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    • Porsche 991 Turbo / S inconsistent performance unofficially explained by GIAC - Car pulls boost at high rpm to prevent limp mode , 40+ whp loss

      The Porsche 991 generation 911 Turbo has many people scratching their heads regarding its performance. PorscheBoost explored this issue in an article earlier this year. For some reason 991 Turbo and S numbers are all over the place ranging from 123 mile per hour trap speeds all the way to 130.


      That is a very big spread in 1/4 mile trap speed indicating vastly different power. What is interesting is that the elapsed times are all fairly good. It is almost as if the 991 Turbo falls on its face after the 1/8 mile.

      The 0-60 numbers are out of this world with Road and Track recording 2.6 seconds. 0-100 is equally impressive coming in 6.8 seconds but then the car sort of just dies registering a very quick 10.9 but only 124 miles per hour in the 1/4 mile. What's going on?

      We think we have an explanation. Earlier we thought the car was pulling timing to protect the motor (a problem GM has been having with the new C7 Z06) due to heat soak and that is partially true.

      Recently, a PorscheBoost member with a 991 Turbo S got his car tuned by GIAC. What is interesting to note regarding his tuning results is that the car loses 40 wheel horsepower from pull 1 to pull 3. Torque drops by 49 lb-ft in that same span:


      Keep in mind this is on a conservative reading Mustang dyno. On a Dynojet someone would be looking at over 50+ wheel horsepower lost. That is massive.

      Now once this was posted PorscheBoost started getting info from various sources who wish to remain discreet to not upset GIAC. The truth is, GIAC knows the car is pulling boost and timing. As does Champion Motorsports who is readying an intercooler upgrade for the car.

      The vehicle is attempting to protect itself when heat soaked and it appears to easily get heat soaked. Some tuners swear the 991 intercooler and cooling is great but how do you explain such a large drop in torque from pull to pull and the car pulling boost as it heats up?

      The owner who got this tune dyno'd has run his car at runways events and from the first run to subsequent runs he lost 6+ miles per hour of trap speed. The dyno runs and inconsistent magazine figures support all of this.

      The GIAC Stage 2 flash is forced to pull boost at high rpm's to prevent limp mode. The stock car is already pulling boost and timing itself. Tuning just exacerbates this problem.

      We will see how tunes are revised and what cooling solutions Champion Motorsports comes up with but clearly the 991 Turbo is not a consistent performer especially when tuned as of right now.

      This article was originally published in forum thread: Porsche 991 Turbo / S inconsistent performance unofficially explained by GIAC - Car pulls boost at high rpm to prevent limp mode , 40+ whp loss started by Sticky View original post
      Comments 11 Comments
      1. ecpchris's Avatar
        ecpchris -
        Wow that is extremely interesting to know. Another great discovery @Sticky.
        Better intercoolers and meth = problem solved. Maybe that's part of the equation why Emre is trapping so high.
        Also, the 991s have smaller turbos than the 997.2s. Boosting them would create more heat, further compounding whatever is going on.
      1. jyamona@motiv's Avatar
        jyamona@motiv -
        Click here to enlarge Originally Posted by ecpchris Click here to enlarge
        Wow that is extremely interesting to know. Another great discovery @Sticky.
        Better intercoolers and meth = problem solved. Maybe that's part of the equation why Emre is trapping so high.
        Also, the 991s have smaller turbos than the 997.2s. Boosting them would create more heat, further compounding whatever is going on.
        +1. It is definitely the meth he is using then. It would keep the DME seeing low IAT's, keeping full timing and boost.
      1. leveraged sellout's Avatar
        leveraged sellout -
        Well this makes sense. I figured it was something like this. Sounds like it needs bigger intercoolers for sure, this is honestly not a problem Porsche usually has so I'm surprised by this. I wonder what a stock car loses over the course of say, 5 dyno runs. Hopefully the 991.2 has a more robust cooling setup.
      1. ecpchris's Avatar
        ecpchris -
        Click here to enlarge Originally Posted by leveraged sellout Click here to enlarge
        Well this makes sense. I figured it was something like this. Sounds like it needs bigger intercoolers for sure, this is honestly not a problem Porsche usually has so I'm surprised by this. I wonder what a stock car loses over the course of say, 5 dyno runs. Hopefully the 991.2 has a more robust cooling setup.
        The 991.1 def have bigger cores so that isn't it. I wonder if it is something to do with the ducting and aerodynamics. Just speculating, but it also looks like the 991s have smaller inlets and outlets on the intercoolers as opposed to the 997.2s. Essentially big cores with smaller endtanks and smaller inlet/outlets. No wonder they are less efficient at cooling
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by ecpchris Click here to enlarge
        Wow that is extremely interesting to know. Another great discovery @Sticky.
        Better intercoolers and meth = problem solved. Maybe that's part of the equation why Emre is trapping so high.
        Also, the 991s have smaller turbos than the 997.2s. Boosting them would create more heat, further compounding whatever is going on.
        The meth becomes even more important now for its cooling use than just as a supplemental fuel additive. In this direct injected motor and with what we know I'm surprised more guys that are tuning are not running it.

        Exactly, tuning a car already having heat soak issues isn't helping the problem.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by ecpchris Click here to enlarge
        The 991.1 def have bigger cores so that isn't it. I wonder if it is something to do with the ducting and aerodynamics. Just speculating, but it also looks like the 991s have smaller inlets and outlets on the intercoolers as opposed to the 997.2s. Essentially big cores with smaller endtanks and smaller inlet/outlets. No wonder they are less efficient at cooling
        Sort of what I was thinking. It seems to be something beyond the intercoolers. Are the turbos smaller and just being pushed harder? Maybe they went overly small for response and the turbos are generating a lot of heat?

        Maybe it's a combo of things. Porsche should know better though shouldn't they?
      1. leveraged sellout's Avatar
        leveraged sellout -
        Click here to enlarge Originally Posted by ecpchris Click here to enlarge
        The 991.1 def have bigger cores so that isn't it. I wonder if it is something to do with the ducting and aerodynamics. Just speculating, but it also looks like the 991s have smaller inlets and outlets on the intercoolers as opposed to the 997.2s. Essentially big cores with smaller endtanks and smaller inlet/outlets. No wonder they are less efficient at cooling
        If that's the case, that would also make sense...and wow I wonder why they would do that. Seems strange, maybe they didn't anticipate this being an issue? Hard to say.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by leveraged sellout Click here to enlarge
        If that's the case, that would also make sense...and wow I wonder why they would do that. Seems strange, maybe they didn't anticipate this being an issue? Hard to say.
        It's hard for me to believe the Porsche engineers would screw up this badly. Then again, GM is having problems with Z06 heat soak so maybe all this pushing power out of forced induction has resulted in cars that are only good for short WOT bursts.
      1. leveraged sellout's Avatar
        leveraged sellout -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        It's hard for me to believe the Porsche engineers would screw this badly. Then again, GM is having problems with Z06 heat soak so maybe all this pushing power out of forced induction has resulted in cars that are only good for short WOT bursts.
        Yeah I was just about to bring that up...we've already seen how the Z06 has been a major failure in terms of heat. In fact MT just did a re-test of the Z06 with a revised suspension setup and noticed that it still had heat problems. Even GM said you should probably take the manual if you want to track it seriously, since the auto box causes it to overheat quickly.
      1. Sticky's Avatar
        Sticky -
        I love the Z06 but it's not topping the ZR1 acceleration wise (handling is a different story) despite more power, torque, and the trick auto box.

        The C6 Z06 isn't even giving up much in that respect and it's capable of repeated laps.
      1. leveraged sellout's Avatar
        leveraged sellout -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        I love the Z06 but it's not topping the ZR1 acceleration wise (handling is a different story) despite more power, torque, and the trick auto box.

        The C6 Z06 isn't even giving up much in that respect and it's capable of repeated laps.
        Seriously if I could have a C7 interior in a C6 Z06 I would drive it. I cannot stand to even look at a C6 interior but the old Z06 was a truly amazing vehicle.