Ken's correct. Until OP gets the octane back up close to 93, the brain will retard the timing but if he pushes hard he'll get detonation at some point and then risk damage. That's why he should just take it easy till then.

 

Which we only dream about here on the West Coast.

Remember the thread I posted a few weeks ago--that it cost me a buck a mile to run the yellow car on merely 95 octane? (Steve's probably cringing if he's reading this.)

 

Seriously... this is a concern?

 

I wouldn't worry about the revs so much around town, just be careful not to load the engine at lower revs....you will hear it rattle, back off quick.
If you fill it with the good stuff next time, should be no problem.

(Nice J3 Kigiin!).

 

I'm curious...does it retard the timing only when the knock sensors tell it to, or is there another sensor that "sees" the octane of the fuel?

Reason I ask is, in Denver our premium gas is 91 octane. But at over 5,000 ft that's essentially the equivalent of 93 octane near sea level, from the standpoint of the engine's propensity to predetonate. So, I'm thinking that as long as there's no pinging going on, running 91 octane at this altitude should allow it to maintain normal advance. Can anyone comment on this?

 

Hi Dave,

First, a correction: there is Detonation and there is Pre-ignition; two different conditions. No such thing as "predetonation".

Here is how the Motronic system works: knock sensors react to cylinder pressures, retarding ignition timing up to 6 degrees when the sharp pressure "knocks" of detonation are detected. Air density drops with altitude so these engines run OK on lower octane premium under such conditions. Cylinder head and air temp sensors all contribute information for which ignition map the engine uses at any given moment.

Now,......here's the rub: unlike water-cooled motors, air-cooled engine operating temperatures mirror ambient and when cylinder head temps are elevated due to higher than normal outside temps, reduced cooling efficiency due to reduced air density and load, the octane requirement can actually go UP, to prevent unnecessary knock-sensor activity. You can watch this happen in real time using the PST-2 or Hammer to monitor timing and knock counts.

To maintain maximum performance in hot weather, 91 octane isn't sufficient, especially when climbing mountain passes (high loads) so a healthy shot of 100 octane unleaded race gas sure helps as the engine will adapt to the better quality fuel.

Lastly, one should not rely on one's ears as the defacto "knock-sensors" as that can have expensive consequences,....

More to read on this subject right here

 

If you need to run the car hard, go get 6 - 8 gallons of 100 octane unleaded and balance it back out. Look for Trick racing fuel near a track...

FWIW, I tried to keep my octane at 93 to 94 by adding a little (5 gals) 100 unleaded.

Cheers!

 

Steve,

I read your article that you referenced. So my question is am I better getting 91 octane ethanol fee or 100 octane with 9% ethanol (Sunoco GT) for driving on the street? These seem to be the only two options in the Denver area other than standard 91 octance 10% ethanol

 

Hi Bob,

9% ethanol is not really an issue as long as you use some Marine Stabil when the car is stored for the winter. Race gas is FAR more stable than pump gas and you need the octane (in warm weather) to retain as much power as possible at your altitude.

Sunoco GT is an excellent fuel.

FYI, Ethanol has a RON of 116 so its no liability in that regard.

 

Steve, thanks for the helpful answers!

Is there a certain method that should be used to mix race gas into a tank of regular gas? Or just dump it in and let it mix itself?

 

Steve, I would think that if you're hearing pinging it's too late. The motronic system, as long as it's still working within it's (correctable) parameters should prevent you from hearing pinging. I would think the electronics should have responded before you get to the "audible" pinging state?

 

I would squeeze in some 100 octane (or higher; whatever you can get that's unleaded). Then after driving (pretty gingerly, of course) add some more a couple times.

Do some googling to find some race fuel near you.

 

lol, have you priced up 100 octane? At $7 per gallon, it's not really much of an option.

 

Dave,

I usually pour in the race gas and follow that with the pump gas fill up. Drive it around the neighborhood and that mixes everything pretty nicely.



Bill,

Indeed, if you hear these ping, the detonation is beyond the system's ability to adjust to it and thats a problem unless its immediately recognized.

There is another condition called "sub-audible detonation" where the shock waves are not discernable to the ear but damage can occur to the rings and pistons if its undetected. These Motronic engines handle that pretty well so its not an issue on these cars.

 

almost as much as normal gas...............LOL