crcain

My understanding is that the stiffness of an external wastegate spring, or of an actuator, effects how the car builds boost. But I don't understand why.

You have pressure (boost) being made post turbo. This pressure arrives at the boost control solenoid. The solenoids job is to bleed off pressure to prevent the wastegate from opening until it needs to.

Let's say you have 10 psi spring, and you are targeting 25 psi of boost. Now my understanding was that the solenoid simply bleeds off a bunch of pressure so that the wastegate does not "see" 10 psi until the boost controller wants it to. Which presumably is when the boost controller's sensor reads just about 25 psi.

So what am I missing? I just don't understand the point of stiffer springs / actuators.

kjewer1

iTrader: (4)

Just like a BOV, the wastegate actuator is not the only thing trying to open the wastegate. You have pressure in the turbine housing trying to blow it open. The general rule of thumb is that you shouldn't be trying to do more than double your spring pressure for boost. It's not uncommon in this situation to blow the WG open before you've built enough boost to reach your setpoint. This is more of an issue on internal gates, but also applies to externals. I prefer to choose the highest spring rate I can safely get away with on pump gas. Usually 20 psi minimum.

hopper

In addition a stiffer spring in a wastegate can allieviate some boost "bleeding" problems and in some cases help a car hold a higher amount of boost to redline without falling off.

CanadianTSi

Exactly, if your wastegate spring is too weak the back pressure built up in the turbine housing will open the wastegate prematurely which will hurt your spool up.

crcain

Thanks for your responses guys. That helps a lot.

Now my next question is ... why exactly is this happening?

My specific example is a GT35R with Tial 38mm with wastegate dump routed back into exhaust. My target boost for this example is 25 psi. I have a 10 psi spring as evidenced by running that boost with boost controller off.

I've looked at the mechanics of a Tial gate here:

http://www.tialsport.com/documents/w3_tial_wg_assem.pdf

Now, are you suggesting that pressure post-turbine housing is coming back through the wastegate dump tube and pushing the valve open?

Ted B

iTrader: (6)

I don't know if that is the situation in your case, but be advised there is no reason to use a WG spring combination that works out to a pressure of any less than what you expect to run on base fuel. In my case, that works out to ~1.5 bar, and that is the tension of my WG spring combo. On pump fuel, the only means of boost control I need is the base tension of the WG springs.

Using less WG spring pressure only invites issues, including less boost control at high pressures.

crcain

Thanks for your reply Ted. It's too bad they can't design a wastegate which works for any boost level you want. What I am experiencing is... the boost will climb to about 20psi very quickly, then gradually reach target boost. And I'm wondering if this is because I run a 10 pound spring.

I'm still curious... based on the Tial diagram. What are the different ways this valve can open? Can pressure strictly coming in to the wastegate entry open the valve? What are typical exhaust manifold pressures? You'd think Tial would publish what exhaust manifold pressure overcomes certain springs?

Ted B

iTrader: (6)

I'd say it's very likely that is the case. I see a revised WG spring combination in your future . . .

kjewer1

iTrader: (4)

Another point worth making is that the larger the WG, the bigger the problem. Force on the WG trying to blow it open equals pressure times [i]area[/]. For a given turbine housing pressure, a larger gate will open more easily. This was a big problem on the EVO16g turbos people run on the DSMs. Upgrading to a larger 34mm actuator would help you run lower boost (lower than anyone would probably need to run with a properly set up car) but would blow right open at high rpm and high boost. The pressure in the WG is affected most by airflow per unit time, which is highest at about peak HP. When you would prefer your WG to stay closed of course.

The valve opens "away" from the manifold. Manifold pressure is what pushes the valve open. Or for internals, pre-turbine turbine housing pressure. Exhaust manifold pressure at high airflow values will be greater than intake manifold pressure.

crcain

Ahh thank you. I was thinking when the valve was seated... it would lift up to open. But you are saying it opens down which I think makes more sense anyway. Ok so is it the case that you can say when exhaust manifold pressure against the valve reaches 10 psi that the valve will open given a 10 pound wastegate spring?

kjewer1

iTrader: (4)

Well, it's not quite that simple. As stated earlier, the force pushing the valve open is a function of both pressure and area. The spring is not going to be an actual 10 lb spring. It will have the required force to hold the valve closed until the boost pressure it's rated for multiplied by the area of the actuator diaphram overcomes it. What force and therefore pressure is required to oppose it from the manifold side for a given valve size won't be a convenient number, but can be calculated. Whether or not we know or can calculate what exhaust manifold pressure it takes to cause problems for a particular spring/valve combination, the effects of it are easily observed, most easily though manifold pressure refusing to rise while the turbo is not out of airflow capacity, and a high freqency boost pressure fluctuation at that max pressure. I remember a particular setup on my 2g running ~30 psi on a 1 bar actuator with a turbo that didn't have a particularly large turbine side. The boost gauge would go nuts somewhere in the high 20s. A stiffer spring is the well documented fix for this problem.

KingTal0n

This is a very interesting topic to me!

I have a bunch of questions about this,
1. If my target boost max was 20psi, and I anticipate buying a 10psi spring and turning it up, how much exhaust gas pressure would it take to start interfering with my intake manifold pressure? Is there an anticipatory ratio of drive pressure like 2:1 I should be concerned about? Do we normally worry about such things, or is it better to just buy and try, then adjust later.

2. Would a good electronic boost controller be able to remove alot of that issue?

3. Does an early opening gate due to high EGP contribute to lower intake manifold pressures, or only slower boost rising rates, or both? i.e. if I buy a 15psi gate spring, and my combo has a LOT of EGP, will it deny me a full 15psi of intake manifold pressure? If so, how much lower could it become?

4. Doesn't a larger diaphragm allow for a larger spring to be used, and thus larger springs/diaphragm combos (although slower) able to resist more Exhaust gas pressure? Is there any company that offers a special "robust diaphragm/spring" combo for this situation?

5. Lets say I buy a 15psi gate but it comes on very very slow due to an early crack because of high EGP. If I buy a stiffer spring, isn't that the same as raising the boost pressure? In other words, if I ever have an EGP problem, buying a stiffer spring will just increase the boost pressure on me- it isn't really a good solution because now I am over boosting (compared to before). How can I buy a stiffer spring without getting more max boost?
Thanks!!!!

LetsGetThisDone

iTrader: (1)

Holy 8 year thread bump, batman.

kjewer1

iTrader: (4)

1) There is a specific back pressure that will open a given WG valve/spring combination. It depends on the spring force and the diameter of the valve. On a 44mm tial gate a 10 psi spring is probably good to approximately 35 psi back pressure. I built a jig to test various springs in a V44 gate years ago...

2) Yes, since most will put manifold pressure to the top of the gate, which stiffens the spring proportionally to the MAP increase. Basically, the "spring" pressure will always be ahead of the boost pressure by the spring's rating.

3) Only if you need that additional EBP that's it's bleeding off to reach your set boost. Which presumably you do, or it wouldn't be trying to achieve it. And again, the limit will be in exhaust manifold pressure, not intake manifold pressure. What you get for MAP depends on the turbo and setup.

4) Yes. I don't know the diaphragm to valve area ratios for any gates, but I suspect most are around 2:1. Some are worse, like the Tial V60. Bigger valves are a pain in the ***.

5) I'm not sure I completely understand your question, but the actuator should still have authority over the valve. So the stiffer spring will allow more EMAP before blowing open, but the boost pressure in the actuator will still open the valve at that spring's pressure (or higher with an MBC, etc).

KingTal0n

Thank you for all the responses! I would like to ask a detail about #5,

Would you say that the reasons manufacturers do not supply jumbo spring/diaphragms is because of the additional space they take, plus the increased mass of parts that move?

In other words, (if) bigger is better for resistance to blowing open, as It helps keep the valve shut until the spring says "ok, im ready to move". But it also costs more money to make larger parts, they move slower because they are heavier, and they take up more space, so there are drawbacks as well, and so designs we see on the market are a trade off between control, size, cost and performance?