Howdy,

My St185 that i recently bought has been fitted with a t3t4 turbo, 3 inch exhaust, aftermarket blow off valve between the turbo and intercooler and a dodgy bosch manual boost controller. The whole setup is basically new and the fresh engine has only done about 3000kms now. It all run great except... it did have a big exhaust baffle in it to limit the boost. With it fitted it could just reach 5-6 pounds and was slow as heck. With this bigger turbo it need about 3-3.5k revs to get it moving. I have now removed the baffle and turned the mbc all the way down and the car still wants to boost way past 15 pounds which with standard injectors is obviously bad news.

It appears to still have the standard wastegate fitted so I am wondering if the factory spring inside cant handle the bigger turbo? Is this logical or not? All the lines are brand new and well plumbed in by the previous owner who is a senior engineer in the airforce here so I am inclined to believe he knew what he was doing.

i am thinking of bypassing the mbc to see if it still spikes right up, is this a good idea or it it clear from what I am saying that it going to be something else?

Would appreciate any help and advice.

Cheers

Rob

_________________
Understeer: when you hit the wall with the front of the car.
Oversteer: when you hit the wall with the back of the car.
Horsepower: how fast you hit the wall.
Torque: how far you take the wall with you.

Hi Rob,

It is common to experience overboosting when running a full 3" exhaust.

The cure? It's all in the turbo... The wastegate cannot pass enough gas in order to slow the damn thing down...

Take your pick - Replacement turbo, ported wastegate or fit an exhaust baffle

This is the path through the wastegate...


And this is the "hole" that the exhaust gases pass through...


The above are pictures from a 165 turbo, yours has a twin wategate - the holes are too small and the path to them won't be as good as it could be to get the gases out of your two small holes...

By far the best aftermarket turbo for all GT4 is the S148 single exit wastegate turbo from Turbo Technics - the DC used to have a discount with them.... great bit of kit!

Hope that helps?

_________________
Kris
95 ST205 - slept for 10 years, now waking up...
Avensis
http://www.gtfours.co.uk
_______________________________________
Turbocharging - the replacement for displacement

I still don´t belive the story of the undersized wastegate. I´m running a stock CT20 in my CS with a full 3" exaust ( Aussi DP, Aussi FP, HKS silent HighPower) and I don´t have problems to keeping the boost at 1.15 bar, controlled by a Blitz SBC iColor spec R and a actuator from Forge, fitted with a upgraded spring.

I think it´s more the weak stock actuator that cause this problems, supported by downpipes that are to small to let the wastegate fully open and the wrong form to let the exaust gases stream out of the wastegate.

For what it's worth I don't have an issue either and I run the stock turbo with a 3" system...

However, a lot of people have troubles with boost, as soon as they swap to a 3" system.


You may well have a point here, I have heard of crappy downpipes that don't let the wastegate fully open

_________________
Kris
95 ST205 - slept for 10 years, now waking up...
Avensis
http://www.gtfours.co.uk
_______________________________________
Turbocharging - the replacement for displacement

Weak stock actuators have been proven to be an issue with boost control but NOT for overboost but underboost. IIRC ATS racing added an additional spring to a standard wastegate and managed to get a standard MR2 CT26 to hold around 21psi to the redline - prior to the extra spring it only held about 12psi.

Any exhaust gas has to flow either through the turbine wheel or wastegate. Reducing boost creep means either increasing flow through the wastegate or reducing flow through the turbine. As such changing to a hybrid that probably has had a turbine wheel clipped to aid flow results in a higher liklihood of boost creep.

IMHO the main issue is that one of the two turbine runners requires the air to turn 90 degrees very sharply to go through the wastegate.

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

Agreed

Thinking about it, a weak spring has little effect except lower boost pressure.

A split diaphragm for the actuator will have more of an effect as the car will tend to overboost or have erractic boost pressure

_________________
Kris
95 ST205 - slept for 10 years, now waking up...
Avensis
http://www.gtfours.co.uk
_______________________________________
Turbocharging - the replacement for displacement

Cheers for the replies guys. Going to pull it all apart this weekend and have a looksee! Will let you know my findings

_________________
Understeer: when you hit the wall with the front of the car.
Oversteer: when you hit the wall with the back of the car.
Horsepower: how fast you hit the wall.
Torque: how far you take the wall with you.

A bad designed downpipe will restrict the opening of the waste gate so worth a check...