl is not very much and maybe adding the extra fuel puts more fuel particles closer together and helps speed flame front speeds? It's just a theory based on my understanding that lean mixtures need more ignition advance because they have reduced flame front speeds while rich mixture have faster flame front speeds and thus need less ignition advance.
Addict
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Joined: May 2001
Posts: 12,830
Likes: 789
Bill,
I have a WB02 setup in the car with a chip emulator to tune the PT IGN and Fuel Maps. This weekend I spent a bit of time tweaking this problem area 2000-3000 RPM at very light loads where the surging is occurring. What you describe as Lean surge seems to be the issue but the mixture is not lean. What seems to really help is to add ignition advance, so I set advance at 40-58* (2000-3000RPMs) and left mixture at Lambda 1.0 and this certainly helped. But then I decided to throw more fuel at it and altered mixture to 0.9 and the surging was eliminated! Then I tried to find the sweet spot for mixture and it's right around 0.94 Lambda.
So your theory on lean surge is correct! But to fix the issue I can't run closed-loop because this brings the mixture back to 1.0 and the surging returns.
One other solution is to try more timing but I need to be careful here because pushing timing more than 50* gets the distributor rotor way out of phase from the post and I think it could result in cross firing the previous post? To really see this I'd need to drill a hole in an old distributor cap to see where the rotor phasing is. I know the 84-89 cars have centrifugal advance for rotor phasing but I'm not sure how kicked in this is at 2000-3000RPMs.
I think that this engine setup with hi-compression does not fill cyl much at these light loads so the actual amount of ingested air is minimal. This means that the amount of fuel/air in the cyl is not very much and maybe adding the extra fuel puts more fuel particles closer together and helps speed flame front speeds? It's just a theory based on my understanding that lean mixtures need more ignition advance because they have reduced flame front speeds while rich mixture have faster flame front speeds and thus need less ignition advance.
I have a WB02 setup in the car with a chip emulator to tune the PT IGN and Fuel Maps. This weekend I spent a bit of time tweaking this problem area 2000-3000 RPM at very light loads where the surging is occurring. What you describe as Lean surge seems to be the issue but the mixture is not lean. What seems to really help is to add ignition advance, so I set advance at 40-58* (2000-3000RPMs) and left mixture at Lambda 1.0 and this certainly helped. But then I decided to throw more fuel at it and altered mixture to 0.9 and the surging was eliminated! Then I tried to find the sweet spot for mixture and it's right around 0.94 Lambda.
So your theory on lean surge is correct! But to fix the issue I can't run closed-loop because this brings the mixture back to 1.0 and the surging returns.
One other solution is to try more timing but I need to be careful here because pushing timing more than 50* gets the distributor rotor way out of phase from the post and I think it could result in cross firing the previous post? To really see this I'd need to drill a hole in an old distributor cap to see where the rotor phasing is. I know the 84-89 cars have centrifugal advance for rotor phasing but I'm not sure how kicked in this is at 2000-3000RPMs.
I think that this engine setup with hi-compression does not fill cyl much at these light loads so the actual amount of ingested air is minimal. This means that the amount of fuel/air in the cyl is not very much and maybe adding the extra fuel puts more fuel particles closer together and helps speed flame front speeds? It's just a theory based on my understanding that lean mixtures need more ignition advance because they have reduced flame front speeds while rich mixture have faster flame front speeds and thus need less ignition advance.
An SC/RS 3.0liter, 10.3cr, S cams(6.6mm timing), only uses 35° of ign timing @6K rpm
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Thread Starter
Racer
Joined: Mar 2008
Posts: 317
Likes: 2
From: Rhode Island
Bill,
This 40-50* advance is only in the Part Throttle Ign table and only at very light loads. Even the 89 stock chip pushes the advance well into the 45* area in this table. This is not the hi-load or WOT ignition, at WOT my max advance is 30*.
These motors need a lot of advance at light loads if you plan to achieve max cyl pressure for the given air flow, keeping in mind that air flow in this case is minimal and TB is only open ever so slightly at like 5% throttle opening in this problem area.
Thanks for all the input and I do feel I'm making progress with the surging issue, at least I know what tends to help.
This 40-50* advance is only in the Part Throttle Ign table and only at very light loads. Even the 89 stock chip pushes the advance well into the 45* area in this table. This is not the hi-load or WOT ignition, at WOT my max advance is 30*.
These motors need a lot of advance at light loads if you plan to achieve max cyl pressure for the given air flow, keeping in mind that air flow in this case is minimal and TB is only open ever so slightly at like 5% throttle opening in this problem area.
Thanks for all the input and I do feel I'm making progress with the surging issue, at least I know what tends to help.
