Twin turbo balancing question

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ptuomov
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Twin turbo balancing question

Post by ptuomov » Mon Jun 18, 2018 5:03 pm

Twin turbo 90-degree cross plane V8 with cast iron exhaust manifolds and a single throttle body. Question: how would you balance the turbos? Electronic boost control, currently in open loop mode. Long run plans to potentially run closed loop turbo operation, hence dual MAFs, dual solenoid valves, and dual turbo speed sensors. In the meanwhile, how would you balance the turbos in the open loop mode? Connect the wastegate operating pressure lines to equalize the pressure and shim the springs to open about the same at the same pressure? Anttthing else to it?
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Re: Twin turbo balancing question

Post by user-23911 » Mon Jun 18, 2018 5:20 pm

In that configuration, the only thing that can unbalance them is having WGs opening at different pressure.

Contrary to popular belief, internal WGs are best as they are easier to adjust.

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Re: Twin turbo balancing question

Post by ptuomov » Mon Jun 18, 2018 5:36 pm

joe 90 wrote:
Mon Jun 18, 2018 5:20 pm
In that configuration, the only thing that can unbalance them is having WGs opening at different pressure.Contrary to popular belief, internal WGs are best as they are easier to adjust.
Right now, the system is so symmetric that the flow is equal to MAF sensors ability to measure when wastegates are fully closed. We don't yet have the control pressures equalized, which is the next thing to do. After that, it's time to start shimming the wastegates. I think that in 2018, internal gates are overall better, but turbos of this size with internal gates couldn't be made fit in the space available. Usually the space requirement make internal gates easier, but this is a special case.
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Re: Twin turbo balancing question

Post by Big Al » Mon Jun 18, 2018 7:05 pm

How much of a problem is it?
If it's a big problem then you have to make a exhaust tube between the cast manifolds to level the sides. A WG could be placed on that tube or you could still have a WG beside(in?) each turbo to sort those gases out.

But I say just set the WGs at same pressure ... It works for most people :)

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Re: Twin turbo balancing question

Post by ptuomov » Mon Jun 18, 2018 7:12 pm

Big Al wrote:
Mon Jun 18, 2018 7:05 pm
How much of a problem is it? If it's a big problem then you have to make a exhaust tube between the cast manifolds to level the sides. A WG could be placed on that tube or you could still have a WG beside(in?) each turbo to sort those gases out. But I say just set the WGs at same pressure ... It works for most people :)
It's not a big problem, and I think I have a solid plan dealing with it. That said, just wanted to get some info from people before doing what I consider the obvious thing to do -- equalize the control pressures for now and adjust the wastegate springs such that the mass flow is equal between the sides.
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Re: Twin turbo balancing question

Post by user-23911 » Tue Jun 19, 2018 1:19 am

One other quite common TT problem with ricers and a big FMIC.........2 turbos require 2 intercoolers, or more but NOT a single one.
If the pipes join before the cooler then the cooler is a bigger restriction than the other turbo (as you come up on to boost) that causes surging of the turbos....usually fixed by undoing the mods and putting it back to the way it was to start with.........2 coolers.

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Re: Twin turbo balancing question

Post by ptuomov » Tue Jun 19, 2018 6:37 am

joe 90 wrote:
Tue Jun 19, 2018 1:19 am
One other quite common TT problem with ricers and a big FMIC.........2 turbos require 2 intercoolers, or more but NOT a single one.
If the pipes join before the cooler then the cooler is a bigger restriction than the other turbo (as you come up on to boost) that causes surging of the turbos....usually fixed by undoing the mods and putting it back to the way it was to start with.........2 coolers.
I don’t have co-surge problem in this engine. Theb Maine has two separate effective intercoolers and two bypass valves and circuits with enough area. Furthermore, we’ve got two turbo speed sensors, two mass air flow sensors, and two IAT sensors so we can tell for sure that it’s not co-surging.

https://www.diva-portal.org/smash/get/d ... TEXT01.pdf
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Re: Twin turbo balancing question

Post by Orr89rocz » Tue Jun 19, 2018 7:49 am

joe 90 wrote:
Tue Jun 19, 2018 1:19 am
One other quite common TT problem with ricers and a big FMIC.........2 turbos require 2 intercoolers, or more but NOT a single one.
If the pipes join before the cooler then the cooler is a bigger restriction than the other turbo (as you come up on to boost) that causes surging of the turbos....usually fixed by undoing the mods and putting it back to the way it was to start with.........2 coolers.
Interesting. I have not heard of this before. On my v8 stuff i used a twin inlet single outlet cooler. No issues that way but turbos spooled fast so very little time for surge, ramped right thru it

Some cooler cores are big like 4-6" thick. Would this co surge thing be reduced by using a bigger cooler? Compressor outlets usually 2.5". Dual 2.5 goin into a single 3.5-4" inlet should transition flow better and be less of a restriction

Lot of the v8 guys that go twins use single coolers and i dont recall them having issues but like i mentioned, they run big inlet coolers and the turbos spool fast so not much time to surge imo

I combine waste gate lines to single pressure source and gates each get same spring. Thats as far as i ever took balancing of turbos. Tried to make system symmetrical as possible

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Re: Twin turbo balancing question

Post by ptuomov » Tue Jun 19, 2018 8:16 am

Orr89rocz wrote:
Tue Jun 19, 2018 7:49 am
joe 90 wrote:
Tue Jun 19, 2018 1:19 am
One other quite common TT problem with ricers and a big FMIC.........2 turbos require 2 intercoolers, or more but NOT a single one.
If the pipes join before the cooler then the cooler is a bigger restriction than the other turbo (as you come up on to boost) that causes surging of the turbos....usually fixed by undoing the mods and putting it back to the way it was to start with.........2 coolers.
Interesting. I have not heard of this before. On my v8 stuff i used a twin inlet single outlet cooler. No issues that way but turbos spooled fast so very little time for surge, ramped right thru it

Some cooler cores are big like 4-6" thick. Would this co surge thing be reduced by using a bigger cooler? Compressor outlets usually 2.5". Dual 2.5 goin into a single 3.5-4" inlet should transition flow better and be less of a restriction

Lot of the v8 guys that go twins use single coolers and i dont recall them having issues but like i mentioned, they run big inlet coolers and the turbos spool fast so not much time to surge imo

I combine waste gate lines to single pressure source and gates each get same spring. Thats as far as i ever took balancing of turbos. Tried to make system symmetrical as possible
I don’t know about a lot of stuff before I measure it and then when I’ve seen something I can’t “unsee” it... two turbo speed sensors and two MAFs and one set of scales drop from ones eyes.

Less restrictive intercooler cores upstream of the merge make co-surge more likely, not less likely. Think of co-surge as a phenomenon where the air tries to flow from one compressor to another. If you make that harder, less likely it is that there’s co-surge. The way to do that without creating unnecessary restriction is to combine the pressurized inlet air paths as late as possible. Also, large enough capacity bypass circuits help, as co-surge is most likely to happen when the throttle is closed.

More restrictive intercooler core downstream of the merge makes co-surge more likely, but if you’re putting the intercooler after the merge “YOU’RE DOING IT WRONG” anyway... ;-)

If you want to use a single intercooler core, just keeping the inlet tanks separated will solve the co-surge issue as the flow now sees too intercoolers. Outlet tank can be combined.
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Re: Twin turbo balancing question

Post by Orr89rocz » Tue Jun 19, 2018 8:37 am

I said less restrictive intercooler downstream of the merge. Not upstream so sounds like we agree there

On my last setup i ran twin water air coolers. Each turbo dumping into one then combining downstream to merge into throttle body. Never noticed a problem. If it was surging i think you would feel/hear it. Based on the size of the compressor, theres noway i was close to surge line anyway and come into boost very quick. If any co surge existed it was no different than regular surge as you come into boost and shoot thru the surge line into the pressure map

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Re: Twin turbo balancing question

Post by ptuomov » Tue Jun 19, 2018 9:43 am

Orr89rocz wrote:
Tue Jun 19, 2018 8:37 am
I said less restrictive intercooler downstream of the merge. Not upstream so sounds like we agree there. On my last setup i ran twin water air coolers. Each turbo dumping into one then combining downstream to merge into throttle body. Never noticed a problem. If it was surging i think you would feel/hear it. Based on the size of the compressor, theres noway i was close to surge line anyway and come into boost very quick. If any co surge existed it was no different than regular surge as you come into boost and shoot thru the surge line into the pressure map.
I have no reason to think that you're system is at risk of co-surge. Mine isn't.

That said, we need to balance the cycle average exhaust manifold pressures by equalizing between banks the mass fraction of exhaust gas that is passed thru the wastegate circuit, or equivalently in the absence of surge, equalizing between banks the compressor mass flow. I think (but do not know) that there's more power to be found by being able to run a little bit more timing and/or more boost.
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Re: Twin turbo balancing question

Post by ptuomov » Tue Jun 19, 2018 4:13 pm

Has anyone actually looked at a (parallel) twin turbo car air flow? How close in percentage terms should I reasonably expect to get the mass flow?
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Re: Twin turbo balancing question

Post by user-23911 » Tue Jun 19, 2018 5:15 pm

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.

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Re: Twin turbo balancing question

Post by ptuomov » Tue Jun 19, 2018 5:34 pm

joe 90 wrote:
Tue Jun 19, 2018 5:15 pm
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.
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Re: Twin turbo balancing question

Post by ptuomov » Tue Jun 19, 2018 7:03 pm

The system is pretty symmetric in terms of external piping, but firing order alone would cause some imbalance in the cycle average exhaust back pressure, even if the wastegates would open exactly the same amount:
Balance.jpeg
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