Regular vs Premium MPG report.

[Gunkk]

That's correct. The higher you go, the less octane you need, unless you run with boost.

 

[427435]

Update: I have driven an additional 850 miles on regular and the mpg is remaining within a fraction of what I got on premium. However, many others have reported a drop in mpg when burning regular gas. I think I know why I am not seeing what others have reported.

If there are any engine techs out there, I would love to get your opinion regarding this theory.

Here's what I think is happening:

The atmospheric pressure at this altitude (7000 ft.) is usually around 11.34 psi as opposed to 14.7 psi at sea level. The compression ratio of this engine is 11.5:1. So, at sea level, the compression-stroke cylinder pressure in this engine would be approximately 169.05 psi (11.5 x 14.7). However, at this altitude, it would be approximately 130.41 psi (11.5 x 11.34).

The reason one would normally experience a degradation in engine performance/efficiency using regular gas in this engine is due to the ECU retarding the timing to prevent detonation. Detonation occurs when the fuel-air mixture in the cylinder explodes abruptly rather than producing a smooth burn. Higher octane fuel slows the burn rate to prevent detonation so the timing is not adjusted by the ECU. Detonation is a function of the pressure within the cylinder. At this elevation, the lower atmospheric pressure results in a lower compression-stroke cylinder pressure (130.41 psi) which is not sufficient to cause detonation using regular gas. Therefore the timing is not retarded by the ECU and the mpg is not effected. So, I believe I can burn regular and save money without suffering any penalty.....except the corresponding reduction in power the comes with the high elevation. I am driving a 340 HP sedan instead a 429 HP sedan....sigh.

What say you?

Thanks for any input.

Bill

First things first. Higher octane gases do not burn slower. It does require a higher combination of temperature and pressure before it self-ignites.

There is some merit to your theory, but I suspect the ECU has enough flexibility that it can advance timing to the point of detonation with either regular or premium gas. The max timing is usually being used at higher engine speeds but light load (high vacuum). The knock sensors are used to constantly dither the timing to just short of detonation.

 

[Gunkk]

There's nothing "theory" about high altitude... it's fact.

Take a bus to Huaraz or La Paz. You'll never forget the smell

 

[bilworth]

First things first. Higher octane gases do not burn slower. It does require a higher combination of temperature and pressure before it self-ignites

Thank you.

I got the wrong idea about the burn rate from Wikipedia:

"Octane ratings are not indicators of the energy content of fuels. They are only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner."

Best regards,
Bill

 

[bilworth]

That's correct. The higher you go, the less octane you need, unless you run with boost.

Thank you.

I appreciate input.

Take care,

Bill

 

[bilworth]

Just a last note to elucidate this issue in case anyone is interested.

The previous "first things first" post by 427435 is not totally correct. It is correct regarding pre-ignition, but not detonation. Pre-ignition is a condition that occurs as a result of the fuel/air mixture igniting prior to the spark plug firing. As the octane rating increases, the tendency for pre-ignition decreases. So, 427435 is correct on that point.

However, my theory was based upon detonation. Detonation is a completely different animal. As I noted, detonation is a condition where the fuel/air mixture spontaneously explodes creating a pressure spike rather than burning smoothly. Detonation always occurs after the spark plug fires. Increasing the octane rating of a fuel helps it resistist detonation and burn more smoothly. This is accomplished because the molecular structure of iso-octane is stable and resists breaking apart in the high pressure and temperature created in a combustion chamber. Iso-octane therefore controls the burn-rate and lets the flame propagate smoothly from the spark plug across the combustion chamber rather than exploding and causing engine knock. Google "what does octane do" if you want more info.

Cheers,
Bill

 

[427435]

Higher octane resists both pre-ignition and detonation. Detonation can occur with or without pre-ignition. I guess I'm missing your point.

 

[craigt]

Took a 460 mile one day trip over the weekend from MA to NJ and back (see other thread). Got 29.2 MPG round trip on 87.

 

[bilworth]

427435 stated:
"I guess I'm missing your point."

Sorry, no offense intended. I just wanted to make sure this thread is accurate.

However, in your original post you stated: "Higher octane gases do not burn slower".

As my previous post explains, higher octane fuels do burn slower. The slower burn rate is how detonation is prevented. Iso-octane prevents detonation by controlling (slowing) the burn rate in order to yield smooth flame propagation across the combustion chamber and thereby preventing an uncontrolled spontaneous explosion (detonation).

And, please, I don't want this to turn into a contest. You win. OK?

Take care,

Bill

 

[bilworth]

Took a 460 mile one day trip over the weekend from MA to NJ and back (see other thread). Got 29.2 MPG round trip on 87.

Wow! That's great. These are amazing automobiles.


Take care,

Bill

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[427435]

427435 stated:
"I guess I'm missing your point."

Sorry, no offense intended. I just wanted to make sure this thread is accurate.

However, in your original post you stated: "Higher octane gases do not burn slower".

As my previous post explains, higher octane fuels do burn slower. The slower burn rate is how detonation is prevented. Iso-octane prevents detonation by controlling (slowing) the burn rate in order to yield smooth flame propagation across the combustion chamber and thereby preventing an uncontrolled spontaneous explosion (detonation).

And, please, I don't want this to turn into a contest. You win. OK?

Take care,

Bill

Again, premium gas does not burn slower. That is a common myth that has been repeated by many. Stop and think about it-------------racing engines are usually high compression and operate at high rpms. A slower burning fuel would not be a good idea due to the speed of the engine.




Let's talk about burn rate or speed for a bit. Higher octane fuels do not resist pre-ignition due to a slower burn rate. While that is a seemingly plausible reason, there is no basis in fact for that explanation. The truth of the matter is that fuels with higher octane levels are blended to handle higher temperatures and pressures without pre-ignition. It is just that simple. For example, a Nextel Cup engine has a compression ratio on the tall side of 12:1, and the engines can turn in excess of 9,000 rpm when the track conditions merit high rpm. I do not think these engines use "slow-burning" fuel. They need to burn the whole of the fuel they are using to get the kind of power they are getting.

Read more: http://www.stockcarracing.com/techa...ing_fuel/octane_burn_rates.html#ixzz3lCPbKvkP

 

[QuantumRift]

In an engine DESIGNED to operate on regular fuel, there is absolutely NO boost in mpg OR performance. Premium fuels contain additives and detergents, which in a non-GDI engine, can keep the valves clean. In an engine designed for mid or prem fuels, you will obtain your best mpg and performance using the correct fuels. Cars designed for premium fuels can, in most cases (like the SmartCar) use midrange or regular fuels, but that use will come a mpg and performance penalty.

If you have been using E10 fuels in your 'regular' gas engine, and switch E0 (no ethanol) fuel, you WILL see a bit of an increase in fuel economy. That's because in E10 fuel, 10% of a given volume of fuel is replaced with a fuel (ethanol) that has about 75% the energy of gasoline. SO the boost is small but noticeable.

Greetings,
I drive a 2014 5.0 R-Spec. I did a little test to see if using regular would adversly affect MPG. I live in the Colorado Springs area so my driving has been in higher elevations (between 6000 and 7000 feet). I don't know if that makes a difference or not, I thought I should mention it though. And, due to the altitude, the octane ratings are lower here than they are at sea level. I checked the milage with each fill up the old fasioned way and didn't use the car's MPG readout. I ignored the first tank full when computing MPG after changing to allow the OBDII to adjust. I drove 1,117 on premium and averaged 21.55 MPG. Then I changed over to regular and drove 893 miles and averaged 21.80 MPG. The slight difference might have been due to variations in traffic conditions. But, as you can see, regular seems to yield about the same as premium.

 

[427435]

I believe that today's top tier regular gas has similar additive packages as premium gas.

Running regular gas in cars designed for premium gas that DO NOT have knock sensors will likely cause problems including fewer mpg.

I'm not so sure about today's cars that have knock sensors. As most driving is done with only a partial throttle, I wonder if premium fuel will actually give more mpg. The numbers reported in this thread would indicate not.

In any event, I'm running my 5.0 on a steady diet of regular gas. That's especially going to be true now that premium is 30-50 cents a gallon higher priced around here.


Edit: Just did a little googling-------------premium and regular gas have similar BTU's per gallon. Thus they should have similar mpg if the engine has knock sensors and a computer to burn both at their optimum.

 

[LandoAWD]

In any event, I'm running my 5.0 on a steady diet of regular gas.

Please fix your signature to reflect the crank HP of using lower grade fuel.

 

[amartz]

Please fix your signature to reflect the crank HP of using lower grade fuel.

+1

 

[427435]

My butt dyno hasn't detected the difference between 429 and 420 yet.