which octane level should I use?

charlieruss · Post by **charlieruss** » 24 Aug 2006, 10:30

I recently bought a 1980 volvo and at first was putting 87 octane gas in it. I realized that volvos usually require a higher level so I checked the manual and sure enough it requires 91 octane. My gas tank is about half full of 87 right now, is it bad if i fill it up the rest of the way with 91? and how much of a difference does using a lower octane level make?

Post by **billofdurham** » 24 Aug 2006, 11:51

Mixing the two octanes should cause no problems.

With the lower octane you may experience pre-ignition (pinking) under load. This, in itself, should cause no long term damage but if the book says 91 octane then use it.

UK Volvos are different from US models and I ran my 1984 240 on 87 octane after a slight adjustment to the timing. This was carried out free of charge by a Volvo dealer who has since gone out of business. He was probably cut off by Volvo for not charging!

Bill.

charlieruss · Post by **charlieruss** » 25 Aug 2006, 10:23

thanks for the reply. When it comes to pretty much anything with cars i'm a newbie, what exactly does pre-ignition(pinking) under load mean? haha, sorry if this a no-brainer question, I really want to learn as much as I can though. Thanks again!

Post by **billofdurham** » 25 Aug 2006, 12:05

For a newbie you ask some technical questions. Put simply pinking occurs when the plug sparks before it should. This can happen if the ignition timing is not set up correctly or, as in your case, the timing has not been adjusted for the lower grade fuel. If you go on to the correct grade it should not happen.

It is not something that happens a lot now because of the modern methods of controlling the timing. When I first started driving it was very commonplace and many hours were spent fine tuning the ignition timing.

Bill.

petershen1984 · Post by **petershen1984** » 26 Aug 2006, 21:15

Is 91 (RON+MON/2) premium gas in America?

Yes, more specifically, UK and European spec volvos didn't have a catalytic converter. Back in those days, adding one implied a decrease in compression and power - but the main reason was the predominant use of leaded gasoline (as an octane booster). It was also the transition period from leaded to unleaded..

I have to use premium gas (Research Octane 98) because of high compression, which is also contributed by carbon deposits.

charlieruss · Post by **charlieruss** » 28 Aug 2006, 10:54

Yes, 91 octane is premium in America. I put a full tank of 91 octane and fuel injection cleaner in this weekend. The engine used to shake a lot, now it shakes slightly less. Though this morning when I left for work as I backed out of my driveway it died. I restarted it and it made the 6 mile drive to work just fine...I'm wondering if it was becuase it was cold that it died? I started it and let the engine warm up for about 5 minutes in my driveway so I didnt think it was that cold. It has never died like that before...

Post by **billofdurham** » 28 Aug 2006, 11:43

Why leave the engine idling for 5 minutes on the drive? This can lead to problems if you continue doing this over a period of time. When the engine is cold it generates a large amount of moisture. This is usually thrown out of the exhaust pipe as you drive and you see this as a greyish white vapour. As the engine warms the amount of moisture diminishes and you will see less coming out of the exhaust. Leaving the engine idling as you did the moisture does not get thrown out so readily. It has to go somewhere so a large percentage will mix with the oil causing a sludge which will appear on the dipstick and the inside of the oil filler cap.

A further point, when you first start your engine from cold, the fuel/air ratio is enriched. The engine is being fed more petrol than it needs and this could have caused it to die as you backed out of the driveway.

Bill.

charlieruss · Post by **charlieruss** » 28 Aug 2006, 12:08

Wow, I guess I was mis-informed, Thank you for the heads up!