Drove the car again yesterday. It's much smoother and the troughs are gone, but I still think the thing feels soft under 5000~ rpm. It's worlds better on good gas—I think I'll start running a mix of 91 and 100 from the track—but it still feels a bit sleepy even on 96.

I don't remember 991-1 Carrera S press cars feeling this slow down low. Part of it has to be the 991's seven-speed gear stack, but there's little doubt in my mind the 3.8 has been neutered. Friends within confirm as much, between exhaust and (maybe) concerns about the transmission with 3.8 torque. This is that rare NA engine begging for a mix of intake, exhaust, and ECU mods to wake it up.

 

Thanks for starting this thread. I'm going to give a 50/50 mix of 91 & 100 a try next track day as well.

 

Amusing to read your post about your experience with better gas, especially considering how underwhelmed you were feeling about the car. As you can now see, there is more performance available from this engine once you figure out how to unleash it.

I am on the East coast and I was unaware how bad your gas is out there. That really sucks! When the shop here was dyno tuning my car after the new intake and exhaust hardware was installed they commented on how sensitive the engine software is to octane. Timing gets pulled back at the slightest whiff of detonation.

 

Yep. To Porsche's credit, I've never heard my GT4 ping, not even once—something I can only say for factory engines and the very best aftermarket setups. There is nothing worse than that sound when you lean into the gas...

 

Just an interesting data point... on my 3.6 GT3 Mezger motor. It dyno wheel on 100 at 408 HP (just for sake of conversation... not trying to compare to "reality"!!). From 91 to 100 saw about 20 HP from he stock tune to cobb 100 tune.

What is clear from the graphs and data: the engine above about 7000 rpm in ALL tunes (stock or cobb) tapers out quite a bit and is intake constrained. (I have 100 cell cup headers on my car)

The tuner feels that with the 3.8 or 4.0 intake i'd see another 10-15 hp and the car pull harder after 7k.

I just shift about 7500 and call it a day :-)

I'll have my stock GT4 on a dyno before end of year for a comparative and dyno some changes we are planning.

 

What gas station and where is it?

 

I am using 102 octane in Germany from Aral (BP). I havent had any problem with the performance last 4000 kms

 

I think that is close to 96 or 97 octane (in US with American octane rating system)

 

It's a VP Racing station in Rancho Santa Fe San Diego. That link posted earlier in this thread shows several locations in SoCal for race fuel, I had no idea how much was available here! I'm going to try to get an account with CFN so I can buy 100 octane right in town and mix it with the 91 as others have noted.

 

Yes it should be RON based 102 octane. It costs 1.4 EUR per liter, which is around 10 cents more expensive than regular 95 RON.

I have checked wiki and it is weird >> https://en.wikipedia.org/wiki/Octane_rating
It says that Aral Ultimate 102 RON is actually 95 AKI.
But BP Ultimate 102 RON is 97-98 AKI.

BP 102 has a MON of 93-94, whereas Aral 102 88. Actually BP is the owner of ARAL and I would be really suprised if they have actually developed another formula only for Germany.

 

Thanks for sharing Pete. I am heading to Watkins Glen to run with CVR this Monday and Tuesday, and it will be my first time on the track with the GT4. I know they have race gas there, and maybe I will mix with some 93 and see what happens.

 

You are probably better off, the engine too, if at the track you run as "pure" 100 octane as you can. Try to arrange to show up with the tank level as low as you dare then add as much fuel as you think is reasonable. A hard running car on the track really works the fuel pump hard and this means it runs hot. The fuel helps keep it cool so I wouldn't want to run the level too low.

The argument against blending racing fuel with street fuel is while one can obtain an octane rating of 93 or something in that region, racing fuel offers more than just a higher octane.

A few years ago there was a very thorough fuel test done by EVO (UK car magazine). What it found was with racing fuel is its performance at elevated RPMs, which of course an engine in a car being tracked would experience a lot, was just incredible compared to street gasoline.

Here's the exact words: Mixes it with BP Ultimate initially, but beyond 3500 RPMs it is well clear of the pump fuels, as expected. Storms to highest peak power, though the flat topped curve [fuels were dyno tested as well as street and track tested too] suggests it is capped. So costly only justifiable in a race care (with a high compression ratio) where a second a lap is a lifetime. Driver notes (tried after 2nd batch of BP Ultimate): "Felt like it kicked hard low down and then kicked again at about 4000 RPMs when it just took off."

So by blending street fuel and racing fuel while you get the higher octane you give up some of the additional benefits racing fuel offers.

Hehe. Let your buddies think you blend it like every one else, but run 100 percent (or very close to that) racing fuel and gain an edge.

 

Didn't have time to get over to the track to get a mix of 100/91, so went back to straight 91 pump gas—and the torque output is back to being super soft until 5000+ rpm. The driveability might even be worse than the tank of 91 before the tank of 96/91, though that may be perception.

Whatever...it's a drag.

There appears to be something about the GT4's ECU mapping that doesn't deal well with (admittedly awful) CA gasoline. But it's still 91 octane, so it's not like it's 87—and I don't remember any 991-1 Carrera S feeling so slow and lazy through the low- and midrange and then suddenly clearing out its lungs up top. It feels like V-TEC circa 1990, and not in a good or fun way. And you say it has VarioCam Plus? Sure, the 991-1 had better gears to work with, for sure, but this seems like far more than a gearing issue. Either this engine isn't breathing, or the timing is being pulled. Or both.

 

Sounds like your car can benefit significantly with an IPD intake and a GT3 TB
and perhaps a tune.

Mine became totally different in a much better way after these upgrades.
Just a thought...

 

I tried a blend of 25% 100 octane with 93 octane at Mid-Ohio last weekend. I'm not sure that I noticed much of a difference. However, my car has run fairly strong from new.

It will be interesting to see how the Dundon headers change the power.