Nissan 370Z Forum - View Single Post - Noob here with some questions. (Wastegate, Blow off)

Nixlimited · 09-09-2011, 03:29 PM

You have about a zillion questions so I will try to answer them all.

First, if you have a wastegate spring set to give you 6 PSI of boost, the wastegate will usually crack open well before you hit your boost target (here 6 PSI). That's because the properties of the spring metal are such that it will start to compress and then reach an equilibrium where the spring pressure trying to close the valve roughly equals the exhaust pressure impacting the wastegate valve and trying to open it. Depending on how far open the valve is (i.e. what ratio of exhaust is being diverted around the turbine versus flowing through the turbine still), you will get your boost pressure. Using a spring is not a very accurate way to control boost, which is why almost all high-end systems use a boost controller. Boost controllers offer much finer grain control over not only target boost, but boost onset, ramp, etc. With all that said, I imagine that running a huge wastegate on a car making low boost would be a waste and would probably make the control of boost pretty poor since even small action of the valve would divert too much exhaust flow. This implies, as well, that the actual flow of exhaust is something to take into consideration when sizing your wastegate.

Regarding the manifold pressure used with a boost controller, that usually comes from a small rubber tube which carries the pressurized air. You will see on wastegates and BOVs alike that they have nipples for receiving boost/vacuum lines which are used to affect the functioning of their valves.

I think you are confusing some terminology. You don't keep the manifold shut, and the manifold pressure is used in various ways. You have to think of the whole system as a bunch of different pressures pushing and in some cases pulling on valves. If you have more pressure on one side of a valve than the other, the valve will open, or stay shut, depending on which side the high pressure is on. When, for example, you lift off the throttle suddenly while in boost, your manifold pressure will quickly be significantly less than the pressure in the intake tract (pre-manifold) and those pressure differences can be used to actuate a valve, such as a BOV. Likewise, you can use positive manifold pressure to increase the force holding a wastegate shut by having that pressure push against the WG valve in a way that it stays shut. Keep in mind that the boost controller modulates the amount of manifold pressure being applied against the WG so that you don't get runaway boost. I think that's perhaps what you are thinking could be dangerous. Accordingly, most boost controllers are meant to fail in a way that they do not apply closing pressure to the WG valve.

You can't use boost to create more boost in the way you are suggesting. However, when you do increase boost then you create more exhaust flow, which will flow through the turbine and create more boost (depending on the action of the WG). However, if you just tried to flow intake tract boost pressure back to the turbo, you would actually be flowing lower pressure towards high, which wouldn't work, since there are losses as the compressed air makes it way through the tract (e.g. through the intercooler).

Yes, you can get a large WG and put a small spring, but that's likely not a great match for the reason I discussed above.

09-09-2011, 03:29 PM	#10 (permalink)
Nixlimited A True Z Fanatic Join Date: Aug 2009 Location: San Diego Posts: 1,235 Drives: 15 GT-R / 11 Prius Rep Power: 26	You have about a zillion questions so I will try to answer them all. First, if you have a wastegate spring set to give you 6 PSI of boost, the wastegate will usually crack open well before you hit your boost target (here 6 PSI). That's because the properties of the spring metal are such that it will start to compress and then reach an equilibrium where the spring pressure trying to close the valve roughly equals the exhaust pressure impacting the wastegate valve and trying to open it. Depending on how far open the valve is (i.e. what ratio of exhaust is being diverted around the turbine versus flowing through the turbine still), you will get your boost pressure. Using a spring is not a very accurate way to control boost, which is why almost all high-end systems use a boost controller. Boost controllers offer much finer grain control over not only target boost, but boost onset, ramp, etc. With all that said, I imagine that running a huge wastegate on a car making low boost would be a waste and would probably make the control of boost pretty poor since even small action of the valve would divert too much exhaust flow. This implies, as well, that the actual flow of exhaust is something to take into consideration when sizing your wastegate. Regarding the manifold pressure used with a boost controller, that usually comes from a small rubber tube which carries the pressurized air. You will see on wastegates and BOVs alike that they have nipples for receiving boost/vacuum lines which are used to affect the functioning of their valves. I think you are confusing some terminology. You don't keep the manifold shut, and the manifold pressure is used in various ways. You have to think of the whole system as a bunch of different pressures pushing and in some cases pulling on valves. If you have more pressure on one side of a valve than the other, the valve will open, or stay shut, depending on which side the high pressure is on. When, for example, you lift off the throttle suddenly while in boost, your manifold pressure will quickly be significantly less than the pressure in the intake tract (pre-manifold) and those pressure differences can be used to actuate a valve, such as a BOV. Likewise, you can use positive manifold pressure to increase the force holding a wastegate shut by having that pressure push against the WG valve in a way that it stays shut. Keep in mind that the boost controller modulates the amount of manifold pressure being applied against the WG so that you don't get runaway boost. I think that's perhaps what you are thinking could be dangerous. Accordingly, most boost controllers are meant to fail in a way that they do not apply closing pressure to the WG valve. You can't use boost to create more boost in the way you are suggesting. However, when you do increase boost then you create more exhaust flow, which will flow through the turbine and create more boost (depending on the action of the WG). However, if you just tried to flow intake tract boost pressure back to the turbo, you would actually be flowing lower pressure towards high, which wouldn't work, since there are losses as the compressed air makes it way through the tract (e.g. through the intercooler). Yes, you can get a large WG and put a small spring, but that's likely not a great match for the reason I discussed above. __________________ Car Sold! Pleasure being a part of the Z community. New ride: 2015 GT-R - 647 WHP / 641 WTQ (e85) & 543 WHP / 519 WTQ (91)