Driving Your MEL Powered Vehicle On Today's Pump Gas

[Chris430]

Hello,
Can you get away with 87 octane, what elevation and climate do you run?
Thank's

[59lincolnrag]

I only use premium fuel in my MELs
Rev A 3/6/22
Octane?
Octane is simply a measure of how much heat and pressure a fuel can withstand before exploding, and in tandem with a properly designed engine a higher octane fuel can increase performance and efficiency while decreasing emissions.
Octane issues are usually related to how the engine behaves. With a newer vehicle, putting premium gasoline into one that’s rated for 87-octane usually will cost more money. You really don't get any benefit.
However, in a classic MEL car, you may want to stop at the pricier pump, regardless of what your engine was originally rated for.
First, Octane is not an additive. Second the amount of octane in fuel depends on how it’s refined. While high-octane fuel sounds like it has super power, it’s actually less volatile than 87-octane. That’s important in high-compression engines, where high heat can cause various points of detonation with lower-grade fuel, or worse, spontaneous combustion (ping or Knock) before the spark plug fires damaging your engine.
Knock is virtually unknown to modern drivers. This is primarily because fuels contain an oxygenate that prevents knock by adding oxygen to the fuel. This oxygenate is commonly referred to as octane.
Premium fuel resists pre-ignition, and creates an even flame that spreads through the chamber (this happens in a split-second). Older engines can be more susceptible to pre-ignition, and using premium fuel can help to prevent it.
As for additives you may want to do some research. Newer fuels do not give the upper engine/ valves proper lubrication. A good additive here will help. I use a marvelous mysterious oil added to my tank. There are others out there. An expensive valve job with hardened seats would help as well.

[Chris430]

So, depending on your area, you run 90-94 Octane. My routine trip is 574 miles from 434 feet above sea level to 4,000 at peak then to 3500 where I usually stay. It's not fuel injected yet so no compensation for knock except me getting out and backingnoff the timing. The FE's AFR drops to 12.1 at 3500 feet from 13.1 at 434 feet. The Quadrajet compensates ok through vacuum loss when going up. I'm expecting similar results with the MEL. With the MEL. 050" below deck I suspect the advertised 10.5:1 compression is more in the mid 9's. 91 Octane at the local Chevron was $4.14 a gallon. See where I am going with this.... What elevation do you drive the most?
Thank's

[59lincolnrag]

The highest elevation near me is 180 Ft.
Premium is 4.42 per gal.

Burnt valves running low octane fuel in high performance engine. 5.4 DOHC 32 valve.

[Chris430]

You definitely ran contaminated fuel for a while. You can curve that by using stations that see a lot of traffic. Those stations tanks are circulated often enough for the fuel not to separate keeping water, Ethanol and contaminants suspended. Premium is refined better but some places around my area still have traces of Ethanol and water in their premium. That is according to a local machinist whom uses testing kit for his dyno gas. If you have a custom tuner on that car slighty fattening up the AFR will cool the combustion chamber some and probably slow down the damage done with this junk fuel we're getting these days.

[59lincolnrag]

Actually the burnt valves (root cause) were caused by an intake vacuum leak causing a lean mixture on the drivers side. The Intake valves were coated with bits of carbon after using Sea Foam. Low octane fuel only exacerbated the condition.

[Chris430]

Update: The last two tanks of gas have been Sam's Club 86 octane with lead additive at around 3,500 feet above sea level. Truck is so quiet I could hear pre-detnation if we're to occur, but I keep octane supplement in the tool box just incase. So far the truck and trailer haven't been north of 9,000 gvw on low grade fuel. Towing 9,000 gvw and up gets premium fuel. I have yet to run a full tank without a trailer so I haven't put pen to paper on the fuel mileage. Rough estimate of one tank came up to 14 mpg.