My point is that some owners can achieve peak advertised boost with just Sport mode, which is what we should be expecting. I'd be really disappointed if I unable to achieve peak advertised boost without having the Sport Chrono option. I would have made more effort to find a car with it if I knew this.

 

 

 

 

I thought about this scenario too but let's look at torque instead of HP since the Torque curve is exactly flat from 1700 rpm to about 5000 rpm and then decreases as rpm goes up. It's possible that all of the additional boost in sport + mode is occurring above 5000 rpm stopping torque from falling (current torque curve) but never exceeding it's max torque that is advertised. I wasn't looking at it that closely to see when the additional boost built in but I believe it was high in the rev range so you may be right. Maybe HP and Torque never exceed the maximum advertised value.

At a minimum, the HP and Torque curves are different in sport + mode than sport mode. (assuming the gauge is reading correctly)

 

For you guys that are doing back to back boost comparisons between Sport and Sport+, are you in a PDK or Manual? Because if it's PDK, it might be holding the gears longer in Sport+ which could affect the peak boost you're seeing. Anyone comparing the 2 modes in a manual?

 

I was in a manual. See post #45.

 

Thanks! Good post.

I'd be interested to hear your boost peak on that same uphill, but in Sport. The uphill may put more load on the engine requiring more boost.

Also, it's good that you ran each trial a few times (i.e. your shift points might be slightly different each time you try, etc.)

However, overall the trend of this thread is that Sport+ has a different peak boost (or at least lets you hit it more easily / more frequently).

 

 

TL;DR:

I suspected when I saw the marketing propaganda for 'sport response' mode that the 991.2s featured a Capital-T Turbo overboost-like mode. The observations in this thread support that there is 'overboost' in Sport+ mode.

The interesting question is why Porsche isn't marketing this as "overboost" but is instead, seemingly, completely silent? My suspicion is that it is due to regulatory/emissions constraints (i.e. if they market it, then they have to test it and C02 emissions go up, CAFE numbers go down, yada, yada...)

The interesting question is NOT:
- why does 'my' 991.2 only show X psi of boost while your 991.2 shows Y? Without knowing all the other conditions (specific test protocol, ambient temperature, engine temperature, altitude, barometric pressure, fuel type, etc.) these numbers are as comparable as observations of the intensity of sunlight when the time, date, and latitude are unknown.

Your post posted while I was writing mine. Your test protocol was by far the best. You attempted to observe maximum sustained boost and you repeated each test several times.

Other data, individual data points gathered by a quick glance at the boost gauge may be data but they are useless data for cross-fleet comparison.

Your data supports the conclusion that there is more boost in Sport+ mode, but your data is not comparable to data from someone else's 991.2 tested until different conditions (except to the extent that 'their' data supports or refutes the theory of more boost in + mode.)

And no where did I dispute this.

My issue is with comparing a single boost value from somewhere in the middle of a test run across cars where ambient temperature, engine temperature, altitude, barometric pressure, and fuel type are unknown.

No. I did not challenge the boost gauge. I challenged the usefulness of drawing sweeping conclusions(*) across a fleet of cars - or for even a single car - where the above additional data are unobserved and the test protocols are not reasonably uniform or repeated.

(*) Such as:
- Porsche is screwing us out of horsepower.
- 'my' car doesn't have the rated horsepower.

To do otherwise would be to risk another class-action. Historically, the rated horsepower of a Porsche engine is a guaranteed minimum. Given normal manufacturing distributions and - for warranty/legal purposes - the need to ensure that 6-sigma of engines meet the rating, it is normal for the average Porsche engine to output a few percent more than the rating.

First, do we know that the MFD display is based upon the reading from an actual pressure sensor? (Or maybe two.) Or, is it a value derived from load, temperature, mass-air flow, and rpm? If it is a (or two) pressure sensor(s) do we know where in the air path they are?

My suspicion is that it is a unconditioned signal from a manifold absolute pressure (MAP) sensor. (I could, but haven't investigated. Most new cars these days use MAP sensors.)

Do we have any reason to believe that this signal is conditioned for pressure and temperature? Remember Pv = nRT? If directly from the MAP it won't be.

Boost doesn't mean that much as an absolute number. The engine management system is trying to determine the MASS (as in number of atoms) of air. It does not care, at all, about the pressure except that that is one input along with others that it uses to determine the air mass. And, of course, the air mass moved by the Turbos will also be effected by Pv = nRT. Last, intercooler efficiency varies with temperature and will play a part also.

All of these are interesting questions, but, one would expect that, absent a failure, any number of 911.2s will display more-or-less the same boost traces if they are tested under identical conditions with the same equipment, fuel etc.


Careful. HP = Torque x RPM.

When we talk about HP it gets wacky because rated HP is made at a single RPM. More boost AT THAT RPM will increase HP above the rated HP. However, more boost in the low rpm range will not necessarily increase HP above the rated HP. More boost in the low range WILL produce more HP, but it will still (probably) be less than the rating.

This is plainly evident, and documented, with overboost on Capital-T Turbos. Overboost increases boost in the mid-range such that the maximum torque is increased by ~30 ft-lbs (I forget the exact number.) However the boost drops off to 'normal' before the RPM at which horsepower is maximized. Thus RATED HP is the same. Maximum torque is higher. And, yes, overboost mode produces more horsepower in the mid-range but at no point is this increased power more than the rated horsepower.

With the above caveat I agree with #2. Specifically, 991.2s with SC are making more HP in the mid-range. But, that doesn't mean that they make more than the rated HP and it doesn't mean that 991.2s without SC are making less than the rated HP.

 

Instead of doing tests comparing what the boost gauge shows in different modes (normal, sport, sport +), why not do some actual timed runs in those same modes?
It all boils down to is the car faster in each of these modes right? I think this would be a better test of what the engine is producing vs. the number on the screen.

 

worf928 makes several good points. And it should be remembered that when Porsche makes its "overboost" claims for the Turbo models, it also clearly specifies the amount of increased torque available in the mid-range. There is no such claim or specification offered for the 3.0-liter 991.2 models.

 

Thanks worf928. Those are all good comments.

I asked the 718 guys in another forum if they were seeing the same thing. One of them posted information from ??? that states boost is higher in Sport + AND sport mode for the 718. I have not seen similar commentary for the 991.2

 

TL;DR: Porsche is absolutely playing games with the engine management operating targets in the two (three with SC) operating modes. Based upon data gather by owners and posted to teh intErwEbs we know this to be true: drive train responsiveness, engine temperatures, etc. all vary between the 2(3) modes. It really should not come as a surprise that we see different boost levels with turbo-charged engines between the 2(3) modes under identical test protocols where only mode is a variable.

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One other navel-gazing item to throw into the hopper is that there are two target air-fuel ratios. Actually, with DFI engines using modern wide-band oxygen sensors and modern cats, there are three:
- very lean for steady very-low load cruising (think 55 mph on the highway where you need about 10 hp)
- stoichiometric (~14.7) for low to medium-high load (think 'round town and when the go-pedal is pressed.)
- power ratio (~12.6) for maximum acceleration (LC starts, pedal mashed through the firewall.)

The first minimizes fuel consumption, but relies on modern cats to clean up excess NOx produced via ultra-lean combustion.
The second provides 90%(-ish) of possible engine output (subject to total combustion charge (load)) at any rpm while minimizing the cumulative combination of C0 and NOx.
The third provides maximum power and knock prevention but results in unburned fuel (higher HC) that speeds deterioration of cats.

Normal mode is - I strongly suspect and seems logical - calibrated for EPA (and Euro/ROW-equivalent) testing where consumption and emissions need to be minimized within the constraints of the test protocol (queue jokes about Diesel Gate(*) and 1) recall threads about how the PDK cars want to shift to 4th at very low speeds in "Normal" and 2) how engine and oil temperatures drop by ~30-ish degrees (F) in Sport and + modes.)

Since Sport and +-mode default to off, Porsche can do whatever it wants to make a product people want to drive and enjoy. The engine management system can target 'better-for-us' operating points without worrying too much about emissions and consumption.

The power ratio is likely reserved for corner cases since the HC-induced cat deterioration has to be managed due to 10/100k emission system warranties.

Ignition timing is - I think - not as 'played-with' in these modes as are A/F ratio and the boost bypass setting. Timing is going to be chosen for each operating point and mode to target the best combustion and minimize knock risk. This is less true for NA motors and more-so for FI motors. You just can't vary timing as much around optimal on an FI engine lest you get knock. Knock bad. Very bad.

(*) If VW had just put a "Sport" button that defaulted off on all the diesels from day one they could have avoided this 100-billion dollar mess. We *know* they already had the software for 'normal' and 'kinda-sport-ish' modes all they needed to do was spend $0.22 per unit for the button. Thus, expect the SJWs to lobby for making "Sport" buttons illegal. But, they might not get that through before they make ICE engines illegal.

 

Chiming in late here, but boost level targeted is likely completely determined by ambient conditions assuming there are no mechanical issues like boost leaks. I don't know how exactly Porsche engine management works, but I found this on COBBs website:
https://cobbtuning.atlassian.net/wik...landVTGControl

So basically, the engine ECU tries to hit a target torque. When I was tuning my 2005 Mitsu Evo, it had a target 'load' to hit which is basically torque. To hit the target torque requires different levels of boost depending on ambient conditions. If it's hotter outside, the air is less dense, so higher boost has to be run to get the target torque/mass of air in the cylinder. Conversely, if it's colder as it is in most of the country right now, less boost is required to get the same mass of air into the cylinder and hit the same torque target.

Elevation plays a huge role as air gets thinner at higher elevations. To make the same torque in Denver as Los Angeles, a lot more boost has to be used in Denver because the ambient air pressure is lower resulting in lower air density. When the turbo suppliers spec out turbos for the engine manufacturers, they have to select a compressor wheel that has 'elevation margin' meaning the engine can run higher boost levels without going off the compressor map when the engine is operating at significantly above sea level. Usually the spec is something like no power derate at 1000m or something elevation; beyond that, it's allowed to have reduced power by some amount.