Poncams causing VCT power dip

[Freebaggin]

I have Tomei Poncams installed and from day one my power/torque curve has dropped off when VCT is switched at the high RPM setting. My tuner has had to fiddle with the VCT RPM switch point to smooth out power as best he can but it's still evident.

He's tried between 4500-5700rpm and has the best result at 5300rpm I think, but I might be wrong....it doesn't matter what exact RPM, since the point is that Tomei promote the fact that the Poncams SMOOTH out the power at this very point, but mine is the opposite!!

Anyone seen this before and fixed it?

Dyno attached, green line is the one that is important.

Cheers.

[R31Nismoid]

Possibly the cams are in incorrectly?

kinda off the topic a bit, what sorta turbo are you using?

[Freebaggin]

So no other takers on this problem?

[RB30-POWER]

Has your tuner tried adding extra timing at the switchover point, to smooth it out?

[Freebaggin]

Has your tuner tried adding extra timing at the switchover point, to smooth it out?

No he hasn't tried that, might see what he thinks.

I just came from his shop and he mentioned he wasn't 100% sure whether the inlet cam timing is at 0° or not. He thinks it's advanced as per factory setting and might be worth making sure it's at 0° as per Tomei recommendations.

[Al]

I assume you have an aftermarket ecu, that can controll the VCT trigger point.

The best way to determine the rpm trigger for the VCT is to complete 2 dyno runs. One with the VCT off (set very high eg:7500rpm) and the second with VCT turned on (set very low eg:1000rpm). Plot the two runs on the same graph and at the point where the lines cross will be the rpm that you should use to turn on the VCT.

[RB30-POWER]

Also double check that the air/fuel ratio's and boost pressure are consistant across the swithcover point, as these two things changing will make the power curve go funny.

Im sure they would have been checked anyway.

[Freebaggin]

All good points!

I have attached AFR chart. Yep, it looks like it richens up at the switch point......petty obvious, should have noticed.

Boost curve is fine, no movement there.

Will also do on and off dyno and overlay.

[RB30-POWER]

If you are running an AFM equipped engine management system.

What you may find is that, once the cam timing changes, it will allow a surge of increased air volume into the engine and overshoot the AFM voltage a little and cause richening of the mixtures at that point.

Lean out those load points to keep it consistant across the switch point and if you still have a dip, look elsewhere for problems.

[Freebaggin]

If you are running an AFM equipped engine management system.

What you may find is that, once the cam timing changes, it will allow a surge of increased air volume into the engine and overshoot the AFM voltage a little and cause richening of the mixtures at that point.

Lean out those load points to keep it consistant across the switch point and if you still have a dip, look elsewhere for problems.

It's a MAP sensor ECU.

[RB30-POWER]

No dramas then.

Smooth it out and let us know how you go.

[Cubes]

Freebaggin,

I remember Tim mentioned to me quite some time ago how it wanted to pin when VCT engaged.

Nice power but.. That snuk up there without me knowing. Last I remember it was around 270rwkw.

Still running the same turbo?

I suspect not as it appears to ramp up around 4k?

[Freebaggin]

Freebaggin,

 

I remember Tim mentioned to me quite some time ago how it wanted to pin when VCT engaged.

 

Nice power but.. That snuk up there without me knowing. Last I remember it was around 270rwkw.  

 

Still running the same turbo?

 

I suspect not as it appears to ramp up around 4k?

Same turbo, it's always ramped up to 17-19psi at around 4000rpm, it's just that the other mods like cams etc have stretched torque curve, it used to drop off at the top.

Tim never told me that re pinging, we'll see when we have another go.

[Cubes]

Nice power.

It was some time ago, right back when you first had the microtech fitted up.

[Sydneykid]

Typical MAP sensor driven ECU problem, when the inlet camshaft timing change occurs there is a change in manifold pressure, which the MAP sensor driven ECU incorrectly interprets as a sudden increase in airflow. So its adds some fuel to match the non existant extra airflow, hence the richness. An AFM driven ECU KNOWS that there is no sudden increase in airflow, because it actually measures the airflow, it doesn't GUESS the airflow.

This can be quite tricky to fix, as similar MAP load points occur at the same rpm with different throttle positions. It will need careful tuning for those other "matching" load points.

[Freebaggin]

Typical MAP sensor driven ECU problem, when the inlet camshaft timing change occurs there is a change in manifold pressure, which the MAP sensor driven ECU incorrectly interprets as a sudden increase in airflow.  So its adds some fuel to match the non existant extra airflow, hence the richness.  An AFM driven ECU KNOWS that there is no sudden increase in airflow, because it actually measures the airflow, it doesn't GUESS the airflow.

This can be quite tricky to fix, as similar MAP load points occur at the same rpm with different throttle positions.  It will need careful tuning for those other "matching" load points.

There is no change is manifold pressure at or after the switch point, going by the boost trace, so I'm not sure what you are saying is entirely correct but thanks for the contribution.

I suspect it will be an easy fix via the various suggestions above.

[Sydneykid]

There is no change is manifold pressure at or after the switch point, going by the boost trace, so I'm not sure what you are saying is entirely correct but thanks for the contribution.  

I suspect it will be an easy fix via the various suggestions above.

You haven't posted up the boost trace:cheers:

[Cubes]

Freebaggin, Still running the stock Bottom end?