Rennteam 2.0 - EN - Forum - 997 GT2 boost pressure - Page1

Sep 22, 2008 4:36:30 PM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002

997 GT2 boost pressure

I just posted this on RL so apologies to those who frequent both forums:
Just been enjoying some youtube footage of new GT2 and was interested to see the boost pressure digital gauge reading the 1.4 bar which Porsche advertise as being the boost pressure of the GT2.

I find this a little disingenuous, is it an example of Porcshe marketing beginning to further dominate over engineering ?

1.4 bar is being produced by the VVTs and I am guessing the measurement is taken around the throttle body, further down the intake manifold where the expansion thingy works its magic and as the (cooled) air enters the combustion chambers the boost pressure will be around 1 bar, same as the 997tt.

1.4 bar in the combustion chamber would give a hell of a lot more than 530PS - try 650PS !
So why do Porsche like to talk about 1.4 bar, is it because the number is big and sexy ? bigger = better ?

I don't like it, and feel it is the marketing tail wagging the engineering dog - the boost pressure displayed in the cockpit should be what is entering the combustion chamber IMO.....

Anyone feel similar or have I really started losing it

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Sep 22, 2008 5:19:43 PM

eclou: Veteran; Loc: Texas , United States; Posts: 1182, Gallery; Registered on: Jun 4, 2006; Reply to: TB993tt

Re: 997 GT2 boost pressure

I am also alarmed by this. What is the point of spinning the turbos and superheating the air to 1.4 bar but only realizing 1 bar in the combustion chambers? It is counter-intuitive to over stress a part for a net gain of zero.

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Sep 22, 2008 5:44:06 PM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: eclou

Re: 997 GT2 boost pressure

But they claim that there is a net gain of cooler intake temps even with the 1.4bar and the "proof" to this is the 15% worse fuel consumption if the expansion manifold is NOT used - but that then begs the question what boost pressure were they testing at to prove this (using a 997tt manifold)

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Sep 22, 2008 8:00:24 PM

eclou: Veteran; Loc: Texas , United States; Posts: 1182, Gallery; Registered on: Jun 4, 2006; Reply to: TB993tt

Re: 997 GT2 boost pressure

Toby they have basically come up with a way to justify a $80k premium for a power increase that could have been achieved with a $3k chip...

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Sep 22, 2008 8:08:20 PM

intouch1: Rennteam Master; Loc: Red Sea , Egypt; Posts: 5754, Gallery; Registered on: May 22, 2004; Reply to: TB993tt

Re: 997 GT2 boost pressure

i guess turbo boost is always measured as the boost generated and not what is actually fed into the combustion chambers.

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Sep 22, 2008 8:31:34 PM

fritz: Rennteam VIP; Loc: BaWü , Germany; Posts: 10248, Gallery; Registered on: Jul 23, 2003; Reply to: intouch1

Re: 997 GT2 boost pressure

Quote:
intouch1 said:
i guess turbo boost is always measured as the boost generated and not what is actually fed into the combustion chambers.

Naturally. Boost pressure is measured at a central point in the induction system to give a meaningful boost pressure reading which can be used by the ECU as a basis for the fuel injection and ignition timing mapping.

Measuring air pressure at one (or maybe all six?) of the induction ports would give a reading which varies wildly, depending on the point in the engine's operating cycle at which the sensor was "read". At up to about 3,500 induction cycles per minute, this would not be the rational way to do it.

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Sep 23, 2008 10:07:59 AM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: eclou

Re: 997 GT2 boost pressure

Quote:
eclou said:
Toby they have basically come up with a way to justify a $80k premium for a power increase that could have been achieved with a $3k chip...

Eclou, I do remember you being slightly ahem "hostile"

to the expansion manifold principle in earlier postings. Do you believe it is total rubbish and that Porsche are lying about its effectiveness ?
I have posted part of their technical explanation below which all seems to make sense to me. I defer to your tech knowledge, can you point out the BS if it is apparent ?

The $3K chip on a stock 997tt motor would indeed do what you say but the claim is that this inovation has brought (quite staggering IMO) effciences. Porsche have released the GT2 motor with 530PS but a similar chip/exhaust job releases 600PS on the RS dyno which indicates to me that the expansion manifold is indeed producing a more efficient engine and some....

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Sep 23, 2008 10:30:59 AM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: fritz

Re: 997 GT2 boost pressure

Quote:
fritz said:
Naturally. Boost pressure is measured at a central point in the induction system to give a meaningful boost pressure reading which can be used by the ECU as a basis for the fuel injection and ignition timing mapping.

Measuring air pressure at one (or maybe all six?) of the induction ports would give a reading which varies wildly, depending on the point in the engine's operating cycle at which the sensor was "read". At up to about 3,500 induction cycles per minute, this would not be the rational way to do it.

Fritz
My understanding is that the GT2 still uses some development of a MAF sensor which basically means the ECU primarily uses air MASS for its calculations and the actual boost number is not really a vital function (maybe only for monitoring of problems ?)
The Porsche blurb I posted above talks about total air flow being the same with and without the expansion manifold (with the benefit of the exp man being the cooler temp of air entering combustion chamber)

So the 1.4bar being measured at the throttle body or whereever is pretty irrelevant to any ECU calculations being made.

This obsession with high boost numbers which Porsche now seems to have embraced (by quoting and displaying the 1.4bar) is really a red herring - having monitored boost levels in my MAF 993tt over the years it can range from 1.1 down to 0.95 bar at the same rev point under the same load with the variation coming from an increased/decreased air mass which is totally atmospheric dependant.

One can easily see this by plugging in a diagnostic reader and reading the kg or air/hr reading at idle speed, it will vary noticably on a cold damp day compared to a warm dry day......

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Sep 23, 2008 11:43:04 AM

Alex_997TT: Rennteam Master; Loc: Reading , United Kingdom; Posts: 3320, Gallery; Registered on: Nov 20, 2005; Reply to: TB993tt

Re: 997 GT2 boost pressure

What I don't understand is why a stock Turbo in 'overboost' produces 680Nm torque at 1.2bar, but a stock GT2 produces the same 680Nm at 1.4bar with bigger VTGs.

...my Turbo with a CG remap and healthier breathing exhaust/air-filter makes more than 800Nm at 1.0 bar without overboost (at 1.2 bar it's even more).

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Sep 23, 2008 11:57:08 AM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: Alex_997TT

Re: 997 GT2 boost pressure

Quote:
Alex_997TT said:
What I don't understand is why a stock Turbo in 'overboost' produces 680Nm torque at 1.2bar, but a stock GT2 produces the same 680Nm at 1.4bar with bigger VTGs.

This "confusion" is sort of what my original post is about - people think that the GT2 is boosting at 1.4 bar which it is when the boost level is measured between the compressor and the begining of the intake runners. If you look at the tech diagram above you see that when the boost air has expanded into the manifold then straight into the engine the boost will have dropped and be more like 1 bar (and cooler hence increased torque vs 997tt bar for bar) - does that make sense Alex ?

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Sep 23, 2008 12:54:23 PM

eclou: Veteran; Loc: Texas , United States; Posts: 1182, Gallery; Registered on: Jun 4, 2006; Reply to: TB993tt

Re: 997 GT2 boost pressure

Quote:
TB993tt said:
Eclou, I do remember you being slightly ahem "hostile" to the expansion manifold principle in earlier postings. Do you believe it is total rubbish and that Porsche are lying about its effectiveness ?
I have posted part of their technical explanation below which all seems to make sense to me. I defer to your tech knowledge, can you point out the BS if it is apparent ?

The $3K chip on a stock 997tt motor would indeed do what you say but the claim is that this inovation has brought (quite staggering IMO) effciences. Porsche have released the GT2 motor with 530PS but a similar chip/exhaust job releases 600PS on the RS dyno which indicates to me that the expansion manifold is indeed producing a more efficient engine and some....

Toby I have no doubts that it works, I just don't see the need to add another level of complexity to the system. Adding additional heat to a turbocharged motor is something to be avoided if possible, especially since the VTG's are known to have greater level of intolerance to it. When the turbos are boosting to 1.4 bar (despite the expansion manifolds being able to cool the charged air back down) the turbos are going to generate heat that will still soak into the turbos themselves. This is something I don't like. The larger VTG's should be able to produce the same level of power at a lower level of boost in the Gt2 (maybe 1.0 bar) using the regular turbo intake manifold without having to push to 1.4 bar.

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Sep 23, 2008 1:49:31 PM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: eclou

Re: 997 GT2 boost pressure

Quote:
eclou said:
Toby I have no doubts that it works, I just don't see the need to add another level of complexity to the system. Adding additional heat to a turbocharged motor is something to be avoided if possible, especially since the VTG's are known to have greater level of intolerance to it. When the turbos are boosting to 1.4 bar (despite the expansion manifolds being able to cool the charged air back down) the turbos are going to generate heat that will still soak into the turbos themselves. This is something I don't like. The larger VTG's should be able to produce the same level of power at a lower level of boost in the Gt2 (maybe 1.0 bar) using the regular turbo intake manifold without having to push to 1.4 bar.

I have to say, the bit on the tech diagram about the compression/expansion "waves" is beyond me but I am wondering if we look into this a bit deeper, they state that this expansion effect is happening at higher power levels implying it happens over what ? 5500rpm ? I would like to see the boost curves below this ? I can see from the RS engine dyno graphs of the stock motors 997tt vs GT2 that there is a lot bigger area under the curve on the GT2 from 6000 to 6750rpm (obviously the 997tt can easily be able to match this with some ECU changes but allowing for like for like Porsche programming parameters) which does point to the expansion thing doing its stuff right at the top - if the high 1.4 bar boost is limited to just this top of the rev band at full load then the effects on longevity of the VTGs is probably well managed ?

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Sep 23, 2008 5:57:40 PM

fritz: Rennteam VIP; Loc: BaWü , Germany; Posts: 10248, Gallery; Registered on: Jul 23, 2003; Reply to: TB993tt

Re: 997 GT2 boost pressure

Quote:
TB993tt said:
Quote:
fritz said:
Naturally. Boost pressure is measured at a central point in the induction system to give a meaningful boost pressure reading which can be used by the ECU as a basis for the fuel injection and ignition timing mapping.

Fritz
My understanding is that the GT2 still uses some development of a MAF sensor which basically means the ECU primarily uses air MASS for its calculations and the actual boost number is not really a vital function (maybe only for monitoring of problems ?)
The Porsche blurb I posted above talks about total air flow being the same with and without the expansion manifold (with the benefit of the exp man being the cooler temp of air entering combustion chamber)

So the 1.4bar being measured at the throttle body or whereever is pretty irrelevant to any ECU calculations being made.

Your understanding is a gross oversimplification because it overlooks the fact that the ECU not only calculates the fuel injector opening times to correspond to the measured air mass flow.
The ECU also governs the ignition timing, and air inlet temperature and pressure are important inputs in defining the optimum ignition timing for maximum performance and minimum fuel consumption. Basing ignition timing on reliable inputs is hardly irrelevant in my eyes.

The benefit of the so-called expansion intake manifold is that it allows earlier ignition timing without pre-ignition, by providing a cooler air charge in the combustion chamber than would be the case with the TT's intake manifold. Earlier ignition means that more of the fuel's energy is used productively in the cylinder, instead of being wasted in the exhaust system. Improved combustion efficiency contributes to both maximizing power output and minimizing fuel consumption.

I think your reference to "total air flow being the same with and without the expansion manifold" is a misunderstanding of the meaning of the original text, due to it not having been written very clearly

.

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Sep 24, 2008 9:46:38 AM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: fritz

Re: 997 GT2 boost pressure

Quote:
fritz said:
Your understanding is a gross oversimplification because it overlooks the fact that the ECU not only calculates the fuel injector opening times to correspond to the measured air mass flow.
The ECU also governs the ignition timing, and air inlet temperature and pressure are important inputs in defining the optimum ignition timing for maximum performance and minimum fuel consumption. Basing ignition timing on reliable inputs is hardly irrelevant in my eyes.

My "understanding" was that the GT2 uses a MAF rather than MAP type of sensor. The only input I was commenting on was your assertion that the ECU uses the boost number as measured and quoted at 1.4bar.
The only thing I said was irrelevant is that 1.4 bar boost reading since I still do not believe that the ECU reads this, instead it reads the mass of air which may start off at 1.4bar then end up closer to 1 bar in as it enters the combustion chambers.
It was YOU who offered that the ECU reads and uses the 1.4 bar boost number (I grossly oversimplified and didn't mention any of the other inputs used on purpose since they are a given and not relevant to what I was asking you to clarify) do you still believe this or do you now agree that this 1.4 bar number is just a nice big number to look at in the brochure and on the dash read out

Quote:
fritz said:
The benefit of the so-called expansion intake manifold is that it allows earlier ignition timing without pre-ignition, by providing a cooler air charge in the combustion chamber than would be the case with the TT's intake manifold. Earlier ignition means that more of the fuel's energy is used productively in the cylinder, instead of being wasted in the exhaust system. Improved combustion efficiency contributes to both maximizing power output and minimizing fuel consumption.

I think your reference to "total air flow being the same with and without the expansion manifold" is a misunderstanding of the meaning of the original text, due to it not having been written very clearly .

My understanding of the "total air flow" bit is that a 997tt engine v a GT2 engine will produce less power for the same amount of air flow (measured at the throttle body MAF sensor) due to the cooling effect of the expansion manifold (as you well explain above...)

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Sep 24, 2008 6:42:16 PM

fritz: Rennteam VIP; Loc: BaWü , Germany; Posts: 10248, Gallery; Registered on: Jul 23, 2003; Reply to: TB993tt

Re: 997 GT2 boost pressure

Quote:
TB993tt said:
It was YOU who offered that the ECU reads and uses the 1.4 bar boost number (I grossly oversimplified and didn't mention any of the other inputs used on purpose since they are a given and not relevant to what I was asking you to clarify) do you still believe this or do you now agree that this 1.4 bar number is just a nice big number to look at in the brochure and on the dash read out

Short answer: I do not agree with your view.

Explanation, kept as short as I can make it:
"Boost pressure" is not a concept invented by marketing people, so they are also not in a position to define it or redefine it as it might suit them (thank God!).
Boost pressure is conventionally measured in supercharged engines using a sensor sited ahead of the throttle valve, because this (when compared with ambient air pressure) provides an important measure of one of the operating characteristics of the supercharging system. This same reading is also used as a primary input value for the electronic control of the engine's operating parameters. I would doubt that the engineers responsible for tuning the world's various turbocharged engines have been getting it wrong all this time by using the wrong parameter.

You quote an in-port pressure of 1 bar in your posts as if it were an acknowledged, stable value. I would suggest that it is just a guess on your part, since the real value will vary wildly with the opening / closing of the inlet valves and the resonance of the air column in the inlet system.
For this same reason, this pressure could also not be used as a basis for the display reading.

I am probably more allergic to marketing BS than you are, but you are just jumping to conclusions when you suggest that marketing people are behind the boost pressure display reading.

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Sep 25, 2008 10:15:47 AM

TB993tt: Rennteam Pro; Loc: manchester , United Kingdom; Posts: 2280, Gallery; Registered on: Jul 22, 2002; Reply to: fritz

Re: 997 GT2 boost pressure

Apologies to Fritz - you are right, I am wrong.....

I used up one of my phone calls to RS and basically the GT2 does boost between 1.2 and 1.4 bar and it is the combination of high boost and relatively retarded ignition (compared to other set ups - like my 1 bar 993tt ) which makes this combination work.
Incidentally the RS Tuning 680hp GT2 engine has a maximum boost of "only" 1.35bar so again as Fritz tried to show me, its a little more complicated than I was making out

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