Is everyone using 91 octane?

[Anfanuts]

There was another thread on this topic initiated by me awhile back. Similar advice. I found on my 69 Spica that it pinged going up hills after two tankfuls of Mid-grade 91. I'm now back to Premium 93. I never even tried 87. I also put 4 oz per 10 gallons of fuel Marvel Mystery Oil in for the rear fuel and Spica FI pump per Wes. Bruce

PS: I'm also running 8 deg advance over this initial stock setting of 3 deg ATDC. I'm at 5 deg BTDC. No cam or other mods otherwise

[Joe Papa Sr]

Quote:

Originally Posted by BlueSpider Very interesting. Question I had for sometime. I am using 91 all the time on my 88 Spider and I know previous owner was running on 91 as well. From what I read it appears I am wasting money more than anything else. I shall try 87

[wilkie]

My '69 1750 definitely will start knocking if I use anything less than premium (91), especially going up hills. IIRC, the owner's manual for these pre-non-leaded gasoline era Alfas recommends the use of premium.

[lgbalfa]

i use nothing but super in all my cars.

[sp949]

Quote:

Originally Posted by Anfanuts PS: I'm also running 8 deg advance over this initial stock setting of 3 deg ATDC. I'm at 5 deg BTDC. No cam or other mods otherwise

[Tifosi]

The Bosch engines have all ignition timing settings BTDC, with the only two marks on many crank pullies being TDC and 11 deg BTDC (the idle point that's checked with a light and all else controlled by the ICU) and no M at all.

[Giovanni2000]

I think I might need a little help, I always thought octane rating had more to do with the compression of ones' engine. I have seen some replys that mentioned altitude. I do know one thing, the older version of my airplane used 80/87 octane fuel at altitudes up to 13,000 ft and my version uses 100LL(low lead) at altitudes to over 14,000 ft, both of which are low compression, so what am I missing here? By the way the push is on from the feds to make low lead fuel a thing of the past for the low compression aircraft engines.

[Andrew]

As I say, I've never gotten a Spica car to ping, even in hard track driving. I use 91 and set the advance so it gets right to, but doesn't exceed, the M mark.

Andrew

[lowmileage]

Now, I may be talking through my hat (and I'm sure someone will tell me so if I am) BUT, IMO a car specifically not designed to run on premium (like one with an under 9-1 compression) should actually run better with a Plus 89 octane or Regular 87 octane fuel. Premium ignites slower than the other to prevent detonation right so therefore you should get faster ignition with the other grades which theorically should give you more ? power. Ok, now looking for my old Nomex racing suit.

[Andrew]

Re altitude, the higher you are the less octane you need. Some high-altitude parts of the US have lower octance gas; I've seen down to 85 for regular. Less oxygen in the air at altitude means less chance of pinging/knocking.

Andrew

[santoli3]

Quote:

Originally Posted by lowmileage ...IMO a car specifically not designed to run on premium (like one with an under 9-1 compression) should actually run better with a Plus 89 octane or Regular 87 octane fuel..

[lowmileage]

Quote:

Originally Posted by santoli3 Later when perusing the owner’s manual I found a note proclaiming that slight pre-ignition is normal and harmless,

[RandallM]

Time for my two cents.

Hi Steve, I hate to say it, but all Fords ping, or rattle. I don't buy Fords, however, my wife does. She says they last forever. That's becuase she has a nut that loves her and keeps her Fords up and running. Anyway, Fords have some anti-knock circuity that does not last long. As the car gets older it just knocks. The best thing to do is run some seafoam cleaner in your tank. Keep the injectors clean and that's about all you can do. You could lean the motor out some, get a chip to eliminate the ping, but that makes the engine run hotter and lowers engine life.

As for lower octane at higher altitude, less oxygen, less fire you need. My Trooper gets 15 mpg at sea level and 22 mpg in Denver. It's mostly because an engine at sea level develops more power, uses more fuel than one at altitude.

My ex-wife (Chemical Engineer) explained the main difference in octane to me as the higher octance mixes better in the cylinder and detonation occurs with a more even burn. Detonation wave is smoother the higher the octane, while lower octance detonation is more ragged. Higher compression engines work better with higher octane because the fuel needs to be more volatile to evenly mix with all that air packed in.

I guess the reason for lower octane availibilty at higher altitude is your not going to pack as much air into the cylinder and the volitility of the fuel does not have to be so high to thoughly mix with the air.

[Giovanni2000]

I know that all makes sense to you all, but because I really don't know that much about cars at altitude(or sea level for that matter) I can only relate back to my plane which is probably operated at twice the altitude as the normal high altitude car. When I am at 14,000 lets say, I am getting the same octane that the engine was designed for(100LL) whether it's at sea level or not. Because the air is thinner the higher I go I have a mixture control that lessens the amount of fuel going into the engine, but at no time does it change the octane rating. Sorry for the hijacking by the way...

[180OUT]

My modified 2L with 10.4 pistons pinged a lot on 91 octane gas. In one of David Vizard's numerious engine performance books, he mentioned that Otto cycle engines all tend to have a 2400 rpm preignition phase. Accepting his premise, I decided to experiment. I installed an MSD6A multi firing ignition which fired over 34 crankshaft degrees (now 20 for new ones) until 3-4000 rpm. As I understand it, preignition, is caused by an uneven burning flame-front. The multi-firing MSD design is intended to induce more complete burning of the combustion process. The experiment worked well: the pinging was almost completely eliminated. If I worked hard I could made the engine ping a little but only on hot, humid days.