For the heck of it, I was considering trying some 100 octane race fuel mixed with my standard 91 octane fuel. I've read posts on adding octane but my question is mechanically how does the 993 adjust for octane automatically? Is this a constant check it does using knock sensors and timing?

 

Yes, and unless you have a chip you get nothing out of fuel above 93 octane.

 

For a brand new motor, yes. But a motor with significant carbon deposits may benefit from higher than 93 octane.

The carbon deposits lead to hot spots in the combustion chamber which can bring about detonation, and if the deposits are severe, they may increase the compression ratio, wich would also increases the chance of detonation.

The mixing is a straight weighted average.

5 gal 100 Octane + 15 gal 91 octane = (5*100+15*91)/(5+15) = 93.25 octane.

 

So excluding eliminating deposits, the chip in a stock 96 993 can only change the timing when it detects knocking but can not keep advancing (hopefully the right term) it to take advantage of higher octane?

 

Yessir, thats true.

FWIW, programs for race gas are available that take advantage of those higher octane fuels but would not run safely on pump (91-93) premiums, especially in hot weather.

It bears repeating that detonation thresholds in air-cooled engines are not static. They change with outside air temperatures and cylinder head temps which reflect engine loads. This is why I recommend a 50/50 mix of race gas and pump gas for those folks that do DE events in hot weather. This mix helps preserve engine performance where the ignition timing would ordinarily be reduced by the ECU.

 

Learn something new every day. Thanks.

 

I'll stick with the regular 91 octane stuff unless I run the mix from time to time to help prevent the hot spotting. I was hoping my chip would be smart enough to automatically and continuoulsy try to advance the timing until knocking was detected then back off some. This would take into account the other variables you mentioned as well.

I don't want to have to run a race fuel mix all the time to take advantage of the higher octane. Maybe I should run just enough race fuel to get the octane up to around 93, since 93 is no longer available everywhere here in Kansas. I'm assuming the stock chip is programmed for 93 octane so running anything less means I'm not getting maximum power out of my car. Would that be correct?

 

As a note, the stock fuel requirement was 93 octane. So in a variety of situations, 91 will not provide adequate detonation protection. Unfortunately, in CA 91 is the top and 100 octane is now being phased out due to MTBE being eliminated .

 

Porsche has specified 93 octane fuels for 993's for maximum performance but the only way you'll know how YOUR car is affected is to observe knock-sensor activity in real time with an OBD-II scan tool.

 

Thanks Steve. I've been thinking of getting one of the OBD II software packages since my other car also uses the same diagnostics. It would be helpful if I could observe the knock sensor activity as you mention. That sounds like the only way to tell if the gas I'm now using is optimizing performance.

Finally makes sense to me. Thanks again.

 

For a pretty informative summary of octane-boosting and when it might make sense, check out:

http://www.idavette.net/hib/fuel/index.htm

As noted by Steve, the only real way to know if it will do you any good is with an OBD tool that will flag activation of the knock sensor(s).

Paul

 

Wow. That's the first time I've seen that article. Excellent and good technical support. My take is that I should be able to use the OBD II software to determine the amount of retard in the spark. If it's significant, I should be able to add a race fuel mix and see the spark advance and feel a HP increase. If it's not being retarded, increasing the octane does me no good.

 

Thanks for posting the link to that article, Paul.

Does anyone on this thread have significant positive / negative experience to share with octane boosters or "race fuel concentrates" such as Torco? I've had one Rennlister recommend Torco to me as a possible solution to the octane-limited gas I'm able to get here in Seattle. (91 or 92 octane is the max; racing fuel is way out of my way so mixing that in is tough.)

I'm interested in finding some solution, as I believe my car has enough carbon build-up to have a negative impact when running on 92 octane fuel. The last time I was at the track I filled up with 100 octane, and the occasional pinging that my car exhibited was pretty much eliminated. (Most other possible causes for the pinging have been eliminated at this point.)

Thanks in advance for any experience.

 

No experience, but my take from the article was that it is better to increase the octane using race fuel than octane boosters because of maintenance issues that could be associated with the boosters and the amount of booster it takes to get a significant increase. I'm sure others with actual experience may chime in.

 

what about if you fitted an ecu to euro-spec ?
it would be programmed to run above 93 as standard .

i run 99 pump-gas on my 93 964 , as 100 isnt readily available at all the pumps