similtanious turbos

For the older twin turbo B4 models

similtanious turbos

Postby Naz » Sun Dec 26, 2010 8:00 am

Hi guys, im around the corner from purchasing my first b4, before the this i had a 1999 WRX.
I have been reading up about the liberty's and have driven a few. I have been told about this "VOD"
and have noticed it while driving the b4.
I have been told that tuning the car to get to of the same size turbo's then getting them similtanious to kick in
is a way to go. Any feedback on this idea? Would this blow the engine?
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Re: similtanious turbos

Postby tangcla » Mon Dec 27, 2010 10:13 am

No point. Just get a retune and you'll get rid of the majority of the VOD.
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Re: similtanious turbos

Postby Naz » Tue Dec 28, 2010 11:08 pm

But by doing what i just explained, would this not add more power across the rev range (depending on what two turbos are installed). But generally add more pull and power gain?
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Re: similtanious turbos

Postby telemekas » Wed Dec 29, 2010 1:36 pm

Converting the car to run both turbos simultaneously or in parallel, would be equivalent to a standard single turbo set up.
If the Syms parallel headers were used this would make it the equivalent to a single twin scroll turbo.
Getting the car to run parallel turbos is not difficult. However getting it to run equal size/flow and type turbos requires more work.
Converting to larger single turbo has good argument for it.
For the parallel set up points
If you have done extensive mods already to your car ie full turbo back,FMIC etc for the twin turbo.
If you want to to able to easily convert it back to sequential.
If you just want to try something different.
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Re: similtanious turbos

Postby jdm rsk » Wed Dec 29, 2010 7:25 pm

dont buy one....buy a gen 4 gt
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Re: similtanious turbos

Postby tangcla » Thu Dec 30, 2010 8:18 am

Naz wrote:But by doing what i just explained, would this not add more power across the rev range (depending on what two turbos are installed). But generally add more pull and power gain?

Yes but at the expense of one end of your rev range.
The idea of different sized turbos is that you get a small turbo for low rpm and a larger turbo for high rpm. Running two matched turbos in parallel effectively makes it the same as a single larger turbo. This will usually equate in more power higher in the rev range.
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Re: similtanious turbos

Postby Jimmy S » Thu Dec 30, 2010 12:28 pm

The B4 runs both turbo's simultaneously anyway... just not until later in the rev range...

I'd be interested to see what sort of results you could get by forcing the valve open permanently so that it is always running both, but I tend to think it wouldn't work especially well? Surely it's been tried before though?
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Re: similtanious turbos

Postby tangcla » Thu Dec 30, 2010 10:47 pm

Jimmy S wrote:The B4 runs both turbo's simultaneously anyway... just not until later in the rev range...

I'd be interested to see what sort of results you could get by forcing the valve open permanently so that it is always running both, but I tend to think it wouldn't work especially well? Surely it's been tried before though?

You'd lose pressure to ramp up the primary (smaller) turbo. The B4's twin turbo setup is pretty sound in principle - primary turbo filled up a second chamber, then second turbo spools. Problem is the chamber is too big before the second turbo spools, hence the VOD. The problem is the not-quite-perfectly-matched turbo sizes.
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Re: similtanious turbos

Postby Hotwire » Fri Dec 31, 2010 7:12 am

if you want twin turbo's in parallel, sell the B4 and buy a galant or Legnum VR4 - twin turbo V6 with one small TD03 per bank. Yes they are each small BUT full boost is achieved by about 2500 rpm and 200awkw is achievable with supporting mods ;)
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Re: similtanious turbos

Postby tangcla » Fri Dec 31, 2010 9:09 am

Hotwire wrote:if you want twin turbo's in parallel, sell the B4 and buy a galant or Legnum VR4 - twin turbo V6 with one small TD03 per bank. Yes they are each small BUT full boost is achieved by about 2500 rpm and 200awkw is achievable with supporting mods ;)

Yeah, but it's still a Mitsubishi :tongue: :P
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Re: similtanious turbos

Postby Jimmy S » Fri Dec 31, 2010 11:59 am

tangcla wrote:
Jimmy S wrote:The B4 runs both turbo's simultaneously anyway... just not until later in the rev range...

I'd be interested to see what sort of results you could get by forcing the valve open permanently so that it is always running both, but I tend to think it wouldn't work especially well? Surely it's been tried before though?

You'd lose pressure to ramp up the primary (smaller) turbo. The B4's twin turbo setup is pretty sound in principle - primary turbo filled up a second chamber, then second turbo spools. Problem is the chamber is too big before the second turbo spools, hence the VOD. The problem is the not-quite-perfectly-matched turbo sizes.


I thought the 2 turbo's were almost identical in size? and there is a valve in the headers that opens to allow flow to the secondary turbo at a set time? I would have thought you could somehow force that valve open at all times meaning that you would have a laggier car down low, but would be a bigger kick later on and no VOD(mostly because you are losing power down low?)...

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Re: similtanious turbos

Postby Kekotic » Fri Dec 31, 2010 12:42 pm

All this is over my head, but I wanted to know what the hell VOD was so I googled it, and when I did I came up with these 3 links, the 2nd of which seems very in depth (hence I'm not going to bother reading it :lol: )

http://www.sl-i.net/FORUM/viewtopic.php?f=59&t=10991

http://www.veloce.pwp.blueyonder.co.uk/ ... opprt2.htm

http://www.veloce.pwp.blueyonder.co.uk/ ... opprt2.htm
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Re: similtanious turbos

Postby tangcla » Fri Dec 31, 2010 1:14 pm

Jimmy S wrote:I thought the 2 turbo's were almost identical in size? and there is a valve in the headers that opens to allow flow to the secondary turbo at a set time? I would have thought you could somehow force that valve open at all times meaning that you would have a laggier car down low, but would be a bigger kick later on and no VOD(mostly because you are losing power down low?)...

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Definitely different sized turbos. The primary (small) one is tiny, and you have pretty much got boost from idle.

From Wikipedia:

Sequential Turbo Staging
The B4 uses a sequentially staged primary and secondary turbocharger to deliver good throttle response and a wide torque spread. The primary turbo delivers boost in the low rpm and load ranges to deliver 278Nm at 2000 rpm, while the secondary turbo joins in above 4000-4500 rpm. With both turbos boosting, a 320Nm torque peak arrives at 4800 rpm and maximum power (190 kW) is seen at 6000 rpm. Note that Japanese-market B4s - running on 100-octane fuel - are rated at 206 kW.

The primary turbo (located at the left rear of the engine) delivers boost in the low rpm and load range, while the secondary turbo comes in to aid mid-to-high range breathing. During the primary turbo stage, boost pressure is controlled by a conventional arrangement of an ECU-controlled duty-cycle solenoid and an internal wastegate.

The secondary turbo remains inoperative during this stage, as a separate exhaust control valve (situated on the right side of the engine) remains closed. This valve prevents exhaust gasses entering the secondary turbine.

During the 4000-4500 rpm transitional stage, however, the exhaust control valve is partially opened, bringing the secondary turbo up to near-operating speed. The ECU - working with another duty solenoid and vacuum diaphragm - determines the amount that the exhaust control valve opens. The ECU calculates this amount of valve opening based on the input of a differential pressure sensor that takes feeds from the intake manifold and the outlet of the secondary turbo.

Any boost pressure produced by the secondary turbocharger during the transitional stage is redirected to the atmospheric side of the compressor inlet (between the turbos and air filter).

With the primary turbo continuing to supply manifold pressure and the secondary turbocharger essentially bleeding off the boost it makes during the transition, the ECU will determine when to close the pressure relief valve and fully open the exhaust control valve. Once this is done, the ECU will again look at the input from the differential pressure sensor and open yet another valve - the intake control valve (which is mounted between the secondary turbo's compressor outlet and the intercooler).

With the intake control valve open, boost pressure from the secondary turbo is allowed to pass through the intercooler (in addition to the boost supplied by the primary turbo). Manifold pressure during the second stage of turbocharging remains regulated by the primary turbocharger's wastegate - there is only one turbine by-pass valve in the whole system.

Subaru claims: "The 'staging' between the single and twin turbo operating range, which was quite noticeable to the driver on the previous model B4, has now largely been 'tuned out' by the careful selection of turbocharger size and the controlling mechanisms. As can be seen from this torque curve, however, under some driving conditions it is still possible to detect a slight reduction in the rate of acceleration in the preparatory or intermediate phase that occurs between 4000-4500 rpm."

But read our Liberty B4 road test to discover how much manifold boost pressure is lost during the transitional stage - it's more than "slight"...

The Turbochargers
Despite being called twins, the B4's turbochargers are not identical. The primary turbocharger is an IHI VF33 unit, which uses a 46.5/35.4mm 9-blade turbine wheel and a 47.0mm/35.4mm 6 + 6 blade compressor. At idle, the turbo spins at around 20,000 rpm and it can go on to a maximum speed of 190,000 rpm. It has a 17mm diameter wastegate opening to bypass excess exhaust gas. The secondary turbocharger is an IHI VF32. On the exhaust side it uses a 46.5/35.4mm 9-blade turbine wheel, teamed with a 52.5/36.6mm 10-blade compressor wheel. It's rated at 180,000 rpm. Both the primary and secondary turbochargers use a floating metal centre bearing - not ball bearings.
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Re: similtanious turbos

Postby telemekas » Fri Dec 31, 2010 11:41 pm

"I thought the 2 turbo's were almost identical in size?"
Yes you are correct with regard to the older turbos.
The paired sets VF16/VF17,VF18/VF19 VF20/VF21 and PE1016F/PE1018F the only difference between each in the set was the turbine housing P16/P18 (almost identical)
Compare that to the VF33/VF32 combo, the only thing common to them both is the turbine wheel size. The VF32 turbo is ball bearing (BB CHRA)
All TT turbos are BB except primarys VF25,VF26 and VF33
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Re: similtanious turbos

Postby XXB4XX » Mon Jan 24, 2011 8:42 am

I'll be the odd one here,

Keep the VOD, it was probably the best part of owning my B4.

Once you tune the ECU / Exhaust / CAI / and Boost, the thrill of the Vod is awesome.

I remember seeing people squirm and freak when I punched the throttle in 2nd gear with just the primary, and then the look of absolute terror once I hit that little gap followed by the MASSIVE rush of the 2nd turbo kicking in.

If I had my way, I would've preferred my GT now to have been Twin-Turbo WITH vod.
Back to Subaru again.. think I would have learnt my lesson the first time round!

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