Cam Overlap Surging, causes?

[scarceller]

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.

[kodioneill]

Why are you so sure the overlap is the problem? Did you just change the timing and the problem started? Did you measure the vacuum with a gauge?

[scarceller]

Timing does affect the surging, if I tune ign on the dyno for the near max torque (in the given PT Ign map lo-load cells) then the surging on the street is real bad. But while on the street if I reduce ign advance by 10* the surging is not as bad but this is most likely because with less ign timing I'm required to open the throttle more to achieve the same unit of work. Opening the throttle more will reduce vacuum in the intake so it sort of makes sense the surging is less noticeable.

The cams are WebCam 20/21 in a euro motor and this engine has always been problematic at lo-RPM lo-load.

I have not tried a vacuum gauge since this issue only happens on the street in 2nd gear between 2000-2500RPMs. What would I see at the gauge in your opinion?

Originally Posted by kodioneill

Why are you so sure the overlap is the problem? Did you just change the timing and the problem started? Did you measure the vacuum with a gauge?