okay, so me and the user formerly known as frestylereaction were chatting, and discussing boost control.



what controls the stock boost valve, the ECU? or do vaccuum line control it? if so, then how does something as simple as a $20 ebay valve regulate boost? a good diagram would be most helpful, and dont put up some FSM crap! i've looked at it, and still dont understand.



i looked over the FSM and saw that two things regulate boost: the "wastegate: control the amount of exhaust gas bypassing the turbine to control boost pressure" and the "twin scroll turbocharger solenoid valve: controls exhaust gas flow area"

 

your wastegate has a plunger inside that is pulled open when boost reaches a certain pressure. A boost controller, manual or electronic, interrupts this pressure to the wastegate and only allows the wastegate to see pressure when it reaches a specified point, this is as long as you aren't talking about the garbage bleeder valve type MBC. I'm not exactly sure what the point of twin scroll is, i think its supposed to help you boost earlier or something but i tossed mine.

 

twin scoll blocks off one side of the divided turbo, under 2500rpms to force all the exhaust gasses thru the turbo at a higher speed. i dont think it works very well, and 89 setup doesnt have that and has less lag. it does do a nice job of keeping the car quiet at low rpms



mike

 

so, up untill ~3K rpm, the twin scroll keeps boost low, and after that the wastegate controls boost my opening at a maximum boost pressure, and is activated by the boost pressure itself? kwel, thanks for the help.



what about boost creep? how does that happen?

 

boost creep occurs when the wastegate opening is physically not big enough to flow enough exhaust to divert away from the turbine wheel and it keeps boosting even with the wastegate all the way open. The gas will always follow the path of least resistance which on a stock car is the wastegate, however on a modded car the aftermarket exhaust upsets the balance and makes the turbine side flow easier up to a certain psi than the wastegate side so that even with the flapper open, it keeps boosting.

 

the twin scroll is trying to get the turbo to boost quicker, or have less lag, by making the exhaust gasses go faster into the turbo. the wastegate is always trying to limit boost to its spring pressure (5.5psi in this case).



think about it like this. the speed of the turbine wheel is what give you boost pressure. the waste gate allows exhaust gasses to bypass the turbine wheel, therfore it will slow down.



mike

 

got it.



but by porting my wastegate, i will no longer get my 10 psi boost? what boost can i expect? i guess i could just port it, then get a mbc to get it back up to 10 psi

 

Porting your wastegate ideally should just let it bypass the turbine sufficiently to prevent boost creep. It won't open any earlier, just that its a bigger hole when it does open. I was under the impression that the factory wastegate actuator is set at 10 psi.