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Mon Aug 25, 2008 5:44 pm

Being the complete vw noobie that I am, I went to the gas station and put in 5 gallons of regular unleaded gasoline. As I started up the long hill to home, the engine started pinging like crazy. I had to limp it all the way home, because I didn't have any tools wherewith to back the timing off.

I noticed after putting the gasoline in, that there was a sticker on the inside of the fuel filler door that said something like "Use only 91 something-or-other". I didn't know if it was trying to tell me "91 Octane", but after the way she acted on the way home, I'm highly suspicious.

Thanks,
Blaine

Mon Aug 25, 2008 7:22 pm

Hi Blaine,

I've only had my car running for a few weeks now but I've had it for about five months which has given me plenty of time to do some reading! If I'm remembering things correctly (and I think I am) 1.) I read that you SHOULD use regular gas when I did a search on the same topic and 2.) for some reason without going outside to check for myself, I think the inside of the tank door says "90" not "91". I think it has to do with unleaded gas.

If nothing else, hope this saved you a search through past forums.

Mon Aug 25, 2008 7:54 pm

Just curious, is your motor rebuilt or original. If the motor was ever rebuilt it may have a higher CR (Compression Ratio) and thus need a higher octane to avoid pre-detonation. When the CR is higher than stock, the lower octane fuel becomes unstable at higher temperatures induced by the higher compression and, in turn, self ignites or detonates. You may try a higher octane fuel and it should eliminate this. But it may also be the timing.

I, having had a similar issue, would set the points, and time the thing, then take it for a drive. If you are still having issues, it's probably the fuel!

Good Luck!!!!

Mon Aug 25, 2008 8:45 pm

Different octane rating system across the pond. I bet the sticker says 91 RON.

If the engine is stock and tuned right, it should run great on the cheap stuff. But after 30-40 years all bets are off as to what state of tune the car is actually in, and as stated above, if the engine has been rebuilt (quite likley) it may have higher compression.

If a normal tuneup including checking the jetting and timing doesn't help it, then that is the time to start stepping up the octane.

Mon Aug 25, 2008 8:55 pm

That's right! It is 91. I knew it was something you can't get. So for some reason I was thinking it must be an even number, but you can't get 91 either. Thanks for the correction.

Tue Aug 26, 2008 6:09 am

Regular unleaded is fine. The 91 RON is for way back in the day, and octane ratings have changed over the years.

Tue Aug 26, 2008 6:31 am

RON is a different octane measurement than what we use here in the states. IIRC the US equivelant is something like 87 octane.

Tue Aug 26, 2008 6:35 am

I've run regular for years with *very* occasional fillups fir premium. IMHO, there is no noticable difference in milage or performance, but a seriously noticable difference in wallet performance...

Tue Aug 26, 2008 11:52 am

Lots of great info -- Thanks!

According to the PO, the engine and trans were both rebuilt 1000 miles ago. It is a SP 1600. I don't know what gas they were running in it, nor how long it was in the tank. All I know is that the gas that was in the tank didn't cause pinging. It was only after I added the 87 octane that it started doing it.

I just bought the tools to do a tuneup. I'm going to gap the plugs and points, and retard the timing just a bit.

I'm also going to change the oil over to castrol syntec 5w50 and see what happens.

Thanks,
Blaine

Tue Aug 26, 2008 2:38 pm

Changed the oil, adjusted the points. Drove it hard through the neighborhood for ten min or more. Went up a steep hill in 4th. Runs smoother, quieter, more power. No pinging. :) Woo hoo!

Points were gapped at about .010 - .012. Set at .016.

Thanks all,
Blaine

Tue Aug 26, 2008 6:00 pm

RON stands for Research Octane Number
MON stands for Motor Octane Number
The octane number on the pumps of of most gas stations in North America is the average of the two and will say MON+RON/2 or similar. If your compression's stock and your timing's reasonably accurate you can use Regular Unleaded from any gas station. As the compression ratio goes up or the timing goes off the engine becomes more succeptible to pre-ignition (AKA "knock" or "pinging") and higher octane fuel can suppress this.

Wed Aug 27, 2008 3:23 pm

back when ghias were built, gas had lead. back then my ghia never pinged on regular. as soon as things went all unleaded, with my engine set to specs, it pinged. i run super now.

Wed Aug 27, 2008 7:52 pm

Higher octane will do nothing to improve the way the car runs. There is no on-board computer to retard or advance the timing. So go with the cheap gas and tune accordingly and you'll be golden.

Wed Aug 27, 2008 8:56 pm

execk2 wrote: Higher octane will do nothing to improve the way the car runs. There is no on-board computer to retard or advance the timing. So go with the cheap gas and tune accordingly and you'll be golden.

Agreed.

Wed Aug 27, 2008 10:22 pm

FYI, this is an example what VW would put into owner's manuals in the later 70s regarding the change in octane ratings: