Ok, seems like there is a little bit of confusion going on here. Lets try and clear some of it up :)
I also don't see how having a 3" exhaust would increase your base boost pressure 8psi beyond the factory boost setting.A worn or stuck diaphragm in the actuator could be the problem. Swapping out to a known working actuator or buying an upgraded version off the shelf is what I'd do. I bet if you disconnected the actuator and tied the wastegate arm to the open position you wouldn't be building much boost, if any at all.
The reason for this is flow. The internal wastegate port has been designed to control boost at a predetermined flow window. So real quick lets go over this. When your wastegate is connected it keeps the wastegate port shut to use the exhaust gases to build heat and momentum in the turbine to spin the compressor to build boost. When the compressor builds the required boost level (7psi or whatever) then the wastegate diaphragm will see the vacuum signal and open the wastegate port. This bleeds of exhaust from the turbine housing and directs exhaust into the downpipe and those gases encounter the gases travelling from the engine and are met with turbulence and backpressure. This combines with the backpressure the turbine is already experiencing in the exhaust housing/downpipe. The combination of the wastegate port bleeding gases from the turbine housing and the backpressure on the turbine wheel keeps it at a set speed which then maintains the desired boost level.
Ok, so when you increase the exhaust size much further than the stock level, especially the downpipe then you effectively remove that backpressure from the turbine and can increase the backpressure on the wastegate port as the gases are now flowing into the massive flow increase from the turbine. So now instead of a nice transition you end up with spiking and boost creep the latter is what Dave is experiencing. This is caused by the stock wastegate diaphragm not being able to overcome the backpressure on the port quick enough caused by the increased flow in the downpipe. You also have the issue of the turbine spooling much sooner than it normally would. This can cause the actuator to stay shut a little longer and then blow the door open at the last second but by then the turbine is already spinning faster than it should be and now instead of making 7psi youre making 15. This issue compounds in that now the door is open at a later and higher boost level than it was designed for and the gases physically cannot get out of the housing fast enough to bring the turbine speed down to the level needed for 7psi so it stays at 15.
Ok second point If you were to force the gate open then yes it would make no boost. This is because turbos work from momentum and heat. Forcing the gate open before boost is made would bleed off the heat and momentum needed to get the turbine to start spooling, this would present itself as a big obstruction in the engine and the wheel will effectively just freewheel in the housing and not build any boost. The problem with small turbos and big exhausts is managing the momentum already there and keeping it at a set level. If you try to manage the momentum before its there then it wont be a reliable test and wont tell you anything and the engine wont make any power as it will be seeing a big exhaust restriction and not increase in intake flow. I had this happen to me when the wastegate pin broke on my 16t and I had less power than I did when I had a sick B21. The engine wouldnt even rev out to redline and used a tonne of fuel. Its like sticking a potato in your exhaust hahaha.
i was reading about the diode mod just before actually , if its all controlled by the maf on lh2.4 i dare say i would be close to hitting fuel cut but im not which is weird , it hits 15psi everytime on WOT and seems to just do its thing haha
The reason why youre not hitting fuel cut is because the MAF isnt reading pressure its reading airflow. A 13c is a very small turbo that doesnt flow very much once it gets above 12psi. If you were reaching 15psi with a larger turbo like a T3/T4 or even a 15g then you would be hitting the airflow governed fuel cut as your flow would be outside the acceptable window for LH2.4. Look up turbo CFM to see what I am talking about. 15psi from one turbo is not the same as 15psi from another if both turbos are different sizes. LH will be seeing the air being drawn in by your turbo and it doesnt care if its at 40psi or 7 psi as long as the volume of the air does not exceed the governed limit for which it can supply fuel for. At 15psi with a small turbo like a 13c your biggest concern is heat as the 13c become severely inefficient at pressures higher than 12psi and the boost being made turns to superheated air. You should resolve this before summer as you will have heatsoak and potential detonation caused by increased intake temperatures.
i dont really want to modify my exhaust again considering it just got done and cost a decent chunk of change , all i do know for sure is i need a wee bit less boost haha
Your exhaust is fine. Both of my redblock turbos had 3 inch exhausts. The problem is going to be the small 13c is a very small turbo and as such doesnt take much momentum to get it spooling and the large exhaust gets it spooled and keeps it spooled very quickly. The wastegate simply isnt flowing enough to control that momentum.
I'd just get a new actuator and go from there. You can either put in a 15psi springs and run it like that or put in a lighter spring and run your MBC to the desired boost pressure.
I doubt you'll be hurting anything by running 15psi on the daily - Especially if your AFRs are good.
If you get another wastegate and it allows you to bring your base boost down, then just use your MBC and play around with a nice combination between power and piece of mind.
I agree that he should get another actuator, but not for the reasons of hoping to resolve the boost creep. I would be getting a new actuator for the increase in boost response and for a more stable boost curve. I highly doubt a new gate actuator is going to resolve the flow issue of the small 13c port in a large exhaust. I would be looking into porting the gate port and go from there. Do some research on turbobricks as this subject has come up plenty of times from people installing large exhausts on 13c equipped cars.
He’s just using the stock 780 Turbo turbo, actuator, and also the Turbo+ kit that activates a bleed valve on the actuator dashpot to achieve higher boost over 3750 RPM IIRC at full throttle only. However, thinking about it he might have a Garrett turbo and not a Mitsubishi...so that could be a difference.
The 780 turbo came with a Garrett turbo which has a much larger wastegate port. In addition to the larger port the turbo is larger and slower to spool. Therefore its mass helps control its momentum when installed in a large exhaust. A 13c is an incredibly small turbo designed for quick spool in a standard exhaust. The two are not comparable.
Sorry for the massive wall of text and I'm not trying to correct everyone. You all are on the the right track and have great suggestions but I just thought id steer us into the more "correct" direction with a little bit of theory added so you guys can all hopefully learn something new. Ive had this exact same issue with a 12b on a B230FX with a 3 inch exhaust. It would spike and overboost if I punched it and had to always roll onto the throttle to keep it remotely controlled. Swapping to a 15g with a ported gate and conical housing resolved the issue as the conical housing introduced backpressure on the turbine and the ported gate helped flow out the gases. Its all good fun at the end of the day and props to you Dave for getting the car up and running. Dont keep discounting your skills and knowledge, you will find you know alot more than you think and at the end of the day you are out there with your tools and time getting your hands dirty and thats what counts.