Which octane?

[MaineShark]

I finally found a Shark that fit my needs - a '79 L82 with just about every option (which is nice, since I will be driving it whenever the weather is good). In a few years, when it's paid off, I will get a '73 to play around with, modify, and generally make impractical, but fun.

So, that brings me to my question: what octane do you recommend/use in these cars? Judging by compression alone, 87 would seem to be fine. However, I have heard some people comment that higher-octane gas is better for use with the carbeuration system, since it will tend to leave less deposits.

Is there a consensus on this? What do people use in their own cars? Locally, I have the option of 87, 89, 91, or 93 octane (don't ask me why a station has four grades, but one of them does).

Joe

 

[Scott81]

Joe,

Not sure what the compression is on a L-82 but on my original 81 motor is was a pathetic 8.2:1, you could use dishwater in it....just kidding. I used all the three grades of fuel when I had a stock motor and none seemed any different, the only thing I made sure of was that I used Amoco. I usually put in the 87 and every now and then filled up with premium. Then I went nuts on a motor and have to use at least 93 octane with about 5 gallons of 108 octane added in (VP Racing C-12) each tank full. :cry

 

[smurfvet]

My 74 L48 takes 91 octane, according to the sticker on the inside of the glove box cover. I run 89 octane in it. I tried Sunoco 93, thinking the car would be happy with it- I don't really notice any difference at all- no knocking with 89 in it. I do notice a slight drop in mileage with 87 (regular) so I keep the 89 in it. Then again, this is a low hp engine.

 

[Bob Chadwick]

87 octane. I would ponly change if there was some pre-ignition knock.

 

[MaineShark]

Thanks. I'll probably put mid-grade in for the drive home, 'cause I don't want to be listening for knocking at the same time as I listen for everything else, then I will put in regular for the next tank, and keep an ear out. If it all sounds good, I'll stick with regular (cheaper gas == drive more )

Joe

 

[Tom73]

Joe,
In my 73 L82 I use 92/93 octane gas. It has 9.0:1 compression as does your 79.

tom...

 

[GerryLP]

Main,

At sea level the octane is more effective than at altitude. For example, a stock vehicle owner at 5,000' would be wasting money since at altitude athmospheric pressure drops about 1" of mercury or about 0.49 lbs/sqr" (inches of mercury / 2.036 (constant) = Lbs/sqr").

The result is that a normally aspirated engine looses 0.49 PSIA for every 1,000 ft of altitude. So an engine looses approximately 2.45 PSI Absolute at 5,000' of altitude per cylinder. This reduces the engine's compression by 2.45 PSIA (not the ratio -ratio remains constant). When this is compared to a standard pressure altitude of 29.92 "Hg (mercury) then we have 29.92 / 2.036 = 14.695 PSIA minus the 2.45 PSIA that leaves 12.245 PSIA. The pressure inside the chamber corrected to account for the athmosphere has LOST almost 17% of pressure absolute. :cry

The pressure inside the chamber is lower and therefore less suceptible to pre-ignition. Back in 1981, the GM cars had an option for high altitude (carb p/n ending in 505 I believe). Nowadays, the computers in vehicles are superior and therefore can adjust to the altitude several times in a minute or even seconds. They also can modify the timing advance as altitude increases and pressure decreases.

My engine's 9.7 compression ratio has a lower compression at this altitude than at sea level. My car runs great with 91 octane and possibly 89 octane as well. If I drive my vehicle down to near sea-level altitude, then the least I can use is probably 91 octane but the better choice would be 93 octane.

Unfortunetely, pre-ignition has detrimental effects on the engine parts (namely the piston). So I would recomend the higher rather than the lower octane you can afford at near sea level atmosphere conditions.

 

[Hib Halverson]

You only want to run enough octane to keep the engine out of detonation under the worst conditions, ie: hot weather, high air density and high load.

Considering your L82 has about 9:1 compression and its an iron head, carburator engine with so-so fuel distribution and not a lot of swirl in the combustion chamber; I'd run 89-oct fuel unless you hear detonation, then I'd run 91.

There is some truth to the part about higher octane gasolines being better at reducing deposits, but that's a localized problem that depends on how the refiner supplying the gasoline selects its additive package.

Some, but not all refiners will add slightly higher levels detergeants to premium unleaded gasolines as a marketing ploy.

Nevertheless, I'd only run enough octane to keep out of detonation. If that means 87- or 89-octane and you're worried about deposits, I occasionally add Red Line Complete Fuel System Cleaner to your gas tank. In the long-run, it will be less expensive that way. A good "dosage" is 4 oz to every 10 gallons of gas at every 3rd or 4th fill-up. RL CFSC is a detergeant-based fuel system cleaner mainly known for cleaning fuel injectors but it works very well in carburators, too.

Lastly, if you haven't done so aready, put a 180-deg thermostat in the engine. That will help it run a little cooler and be more resistant to detonation.