DaX

iTrader: (1)

Here is what I've come up with/been wondering:

Internal and external wastegates function slightly differently. All the internal gates I have seen use a dashpot with an actuator arm to open the flapper on the internal gate. There is a spring inside the dashpot to hold the gate normally closed. These dashpots have only one nipple on them and the nipple is on the side of the diaphragm opposite that of the actuator arm. If you can't comprehend this, the nipple is on "top" of the dashpot, or you could say the dashpot has only a "top port."

All external gates I have seen use a high quality "dashpot" with a diaphragm inside that is directly connected to the gate's valve. These gates have two nipples - a "top port" that feeds boost to the top of the diaphragm and a "bottom port" feeding boost to the bottom of the diaphragm. There iis also a spring inside that holds the gate normally closed. When set up without a boost controller, the external gate leaves the top port open and vented to the atmosphere, and a boost source is attached to the lower port.

Without a boost controller in the mix, the two gates function similarly. On an internal gate, as boost increases, it is fed to the top of the dashpot which helps counter the spring's force, thus opening the gate as more boost is applied. On an external gate, as boost increases, it is fed to the lower port which aids in countering the spring's force, again opening the gate. Internal gates actually have a small hole on the lower side of the dashpot that vents to the atmosphere, much like the external gates.

Now, let's add a boost controller...we'll just say an MBC for now. A proper MBC [like the TurboXS] will bleed boost to the atmosphere, instead of just restrict the boost [like the cheap eBay MBC's].

On an external gate, we already have the lower port being fed boost. Now we'll feed the MBC boost through its inlet, and run the outlet to the top port on the gate. Now instead of simply venting the top port to the atmosphere, we're applying a calculated amount of boost. If you close the MBC all the way [no boost is bled off to the atmosphere] you are applying the same amount of boost to the top and bottom of the diaphragm inside. A simple crack of the physics whip will show that the two forces supplied by the boost will cancel each other out, and only the spring's force is left in the equation. As boost increases, it doesn't matter, the spring is doing all the work and the boost is not aiding in opening the gate. You have just "turned the boost all the way up." This is similar to having a boost line connected directly to both ports [no MBC]. Plugging off the top port is very bad, and will cause even more boost, because now you have trapped air on top of the diaphragm and it has nowhere to escape to when the boost on bottom tries to push it open. You might as well not have a gate at all. Now, as you begin to open the MBC, you begin bleeding boost off the the atmosphere. Less boost is seen on top of the diaphragm than on bottom, and the gate opens under higher boost than with just the spring, but less boost than when the top and bottom ports see the same amount of boost. This "ratio of boost" on top and bottom of the diaphragm is how you control your gate.

As stated previously, on an internal gate there is no bottom port [the side with the actuator arm], only a small hole that vents to the atmosphere. Let's put an MBC on and see what happens. The only way I can think to put one on an internal gate is by running it inline from a boost source to the nipple on the dashpot. Closing the MBC all the way, you don't bleed any boost off to the atmosphere, and it's like not having the MBC at all [boost fed directly to dashpot]. In the case with the external gate, on this setting you're cranking the boost "all the way up." Here, you're letting the dashpot see the pressure it normally does and in essence, the boost is turned all the way down because as boost is applied, it is helping counter the spring force and open the gate. As you open the MBC up and begin bleeding boost off to the atmosphere, you are limiting the amount of boost the dashpot sees. In the case with the external gate this is "turning down the boost" to the limit of having an open top port and running the "spring rate." In the case here with the internal gate, we're starving the dashpot of boost, in essence turning up the boost because there is less boost pressure to counteract the spring's force. You can do this to the limit of having the dashpot fed zero boost, which would be maximum boost on an internal gate, short of wiring the flapper shut.

So from what I understand, using an MBC on and internal vs. an external gate work opposite of each other. Opening the MBC up to bleed off more boost "turns up the boost" on internal gates and does the exact opposite on external gates. External gates with MBC's work on a pressure ratio, while internal gates with MBC's work on starving the dashpot of boost.

All this is simply observation...I have no emperical evidence to back any of this up...this is just me sitting and pondering. If I'm wrong about something, please correct me in an intelligent fashion.

Sunrise City Rider

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DaX &raquo;</TD></TR><TR><TD CLASS="quote">Here is what I've come up with/been wondering:


Now, let's add a boost controller...we'll just say an MBC for now. A proper MBC [like the TurboXS] will bleed boost to the atmosphere, instead of just restrict the boost [like the cheap eBay MBC's].

</TD></TR></TABLE>

A "proper" boost controller such as the Turbo XS type does restrict boost and is known as a ball and spring type controller...The Bleeder Valve is the type of controller that made cheap and I've seen people run a Vacuum Tee with one end open and just melt it down to allow for increased boost without any real way of controlling it...

You hook up the MBC inline to the Lower port on the External gate as you would hook it up inline to the port of the Internal gate...Feed the MBC from after the turbo's compressor and feed the gate from the MBC's outlet...The MBC adjusts the same way, tighten it down to raise the boost and loosen it up to lower the boost...

Now with an internal gate and possibly an external gate, you will get, what you hinted at, the valve opening as boost builds...This will lead to slower boost response and a loss of power throughout the RPM band...With the use of a MBC, you will prevent all or most of the boost signal from pushing the valve open before Max boost is reached...This will lead to a big jump in power in the lower RPM band and make for a more powerful engine...Even at the same peak boost...Your peak HP will probably stay the same but your torque curve will peak much sooner and give you quicker acceleration...


Modified by Sunrise City Rider at 1:05 AM 6/3/2004

DaX

iTrader: (1)

ah, i didn't know that. I thought my Turbo XS MBC was just a bleeder...I can blow through it and it's easier/more difficult depending on which way I adjust it. So the MBC can raise the boost pressure and at the same time it prevents "creep?" Is creep when the valve cracks open then opens gradually?

Sunrise City Rider

That premature opening of the valve is called Wastegate Creep and is common on Internal Gated setups...

EE_Chris

I thought wastegate creep was bad. If its creeping, boost isn't being regulated....its actually creeping up?

I thought 'premature opening' was just normal wastegate operation.

Sunrise City Rider

Well, I see your point...But I think the creep you mention as in "creeping up" is what I call boost creep...The creep that I mention that is referring to lower boost response is what I call Wastegate creep...