Why my RB20T drop boost when reach high RPM? Please HELP!

[kelvin_goh999]

Guys,

Can anyone help me the problem above?

Im driving a Cefiro with Skyline GTS engine RB20T. The problem that I'm having is when I test my car on the straight road (DRAG), 1st gear when I press full on the fuel pedal, it goes up to 0.8bar & when it reaches around 5k-7k rpm, the boost drop slowly until 0.6-0.7bar. It sames goes to 2nd gear, 3rd gear, 4th gear & 5th gear. (I try it in d afternoon & night as well).

Anyone experience this before or can help me to figure out what is the problem? I think a normal turbo car should be maintain the boost. Am I right?

By the way, my RB20T is standard. I just modified air-filter & exhaust only.

Your help & advice would be appreciate. Thanks.

P/S: Got a friend told me that the actuator spring is not strong enough anymore, so it opens more & the boost drop when it reaches high RPM. Do you guys think so? He ask me to modify & add additional spring on the actuator.

Kelvin

 

[GSR_Freak]

best way is to find another actuator, drop it in test it, to confirm whether the actuator is the main problem or not. I would suspect the actuator.

 

[defcon1]

Guys,

Can anyone help me the problem above?

Im driving a Cefiro with Skyline GTS engine RB20T. The problem that I'm having is when I test my car on the straight road (DRAG), 1st gear when I press full on the fuel pedal, it goes up to 0.8bar & when it reaches around 5k-7k rpm, the boost drop slowly until 0.6-0.7bar. It sames goes to 2nd gear, 3rd gear, 4th gear & 5th gear. (I try it in d afternoon & night as well).

Anyone experience this before or can help me to figure out what is the problem? I think a normal turbo car should be maintain the boost. Am I right?

By the way, my RB20T is standard. I just modified air-filter & exhaust only.

Your help & advice would be appreciate. Thanks.

P/S: Got a friend told me that the actuator spring is not strong enough anymore, so it opens more & the boost drop when it reaches high RPM. Do you guys think so? He ask me to modify & add additional spring on the actuator.

Kelvin

You are experiencing what is called "boost creep". There are a number of causes, but since I have no knowledge of the capabilities of the RB20 turbo, I will not try to venture a guess. I will, however, list the possibilities for you...

a) Your actuator. Your friend could be right. A lot of stock turbo set-ups tend to lose some boost pressure at high rpm because the actuator spring is too soft. Most of the time, this is intentional since the car is sold as a street vehicle. There are a couple of ways you could solve this.
i) Change the actuator spring.
ii) Add a shim or some washers under your actuator mounting to push it out i.e. compress the current spring a little more.
iii) Install an aftermarket electronic boost controller.

b) Your turbocharger. If you exceed your turbocharger's maximum flow capacity, your boost pressure will begin to drop as your engine demands more air than your turbo can deliver. As I have admitted earlier, I have no idea what turbo the RB20 uses. My experience has been with Toyota motors so far. But in my old car, the stock 3S-GTE came with a CT-26 turbocharger (ceramic turbine wheel). Without an electronic boost controller, I'd experience boost creep no matter what I did with the stock actuator. With an electronic boost controller, it hit 1.0 bar and held steady. After re-machining and modifying in a Garett TO4-50 compressor wheel, it held steady at 1.35 bars. There was headroom for up to 1.5 bars, but I didn't want to blow the motor...

 

[a31gtst]

RB20det stock boost is 0.6bar.
Your boost is creeping by 0.2 bar, check if the vacuum hose has a leak or is damaged just to make sure thats not the problem as that is the easiest to check.
Quite likely it could be your actuator, if you have a mech who does conversions you might be able to get a spare actuator and put it in to try. I have a spare RB25 actuator which is also 0.6bar, if you're interested let me know.

An option for upgrade is a HKS adjustable acutator, the spring is much stronger and it holds boost much better than a stock actuator wth a boost controller.

Cheers!