Overboosting!

[Sampo]

Hi,

Im wondering if i can get some advice, The car was running fine with a 3" exhaust straight off the turbo leading into a 2 1/4" rear muffler (used to pass epa). I have now changed the rear muffler back to the 3" muffler i have run in the past but my boost levels have sky rocketed.

The car has a peice of brand new rubber hose running between the compressor housing of the turbo straight into a 5 psi actuator on a GT35/40R internally gated turbo. The dump pipe is a split type setup and the wastegate flap does not foul (easy 90 degree movement).

The engine specs are:.

RB30 bottom end, RB25 head 8.0:1 comp ratio.

Standard rb25 exhaust manifold

internally gated GT35/40r from a BA XR6T

5 psi actuator

3" exhaust from the turbo back.

Any ideas?

[89CAL]

Thats a small actuator on such a big turbo, so has it run this setup previously with no boost issues?

[scotty nm35]

The internal gate can't keep up, you would probably need a 60mm external gate in order to safely keep the boost that low. Why don't you just get a tune and bump it up?

[Sampo]

it has, kinda. The injectors and plenum have been changed, but yes this setup has run before.

[kspots08]

How much boost is it supposd to be running and what does it spike too?

Have you had the flap in the rear housing machined out and a bigger flapper put on. This will have some effect on over boosting.

Also i would get a stronger actuator. something up around 16psi. It will help to control boost a bit better.

[GTRNUR]

Have you tested the actuator with an air compressor to see if it begins to open at 5psi?

1. Dial back the regulator on the air compressor to fully closed.

2. Connect the air hose with an accurate pressure gauge, to a hose running only to the actuator.

3. Dial up the regulator slowly and observe the pressure that opens the gate. It should be very low. If it hasn't started to move by 5psi then its stuffed.

4. LISTEN for leaks in that actuator. It might have some pinholes in the diaphram.

I can't see how an internally gated 3040 from an xr6 with the standard actuator and 5psi boost setup would overboost on an engine that has 1lt less capacity. It just won't happen. So either the actuator is seized or has an internal leak (pin hole), or the wastegate mechanism has seized in the closed position

[Sampo]

Thanks for the responses, Its fitted with a 5 psi actuator from the rb25, but would climb to around 16psi at 7000rpm (became doughy in patches, then would clear up and continue to rise. Loading a different map onto the ecu has brought this figure back to the 9psi mark which seems more manageable and improved torque curve.

Given the size of the waste gate is apparently an issue when running so little boost, is it likely to become more of an issue when i actually have this setup tuned for around the 300+kw mark?

Does anyone on here have much experience tuning these engines, not really interested in playing with the fuel map myself but would like a better understanding of what is actually going on. I can include a log from the last power run, but whats happened is the afr's would fluctuate between the high 10 - low 11 mark through the entire rev range, however at 5740rpm the afr shot to 20.8:1 the car surged then it dropped back to a safe ratio just as quickly as it happened.

[KiwiRS4T]

What kind of ecu do you have? Have you had it tuned on a Dyno by a competent tuner?

[Rolls]

Given the size of the waste gate is apparently an issue when running so little boost, is it likely to become more of an issue when i actually have this setup tuned for around the 300+kw mark?

The wastegate is used to control boost, so the more exhaust gas that is diverted out of it the less that goes through the turbine, eg the less boost the turbo generates, if you make more power (run more boost) then the wastegate will be exhausting a lower ratio of gasses, however you will now be making more gasses overall. It will almost certainly be flowing less as you'll be going from 5psi to 20+ psi but I personally haven't had experience with that size gate/turbo to tell you for sure, if I had to guess I'd say it would be far less of an issue though.

My advice is to find a competent and experienced tuner familiar with skyline engines and get them to sort it out for you.

Edited July 26, 2012 by Rolls

[Hybrid1310]

If you want proper boost control i would say 44-50 mm gate you shouldn't have any issues,