Does anyone have a tried and true method to determine the amount of Octane required for an engine??? I have an L-82 with 9:1 compression, should I be using 87 or 89 Octane?
Mike

 

The tried and true method is to run whatever does not ping. Run 87, if it pings, go to 89, if it pings go higher. I have to run 92 in mine because my engine probably has carbon deposits with 103,000 miles on it. It should be able to run lower octane but can't without ping or run-on. Depending on the condition of your engine, you should be able to run at least mid-grade, but again, it all depends on your engine.

 

The previous post is essentially the correct answer. Your L-82 was originally designed for 87 PON unleaded fuel and if it is unmodified it should perform without detonation. If it's been "modified" with a quicker ignition curve it might have developed more octane appetite. Deposit buildup over the years in the combustion chambers can effectively raise the CR and have the same result.

The octane scale is non-linear and lead has a decreasing marginal benefit, so it's very difficult to "compute" the octane of a blend or determine what octane a specific car requires. The best you can do is find a fuel or a blend of fuels that will allow detonation free operation and stick with it.

Duke

 

Thanks guys!!! My rebuilt engine still has less than a thousand miles on it and I just wanted to be sure I was "feeding" her right! I've used both 87 and 89 so far without any knocks. Thought that I might continue with the 89 until she's good and broken in!

 

Just FYI, in terms of "cleanliness" there is no difference between octanes. If your engine runs well on 87, save a few pennies and run the 87. Octane is a measure of how fast the fuel burns. However, there are differences between fuel manufacturers. My car runs like crap on Shell no matter what octane. Many here will tell you that Chevron has the best product due to their additives and that the major car companies use Chevron when testing their engines. Just a little additional info to ponder. :cheers:

 

You could get better low/midrange performance by setting up the ignition curve more like SHP engines from the mid sixties. Chances are your centrifugal advance is fairly slow to keep the engine out of detonation with 87 PON fuel. If your willing to use premium, you could have all the centrifugal in by about 2000 to 2500 and experience much improved low end torque.

Duke

 

"Octane is a measure of how fast the fuel burns."


This is a myth - probably based on an oversimplifiction of what octane rating means and what it does. Normal flame propogation speed is about the same for a wide variety of gasoline blends, but if the gasoline is of insufficient octane, the "end gas" - the very last portion of unburned fuel air mixture can react almost instantly rather than being consumed by normal flame front propagation. As the flame progesses, the pressure and temperature of the unburned fuel air mixture increases, and if the autoigntion conditions of the unburned mixture is reached, detonation - or the sudden (near instantaneous) reaction of the end gas can occur.

Retest with sufficient octane fuel with a higher autoignition temperature, and the end gas will remain unreacted until the flame front arrives and consumes it. This is "normal combustion". Detonation is "abnormal combustion."

The "knocking" we hear when detonation is present is the sound of supersonic shocks waves bouncing around in the combustion chamber that result from the overpressure caused from the sudden reaction of the end gas.

The primarly problem with detonation is that these shock waves will dramatically increase the rate of heat transfer to the combustion chamber boundaries and rapidly overheat piston crowns and valves to the point where they loose mechanical strength an fail.

If you've ever disassembled an engine and the piston crown looks like someone took an ice pick to it, you are looking a the effects of moderate detonation over a long period of time. Local overheating of the piston crown causes small bits of aluminum to melt, leaving behind small craters. Sustained heavy detonation can lead to broken piston crowns and broken exhaust valves.

Duke