So I'm having a discussion with someone on another car forum about whether the 5.0 will get better fuel economy when running 93 octane instead of 87. My reasoning is if the car has less power when running the 87, it will get worse fuel economy. I have observed (supposedly) this in my own experience, although it could be some kind of statistical error; and I have also read a lot of posts from people agreeing. Am I wrong? The other person is saying the car should get the exact same highway mpg (in theory) whether it is running 87 or 93.

 

Octane has nothing to do with MPG. The Higher the octane the harder it takes to burn. Meaning it has additives to make sure it takes more energy to ignite it. So if your car is not pining going down the highway then you are just wasting money on 93. 93 gives our cars more power only because it can increase timing with out pinging. More timing more power.That is the simple answer. Most cars once at speed only need a little horsepower to stay at that speed. Hence why some manufactures turn off cylinders to save on gas.

Hope this helps. My family use to own a Union Oil station for many years.

 

but wouldn't less hp in the same car generally equal worse fuel economy?

 

The only time you are really getting more HP is when you are at full or almost full throttle. When going down the Highway at speed both cars are making the same HP. Timing is then decreased. Only when timming is increased will you need more octane so it will not preignite. High compression also plays a part.

 

It’s probably worth a preamble on fuel and some of the related terminologies before delving into your questions…my guess is they will be answered by them!

· Octane Rating – This is simply an expression or measure of the fuels ability to withstand the pressure and heat placed against it before it spontaneously detonates as compared to isooctane (Which is considered 100) through research (R), motor (M) method or in most cases both (Ron+Mon/2) . (Ron+Mon/2) is how pump gas octane is derived. One more note about octane: Urban legend carried forth by those who refuse to do fuel science research will have you believe the higher the octane of the fuel, the slower it will burn. This is simply false. The fuels burn rate is largely adjusted by addition or subtraction of the light aromatics it contains such as toluene and benzene as examples.
· Normal Ignition – The desired, correctly timed ignition of the intake charge by the spark plug. When the engine is operating normally, the air/fuel charge is burned in a rapid but controlled manner.
· Pre-Ignition – The unintended ignition of, in this case, our as yet unignited intake charge (By normal ignition). This is sometimes referred to as “Dieseling”. It’s caused by hotspots in the combustion chamber (Typically an overheated: spark plug, valve head, spot in the combustion chamber, a chunk of hot carbon, etc.). Make no mistake about, pre-ignition is quite different than detonation but when pre-ignition does occur it often leads to detonation.
· Detonation – The uncontrolled, simultaneous combustion (Explosion) of, in this case, our intake charge when its octane rating has been exceeded. There are a lot of factors that will contribute to, and in some cases out and out cause detonation by themselves. Here are a few of the leaders: high intake charge temperature, lean intake charge, high engine temperature, high combustion chamber component temperatures, too much spark advance, high engine load/low operating speed, etc. and they can either act singularly or in consort with each other. Ultimately the fuel is unable to handle the heat and pressure placed against it and it detonates (explodes) in an uncontrolled manner.

When the engine is right on the edge of detonation, the combustion cycle will often begin as intended by ignition of the charge by the spark plug, however as the charge burns, if the heat and pressure rise in the chamber to the point where the fuel can no longer contend with them, the balance of the unburned charge spontaneously combusts (Explodes). In modern computer controlled engines, the computer has the ability to detect detonation by means of a “Knock sensor” (A piezo electric sensor tuned to the same frequency of that produced by detonation). When the sensor detects detonation, its normal voltage signal to the computer is varied and the computer retards the timing until the sensor no longer detects detonation and returns to its normal range.

Cylinder pressure equals horsepower and eventually torque, right? It doesn’t matter how you get it as long as you get it. The gasoline stock we call pump gas has about 19,000 BTU per pound no matter what octane rating it has. That’s critically important to remember! Another important factor is, the octane rating by-in-large has no bearing on the fuels energy content. That means 87 octane fuel has the same potential heat as 93 octane does. The difference in potential power output between the two comes from the 93 octane’s ability to withstand more heat and pressure prior to detonation. That means you can run more ignition advance. Lighting the intake charge earlier builds more pressure inside the cylinder, right? Remember cylinder pressure equals HP. The greater potential cylinder pressure per equal unit of fuel between the 93 as compared to the 87 makes 93 the winner right?

So you’re asking yourself, when in he11 is this dufus going to get around to the point? OK, your ‘lill 5.0 is turning ~1700 rpm cruising down the freeway. On 93 octane fuel it’s using more ignition advance than possible with the 87. Remember, ignition advance equals more cylinder pressure and cylinder pressure is HP. If you’re building more cylinder pressure than the drag placed against your trusty Musty, you’re going to accelerate, right, because the crank speed is going to increase since the resistance against the car is less than the cylinder pressure being created and placed against the crank. Unless you want a speeding ticket, to balance (Reduce) the cylinder pressure (And pressure against the crank as a result) to the car’s drag, you’re going to get out of the throttle right? Getting out of the throttle saves gas last time I heard.

I’m sure there are many who will read this and refute it. To them I say, “Do the research”. If you have access to a petro chemist, bend his ear for a while if he can dumb down the terms far enough for a normal human being! Otherwise, talk to the fuel manufactures and blenders like VP and Unical and make friends with a good sharp engine builder and they’ll all tell you the same thing. Don’t ask other racers or kids in their parent’s basements on the computer ‘cause 99% of them don’t understand it and have it wrong too!

Cheers,
John

 

Good write up.

Italicized part is the key. If the computer sets the timing to fire at 20 degrees advance and the sensor detects detonation, the computer will back the timing off and therefore the result is less pressure/less power and more fuel would be required to make up for it with a decrease in advanced timing.

However, I have run 87 in my 5.0 since I got it a year ago. I ran 93 in it for two tanks and could really not tell a difference in power or mileage at all but my driving may have not been as consistent either for a true test. Most of my driving is below 2000 rpm and some traffic congestion at certain points.

 

Thank you!

I ran 87 in my GT for the first year as well and consistently pulled 23.5 mpg on average. On mainly highway/freeway road trips it would pop up to 25.6. The second year I did a few mods but stuck with the 87 and the average mileage dropped to about 22.1. I didn't do any road trips that year so that data point is missing. This summer I decided to run 92 which is the best we have in this area. The average mileage jumped back to 22.9. On highway/freeway road trips it pops up to 24.4.

The proof is there. I guess it's up to the individual to decide if paying between 30 and 40 cents more per gallon of premium is worth it. It's definitely going to cost more. But, as side benifits, premium has cleaning additives that the 87 does not, the additional 30 ponies from 92 octane at WOT is nice, and you can feel all good inside that you're doing your best to conserve the earth by getting the best fuel ecomony possible...if that matters to ya!

John

 

When I ran the 93, I realized I only benefitted from the octane 3 or 4 times (not many places to run a WOT to and from work where I live). For the extra $5+ per tank it just wasn't worth it for me so I went back to 87. If I had seen a 5 to 7% increase in mileage I would have been more convinced to use the higher octane.

 

The efficiency of the making of the horsepower affects the gas mileage, not the measurement of maximum horsepower.

 

Do you have a total stock engine / Tune / any Mods?

 

No…you benefit from 93 anytime, not just WOT, because the computer has greater latitude to increase ignition timing beyond what it can with 87. Remember that timing is most advanced when the engine is least loaded, meaning cruise. Under acceleration or anytime the load placed on the engine is increased, timing is pulled to keep the charge from detonating. If you’re old enough you’ll remember the old vacuum modules that were on the distributors back in the day? They added timing under high vacuum conditions (Low pressure is more accurate because the world we live in is pressurized. It’s that pressure that forces air into the engine). Low manifold pressure (High vacuum if you insist!) exists when the engine is least loaded, right? Each jug pulling against a mostly closed throttle plate creates low pressure in the intake tract. Stomp the throttle plates open, and ambient pressure at 14.7psi is free to flow into the engine. In the old antique engine the spring in the ignitions advance module retards the timing again.

Incidentally, in a normally aspirated engine you can easily determine the intakes volumetric efficiency by simply adding a pressure gage (Or vacuum gage) to the intake. If under full throttle the manifold pressure is at say 12.7psi and you know ambient pressure is at say 14.7psi when you did the test then: (14.7-12.7=2) (2/14.7=0.13605 or 13.6%) (100%-13.6%=86.4%VE). If you’re a good engine builder and know what makes the world go ‘round, VE’s of 110% on a normally aspirated engine are possible, not easy but possible…the majority of the magic is in the cam and port design. Most production engines are about 75% from the factory.

Cheers,
John