The Project Pad

So while waiting for Marc to figure it out for me (lol) I decided to see if I can wrap my head around a second N75 controlling the QSV. What I want is for it to start opening at say 250kpa and finish opening by 280. I figured that modifying it the same way as you modify the refDC in VEMS would be best - except that I wanted to control it by boost, not RPM.

So I figured out how to go into Outputs -> Assign Outputs and set pin 4 or 5 (34 or 36) to alt boost. Then I was looking at the secondary PWM settings and have the option of either Relative or Absolute. I'm not sure what the difference is, but the map looks the same for either one. And actually, I think the map is better than the refDC line.

Here's how I have it mapped - assuming 0 = N75 off and thus QSV shut or in small turbo mode. 25% should crack it at 250kpa (21psi) since the QSV starts opening at 5psi. Then as boost climbs it opens more and more. This obviously may need some fine tuning but I think it's a pretty good starting point. If I'm doing this right

lol. have your map open right now, but thats pretty much it exactly.

as to the channel type, its I259 or Ignition Output you are looking for.

Oooh wait. on your QSV, is it a dual port wastegate or just a single chamber on the "can"?

If its a single chamber, thats actually backwards from how you want it.

Small turbo mode means the QSV is 'closed' right? that means no air to it, which means N75 needs to be ON.

So, you'd have 100 in the lower cells, tapering to 0 in the upper cells.

Matt, you want the setting to be absolute... it makes a big difference even if the map doesn't look different. Relative uses your boost target while absolute uses your true MAP value.

Also, don't make any assumptions about when the N75 starts to operate in terms of duty cycle. It may open at 15% or 40%... you gotta test it. You can do this on the car by setting the map to 100 kPa and increasing duty cycle until you hear the valve start making noise. Make sure your PWM period is set correctly (should be the same as it is in the primary settings).

Now that I read over your logic again, I'm not sure if you understand duty cycle or not. You are saying 25% of 21 psi should net you ~5 psi right? Well... that isn't really how it works. The only really good way to do this would be on the bench with pressure sensors and such, or just some good trial and error with a position feedback, but you are on the right track... just don't make any assumptions about when the N75 starts to operate and what duty cycle might equate to 5 psi. Your method might get you relatively close though... so basically just try it lol.

Oh and what Marc said about the reverse duty cycle thing.

thanks guys. Yea Marc and Chris, just one chamber on the actuator. So I'll switch it around then.

Oh, Chris, you're right in that I don't really know how DC works. Basically what I'm after is just a smooth opening of the qsv, over, say, a second or half second. I'm pretty stoked to start tuning this thing now. Do you happen to have any reading material on DC that I could understand? I like your method of at least figuring out when the n75 will start operating.

You could pick up a controls book, or look on wikipedia, but they are a little difficult to get the grasp of I think. I'll be your reading material lol.

OK, so basically what this boils down to is pulse width modulation, also know as PWM. Lets think of this in electrical terms first. Say you want to control the power to a light bulb so you can dim it. Historically, this was done with a variable resistor. The more resistance, the less power would get to the bulb. This creates heat (that is the byproduct of resistor resisting) and this is not only a waste of energy, but it can burn your house down. As a result of this, we now use pulse width modulation for things like dimmers.

The idea is that you turn the power on and off at a fixed interval which won't affect the load (say 120 Hz or 120 switches/second for a light bulb). In other words, you can't tell that its being switched because it is being switched so fast. The length of time that the power is sent to the load (the bulb) in this fixed switching interval is called the duty cycle. If it is on for the entire interval, then you would call that 100% duty cycle and the light bulb would be fully on. If it is on for 0% of that interval, then you would call that 0% duty cycle and the light bulb would be off. Anywhere in-between will produce an in-between result.

Now, you can control a solenoid with pulse width modulation as well. However, solenoids will often not start to move until a certain duty cycle is reached. You need to figure out at what duty cycle the valve starts to move. As far as how pressure relates to duty cycle, that is something likely better done with experimentation. I could get into flow rate through an orafice and how that changes to the fourth power with respect to diameter, etc... but I won't. It isn't worth it at this point in time. I think you are just better off getting an idea of when the thing starts moving and then just putting it on the car and playing with it a bit.

So I have it setup using Pin 4 (34). Then in Secondary PWM I have Boost Alternate channel set to I259. What channel is it? My options are 0-7 and then 0-7 inverted. Or does this matter?

That would depend on how your outputs are set up. Marc should know better than me.

Good suggestions Chris.

Additionally, you may want to spread the valve opening event out over a larger range of boost pressures. Of course you will have to play with it to find out, but going from fully closed to fully open in just 30kPa is going to make it an on/off switch - that is just ~4psi so your car is going to blow through that in a split second. I think you will see better results from the QSV by having it open at a slower, more modulated rate...just a hunch.

chaloux wrote:So I have it setup using Pin 4 (34). Then in Secondary PWM I have Boost Alternate channel set to I259. What channel is it? My options are 0-7 and then 0-7 inverted. Or does this matter?

Pin 34 is ignition output channel 2. You want to use the regular channel, inverted would default to "ON" - I suppose you could set it up either way though.

So VEMS is smart and automatically sets the channel depending on the pin you choose in Assign Outputs.

But we have a problem - it seems the N75 is always open (off?), allowing air to go through it and making the car feel like a dog turd until 5krpm. I tried both 100% DC and 0%, exact same results.

I am 100% sure one wire goes to power and the other goes to pin 4. Also 100% sure that I have it set to pin 34, channel 2, absolute, I259, 32hz. Perhaps that's it? What is the N75 hz rating? We are going to try hooking up another N75.

Does the car have to be running for the n75 to operate or can we test it with the key turned and changing the map? We'll find out soon enough I guess

Hrmmm. Could also have the N75 hooked up wrong. I have air coming in the bottom (short), and then short to the QSV.

f me. that's wrong. that also means that dad's wastegate has been clamped shut for... oh... 10k km?

switched it, but we're going to have to make two plugs, one for the QSV and one for the wastegate.

jeepers.

OK hang on a second now. What do you have "boost valve off below this pressure" set to? If it is at 102 kPa, then you need to lower it to see if it will come on or not at atmospheric pressure (101.325 kPa as standard).

Hm.

I have to think about this.

I may be misunderstanding how the N75 works. First I have to figure out the correct orientation and go from there.

But Chris, in Absolute mode the "boost valve off below this pressure" is greyed out. I can only change it in Relative mode.

loxxrider wrote:OK hang on a second now. What do you have "boost valve off below this pressure" set to? If it is at 102 kPa, then you need to lower it to see if it will come on or not at atmospheric pressure (101.325 kPa as standard).

doesn't matter. its a wastegate actuator, so it wont open till around 8psi anyway even if "off below this pressure" is a high number.

I'm just saying to be able to "bench" test it he would need to have that set appropriately. I don't know why it would be greyed out though...

What do you mean it wont open until 8 psi? What wont open? The valve? It will open at whatever duty cycle makes it open regardless of boost wont it?

loxxrider wrote:I'm just saying to be able to "bench" test it he would need to have that set appropriately. I don't know why it would be greyed out though...

What do you mean it wont open until 8 psi? What wont open? The valve? It will open at whatever duty cycle makes it open regardless of boost wont it?

nope.
its got a spring in it.

until there is enough boost to overcome the spring pressure (guessing between 6 and 8 psi) it will not open.

Huh, didn't know that. That is good to know. Still, one should at least be able to hear it cycling. That would rule out a bad valve.

So Marc (or others), as per the discussion on Quattroworld and the operation of the N75 - because the positive pressure defaults to the shorter connector on the N75, I should be hooking the QSV up to the longer connector on the N75, otherwise it will always be getting pressure. If N75 doesn't start working until 6-8psi to divert air away from teh QSV, the QSV has already cracked open.

SO I should have it hooked up to the long connector so that NO air is going to the QSV, and as the N75 DC ramps up, it diverts air towards the QSV. Having the short connector plugged should not cause any harm to the N75 as far as I can tell.

I'll do that today.

OK so on the wastegate, we want pressure going to the bottom port of the wastegate in order to limit boost (open the wg). In this condition, the valve is essentially at 0% duty cycle (or off) to cut boost to wg spring pressure. So at low boost, we want no pressure to the bottom port on the wg such that it stays closed for as long as possible such that boost ramps up quickly. This will be the 100% duty cycle position (or fully on). Then when you want to limit boost, the valve will limit duty cycle (start to turn off or lower duty cycle).

For the QSV, the normal position is closed and this requires atmospheric pressure at the QSV. We want the QSV to open at higher boost which requires pressure at the valve. If you want pressure at the valve, then the N75 needs to be off in its normal orientation or on in its reverse configuration. So, in the regular configuration, you could write your map such that the valve is at 0% duty cycle (off, and thus full pressure at the QSV) when you want the QSV to be fully open and 100% (on, and thus zero pressure to the QSV) duty cycle when you want the QSV to be fully closed.

Does this make sense? It should be that simple except the pressure at which the QSV actually opens will have to be factored in. I'm getting on a flight now, so I won't try to think about that at the moment.

I'm not sure you can really rely on the N75 working correctly when plumbed backwards (the 'vent' connected to the device, your quick spool valve).

just use reverse logic on the table. In other words, energize the valve (100% duty cycle) at the bottom of the table under low/no boost, then take away duty cycle when you want the valve to open.

Yeah that's how I had it set up, but it spooled like shit (5k). So then I tried 0, same thing. I am thinking that either the N75 is bad, or the fact that the N75 lets 6-8psi go through before operating is what is opening the QSV, as it opens at 5psi. That's why I wanted to try it backwards, as it only lets air through when it starts cycling.

I'll swap it with another N75 using the same configuration we have now, and if it still doesn't work, I'm going to switch it around and see what happens.

Try using a solenoid over an N75 if anyone has one near you?