Nixter

I'm hearing many different things. My buddy has the owner's manual and he can't track it down at the moment so I'm unable to check. It's a 1980 911 SC. California car.

Anybody have the final word on this? I just filled up with 94 Techron and I swear I can feel a bit more punch, probably just in my head though!


Thanks,

Nix

Steve Weiner-Rennsport Systems

Nix:

You asked a GREAT question and there is a lot of misinformation out there on this subject.

You can review some useful reading on this at http://www.rennsportsystems.com/2a.html.

I will tell you that detonation thresholds in aircooled engines do NOT mimic their water-cooled counterparts and people should not make generalizations based on what their Honda-Chevy-Ford-BMW-Toyota etc, works with.

Since cylinder head temps are a huge variable in a 911, detonation thresholds are therefore dynamic, not static. One really should use Premium fuel in these cars since their requirements for higher octane fuels change with outside air temperature, cylinder head temperature, oil temperature, throttle position, and fuel mixture.

The human ear makes a terrible "knock-sensor" and one should never rely on those to determine if the engine is detonating or not. By the time you hear and recognize what is going on, the damage (broken rings) is already done.

CDiercks

After reading that article, you have gotten me scared to drive my car! My CR is 9.8:1, and I have been using no less than 93 (R+M)/2 since I got it. I have had problems in the past with the car bucking when the rpms are low and I push the peddle down to accelerate. It only happens at low rpms and only when the engine is lugged and up to full operating temp (after at least 20mins of driving). It doesn't happen anymore because I know how to avoid it by keeping the revs up. Could this be pinging and not a vacuum leak like I thought? It used to be very hard to start the car rolling from a dead stop unless I reved above 2500 and slipped the clutch. Then I realized that the vacuum advance had been disconnected from the distributor. Since reconnecting it, it has been fine.

Steve Weiner-Rennsport Systems

Hi Casey:

Sounds like you found and fixed your drivability problem,...good job.

I will tell you that the vast majority of Porsche engines do not meet their advertised compression ratios and in fact, they are lower than spec.
9.8:1 is marginal on single ignition & pump gas, but if you are running 93 or better, you should be OK as long as your total timing is at spec.

If you do any DE events, I always suggest a 50/50 mix of unleaded race gas and pump premium. You can feel the difference under those conditions.

Dan in Florida

Steve,
What octane of unleaded race fuel in the 50/50 mix? Same 50/50 mix for either 100 or 110? Or would it make any difference, as long as 50% is race fuel?
Dan

Steve Weiner-Rennsport Systems

Dan:

I use unleaded race gas which ranges between 100 & 104 octane.

110 is the leaded stuff and a poor choice for any oxysensor/catalytic equipped car,........

Bill Gregory

With a recommendation here to use premium for street use, I wonder why the factory might recommend regular unleaded [for street usage] in certain models (1981 911SC, for example, factory recommendation was regular unleaded)? Through the years, the factory did recommend premium here and there (964's, for example, needs premium).

Steve Weiner-Rennsport Systems

Bill:

The factory is NOT infallible and doesn't appear to understand that the USA is the ONLY country with such wide variations in fuel quality and octane availability.

Unlike other countries, fuel testing and standards are done at the state government level and there are 50 different standards. Some states don't
even inspect and test motor fuel for quality, only pump volume.

Further, fuel formulations and octanes vary by season.

Given the cost of new pistons & cylinders, using good gas is cheap insurance as one cannot be assured of what you get. I cannot count the number of times that we've disassembled an engine only to witness the rings hitting the floor in pieces. The owners never hear a thing,........