So the condensed version is that you cannot run as much boost safely on lower octane gas. I have been advised by Todd K to only run 1 bar, 1.1 max on 93 octane (I would run up to 1.2 when I had smaller turbos). I can run up to 1.5 on C 16.

I would not risk more than .9 bar max on 91. Each .1 bar =35 rwhp on my car so there is a 175 hp difference between 91 craptane and C 16.

 

Sean "2 kicks" do you have a dyno sheet on your car?

 

I agree, and contrary to what alot of folks believe, Todd Knighton
is very conservative in the boost pressures that he tunes for and recommends. My car was actually tuned for a peak boost on his dyno
of 1.34 bar (PEAK) on MS103 (only 99 octane) but it runs lots of fuel
(2 fuel pumps, ). Todd told me that the higher Octane that I run is not
improving my performance becuase the tune doesnt take advantage
of the Sunoco GT260 PLUS (104 oct, I believe?). I will be doing some
data logging and emailing to Todd for further info on this.

 

Depending on what set up...Turbos, etc. Range on stock internals 400 to upper 600, although I am not a big fan of running 91 octane

 

Dennis, excellent write up and right on. I want to add a few things in here that need to be cleared up when speaking of all this stuff.

First, in the beginning of the discussion horsepower was asked. We need to be clear on whether we speak of Wheel or Brake. Huge difference. Presuming one or the other will make a huge difference.

The second is 91 octane. When you speak of running 91 octane, that means you are running a 91 octane tuning file. This type of tuning file has less timing, boost, etc, to maintain power at WOT so that the engine will not go into limp mode or better yet, detonate. This kind of file will adapt slightly to higher octane, but if you run a 91 octane file and put 93 or higher, your still running a 91 octane file. The file will adapt a bit, but it will never run like a file suited to a higher octane, say a 100 octane race file. So when we speak of 91 octane, the presumption should be that it is done with a 91 octane file.

Another thing to note is that fuel looses octane at about 1 point per month, and I have heard as high as 2 points per month. Couple this with the fact that hardly anyone uses 91 octane these days, and you have fuel sitting at gas stations for may be a month or more and that fuel is no longer 91.

just some thoughts.

 

Remember that your .1bar = 35 rwhp is a sliding scale as the turbos move through their compressor map. At the low end and high end, it'll be much less.

I think that the advice from your tuner is solid. The only caveat I'd add is on pure C16, you could cap the wastegates. I've, and countless others, have run upwards of 35 PSI (2.3 bar) on C16 without issue. Remember that C16 is 116-117 MON octane. Incredible fuel.

Of course capped wastegates raise other issues which may be problematic for your car. But the fuel will support it.

 

Excellent explanation!!!

You can run a higher amount of boost if you having timing set right. I can run 1.25-1.3bar on 93 oct. if I want to only because my car has VERY little timing in it, just so it will be safe. Some tuners prefer to run a higher amount of timing to make their power with a lower amount of boost. I had Tony write my tune so I can run a decent amount of power on 93oct. and then run a high amount of boost on MS109/C16. I'm sure I could get a decent amount of power from timing alone on my car. I just know on MS109 and 23psi there was NO sign of knock or anything and the a/f's were in the 11.8:1 area or so.

 

Thanks for the nice words! I was worried how lucid I was at 12:30.

You're obviously correct about fuel degradation over time. I would submit, however, that 91 vs 93 is a distinction without real world meaning. I would not be any more aggressive with 93 over 91. Suffice to say, if you're on the ragged edge of 91, you're on the ragged edge of 93 also.

I like what you had to say about timing. That's true, the ECUs timing is tuned for specific octanes. I have not seen the map controllability possible in a Motronic flash, but if it's significant, it should be possible to make one map that works will all gas and octanes. How?

In traditional 3D fuel and ignition maps, you have load and RPM axis (x and y) with the fuel/timing data point your z axis. Thus, you have full control over the entire load and rev range. By making certain assumptions, e.g. proper octane at various boost levels, a map can be constructed with different ignition points. Most don't realize that fuel is not tuned for octane. Higher octane does not change the A/F ratio. So, it's irrelevant when discussing tune. Its only relevant when discussing timing. And to that end, the higher load sites representing more boost can be dialed in for higher octane settings.

On low octane, dial the boost back and you never get into the aggressive settings of the higher boost levels. High octane, raise the boost and get into the more aggressive settings.

Again, I don't know if the Porsche ECU offers the layman such flexibility. I'd assume it does, so I'd further submit that the "100 octane/93octane/etc" tunes are largely snake oil for separating you and your money with additional flashes.

 

Thanks Ralph! I passed by ME (I'm an EE) scrutiny! I never practiced as an engineer, and I am now an attorney, so I'm happy to defer to "real" engineers.

Theoretically, 93 affords more protection. However, as I mentioned earlier, I don't think it's a practical or meaningful distinction. In the real world, two engine pulls are never the same. Different temps, pressure spikes, ignition events, air intake temps, air intake turbulence, etc ensure that. Therefore, if you are running on the edge of 91, I assert that 93 will not come to your rescue.

Further empirical proof is in the difference of 100 octane. I've generally found that it's good to 15.5:1 (again, the specific number depends on many factors, including combustion chamber designs in different engines). Running a rough and dirty linear extrapolation, that's a little under 1PSI per unit of octane. For two points in octane, that's under 2 PSI. Typical boost spikes are within 2 PSI, especially Porsches running internal wastegates. So that's why I say that I don't think it's a meaningful distinction.

 

I'm long time friends with Nero at Titan, so he'll be helping me with my build too. Cheers to that!

I don't know how the Porsche engine responds to timing, but generally people are surprised how little aggressive timing helps in most engines. Assuming that the timing is in the ballpark, often a "safe timing" and "aggressive timing" is 3% in horsepower.

I am not comfortable running more boost because my timing is more retarded. It's a modest reduction in cylinder temps, a modest reduction in "beating" pre-ignition to the punch (the spark is coming before the cylinder has sufficiently compressed the charge), but a significant gain in pressure ratio and associated heat from the higher boost.

 

Dennis - 93 just gives one a little extra "insurance" to prevent dreaded denotation. I am happy to be able to use pump gas at 93 and not 91.

 

450-500 ish

 

So under this discussion, those of us in California stuck with 91 regularly is a flash to 1 bar a bad thing for our motors?

Theres a couple places in the bay area you can get better fuel but nothing in or around SF proper. Please correct me if somebody knows of a place...

 

Great write up Dennis, great to have you on board now here on the "darkside". Can't wait to start turning the wick up on your car.

 

Hey Wes, I guess Nero didn't get a chance to get any numbers on my car. I can't wait to come pick it up next week.