How (exactly) does the actuator work?

[JohnA]

I'll elaborate:

 

Normally designs of this generation use a solenoid (VSV in the supra's case) to redirect pressurised air (boost) to the back of the actuator diaphragm. This is designed to combine with the strength of the actuator spring and stock exh backpressure and open the wastegate.

Usually it's pulsed by the ECU, I don't know if that's the case with the supra ECU --- hopefully someone else here knows this.

 

Some other designs even use two vac connections to the wastegate, one on each side of the diaphragm. The ECU can then regulate the pressure differences inside the actuator and thus have an extended 'dynamic' range, without having an excessively stiff spring at high boost pressures.

 

Now looking at the supra's setup, I see TWO vac connections at the back of the diaphragm, one towards the intake (straight to boost then) and the other one towards the VSV (ECU-controlled boost flow, I would imagine)

Looking at the schematic (Ian C's excellent posts at FAQ) this is verified as indeed being the case.

 

So what is happening here? I want to fit a 'ball and spring' MBC and I think I might have hit onto something now. I've seen no mention of this setup anywhere here or MKIV.com

 

If someone understands this system to a sufficient degree, please enlighten me

[RICHARDA]

Although I don't know the answer, good question.

 

Rich

[JohnA]

I'll try to make the question clearer:

 

If the VSV were to be redirecting boost via the ECU's instructions, then the 'other' actuator vac connection (going upwards as you're looking at it) would be overwhelming the chamber with boost anyway, since it is permanently connected to the pressurised side of the intake.

...so the VSV doesn't appear to be doing what I thought it was doing.

Both hoses seem to be on the same side of the diaphragm, too.

 

Clear as mud?

[JohnA]

Here is the schematic from the FAQ.

Look carefully at the back of the actuator ( #12 )

[JohnA]

http://www.max-boost.co.uk/max-boost/Gifs/tumbleweed-silence.gif

[eyefi]

the wastegate vsv is open to close the wastegate, pressure (from13) is allowed to escape the actuator by the vsv. vsv closes, pressure builds from 13 opens the wg.

 

i think this answers yr question. the vsv is not supplying pressure but relieving it.

[JohnA]

OK, let's look at this more closely then:

 

As the engine starts making positive boost, the WasteGate VSV is shut, so it's valve is open, so the pressurised air (boost) being dumped at the back of the actuator is being diverted to the atmosphere (well, between airfilter and any compressor) via the VSV 'relief'.

 

Then as boost starts approaching the stock boost limit, the wastegate VSV starts opening, thus shutting its valve, and trapping pressurised air (boost) at the back of the actuator, which starts pushing the diaphragm and that pushes the actuator rod, cracking open the wastegate.

 

You have to be following this sequence of events while looking at the diagram or else it doesn't make sense.

 

The question now is: In it's idle state, does the wastegate VSV default to being open or closed? I haven't held one in my hands so I can't tell.

This is very important, as it will indicate whether the above theory is correct.

Even so, it would be an unconventional way of operating boost control (But if that's the case I see a silver lining, where the spoolup time of stock twins can be increased )

[JohnA]

A couple of other relevant observations:

 

All VSVs appear to have a link to the pressure tank, one way or another. All apart from the actuator VSV.

This is sensible in a 'failsave' design as a pressure tank failure should not be able to cause overboosting damage.

 

However, this implies that the wastegate VSV is always SHUT by default, and it takes effort to open it. So if all else fails (cracked/popped hoses, whatever) then the back of the actuator is always under boost pressure and will open at stock boost pressure, or even slightly earlier (better-safe-than-sorry school of thought, which I'm sure Toyota designers subscribe to)

[eyefi]

vsv valve open in idle state

[JohnA]

If the wastegate VSV is open in idle state, then the 'relief' theory doesn't make sense.

A knackered VSV (or ECU wires to it or vac hoses) would allow the engine to go way beyond the stock max boost limit*, and that's a serious design flaw that Toyota engineers would never allow.

 

 

 

*

how much would depend on the strength of the actuator spring

[eyefi]

i may have missunderstood what you meant by idle. the vsv valve is closed if not energised. energised, it's open which is the state when the engine is at idle.

[JohnA]

Ah, that explains it then.

Yeah, I meant 'idle state' as in 'not energised', while you meant 'engine is idling', not the same thing at all

 

So the ECU is meant to keep the 'relief' open while boost builds up and then shut it as it approaches 11 psi.

 

....therefore a ball-and-spring MBC should be inserted in the hose connecting the pressurised inlet and the actuator, NOT the VSV hose. [/mysteries of life]

 

Nice one eyefi, thanks for forgoing the txt-lingo on this thread.

[eyefi]

(re shorthand) a bit still did creep in. it's a habit i'm trying to break but 15 years of abbreviated typing doesn't want to end overnight.

 

what is a ball and spring BC?

[JohnA]

Here is a relevant link off my website (scroll a bit down)

 

You can get these for £20 via eBay nowadays.

If connected and adjusted properly, it doesn't allow pressurised air to reach the back of the actuator until the very last moment. It can help build up boost quicker, as the ECUs usually play it safe and start cracking up the wastegate 5-6psi before max boost.

They might have a profound effect, depending on the 'aggressiveness' of the ECU code.

 

As for the txt-lingo, I typically refuse to read or respond to any posts in txt-speak. U no m8, me iz kewlthat sort of garbage, to me it's an indication that someone is too stupid or lazy to deserve a response.

I never thought that it could be a remnant of stenography or such, I think you're a one-off case on this one

The vast majority of people using txt-lingo cannot even spell their name properly, let alone articulate a proper sentence.

[eyefi]

ahhh, a dawes device.

[JohnA]

sort of, this one is cheaper and less hyped up.