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2.5 boost levels observed

12984 Views 38 Replies 19 Participants Last post by  Semitone
Porsche states 1.1 bar or 15.95 psi as max. boost level for the S/GTS.

In wide-open-throttle runs to redline I'm seeing indicated boost steady around 8 psi in the upper third of the rev band. Sometimes it actually decreases as the tach needle approaches peak power 6500rpm, going 8-7-6 even to 5 sometimes. Higher boost levels (10 psi) occasionally seen at low rpms 'pulling out of the hole'. But it's the behavior from 5k to redline, w-o-t, that intrigues me.

This is not what I expected to see....how does it compare to others with the 2.5/VTG?
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When I am accelerating at WOT, I have to look ahead. I have read that under wot that the turbo psi reduces at higher rpm's by design.
Under full acceleration and using sport+ mode I have seen a max of about 14 psi. It isn't always that high though - mostly in the 9-12 psi range when on a track. Around town it hardly ever gets past 0 or 2 psi, unless I'm accelerating onto a highway.
Yup, perfectly normal for boost to trail off towards the red line. Likely also different boost level based on gear. Lower gear may have a lower boost limit.
That's interesting, somewhat counter-intuitive.....I'd have expected to see something like max. boost at peak power band under WOT.

Does this imply there is more boost available from the turbo but DME has been programmed not to tap it (in this specific set of conditions wrt rpm/heat/load/etc.)? Or more like: the turbo is maxed-out even at the lower boost level (for this specific set of conditions)?

Again, I'm observing 8psi (sometimes decreasing to 5psi) for long WOT pulls to redline in third gear. That seems like a very under-stressed turbocharger....yet a tuner site (PorscheBoost.com) states turbo size is the constraint to higher peak hp for tune-only (i.e. higher boost) flashes of the S. How can the 64/55 VTG turbo at once be under-utilized at peak power stock, and the limiting factor on boosted tunes?
The power curve peaks at 6500 rpm so a fall in boost after that would be expected.

I'd be assuming that Porsche has got pretty good at balancing performance and reliability so there must be reasons for what you've observed. I do know that my knowledge of turbo engines is miniscule. Hopefully someone can help educate us.

I've had good responses from John Cadogan on other topics (such as why tyre grip is only partly friction), but I've bothered:) him a bit often right now. Searching his Youtube channel may provide answers, or at least his email address.

WARNING--John can be quite crude and his Australian sense of humour has been known to cause agitation in sensitive persons.
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That's interesting, somewhat counter-intuitive.....I'd have expected to see something like max. boost at peak power band under WOT.

Does this imply there is more boost available from the turbo but DME has been programmed not to tap it (in this specific set of conditions wrt rpm/heat/load/etc.)? Or more like: the turbo is maxed-out even at the lower boost level (for this specific set of conditions)?

Again, I'm observing 8psi (sometimes decreasing to 5psi) for long WOT pulls to redline in third gear. That seems like a very under-stressed turbocharger....yet a tuner site (PorscheBoost.com) states turbo size is the constraint to higher peak hp for tune-only (i.e. higher boost) flashes of the S. How can the 64/55 VTG turbo at once be under-utilized at peak power stock, and the limiting factor on boosted tunes?
Boost is going to fall towards redline, just how turbos work, it's not programming..... unless.... what map are you in? If it's normal mode there may be a boost limit. WOT in 3rd gear, 5k ish RPM and Sport+ mode i definitely see 14ish PSI maybe more, don't stare to long at it. You should being seeing more than 8psi if not in normal mode.
Boost is going to fall towards redline, just how turbos work, it's not programming..... unless.... what map are you in? If it's normal mode there may be a boost limit. WOT in 3rd gear, 5k ish RPM and Sport+ mode i definitely see 14ish PSI maybe more, don't stare to long at it. You should being seeing more than 8psi if not in normal mode.
Ha, yeah better sneak a quick peek at the digital boost gauge at those speeds!

I'm in Sport mode, manual S, around sea-level, ambient temps in 70s, 91 octane, lightest weight (low fuel load, no options, driver-only).

I'm observing 8psi pretty much all the way from 5000 rpm to peak 6500 rpm at WOT (so this is definitely not about drop-off between peak and redline). I have seen higher psi briefly (into teens) at times when lower in the rev-band (like calling up V-8 style 'grunt')...so it's not as if turbo won't make the higher boost, more like DME doesn't need it to draw the power curve up top it wants.
One consideration for declining boost at high rpm is exhaust system back-pressure, where the exhaust plumbing ultimately restricts the amount of flow. My first turbo car was designed with a certain backflow resistance because it had no waste gate. Manifold vacuum would be replaced by positive boost around 3500 rpm, and get up to around 10psi maximum and then begin to fall off. Replacing the stock muffler with a section of straight pipe allowed about 1 bar of boost at redline in top gear and still pulling but enough is enough!
One consideration for declining boost at high rpm is exhaust system back-pressure
Good point, I wonder how that factors in here. Newer VTG technology in the 718 has integral -- rather than external -- wastegate (apparently). Just found this:

"I hope that Porsche have gone one step further (with the new 991 GT2 RS turbo) and introduced wastegates as on the new Boxster S. VTG with wastegate is really nice since you can remove some exhaust flow from the turbine thereby allowing the VTG vanes to be set at a position for best turbine efficiency. This gives lower back pressures for a given boost level, and with the data Cobb provided us here re: back pressure over boost on 997.1 TT I think we all understand that it is *not* optimal as is (997 has first-gen Porsche VTG). Also the data from BorgWarner re: this technology (integral wastegate VTG) says a lot about its potential (I am not sure if it is official and what I can share)."

slightly edited for readability, here is the source thread
https://www.6speedonline.com/forums/997-turbo-gt2/408324-new-991-gt2-rs-new-turbos.html
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I was wondering how much impact 91 octane would have on max boost. I’d think that 93 octane could tolerate greater boost pressure.
I was wondering how much impact 91 octane would have on max boost. I’d think that 93 octane could tolerate greater boost pressure.
I wondered about that too. 91 is all we have in California, but many of the Car & Driver track sheet tests are done in Calif. with pump gas (I believe). So I think to the extent it's a factor, Porsche (and other manufacturer) DME programming takes it into account for consistent performance with R-O-W.

Guess I could add some octane boost to the tank and do another set of observations. First I'd like to hear from more 2.5 drivers on the forum, we have a really great raw data collection capability going largely untapped.
I wondered about that too. 91 is all we have in California, but many of the Car & Driver track sheet tests are done in Calif. with pump gas (I believe).<snip>
The C&D test record sheets show a tick for PREMIUM. Is 91 premium? (We use a different rating system in Australia.)

I also note that in their YouTube video their main testing ground is in Michigan.
Good point, I wonder how that factors in here. Newer VTG technology in the 718 has integral -- rather than external -- wastegate (apparently)...
afaik 718's 2.5L and its VTG is augmented by an external wastegate.


fwiw mine is also down to ~6-psi at the 2-3 WOT upshift...off-highway use only, of course.

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afaik 718's 2.5L and its VTG is augmented by an external wastegate.
Text from your attachment: "A turbocharger....with additional wastegate is used on the 2.5". "Additional" meaning integral, as opposed to external? The illustration also shows an integral wastegate (I believe).

It's been difficult to get this level of technical detail on the VTG. The blurb I quoted from 6SpeedOnline just struck me as someone (an engineer; in Sweden?) who had some inside knowledge in this context. Plus it jibed with something I'd read somewhere previously (filed in memory-bank but no link to recall unfortunately).

fwiw mine is also down to ~6-psi at the 2-3 WOT upshift.
Thanks for sharing this, at least I know I'm not alone in not seeing the higher psi numbers reported in this thread. You say "down" to 6psi at the upshift....what max. psi are you seeing on the full-throttle run-up in 2nd gear?

Others?
I was wondering how much impact 91 octane would have on max boost. I’d think that 93 octane could tolerate greater boost pressure.
My GTS with the 93 octane we have here was showing 17 psi this morning on the interstate going to cars and coffee. It flipped to another number momentarily - not sure if it was 16 or 18 - it happened so fast.
Just did a few 3rd gear pulls, in sport+, 16psi max, 13psi at the shift. Sure your full throttle? Something has to be wrong with the car otherwise.

Generally the car wont adjust boost for octane, it would just dial back the timing. Base timing table value minus compensation values for temp, altitude, knock, lots of other things, gets complicated. Possibly could be dialing back boost, very fancy ecu.

If thats the case would need to pull the battery and let it reset ans then try some pulls.
Fosgate769;138380Something has to be wrong with the car otherwise.[/QUOTE said:
I don't think so, @splinter (another Ca. car) reports similar numbers to what I'm seeing. And my S sure feels like it's pulling strong, fast and linear all the way to redline (and way more powerfully than my 315-330hp 996 flat-sixes as a frame of reference).

I *am* seeing 11 psi on rare instances at lower rpms but even that is hard to provoke on-demand in most runs -- so it's as if the turbo is clearly capable of more boost *if needed*, the ECU is just not calling it up very often for ambient conditions and loads. Humidity is low here in Northern Calif., temps have been moderate 70-80s. Oil temps reading just around 205 degrees under load. Car is a manual S, no SC, in Sport mode, WOT for the full run, on 91 octane.

Dealer just had the car for one-year service and would have pulled any error codes off the ECU I'd think. But I've had a 'sick' turbo-four (Mistubishi) before, know how it feels, this one feels stout....

Let's get some more data from Forum cars, please!
Continue the ecu/vtg conspiracy
Did some 3d gear pulls with my tuned gts in sport+ 22psi Max, so it seems that porsche have left some boost on the table..
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