One thing that the 323 GT-R can really suffer from is boost creep. This is where the boost rises as the rpm rises. On a standard car, this never happens, as the restriction in the exhaust and intake paths prevent it from occuring, however once a 3 inch exhaust is fitted, it is very easy for the boost to spiral out of control!
On a standard GTR, the boost pressure is 0.5 bar up to 5300 rpm and then 0.6 bar from 5300 rpm to the rev limit. This is controlled by a solenoid mounted on the turbo, switched by the ECU. Even fitting a 60mm bore exhaust and still retaining the cat, or just gutting the cat will cause the boost to rise from this level, typically to around 0.7 - 0.8 bar. A 2.5 inch exhaust with no cat normally sees the boost reach 1 bar, and a 3 inch bore or bigger will see boost levels over 1.3 bar unless something is done to reduce the creep.
On my own GTR, I had seen boost levels creeping to 1.8 bar. Following my engine rebuild, I needed to have better boost control, hence I needed to find why the boost was creeping and stop it happening!
The theory is that boost creep happens because the turbo wastegate is to small, and hence cannot flow enough air to bypass the exhaust turbine. This means most of the exhaust gasses pass through the turbine, causing the boost to rise, and this increases with engine revs.
The first step to resolving the problem should be to enlarge the wastegate, and this is what I tried. The pictures below show the wastegate after enlarging. The circular mark on the flap shows the original wastegate size (around 20mm), whereas the diameter of the flap is 30mm. Using a TCT cutter, I enlarged the hole to 26mm, and smoothed the entrance path to the wastegate from the exhaust manifold.
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The results of this attempt were dissapointing. Yes, it had improved, but not by much, with boost still creeping to 1.5bar...So, what next???
It was time to do some testing and find out what was really happening! The first step was to make up a test rig, consisting of a foot pump and a boost gauge...
With this, I carried out tests to see how far the actuator actually moved at various pressures. The actuator should open fully at 0.5 bar pressure, however my tests showed it needed quite a lot more!
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As the pictures show, the wastegate only just begins to open at 0.5 bar, with the flap only a tiny fraction off the seat. As the boost level rises, the flap continues to open, and at 1.5bar, it's around 12mm from the seat at the widest point.
This shows that the boost creep on my car is mainly caused by the actuator, which should open at 0.5 bar and move the flap far enough from the seat to maintain that boost level.
My first idea was to replace the actuator, however I have been unable so far to source an original replacement or find something that might fit, due to the tight mounting space. I am still hopeful of making a garrett unit fit...
In the meantime, I decided the best option was to port the wastegate to improve flow as far as possible. Firstly, I had a larger flap made to cover the hole, 34mm diamter. The actual wastegate hole was then drilled to 32mm, and the inside of the turbo enlarged as far as possible to maximise flow to the wastegate. This was all smoothed and blended. I also modified the exit side of the flap, adding a slight radius to the hole, and carefully removing any casting that may cause a shrouding effect on the wastegate. If you compair the pictures below to those at the top of the page, you can see how much metal has been removed. Originally there was a lip where the seat had been machined, and I have removed this as far as possible to help flow. The work was carried out using TCT and stones with a die grinder and flexi shaft.
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