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Old 11-20-2008, 10:10 PM   #1
PitwrkzZ1
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Default Subaru WRC boost pressure

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Small engines and big performance. We take a look at the role of the turbocharger in the world of rallying. 

A mainstay of Subaru’s model lineup, the turbocharger in its simplest form works by forcing more air into the combustion chamber. Put simply this causes the ECU (engine control unit) to react by injecting more fuel, which with more air means a bigger bang, and more power. 

But how does the turbo do this? It works by compressing the air before it enters the combustion chamber, forcing more into the engine. As the exhaust gasses leave the engine, they pass through the turbo and spin up the exhaust turbine. 

This turbine drives a connecting shaft which spins a compressor wheel. The compressor wheel sucks in air through the air intake at the top of the turbo, and compresses it within the compressor housing, increasing the speed at which the air enters the intake manifold. 

As you probably know, turbo charging technology plays a crucial role in any World Rally Car, by allowing relatively small engines to produce substantial power. For example; the WRC2008 produces around two and a half times the amount of torque of a roadgoing Impreza STI. This means that, in spite of the power output of Subaru’s two-litre boxer engine being capped at 300bhp by FIA regulations, the latest rally Impreza is still able to accelerate to 60mph from standstill in under three seconds even on gravel. 

To put all this into perspective, a performance road car turbo produces around 20psi (1.3bar) of boost pressure, and a standard turbo between 10 - 15psi (around 0.8bar). By contrast, the turbo on the Impreza WRC2008 produces up to a monstrous 60psi (4bar) of boost, and this is where the performance comes from. 

Whilst incredibly impressive, it helps to bear in mind that the engine alone is worth around three times the cost of a new Subaru Impreza WRX STI, and the cost of the turbo itself is more than enough to buy yourself a Subaru Justy! 

But don’t go raiding your bank just yet; even if you are willing to spend the cost of a small hatchback on a single turbo, they are supplied specially to the Subaru World Rally team by Japanese manufacturer IHI. Operating at a temperature of 1000 degrees Celsius, they would also readily melt most of your engine bay, battery, fuse box and all!
http://www.racecar-engineering.com/a...bocharging.htm
Alrite, I know this is an old article, but 4 bar of boost and only 300 hp? How is this possible?
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Old 11-20-2008, 10:15 PM   #2
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They are not allowed to break 300hp, but torque is not limited. Since HP is a RPM based function of torque you simply have to limit the torque at higher RPM. Any rpm below 5250, using ft lbs and hp, will yeild more torque than HP. Above 5250 and HP will always be higher than torque. So, make 500 ft lbs at 3000 rpm and you are not breaking the rules, however by 5250 you had better be below 300 ft lbs and continue to drop with increasing RPM.
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Old 11-20-2008, 10:17 PM   #3
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on the inlet of the turbo there is a restrictor that limits the amount of air that is used by the engine.
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Old 11-20-2008, 10:27 PM   #4
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Quote:
Originally Posted by bmxpunk View Post
on the inlet of the turbo there is a restrictor that limits the amount of air that is used by the engine.
... which effectively starves the turbo of the air at higher RPM's.
So while the power is relatively small, the acceleration from lower RPM's utilizing huge torque combined with antilag makes these cars a riot.
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Old 11-20-2008, 10:29 PM   #5
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The restrictor just makes it harder to make power at higher RPM. Since it restricts total flow it is easier to push big boost at low RPM than at high RPM. Think about the liters of air going into the motor at 1000rpm vs 5000rpm. Ignore VE for a minute and you realize you can push 5x the boost pressure at 1000rpm as you can at 5000rpm and the restrictor has the same air flow going through it. If anything it makes it easier to not break the 300hp rule. On the US Rally cars we have tuned the fastest cars are the ones that can spool the turbo at the lowest RPM. Subiegal with a TD04 often is faster than most/all of the open class cars. A big part of that is driver skill, but take away the restrictor and she would not be able to keep regardless of skill.
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Old 11-20-2008, 11:04 PM   #6
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Two and a half times stock STi torque????? Does anyone else find this a little dubious. Even if they are talking the EDM STi that used to have 250 lb ft and even if they are talking on the dyno which might read 210 maybe that still means they would have to make 525 ft lbs. Okay, maybe 525 is believable.
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Old 11-20-2008, 11:13 PM   #7
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Originally Posted by pegdrgr View Post
The restrictor just makes it harder to make power at higher RPM. Since it restricts total flow it is easier to push big boost at low RPM than at high RPM. Think about the liters of air going into the motor at 1000rpm vs 5000rpm. Ignore VE for a minute and you realize you can push 5x the boost pressure at 1000rpm as you can at 5000rpm and the restrictor has the same air flow going through it. If anything it makes it easier to not break the 300hp rule. On the US Rally cars we have tuned the fastest cars are the ones that can spool the turbo at the lowest RPM. Subiegal with a TD04 often is faster than most/all of the open class cars. A big part of that is driver skill, but take away the restrictor and she would not be able to keep regardless of skill.
well you have things kinda jumbled...the restrictor causes choke flow and the given size of the restrictor dictates the total airflow into the engine. Choke flow is a gas dynamic issue but put simply it happens near 2:1 pressure ratio (changes pending the air's conditions) on either side of the restrictor.
For this given reason the motor approaches this choking point as it needs a great flow rate in and hits the point where the atmospheric pressure is roughly twice that of the backside of the restrictor (turbo in this case).

This is why they tune for total area under the tq curve and try to make the most airflow at the bottom end. Also with the slower spinning motor, the cylinder in effect has more time to fill with air.

Turbo sizing also has a great deal to do with how effectively you can get that big boost at the low rpms and prolong the choke point. Its hard to make lots of pressure with little air coming in

We won't even start about the changes to internals to help improve the tq curve...we're talking about boost here
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Old 11-20-2008, 11:14 PM   #8
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60psi.....come on now. this can't be for real
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Old 11-20-2008, 11:16 PM   #9
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60psi.....come on now. this can't be for real
no, that's pretty much spot on...I thought is was 53psi from what I had last read, several years back.
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Old 11-20-2008, 11:34 PM   #10
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Originally Posted by Homemade WRX View Post
well you have things kinda jumbled...the restrictor causes choke flow and the given size of the restrictor dictates the total airflow into the engine. Choke flow is a gas dynamic issue but put simply it happens near 2:1 pressure ratio (changes pending the air's conditions) on either side of the restrictor.
For this given reason the motor approaches this choking point as it needs a great flow rate in and hits the point where the atmospheric pressure is roughly twice that of the backside of the restrictor (turbo in this case).

This is why they tune for total area under the tq curve and try to make the most airflow at the bottom end. Also with the slower spinning motor, the cylinder in effect has more time to fill with air.

Turbo sizing also has a great deal to do with how effectively you can get that big boost at the low rpms and prolong the choke point. Its hard to make lots of pressure with little air coming in

We won't even start about the changes to internals to help improve the tq curve...we're talking about boost here

I am not sure where I have things jumbled, we are in agreement. I did not go into the detail of choke flow, and was trying to keep it light and easy to understand. The OP obviously does not have a tremendous amount of technical knowledge on the subject. In simple terms running 60 psi is only possible at lower RPM with the required restrictor in place. A restrictor has a greater impact on ones ability to make peak hp than it does on peak torque, that is all I meant when I said that limits the ability to make hp.
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Old 11-20-2008, 11:37 PM   #11
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sorry maybe i just misread what you typed...my bad
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Old 11-20-2008, 11:57 PM   #12
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60psi.....come on now. this can't be for real
this has been proven before and before, yes it is legit, the turbocharger alone costs around $60,000 go onto SWRT.com website, its where the information is posted
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Old 11-21-2008, 12:31 AM   #13
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Please note not every car runs at 60psi (which includes the atmos pressure so then its really 60-14.5psi) .... yes pro built rally cars do make disgusting because of boost pressure and engine compression. The staggering spool is also thanks to the restrictor that acts as a velocity funnel while limiting the amount of air past the restrictors effective section. Secondly the restrictor is a BOOST LIMITER because you can only draw x amount of air through a straw.

one known subaru wrc S12 had made just a very small touch over 300whp and generated in the area of 480lb/ft at approx 3500rpm.

Please note it was just recently that WRC found a need to limit engine compression for team engines.
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Old 11-21-2008, 12:36 AM   #14
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Please note not every car runs at 60psi (which includes the atmos pressure so then its really 60-14.5psi) .... yes pro built rally cars do make disgusting because of boost pressure and engine compression. The staggering spool is also thanks to the restrictor that acts as a velocity funnel while limiting the amount of air past the restrictors effective section. Secondly the restrictor is a BOOST LIMITER because you can only draw x amount of air through a straw.

one known subaru wrc S12 had made just a very small touch over 300whp and generated in the area of 480lb/ft at approx 3500rpm.

Please note it was just recently that WRC found a need to limit engine compression for team engines.
the 60psi doesn't include the absolute pressure, so no its not 60psi - 14.5psi

It's 60psi created by turbo, absolute would be 74-75psi in the article it compares it to normal factory cars making 1 bar or 1.3bar while it makes 4bar.
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Old 11-21-2008, 12:52 AM   #15
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Secondly the restrictor is a BOOST LIMITER because you can only draw x amount of air through a straw.
This is only true when considering a given displacement engine at a particular RPM. If you take this to the limit of 0 rpm you could run infinite boost pressure, in theory. Since pressure does not equate to flow saying it is a boost limiter is actually misleading. In a practical application running 40 psi boost on a 2.0L motor at 4000 rpm may be limited by a given restrictor, while the same 40 psi on a 1.0L motor at 4000 rpm will not be limited by the same restrictor. As previsouly stated the restrictor is a flow limiter, as in lbs/min, the actual boost pressure that flow occurs at is irrelevant, at least in a theoretical discussion.
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Old 11-21-2008, 12:58 AM   #16
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sorry to be a bit off topic, and I already pm'ed homemade wrx, but does any one have any suggestions for reading (books or other) on designing/tuning/theory etc of engines with restrictors? please post or pm me.
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Old 11-21-2008, 07:15 AM   #17
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Originally Posted by BHawk View Post
Two and a half times stock STi torque????? Does anyone else find this a little dubious. Even if they are talking the EDM STi that used to have 250 lb ft and even if they are talking on the dyno which might read 210 maybe that still means they would have to make 525 ft lbs. Okay, maybe 525 is believable.

Why is that dubious? The engines last for, what, 20 hours of high output *maybe*?
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Old 11-21-2008, 07:50 AM   #18
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Where i just wonder where is there a rule in FIA WRC specification abotu 300 hp...... There is no such rule it is onnly Group A restrictor 34 mm and that is it .C4 pushes about 370hp.... hell how does that thing go as i watched it on saturday in monza it is much faster under acceleration than the fords or 2007 Imprezas. And yes after the ban of methanol injection there has been a tendency of high compression a bit lower boos engines .
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Old 11-21-2008, 08:23 AM   #19
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The compression ratio has been capped for a while and pressure and flow are directly related...especially when the restrictor is pre-turbo. that means the turbos has two flowrates to consider...flow rate into the compressor and flow rate demanded by the engine. These flowrates also then have an associated pressure with them.

But yes, as mentioned the pressure ranges completely by that needed flowrate...ingoring VE, a 1L at 10,000rpm is the same flowrate as a 2L at 5,000rpm...

As for books and reading...I'm a mechanical engineer (motorsports and aerospace background) so this knowledge comes from several courses that I've taken as well as studying restrictors for Formula SAE for several years. Also a great deal of experimenting in an engine lab I use to operate/manage for an internship/TA position through my schools Research Foundation and NASA. So if I had questions, I had genius' (my bosses) to ask for input.
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Old 11-21-2008, 10:31 AM   #20
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Impreza WRC 2007 headers and turbo + antilag system, photo taken from prodrive open days. You can see the 34mm restrictor (black part) before the turbo. Lots of good info on previous posts. There's no "300" hp rule, you just cant make more with the restrictor. Prodrive says that they make around 700nm / 516 lb/ft of torq. Cost of the turbo alone 15000



moar pics from the prodrive open days: http://www.clubgti.com/forum/showthread.php?t=176467 I think its posted somewhere here earlier but no harm in here also
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