Noob here with some questions. (Wastegate, Blow off)
 

First, I did use the search feature, and looked at the noob sticky post.

I am a complete noob to forced induction. I have been reading rather extensively on the subject. I'm not mechanic but I do my own car work. IMO most mechanics charge way to much per hour. So I basically learned everything I know about cars by searching and experimenting.

So my question about the wastegate is what size you should get. On this site

GTM Motorsports*::*FORCED INDUCTION*::*GTM-370Z-(TUNER) TWIN TURBO KIT

^my planned stage 2 twin. It has options for the wastegate. Is this tunable(adjustable PSI)? If it is tunable is there a reason not to get the biggest one and tune it down to what you need? I would suppose larger ones could divert more exhaust away from the turbo. Is there a disadvantage somehow to having a larger one? If you guys could give me some info on this it would be great.

I have been reading on here for some time in preparation. I will be ordering my 370Z Nismo in January 2012 (hopefully '12 model by then) and hopefully ordering my turbo kit shortly after. I plan on making an entire build thread out of it like other with millions of pictures and videos :) My goal is 500WHP. Also plan on doing all the work myself (aside from tuning which I will go to a shop for). So as a noob I have my work cut out for me.

Thanks for all your help, and I hope to be a good addition to the community eventually. :D

too small = boost creep too big = a wavy boost curve. The less exhaust you have to bypass the smaller your wastegate can be, so if you have a turbo setup that can make 30psi but you are going to have the boost controller set to 8 you need a big motha waste gate. If your turbo's are gonna max at 13psi and your set to 8 it won't need to be that big. Give Gtm a call about sizing for your particular turbo choices and desired boost levels.

You cannot turn down the boost on the the waste gate spring.
EXAMPLE: If you have a 8psi waste gate spring, the lowest you can turn the boost down with a boost controller is 8psi. However, you can add pressure to that waste gate with a boost controller, you just cannot go lower than what the waste gate is set to. Also you do not want to go too much higher than the stock either. If you have 8psi spring and plan to boost up to 30psi all the time, you will have issues holding that boost. Not that any of us will be boosting to 30psi except "maybe" Bullitt, lol.

Ahh thanks very much for the information. I guess I will give GTM a call and see what they recommend.

I would think since it is kind of a pressure valve that they would just make big ones with a variable setting. But then again, I don't know anything about turbo's yet.

I would also think you could bypass all this exhaust stuff, taking power from the engine (even if small), and just make an electrical compressor? Has anyone ever tried this?

actually you just change out the spring if you want to lower or higher the stock boost setting.

Also remember that external waste gates play nicer with electronic boost controllers than internal waste gates. And they sound much better too.

See you just confused me again. Is the wastegate solely responsible for how much boost the turbo builds up (PSI), by just letting certain amounts of exhaust passed (or not in the case of higher PSI)?

This wouldn't make sense to me as the larger wastegates show higher PSI on the site I listed. You would think the smaller wastegates would allow less exhaust to pass, getting more in the turbine, allowing a higher boost(PSI). Unless the larger wastegates, mean larger TO the turbine. Maybe I'm confused how the size relates to the wastegate, I assumed it was the size of the overflow(letting more pass back to exhaust pipes). Also thanks for the info on the external wastegate.

I'm sorry to ask but can you be a bit more specific on the exact functionality of the wastegate in reference to exhaust coming in and turbocharger (springs, compression, tuning, etc etc).

Not sure how much you know (and wikipedia is a good general reference), but a wastegate is used to divert exhaust flow around the turbo so the turbine speed is limited thereby limiting the boost pressure. Some wastegates are internal (i.e. built into the outlet side of the turbo) and some are external (e.g. Tial wastegates). Wastegates have sizes, usually in mm (e.g. 38mm) that refer to the size of the exhaust path through the wastegate. The bigger the wastegate, the more exhaust it can flow. Wastegates also have springs that limit the wastegate valve from opening before a certain pressure. The springs are typically rated in PSI of pressure. E.g 6 PSI spring. In concert with the spring, a boost controller can be used, which siphons pressure off the positive manifold pressure to push against the wastegate valve and keep it closed (in some designs). This is how you achieve boost pressure above a wastegate spring pressure. Typically speaking, if you want fast response, you want to keep the wastegate closed as long as possible before hitting your boost target so you use both a spring and a boost controller of some sort. While you don't need to buy a huge wastegate for no reason, like anything else, the wastegate is an exhaust flow restriction and bigger can be better. Also, typically, you will have a wastegate for each turbo. You don't really tune a wastegate other than choosing the right spring. You tune the action of the wastegate with the boost controller.

The blowoff valve serves a different function. As you create boost with some sort of compressor (e.g. SC or turbo) it forces air into the combustion chamber. However, when you suddenly let off the throttle, the throttle plate (if your car has one) closes. Because of all the pressure in the intake charge rushing towards the throttle plate, you want to vent that pressure rather than damage the throttle plate. Accordingly, a blow off valve opens to vent the pressure to the atmosphere or back into the intake tract depending on your setup (OEM is usually back into the intake because the MAFs have already metered the air). When they are vented to atmosphere (VTA), then you get the typical "blow-off" noise, which may be a whistle, whooosh, pshhh, or otherwise, depending on design. A blow-off valve typically uses a spring too (though not all designs do) to force the valve shut (i.e. to not blow-off). The blow-off valve may be triggered by an imbalance of pressure between the compressor and the manifold.

So with a wastegate, the spring, say 6PSI in your example, won't open until 6 PSI is reached. Meaning it won't divert exhaust. Until then all exhaust is going to the turbine. IS this pressure from the manifold to the wastegate? Or from the wastegate to the turbine. Or is this one continuous track (until wastegate opens)?

SO when picking a wastegate you're really only limited to what you can fit? You just put in different springs to achieve the results you want?

You mentioned a boost controller. This takes from PMP, is this to say the intake manifold pressure?

If so then, assuming your turbo is working and putting large amounts of air into the Intake manifold, why would you take from that to keep the manifold shut? Is this just to rev the turbine faster and achieve higher boost? I would imagine that could be dangerous? Also if what I said is true, won't the diversion of intake manifold pressure increase the boost PSI beyond what the wastegates spring is rated at? This situation would seem like a supercharger, using boost to create more boost ><

Do springs have limitations on them? Can you get a large wastegate and put in a low PSI spring and use the controller to get the PSI higher, or to your desired PSI?

Also ty everyone so much for dealing with me. I did read the wiki, sometimes hearing it a different way makes understanding it easier.

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.

Thank you very much for that detailed explanation, it's much clearer to me now. Had I been thinking I would have realized the WG doesn't open and close statically, being a spring and all.

In your example of holding the WG shut on releasing throttle, this is intended to not lose turbo pressure as your revs decrease? Or to at least slow down the loss? Is that correct.

Seems to me there are a ton of considerations going into this one part. Most of the other parts of the turbo kit seem very straight forward compared to this specific part.

You need to really read what I have already said much closer because you are not understanding it. In my example of lifting off the throttle, I am talking about opening the BOV, not holding the WG shut. I am not sure where you got that. There really isn't that much consideration for the WG usually. It's usually one of two predominant size offerings from Tial and you usually get a spring according to what your target boost is. I think you need to do some more basic research before asking anymore questions.

Yes I am buying a book on it. Sorry :/ I'm trying :)

After doing some searching, and reading comments I got this one
http://www.amazon.com/gp/product/B00...=docs-os-doi_0

If I can't answer my own questions I'll come back :)

Also, to be fair, this is not an entirely easy thing to get into. Having not grown up around shops, or knowing anyone at all who does their own car work. But I am determined to get it :)

There ya go. It's all for your own good if you are planning on investing in a turbo kit at some point. It's a lot of money to spend and you should be well educated on what you are getting into.

General wastegate operation:
Wastegates are a valve that allow exhaust gas to bypass the turbine.
Without a wastegate, boost pressure would continually rise until your motor pops or until the overall system just can't flow any more.
When a wastegate is fully closed, all air is flowing through the turbine, your turbo will be spooling up.
A wastegate never directly "restricts" the airflow path into the turbine at all, it only allows a secondary path of less resistance for air to bypass your turbine.

Wastegate sizing:
If a wastegate valve is too small, it will not be able to divert enough air away from the turbine, and your boost pressure will continue to rise as rpms increase. This is known as boost creep.
As long as your wastegate is large enough to avoid boost creep at your desired power levels, there is no reason to go to a larger valve.

Wastegate signal:
The wastegate actuator typically gets it's "signal" from a vacuum line(well, not really vacuum when you are in boost, but that's what they call them) that connects to the intake manifold, though you could technically tap into any point after the compressor.
Tapping in closer to the compressor will result in overall less boost since there is some pressure drop through your intake piping, intercooler, throttlebody. For example if you ran a vacuum line straight from your compressor outlet to the wastegate, the wastegate might "see" 8psi while the pressure in the intake manifold is only 6psi.

Boost control:
Without a boost controller your wastegate spring pressure is the main thing that will determine your psi.
Boost controllers work by intercepting the boost signal before it gets to the wastegate actuator. They can make the wastegate see less pressure than is really in your intake, keeping the wastegate closed longer. They cannot make the wastegate see more pressure than is really there, which is why they can only raise boost and not lower it.
If you plan to run a boost controller, you can get a wastegate spring that is lower than your desired psi. If not then just pick the spring psi you want to run.