boost creep

[Ender]

Originally Posted by JBurer

Boost,
My S5 has a ported motor, and this is exactly what I'm seeing. 2nd gear hits 11psi and then slowly drops to 5psi by around 8k rpms.

The statement I keep reading is that the turbo can't keep up - this makes no sense to me as the wastegate would completely close if boost pressure dropped below the target.

First gear is different for me, I see only 8psi of boost, which also drops to 5psi. Anyone have an idea whats going on here?
Best,
John

In my case at least, and I'm sure a lot of other guys use these, too, it's the manual boost controller that's part to blame. Unless electronically monitored I don't think there's a real smooth way for a boost controller to hit peak boost and simply keep it there unless it's set at or only slightly above the stock boost level. A bleeder valve simply isn't the best solution, but since it's very cheap we do it anyway. At 4000 RPM and 12psi there's a lot less air moving than at 7000 RPM at 12psi, that little bleeder valve can only release so much air so fast. What does this mean? It means the wastegate actuator ends up seeing more pressure as the RPM rises while boost is up, which will unfortunately bleed boost through the wastegate.

Another factor is the turbo itself. For one, the stock turbo is built for holding stock boost levels on a stock motor. It can perform better than that, but we can't expect too much from it. It can only be pushed so fast before it just becomes a heating fan and stops helping you make more power. What I've read is that the stock turbo is pretty useless past 12psi and many people have blown stock turbo's or apex seals by boosting higher on the stock turbo. You may help the situation a little with a FMIC, but not by a lot. The stock turbo simply can't push a ton of air at high RPM. You might look into BNR turbo's if you're looking for a cheaper turbo upgrade. I've heard a lot of people are happy with that route.

I have all my intake and exhaust ports mildly modified. The funny thing is that even if my boost peaks around 5000 RPM I can tell I'm making more power at 6000 RPM at ~3psi below peak boost than at 5000 RPM at peak boost. I'm aching to get a good electronic boost controller to see how much improvement I can make over the stupid MBC. I'll probably start roasting tires at the top of second gear...