Twin turbo balancing question

[ptuomov]

This is where the sides are without any special optimization with sane nominal wastegate springs and same solenoid valve duty cycles. Is there any point spending time to get these closer?

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[JodyB]

Have you tried running with open downpipes to verify it is an "upstream" issue?

[user-23911]

If you've built it right then each turbo gets half the total flow at all times and all you need is 1 MAF which meters the air to one turbo.That's a 50% bypass and it DOES work.

You’re doing it wrong?

In a cross plane V8, you’re not going to get exactly same exhaust flow from the same size wastegate hole. So you have calibrate it. How close is close enough?

It’s ok to say I don’t know or not answer at all if you don’t know.

It's NOT really about getting exactly the same flow for each side.
It's about it staying the same all the time and tuning for it, no different from tuning any other engine.

Once it's tuned and it's running and it's not going to change then it's not a problem.
If the balance between the 2 sides changes for no reason....it's broken.
In which case you did it wrong.


The whole reason I know it works is because I've done it.
Like I've done lots of other "out of the ordinary" things too.


External WGs........the springs won't be identical.
Just like valve springs, they have a "spring rate".
A closed seat load.
As well as a fully open load.
But it doesn't matter so long as the relationship between them doesn't randomly change with the weather.

[ptuomov]

Have you tried running with open downpipes to verify it is an "upstream" issue?

No, but it's not an issue downstream of the wastegate. The area ratios and simple logic tell me that.

I don't even know if there's any issue. I know that in principle I'll make more knock limited power if the exhaust manifold pressures are equal. I was looking for some quantitative indication from others whether this is at all worth it to balance. My instinct says no, but I'll do some back of the envelope math on it.

[Big Al]

This is where the sides are without any special optimization with sane nominal wastegate springs and same solenoid valve duty cycles. Is there any point spending time to get these closer?

Apply some more WG spring pressure on the blue one.

[ptuomov]

This is where the sides are without any special optimization with sane nominal wastegate springs and same solenoid valve duty cycles. Is there any point spending time to get these closer?

Apply some more WG spring pressure on the blue one.

Conventionally, I take it that applying more wastegate pressure would mean opening the wastegate more and bypassing more exhaust gas, thus flowing less mass thru the compressor. So I think you mean the opposite?

If you mean the opposite of what you wrote, then I agree.

In any case, I've got control pressure on both sides of the diapraghm. I'm thinking that the best way to calibrate, should I try to calibrate it, this is to restrict one of the solenoid valve discharge ports with a calibrated orifice until they are close (blue side). Since this is in open-loop control in terms of side-to-side balance (closed loop in terms of boost overall), this restrictor would bleed slightly less pressure out of the side of the wastegate that tries to keep it closed.

However, what's the so-far-unanswered question is whether spending any time balancing the sides is worth that spent time.

[ptuomov]

Wonder how balanced these guys got their turbos back in the day?

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[MadBill]

I'm no turbo guy, but how can waste gates affect a progressive flow disparity that starts at 1,000 RPM and grows rapidly larger with revs?

How do the bank-to-bank inlet and exhaust pressures compare through the range?

I would think that even with individual cylinder or bank spark control, a ~10% difference like this would significantly compromise power. It's like running 12:1 CR on one bank and 10:1 on the other.

[ptuomov]

I'm no turbo guy, but how can waste gates affect a progressive flow disparity that starts at 1,000 RPM and grows rapidly larger with revs?

I don't think it can. The wastegate in the early graph starts operating at 3500-3700 rpm. Below that, it's closed.

How do the bank-to-bank inlet and exhaust pressures compare through the range? I would think that even with individual cylinder or bank spark control, a ~10% difference like this would significantly compromise power. It's like running 12:1 CR on one bank and 10:1 on the other.

The inlets both in my car and in the ancient Porsche marine twin-turbo engine combine to a plenum. So the intake pressure that the both banks see is identical (with some variation cylinder to cylinder due to the intake manifold). So I don't think there's a 10% issue there.

The only way this is going to change the cylinder filling is that the two banks have slightly different exhaust back pressures. That's an effect, but it's a second order effect compared to what would happen if the fresh pressurized charge paths wouldn't combine on the intake side.

I don't have exhaust pressure measurements for this iteration yet, as it's a little involved to set up because of the sensor cooling. Once we got some more pressing issues logged, we'll check that. I could just compute it from the intake flow mass flow and pressure ratio, though.

[user-23911]

Pressure and flow are 2 different things.

The pressure can be the same but the flows can differ.