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  • Writer's pictureDave LeGear

Just what is Octane and how much do I need?

Octane rating measures a fuel's anti-knock properties, indicating its resistance to engine knocking. Higher octane fuels are used in high-performance and turbocharged engines. But your engine may need more than what is called for in your manual to operate at its best and most efficient performance at various times.

Maverick HPX-18 trimmed out

Now, most people seem to think that the higher the number on the pump, the hotter the fuel burns well, not really… In the simplest of terms, the higher the number, the “Less Prone” the fuel is to explode on its own (very important) under Heat and Pressure.

Pump octane rating sticker

Picture somebody using a torch and passing it back and forth over a piece of metal they are working with; remember seeing those tiny little red glowing hot spots even though the flame is not currently at that exact spot? Now, picture the inside of your engine and the piston going back up after a power stroke of roughly 1800 degrees. Picture all those little carbon deposit spots now glowing RED HOT heading upwards in the cylinder, and we are about to send it a tiny mist of Air and Fuel, and BOOM, it just exploded, but wait, the spark plug had not fired? Welcome to the world of knock and ping…

Knock and Ping can be very destructive on an engine since the fuel and air explode incorrectly! I have seen cracked pistons from the long-term effects of (not good) knocking and pinging. Weather conditions, Dirty Engines (both inside and outside), and even your Truck and Outboard color all impact how much of an Octane rating you need at that time. Let’s go down this list and explain how each could affect your engine's fuel octane requirements.

Maverick HPX-19 with a 115 HP outboard

  • Weather Conditions: ever notice how good your Truck runs when driving down the highway in the rain? I call rain a low-tech water injection unit! Ha-ha, but all that cooler, humid (water-filled) heavy air going down your intake manifold has a nice cooling effect on your engine. You could run 80 octane fuels, which would not explode until the spark plug fired. This is why your SUV could run (if high-octane fuel is recommended) on regular 87 if you knew that it was going to rain and be cool during most of the time it would take you to burn through the next tank of fuel.

  • Dirty Engines: All those carbon deposits covering the valves and pistons can make a mess. With the valves (and the more valves, the worse) covered with carbon deposits, picture in your mind trying to jog with your hand over your mouth. It's hard to take in a breath or out under such conditions. This also applies to a plugged Air Filter on your tow unit, by the way. Now, the less airflow, the leaner the air/fuel mixture can become, and the leaner the mixture, the HOTTER it burns, the hotter it burns, and the HIGHER the Octane rating needed to prevent it from exploding before it is supposed to. And when you get too low of an octane-rated mixture, you can get knock and ping! And think, we are only talking about dirty valves here… Now, here comes that carbon-covered piston with its combined negative effects of red hot carbon spots AND the effect of a higher-than-designed engine compression ratio due to the less volume (tighter air squeeze) between the top of the piston and the bottom part of the head from all that carbon build-up. This all leads to one very volatile mix when you shoot in air and gasoline… What about the outside of the engine, you ask? Well, how cool would you stay if you had a coat on in July? It only takes about 10 mm of crud and dirt on an engine to raise it 5 degrees. Over time, that leads to more work on your cooling system and can also add to the need for a higher-than-designed octane-rated fuel in the tank. Simple fix: the next time you are at the car wash, gently rinse off your engine, radiator, and A/C condenser while (if needed) the engine is running. It also has the side benefit of being able to help you spot and find engine fluid leaks faster AND helps keep your resale value by having everything all nice and clean. 😉

  • Truck and Outboard paint color; hopefully, by now, you are starting to see that HEAT is the enemy here. Which is cooler to the touch in July, a Black or White hood? How much more demanding (for your truck, that is) is the A/C unit going to have to work and add drag (thus burn more fuel) on the engine, a Black Truck or a White one? Pretty simple, is it not… What about the outboard? Well, it's not as critical, but if the water you are running and cooling your engine with is at the 80-plus degrees mark (Gulf of Mexico) and painted black on the outside, then you will not transfer heat as well. So, the engine is now running hotter, so the octane requirements will increase. In the mid-1980s, many Black Max Mercs would vapor lock and not run due to fuel boiling in the lines. That issue seems to have improved over the years, though, with fuel injection and more stable fuels naturally.

The last thing to understand about fuel octane requirements and how they affect modern engine performance is that many (if not most) modern computer-controlled engines now have knock sensors.

When the engine’s ECU (electronic control unit) detects it is knocking (many times long before you even hear it, making it a silent power killer), it backs off the engine timing. It tries to bring it back up again till it hears (detects) another knock. This is not good and can kill the power, which means you will keep pushing down on the accelerator or throttle to gain more power or speed and thus (you got it) burn/dump more fuel into the engine for the same amount of work. Here is where octane requirements come into play. You may THINK you are saving money by buying/running that lower fuel grade when it hurts both performance and overall fuel burn rates! Burning more of a lower-grade fuel may cost even more than burning one with a higher level of (or boosted) octane rating.

This is why I believe the detergent packages in the gasoline, like Techron in the fuel of your choice, are as much, if not more, critical than the actual octane rating. If everything stays cleaner (remember these engine's performance and emission outputs are tested under new clean conditions), then naturally, the octane requirements for your engine will stay lower.

Techron example

Techron is a patented fuel additive developed by Chevron to improve engine performance and reduce carbon deposits in the combustion chamber. It cleans and protects critical engine components, enhancing fuel efficiency and reducing emissions. Techron is widely used in gasoline-powered vehicles to maintain optimal engine health and prolong lifespan.

A couple of things about Techron from my days at Porsche/Audi as a Service Director that you should be mindful of when using it.
  1. Several product versions, including what you find in Chevron fuels at the pump. The clean-up version seems to work the best.

  2. What (was) recommended was using three bottles in a row with three tanks full of fuel to help clean up deposits and get it as clean as possible. Naturally, it can be added to any fuel brand when you go to fill up the Truck or Boat.

  3. After running three bottles with three consecutive tanks, it was recommended that you change the engine oil. All that hard carbon it cut free is now circulation in the engine oil, and you should get it out so other components do not get scorned while cleaning up the engine's top end.

  4. Yamaha Ring Free at its base is Techron, so shop-wise, and do not let specialty labels possibly add even to the maintenance costs when not needed. 😉

Here are some essential tips for getting the best fuel economy in your Boat…

1. Keep the hull clean.

2. Keep the boat weight as low as possible; load/add only the fuel needed for the trip.

3. Most Outboards are designed to return the most work for X fuel burn between 3000 to 4500 RPM. Running them any faster than this, most times, only produces a much higher fuel burn for a lot less range, and though speed is excellent, mid-range cruise distance is what we are looking to gain. An inspection of many performance charts will show that once above 4000 RPM, your speed increase is not as significant as your fuel burn will be. One must shift from the GPH mode (as strange as it may sound for a boat) into the MPG thinking when looking at range and performance charts.

4. If one cannot find a performance chart for their engine and hull combo and you do not have fuel management gauges, the lowest RPM you can run and still stay on a plane most times will give you your greatest range.

5. Keep the induction system clean. A can of Throttle Body cleaner (not carburetor cleaner) can go a long way in helping maintain performance since many such parts cannot be cleaned by fuel injection cleaners poured into your tank (because) they are located above and before the injectors.

6. Spend time finding the best prop for your boat and your needs. Speed (though great to have) is not everything.

7. If needed, install a quality lower unit stabilizer plate. And just because you find one wider does not mean it will work better, for the increase in hydrodynamic drag could offset some of the benefits. Click here to read about one we have found to work well!

8. For engines after a break-in, I believe in using 100% synthetic oils for the engine and lower units. The plugs will stay cleaner (an old issue with two stokes), and reducing metal friction helps reduce fuel burn. Again, we track such things in a spreadsheet (see below for a copy of ours) to see which gives you “your” best return. We will have a follow-up article on how to use this sheet soon!

Boat Fuel Burn Template
Download XLS • 57KB

9. Flush the engine after each use to help keep the cooling system clean, and this not only applies to saltwater use. Many lakes have a ton of mud and crud that I would not want to leave sitting in my engine…

10. More tips for both the Boat and Truck on how to save on fuel costs can be found by clicking here.

We hope this helps you save on both fuel expenses and maintenance costs at the same time by having a better understanding of octane and engine cleanliness. 😎

In the meantime,

We invite you to take in a few Flats Nation Podcast Episodes to help "Scratch that Fishing Itch" when working or traveling and you cannot hitch up the Skiff and go, on these links:

Our main podcast page works well for your mobile devices since it will stream (or download) right on your phone/tablet. It can be found by clicking Here.

Our newer Flats Nation YouTube Channel can be found by clicking Here and sharing them with family and friends if you find them Entertaining and Educational! We have some great guests in the works and planning stages on a wide range of topics and product coverage soon.

Plus, our store has some new clothing and items you can review and order by clicking here.

Tight Lines, and God Bless!

Dave and the Team

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