Someone told me that when the fuel light is on that power is reduced via dropped timing and leaner AFR than normal targets. Is this true? Does that mean all of you run at least 1/4-1/2 tank of fuel at track events?!

Sounds very suspect to me, but maybe someone has objective experience they can share supporting or refuting this.

 

yep. it goes into limp mode when the light goes on.


at the track you want as little fuel as possible, but you have to keep the light from going on. so 1/4 tank is min.

 

I don't believe this is true. I have ran the half mile with the fuel light on and still ran 172 mph.

Jeff

 

^ I agree..I'm quite sure I ran with light on at the track for minimal weight and with no issues...

 

I actually just experienced this yesterday. Noticed some major power loss after low fuel light went on. As you can see, was only getting 5 lbs boost max and I'm normally around 25. Once I fueled up, I was good to go.

 

Yes it's true, because of the way fuel levels are measured in the 911 cars. When you're tank is full, your fuel level is measured by an actual float in the fuel tank, just like every other car. However, because of the shape of the 911 fuel tank, the bottom 1/4 (approximately) is not measured by a float, but is calculated using the cars onboard trip computer. Although it's fairly accurate, it's not an actual physical measurement of fuel level.

Because of this, there are failsafes built into the factory programming that reduce power slightly when your fuel light is on. It's a protective measure to prevent the fuel tank from running dry or causing any lean running conditions.

I'm sure it's possible to bypass this failsafe with programming but I'm sure that in most cases it's left intact. I can't speak for every tuner, but I know that when we create our custom programs with GIAC, it's left in place.

 

^ Hmmmph...learn something new every day...good to know

 

This is new to me. Does this happen when the red light portion of the tank level indicator lights up at just under 1/4 tank, or what is the low fuel trigger?

 

It's not noticeable in my experience. I have a video of me at COTA and I say to the instructor I have to go in because the fuel light came on but I ran two more laps and one of them was my fastest of the day. I finished the run group and then filled up. I think at the track it puts the light on sooner because the consumption is so high. You only get about 5mpg running at the track so maybe around town it does more than at the track. Honestly I have never noticed any power loss but around town I always tried to keep it full. At the dragstrip I always try to keep it around 1/4 and I can't remember the light coming on. If there is a power loss, it's not like limp mode which I have definitely experienced at SAR when I had the secondary coolant pump issue and the diverter valve problem.

 

It happens when you're low-fuel light is on. But not in all circumstances (see next reply)

Correct, it's not like a traditional limp mode at all. And in fact in most cases, most people would not notice it. The only times it's really something that effects the car is in very carefully measured events, 1/4 mile, 1/2 mile, etc. Also it may not always effect you. It's a protective measure, so if the ECU thinks it's safe at that moment, you might be fine.

 

I have never seen any loss of power due to low fuel levels. When I track my TTs the low fuel lite comes on at approx 40% tank! 4.6 mpg

 

It rarely happens and has to get pretty low, then sense a pressure drop.

 

Ditto

 

This must be a 991 TT/S phenomena, I've been on track in my 997.2 TTS when the fuel light came on and knowing that I had maybe 2-3 laps to checkered flag I stayed on with no noticeable difference in power output (especially with pulls on front or back straights, and I'm talking about VIR, for those who know...that's a 160mph back straight at WOT). Interesting factoid nonetheless...