Octane and Your Engine
It’s a common misconception that using a higher-octane fuel in your
vehicle will give you better gas mileage. This isn’t necessarily true.
The long and short of it is that the RM2 method of octane rating is a
rating of resistance to volatility. The higher the octane, the less the
fuel wants to burn. They do this to accommodate the high combustion
chamber temperatures you see in high performance engines. High
performance engines often use high compression or forced induction to
create more power. This increased pressure in the combustion chamber
equates to heat. This heat can pre-ignite the air fuel mixture in the
combustion chamber. This is bad, as it equates to a loss of power and
possibly engine damage. On these engines, you use a higher octane fuel
to prevent this pre-ignition from happening.
If you use a higher octane fuel in an engine that doesn’t call for
it, you’re wasting your money in my opinion. Yes, some premium fuels
come with a better additive package, but I don’t think you’ll see much
benefit from them in your ordinary non-performance engine. In other
words, you likely won’t go any faster and you won’t get better fuel
economy when using premium fuel if your vehicle doesn’t call for it.
Conversely, if you run regular low-octane fuel in a vehicle that
calls for premium fuel, you’re making a mistake. As stated, you run
high-octane fuel to help prevent pre-ignition and misfires. Running
low-octane fuel in an engine that calls for high octane will not only
compromise power and cause a drop in MPG, but it can also damage your
engine in some cases. The takeaway is to put in the fuel the
manufacturer calls for and you should be just fine.
Oil and Fuel Additives
My knee-jerk reaction here is to call all fuel and oil additives
snake oil, meaning they really aren’t worth it. I think fuel and oil
additives are a case of perceived value. You go to the auto parts store,
pick up a bottle of “make your engine happy,” dump it in, and you
suddenly start getting more power and better MPG out of your vehicle.
This is why I say “perceived.” I’m sure there are people out there who
swear by product X. I’m not saying product X isn’t a good product. I’m
saying that product X is different things to different people. It’s
money in the pockets of the people selling product X more than anything
else. Am I opposed to this? No, I’m a capitalist. But as a capitalist I
like to save money. For me, I’d rather save my money than put “make your
engine happy” in my tank or crankcase. I prefer old-fashioned service
and maintenance to keep my engine efficient. Dumping stuff in my tank
just seems like a shortcut that will get me lost rather than to my
destination. You, however, can do what you like. One thing I will say is
that when used properly, fuel and oil additives don’t do any harm. So
if they make you feel good, use them. If you want to save money, don’t
spend it on fuel and oil additives.
Forced Induction Problems
Forced induction is anything that adds pressure to the intake on your
engine. The most common forms are superchargers and turbo chargers.
Both of these do pretty much the same thing, but in different ways. A
supercharger is a compressor driven by a drive belt connected to the
crankshaft of the engine. A turbo charger is also a compressor that
forces air into the engine, but instead of being belt-driven like a
supercharger, it’s driven by the exhaust gases coming out of the engine.
Both of these systems help increase the power output and efficiency of
the engine.
Superchargers produce max output at lower RPMs, which makes getting
off the line quicker. Turbochargers, on the other hand, have a delay in
power delivery. They need to “spool up” before they start increasing the
engine’s power. A problem with either of these systems can cause poor
fuel economy. Since they create pressure in the intake, you need to
employ different testing methods when diagnosing these systems. For
instance, instead of spraying carburetor cleaner or water on the intake
when looking for leaks, you might try a mix of soap and water. When the
soapy water reaches a leak, it will start to bubble when the intake is
under pressure during boost, indicating the leak’s location. Other
problems, such as bearing failures or control issues, can hamper the
ability of these systems to operate properly. When these systems fail,
you will see a decrease in power and fuel economy. Consult your
vehicle’s service manual for testing and evaluation of the system on
your vehicle.
Engine Mechanical
I’ve spent a lot of time talking about the systems that manage the
fuel delivery to your engine. We haven’t said anything about the
mechanical integrity of your engine yet, and we should. All of the
aforementioned systems don’t mean diddly if your engine has a mechanical
problem. It’s been my experience that engine mechanical is often
overlooked in diagnosis. It shouldn’t be. If your engine has a
mechanical problem, no amount of parts or sensors you throw at it will
cure it. It is the heart of the system, after all.
One of the most basic tests you can perform to assess an engine’s
mechanical integrity is a compression test. This test measurers the
engine’s ability to draw in and compress air. In truth, your engine is
nothing more than an air compressor. If it can’t do that efficiently, it
won’t run right, and fuel economy will suffer. Here’s a video on
compression testing that can walk you through the process.
Driving Habits
I saved one of the easiest and best for last. Many times poor fuel
economy is your fault. Yep, that’s what I said. The way you drive has a
direct effect on fuel economy. If you have a lead foot, make sure you
have a high credit card limit, because you’ll be spending a lot more at
the pump. If you want an easy way to save fuel, slow down, don’t race at
stop lights, and take it easy on your vehicle. Do it for one tank of
gas and see if you go farther. I bet you do.
SUMMARY
As I stated in the beginning, there are many things that can affect
your vehicle’s fuel economy. The main takeaway is anything that makes
your engine work harder will compromise fuel economy. Therefore, in your
search for a cause of poor MPG, be sure to look for things that will
drag your engine down and make it not perform as well as it should. Take
a common-sense approach. Listen for new noises, keep your eyes peeled
for leaks or anything out of the ordinary, and generally feel how your
vehicle is running. Be aware of its operation so that when something
does go wrong, you’ll feel it before it gets to a critical point. If it
feels weird or unsteady, look into it. Don’t be afraid to be intimate
with your vehicle. Think of it as a modern-day horse. Treat it well and
it will take care of you for years to come. Good luck in your search for
better MPG, and thanks for reading.
