Power Drop at High RPM Range

Hi there,

I recently made some modifications to my GR Yaris, including installing a high-flow turbo, cylinder head studs, and an HKS gasket. Yesterday, I took it for a test drive with a target boost set to 1.9 bar. During the test, I noticed that the engine was hesitant to rev in the high RPM range. It would only reach 6500rpm and couldn't reach the redline of 7000rpm. Today, I went for a dyno run, and the power curve showed a significant drop in power from 5000rpm (which was noticeably lower than a previous dyno run I did two months ago). I conducted tests with different target boost levels (1.5, 1.7, and 1.9 bar) and different cam angles (0, 10, 20, 30, and 40 degrees), but the power loss issue persisted in the high RPM range. Additionally, I observed that the power output for different cam angles was nearly identical, as if the VVT (Variable Valve Timing) was not functioning properly. However, upon checking the datalog, I confirmed that the cam had followed the cam angle target.

PS: In addition to the modifications mentioned above, my Yaris is equipped with Kelford cams and springs kit. It also has an aftermarket intake and a 3" decat exhaust system.

Does anyone have any insights into what could be causing these issues?

Thanks in advance.

What is the exhaust pressure with the new turbo? Perhaps you are just falling off the efficiency curve of the turbo. Can you re-install the original turbo and test it?

I don’t have sensor to measure the exhaust pressure.

The supplier (Fensport) states that the high flow hybrid turbo has larger turbine wheel that can hold 2 bar of boost to the redline and is supposed to make more power for entire range. However, it makes less power than the stock turbo (approx. 30whp loss) at high rpm range. I tried running with lesser boost (1.5 and 1.7 bar), the power still drops rapidly after 5000rpm.

So I think the power loss not caused by the turbo right?

According to the datalog, more information to provide.

Boost: able to hold the boost to the target (1.5, 1.7, 1,9 bar). IAT stable at around 50C.

Fueling: the afr is close to perfect.

Ignition advance: was tuned close to the threshold. No knock and no ignition retard was found.

Ingition system: stock coil on plug and spark plug. Battery voltage 13v. Dwell time 3.5ms

How sure are you the cams were timed correctly after the head was off to replace the studs? Was it done in the car, or was the engine removed?

It was done in the car. Will check again if the cam is exactly aligned with mark.