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Best when using expensive gas ...

42K views 116 replies 33 participants last post by  Goodwin83 
#1 ·
Yahoo article ... although 87 octane can be used best/optimum performance when using 91 or better. Fiat, Dodge Dart Aero, Chevy Volt among the contenders. Yeesh.
 
#8 ·
That depends. The Fiesta has a high compression ratio of 11:1. Recently, with the addition of variable valve timing and knock sensors to delay ignition timing, it is possible to manufacture gasoline engines with compression ratios of over 11:1 that can use 87 (MON + RON)/2 (octane rating) fuel. Such engines may not produce their full rated power using 87 octane gasoline under all circumstances due to the delayed ignition timing.

Quote:
Originally Posted by FORDSVTPARTS
These modern high compression engines do indeed benefit from higher octane fuel, the car will adjust itself to the fuel you run as others have noted with the effect that lower octane fuel will cause the PCM to dial back timing and reduce power and efficiency.
.
 
#7 ·
Again, I'm not so certain...
Higher octane fuel burns slower than low grade, and that's all.
If our Fiestas can adjust timing to make use of the premium gas, then again, you're just wasting your money.
If it were higher compression also, it could extract more power from the premium gas.

If you want better mileage, buy gas with no or low amounts of ethanol...
Ethanol has a lower specific btu per pound than does gasoline.
In other words, you have to burn more to get the same power (higher fuel consumption).
But I dunno of any major refineries that Aren't adding ethanol to their fuels...
 
#10 ·
11:1 compression isn't enough auto-ignite the compressed fuel/air...
Unless maybe you have an exhaust valve not seating or the combustion chamber design is a complete messup.
Pretty sure the engine wouldn't adjust valve timing if it detected knock. It's all in the spark advance.
If you run high test, the engine can spark sooner to make more use of the ignition pressure (without knocking). Doing that on lowgrade is asking for trouble.

If I understand the whole process right!!
 
#11 · (Edited)
You guys are correct. The knock sensor is a small microphone if you will. When it detects PING (pre ignition) it sends the signal to computer to retard ignition timing (loss in power) untill the knock stops.


Now the scary part, do you know what makes the pinging sound from pre ignition?

In preignition the fuel mixture ignites before the spark occurs. The mixture starts its burn and then the spark ignites the rest of the mixture. The 2 burning mixtures meet (crash) together if you will. This twist sideways the piston and causes the skirt of the piston to knock on the cylinder bore. Severe cases will cause pistons to crack and come apart or develop a hole in the top.
 
#12 ·
From what I've read on pinging, it's actually the preignition of the mixture causing it.
Gas burn relatively slowly...it isn't an explosion as we know it but rather a quick burn (compared to something like C4 with a shock wave traveling thousands of feet burn second)
When the gas preignites, it's flame front is traveling much faster than it should...and this flame front "hits" the piston like a hammer blow instead of a solid "push".
And this is where the knock sound comes from.
Good discussion!!
 
#15 ·
Another thing to consider is your driving style. If you never go beyond 3500 rpm, you probably will not benefit from higher octane fuel. In general knock or premature detonation happens higher in the rev band. It's in the high revs that you will benefit from 91/93 octane fuels.
Most luxury car manuals will say that in emergency they can use 87 octane, but it will warn to keep the engine revs low. I think I saw this at BMW when I worked for them. Cheapo dealer filled all the new cars with 87 octane. even the M cars (Sleeziest dealer I ever worked for)
 
#19 ·
I use to use 92 all the time.. it did give it a little more pep in its step but mpg stayed basicly the same. I just use 87 now cause I'm broke and I get the same mpgs as the 92. But I will reset the ecu and use 92 a week prior to any race event to squeeze all the power I can get out of that little 1.6. ;P
 
#20 · (Edited)
This thread is hilarious. You know that bowl of suckers in the waiting room of a pediatricians office? That's what this thread reminds me of, a bowl of suckers.

93 will do NOTHING for your Fiesta. It does NOT:

1. Burn better
2. Burn cleaner
3. Burn faster (it actually burns slower)
4. Prevent knocking
5. Cause knocking
6. Get you better MPG (in fact, you will get slightly worse, use ethanol-free gas for better MPG)
7. Give you more "pep"

The engine in the Fiesta, for the 10th time, CANNOT TAKE ADVANTAGE OF HIGH OCTANE FUEL! It does NOT have high compression or a turbo, so its a waste of money. Vehicles that will benefit from it will have "premium fuel only" on the dash next to the fuel gauge, like my old Mercedes did. And if you ran regular in it, the world did not spin backwards! You just did not get full rated horsepower or the best mileage as the engine would begin to knock and the computer would thus retard the timing and kill your power. You would NEVER notice a change unless you got on it with a stopwatch and timed your 0-60 time.

All 3 grades of gas come out of the same batch of base stock. Additives are added to increase the knock resistance of the fuel, thus raising the octane level. It's not refined in any way differently than any other grade of fuel.

Guys please, I don't know everything, but I am an FAA licensed airplane mechanic. I've worked on turbine engines worth several million dollars. I've worked on small piston airplane engines. With airplanes, they use 100 octane fuel. But they have a few airplanes out there who are turbocharged, and it is these planes that need the high octane fuel. Aircraft engines are also 50 years behind the times as far as technology is concerned so comparing airplanes to cars is difficult at best.

The Fiesta is a great little car, but it is not a high performance racing car so it does not need fuel suited for racing applications and adding that fuel to the car will do NOTHING to performance. Ford would recommend medium or high grade fuel if it would benefit the vehicle at all. It does not.

Please stop wasting your hard earned money on fuel that is not giving you any benefit whatsoever.

This last little tidbit should scare you AWAY from using premium fuel. I read that one gas station was having trouble with the premium tank in the ground because of the price of premium and the fact that so few cars require it, nobody was using it and the tank in the ground was collecting water and was actually pumping contaminated fuel into vehicles.

Like I said, leave that fuel to the Mercedes guys. We just take the 87. That's perfect. You want the best performance out of your Fiesta? Use 87 octane, ethanol-free gas. That will give you the best performance and best MPG possible.
 
#36 ·
Actually, I use 87 octane. The rolled eyes emoticon is simply a reference to how terribly rude you are to other forum members in your rants. You've gone off on people a number of time now and it's getting tiresome.
 
#34 ·
Umm...you conveniently neglected to include a part from Wikipedia that actually supports against your rant so I bolded them for you.

It is possible to manufacture gasoline engines with compression ratios of over 11:1 that can use 87 (MON + RON)/2 (octane rating) fuel with the addition of variable valve timing and knock sensors to delay ignition timing. Such engines may not produce their full rated power using 87 octane gasoline under all circumstances, due to the delayed ignition timing.
 
#40 · (Edited)
Really? Well then explain how it goes against my argument when in my VERY FIRST REPLY in this thread I stated this:

Vehicles that will benefit from it will have "premium fuel only" on the dash next to the fuel gauge, like my old Mercedes did. And if you ran regular in it, the world did not spin backwards! You just did not get full rated horsepower or the best mileage as the engine would begin to knock and the computer would thus retard the timing and kill your power.
And did you read your own bolded part? It says OVER 11:1!! The Fiesta is 11:1. You want to talk about a major fail?

The full rated HP of the Fiesta is with 87 octane fuel. It says MAY NOT PRODUCE FULL RATED HP. The Fiesta does. In fact the part you bolded I stated in my reply several pages back, that my Mercedes would not produce full rated HP if I didn't put in the required premium fuel. So you going to say that your bolded part goes against what I said when I basically verbatim stated what your bolded part said?

Sorry, MAJOR FAIL.
 
#41 ·
Guy's cool it. We have two very knowledgeable people here who are not going to agree as to how a statement is being interpreted. One is a qualified and apparently very knowledgeable aircraft mech. and the other extremely knowledgeable about his car and auto engines and fuel combustion.

From both your statements I've gained interesting insights to how and why fuel burns the way it does.

I suspect the only way to resolve the argument is with imperial data. Now lets see who is willing to put their money where their mouth is.
 
#42 · (Edited)
Data on what? I have nothing on the Fiesta specifically, but I have tons of information I can post about compression ratios and high performance engines, etc. I see no reason why the Fiesta engine would break any of the known rules of internal combustion engines. I already posted tons of evidence. All I've heard in reply to these facts are little childish things like roll-eyes smileys and such. Me thinks that you should be asking for things that refute all the evidence I've posted so far.

By the way, its empirical data, not imperial.

Here is tons of data on compression ratios and such:

http://en.wikipedia.org/wiki/Compression_ratio

Internal combustion engines are heat engines. The thermal efficiency of the engine increases with higher compression ratios. This is good, we want the most power out of each drop of very expensive fuel. High CR's help achieve that.

http://en.wikipedia.org/wiki/Thermal_efficiency

The new Mazda line of engines, Skyactiv, with 13 and 14 to 1 compression ratios, do not, and do require high octane fuel, respectively. But 14 to 1 is much higher than 11 to 1. That's almost getting into the diesel realm.

http://en.wikipedia.org/wiki/SkyActiv

I'm not saying that the full POTENTIAL horsepower of the Fiesta engine would be had with 87octane fuel, I'm saying the full RATED horsepower is. Would you potentially gain a few more by remapping the computer and using premium? I'm almost certain you would. But that's not what the average Fiesta owner is going to do or experience. Just simply putting high octane fuel in the tank will do nothing but empty your wallet faster. I posted 5 links to 5 articles that state the same.
 
#44 ·
Wow. This has gotten heated.

Guess what? I use 91, always have, always will. I drive 200 miles a day, and if there is a snowballs chance in hell that it will help the vehicle that I spend 10 hours a day in, it's worth $0.20. Go ahead, throw some math. ~45000 miles per year, and go about 350 miles between fill ups, for simplicity, we'll call that 130 fill-ups. It's typically ~9.5 gallons, but again, for simplicity, we'll call it 10. That's $2.00 for per tank. I spend an extra $260 per year on fuel. Considering I spend over $5,000 per year on gas, that's chump change. We'll chalk it up to maintenance cost.

Every forum out there has a similar debate that is just as heated. So is it beneficial? Maybe, maybe not. I don't really care. Because I have never known of anybody harming an engine by use of 91 octane.

:)

Now that I said that, I do have a serious question that I really don't know what the answer is. If a motor is tuned to run optimally on 87 octane, what about the parts of the country that classes their 89 as regular, when that's mid-grade in my parts?
 
#45 ·
I had a '92 Tempo that had an engine that clattered at high RPMs when struggling up a hill or something like that. I stopped to fill up gas once and all they had was the highest octane. Not wanting to go elsewhere I just filled it. Did it ever make a difference. Engine stopped clattering, became quicker (for a Tempo), and all around the engine seemed to drive better until I filled it up with the lowest octane again. Didn't matter what brand gas I put in. I sold that car to my brother and I told him to do the same and he said it had the same results. It's not just me.

My '03 ZX2 started knocking every now and again. I went ahead and put in the next octane up just to see if it would stop. Several fill ups of the next octane and I didn't hear the clattering again. I went back to the lowest octane some time ago and swapped out the PCM so I suppose the new PCM learned or something since I haven't heard any noise in a long time.

I know neither of those are Fiestas, but based on my personal experiences putting in higher octane gas will stop engine knock and in some cases it may perform a bit better (at least an old engine will). But really, putting anything higher than the mfg recommended is generally a waste of money for a newer car like the Fiesta.
 
#47 · (Edited)
I had a '92 Tempo that had an engine that clattered at high RPMs when struggling up a hill or something like that. I stopped to fill up gas once and all they had was the highest octane. Not wanting to go elsewhere I just filled it. Did it ever make a difference. Engine stopped clattering, became quicker (for a Tempo), and all around the engine seemed to drive better until I filled it up with the lowest octane again. Didn't matter what brand gas I put in. I sold that car to my brother and I told him to do the same and he said it had the same results. It's not just me.
Your 1992 Tempo most likely had large amounts of carbon deposits on the pistons and that changed the compression ratio to where the engine would knock at high throttle settings. It could have also been caused by a hot spot on the pistons from the carbon that would cause the premature ignition. Without a knock sensor, it won't correct itself. You used high octane fuel which stopped the knocking, but it didn't clean out the engine.

Believe me, your Tempo knocking was not good and using premium to get it to stop knocking was just a bandaid.

The Causes of Internal Engine Knock, and How to Eliminate it. By Ray T. Bohacz

Carbon deposits that build up in the combustion chamber or on the piston top will have the same effect, along with the decreased volume at TDC, which in turn raises the compression ratio
All of us gear heads and "professional" mechanics can spout our opinions based on what we've been taught or have learned in the course of our lives, but I've seen no evidence that anyone here is qualified to tell us what Ford's Design Engineers intended for this engine.
Of course you can tell what they intended. If the engine is rated for premium fuel, then they are utilizing the engine to its utmost to gain every ounce of HP available from the engine. 87 shows me they rated this to be a commuter car. High performance cars like the Veyron use premium only, with 100 recommended I believe. THAT is a high performance car. Same thing with Mercedes, those engines are high output engines that require premium. To think that the little 4 banger in the Fiesta requires or would benefit from this is hilarious.

http://www.roadandtrack.com/go/news/new-technology/premium-fuel-futures

If your car would benefit from premium or be hurt (performance or MPG from regular), you would see something like this in the owners manual:

"lower octane may yield reduced performance or inferior mpg.
To those who continue to use premium despite the OVERWHELMING evidence that it is doing NOTHING for you, I guess the old saying goes, "You can lead a horse to knowledge, but you can't make him think." Exxon says:

 
#46 ·
We should all keep in mind that the statements in this discussion about Ford's design intentions are beliefs and speculation, not facts. Although during the first year of production, several Ford Fiesta design engineers participated in discussions on this forum, it's evident from the content of this thread that they're not here now.

All of us gear heads and "professional" mechanics can spout our opinions based on what we've been taught or have learned in the course of our lives, but I've seen no evidence that anyone here is qualified to tell us what Ford's Design Engineers intended for this engine. After all, the Fiesta engine is what's being debated. If you have evidence, then let's see it. And I don't mean quotes from Wikipedia or college textbooks. How about several back-to-back dyno runs using regular and then premium? Which makes more power? Which gets better mileage? Anything else is just hot air. :)
 
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