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GTS25t: Boost falloff with revs (Powerdigger EBC)

sloth

By sloth,
in Engines & Forced Induction

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sloth Apprentice

sloth

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I notice from the boost vs rpm plot I received from my dyno tune that the boost gradually falls off from about 10.5psi down to 8.5psi at redline. I am using the Powerdigger Hybrid EBC. This took me by surprise, because I've never noticed this falloff on the factory boost gauge.

Is it correct that the factory ECU creates the boost level signal from the airflow meter signal? If so, could the falloff be due to a non linearity in the conversion of airflow to boost pressure? It just seems to me that the EBC might be doing it's job as best it can, using the boost level signal from the ECU, and that the only way to fix this would be to install a MAP sensor. (although I have no idea whether the Hybrid EBC could directly make use of a MAP sensor).

Greg.

sloth Apprentice

sloth

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Could be that the stock wastegate actuator isnt strong enough to hold that amount of boost.

I had a similar problem, I changed the wastegate actuator for a garret adjustable item. Now it holds boost right to redline.

Adam

The EBC's solenoid fits in *series* with the pressure line to the wastegate actuator, right? (i.e, it's not a bleed type solenoid) This means that when the solenoid is in the closed position (blocking all pressure from reaching the wastegate actuator), the wastegate would have no pressure on it, and the boost would rise to whatever the wastegate would allow. Do you mean the actual wastegate actuator, or the wastegate itself?

Greg.

Zombie Collaborator

Zombie

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Powerdigger are telling me the car *does* have a seperate boost sensor - i.e - seperate to the airflow meter. Now I'm confused!

Greg.

IIRC the stock boost gauge takes its reading off the back of the inlet manifold. AFM does not read boost pressure.

Zombie Collaborator

Zombie

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Posted
The EBC's solenoid fits in *series* with the pressure line to the wastegate actuator, right? (i.e, it's not a bleed type solenoid) This means that when the solenoid is in the closed position (blocking all pressure from reaching the wastegate actuator), the wastegate would have no pressure on it, and the boost would rise to whatever the wastegate would allow. Do you mean the actual wastegate actuator, or the wastegate itself?

Greg.

A combination really. The stock wastegate actuators are sometimes not strong enough to hold the wastegate shut, so the wastegate opens more and boost drops off.

Adam

sloth Apprentice

sloth

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A combination really. The stock wastegate actuators are sometimes not strong enough to hold the wastegate shut, so the wastegate opens more and boost drops off.

Adam

Ok, thanks. I do know that if I crank up the EBC's boost setting, the boost skyrockets (to the point of inducing a fuel cut), so if the problem is with the wastegate actuator, it must be a problem that only exhibits itself after being on boost for a relatively long time, because it is definitely strong enough for a short burst.

Greg.

Kinks Grand Master

Kinks

Posted
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Skylines certainly do have a MAP sensor, it's used to provide the fuel cut if you overboost the motor. The AFM is used for fuelling though.

Sometimes the turbo can't flow the extra boost at high rpm. When you crank it up and hit boost cut, is it still holding the higher boost near redline? Or did you only test at ~3k RPM... my turbo will hit 15psi in the midrange but drops back to 10-11psi by redline, using a bleed valve.

sloth Apprentice

sloth

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Yes, it is the stock turbo, but I do have a dump & 3" exhaust.  

Thanks,

Greg.

For what it's worth, here's my dyno plot - looks like I slightly exaggerated the boost dropoff - it only drops to about 9.5psi, not 8.5psi as I said earlier. Is this small dropoff normal/expected with my setup? (air filter is a K&N panel by the way)

Greg.

sloth Apprentice

sloth

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Skylines certainly do have a MAP sensor, it's used to provide the fuel cut if you overboost the motor.  The AFM is used for fuelling though.

Sometimes the turbo can't flow the extra boost at high rpm.  When you crank it up and hit boost cut, is it still holding the higher boost near redline?  Or did you only test at ~3k RPM...  my turbo will hit 15psi in the midrange but drops back to 10-11psi by redline, using a bleed valve.

Ok, thanks for the info. I called a performance workshop and they too said that the standard turbo may have trouble flowing 10.5psi at high revs, although if the boost is set to, say, 15psi, it will drop to about 10psi at high revs - this agrees with your observsations.

I certainly did not test the high boost to redline - as soon as I heard the engine splutter, I pulled back. (I didn't even know about fuel cut overboost protection when I did this little test - I thought I may have blown my engine and was very relieved. I had assumed that the workshop would probably have calibrated my EBC such that it was not possible to crank the boost up so high - boy was I wrong!)

Greg.

p.s I can't seem to figure out how to get the attachment to upload - sorry.

sloth Apprentice

sloth

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Powerdigger helped me find the MAP sensor - it's located behind the driver's side top suspension mount, just in front of the firewall. I can clearly see the hose going to the manifold. I can also see a seperate wire spliced in to the cable that runs in to the cabin - this must be the pressure signal for the EBC. (the Powerdigger Hybrid EBC has an open loop mode - I thought *perhaps* my EBC had been wired for open loop mode, but fortunately that's not the case, unless of course there's a fault in the cabling). If I do decide to monitor this boost signal, it's going to be fairly easy to access it. I'm probably not going to bother though, because it seems that the slight boost falloff I'm experiencing is probably entirely normal.

Greg.

Kinks Grand Master

Kinks

Posted
Posted

Hmm interesting. I would have thought a MAP sensor would be better but I guess if a hose pops off the MAF sensor method will still stop the turbo from overspinning..

I guess it follows then that if you increase airflow (intake exhaust and head/cam work) then the stock ECU will stop the fun at lower boost levels because you've got more flow. So the 'boost cut' will be limiting your total power output rather than boost per se...

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