Well, it's official for me that regular bags me less power for sure. Tonight I tried running my AT RX-8's first two gears to redline at near WOT. First gear doesn't really show any huge differences in performance, partly because it's over so fast, but because 2nd gear is so long, any differences in how fast it gets to redline is very noticeable. After 5000 RPMs the car started to exhibit signs of not revving very fast, and I had to go WOT in hopes of getting more power, something I normally don't have to do at all to get 2nd gear to redline with premium. After 6000 RPMs, the car just didn't feel like it was going anywhere and it took forever to get to 7200.

Pretty interesting overall, at least the car's totally driveable up to about 5000 RPMs even on regular. It's after that that things start slowing down.

 

Thanks for this *extremely* informative and useful post. I confess that I too have been influenced by the naming into thinking the lower octane stuff was poorer quality. This thread has *almost* convinced me to try lower octane. But I'm troubled by just one thing:

When I got the recall notice recently I had to call my service department to see when they could look at it. While I had the guy on the phone, I asked him about the octane thing. He insisted that Mazda *strongly* recommends premium, that that's what they tell him. They must have some reason they think it's better, what are they thinking? It's not like they're selling the gas.

Krankor
05 Shinka 6MT

 

This is not entirely correct. First of all octane is a rating of a fuels resistance to ignition/detonation. THe higher the octane rating the harder it is to ignite a fuel, but the more potential energy it stores. THis has nothing to do with the "effectiveness of the fuel". (In fact i'm not sure i've ever heard that phrase before and would be intrested in how you came to that conclussion and how one goes about rating a fuels effectiveness. but thats beside the point) Higher octane fuel is not designed to combat compression as you state but rather heat. Heat (which is the byproduct of compression and a number of other things) is what causes preignition/detonation not compression. YOu are sorrect though in that more compression = more power.

What i think is happening in the case of the RX8 and 87 octane is that the knock sensor is picking up the preignition (the pizo-electric crystal inside will detect preignition/detonation long before you can) and retarding the timing. It is the retarded timing that is giving better fuel ecconomy ect. There is a power loss from this weather it is noticeable to you or not. Theoreticly this could cause damage. Theory and practice are very rarely the same thing though. That is mostly determained by the ECUs programing and weather or not it ca nreach base timing on 87 octane and weather it continually attempts to advance the timing untill it pings/knocks then backs off.

The gist of your post was mostly correct i just wanted to clarify i few things. I hope i didnt offend you

 

no offense taken. I was just trying to dispell the belief that lower octane = dirtier or lower quality fuel. Thanks for clarifying further.

What I meant by speed of burning was essentialy what you were saying: low octane fuel ignites at a lower temperature than high octane fuel, thus starts burning earlier in the cycle, and can sometimes ignite under the heat created by compression - basically if you take a gas at a certain temperature and compress it to 1 tenth of its original size, it still has the same amount of heat energy but in a much smaller space, thus the temperature is increased by a large factor. Some gasoline formulas can be so high octane that if a lit match is dropped into them, they will not ignite. What I meant by the effectiveness of the fuel is that a lower octane fuel will burn faster than a high octane fuel, converting the fuel air mixture into expanding gasses faster. This is because at a molecular level, the properties that make a fuel resistant to heat slow down the front of the flame wave that travels through the chamber.

You could demonstrate this yourselves by taking an equal portion of each of the grades available to you and evaporate them inside a clear combustion chamber, with a high speed camera. put a temperature controlable ignition source in the center at the bottom of the chambers and set all three off at the same time. Then play back the high speed camera, and the "explosion" or flame wave patterns in the three chambers should show the characteristics I described. the lowest octane should ignite first and burn fastest. The highest octane should ignite last and burn slowest. In practice it may take several tries to get any sort of statistical difference between the three since there are so many other factors involved. However, remember that there are as many as 15,000 combustions taking place per minute in your engine, and you can see how the slight differences between the octane grades can have greater impact.

As long as the gasoline does not ignite before the spark plugs fire, you should be fine however. Pinging or knocking sounds happen when the ignition takes place before the rotor is in the correct place for it to happen, and undue stress is placed on the rotor and since it has to continue to compress the mixture because of the rotation of the engine, but the burning gasses are increasing in pressure exponentially with no where to go. This could over the long term cause damage or failure in the engine.

This is why Mazda recommends Premium grade 91 octane. The RX-8 is rated at a relatively high 10:1 compression ratio. But the knock sensor will catch any predetonation that occurs and compensate to prevent damage, so that is why the manual states that 87 can be used, but may degrade performance. If it does not degrade performance or cause knock or ping in your engine, you may as well use it all the time, as at that point you're paying extra for a property in the fuel that you do not need.

I think that about covers it, if anyone else wants to add to this information feel free and don't worry about offending me. I'm not a mechanic, just very interested in physics and science, hence I have read a lot about how engines and fuels work.

 

No way i would use regular on an rx8, hell, don't even use it on my 02 corolla, have you ever seen an outboard motor on a dinghy (Translation for Americans [Aluminium] Aloominum boat) run on both, literally you can put a coin on the same outboard motor runnning normal unleaded and it less than 10 seconds it's in the water, run that same engine on premium unleaded and that coin won't even move, not too mention the better mileage you get from it.

No newish engine should be run on the regular unleaded, or octane whatever Americans call it, 87? Over 93 for premium? Just trying to get some sort of cross transaltion working, lmao. Lucky bastard rx8 owners... Soon, my pretty, soon.....

 

Personally I would run at least Mid-Grade. While it is true most cars since about 90' have and use knock sensors... I would not necessarily look at it as a free pass to use a lower octane fuel on an engine really designed and suited for higher octane fuel.

 

EH??? You don't seem to understand what raz0rama is trying to say... Basically if your engine can run on a lower octane gasoline then it's not getting any benefit from running on a higher octane. Since the car engine is hand-built some degree of variations may be expected. Some cars will run fine using 87octane while some can never go below 91octane. The only way to know is to try it, go ahead it won't kill your engine on a tank of 87.

On your "dinghy" example, obviously the motor runs better on premium so go ahead and use premium on it. However, such example is a bad case of "what works for John should work for me" mentality. A prime rib steak may taste better but it does not offer me any more calories than a comparable cut of sirloin.

 

You can always find plenty of verbage, most of it opinion and not based in fact, to justify your decisions. Whether you choose premium or regular or mid-grade you will always seek validation for that decision.

I use premium because I know a little about rotary engines. But that knowledge is not even close to the knowledge of engineers at Mazda and other mechanics. I will take the word of those people before I ever listen to Bobby Car driver who has used regular gas for two tankfuls with no "long term" damage.

People have owned RX-8's for slightly over 2 years max. And they claim that regular gas has no long term effects? Two years is certainly not long enough for a long term case study.

Sure, Mazda says you can use 87 to 90 octane but the manual says "this will slightly reduce performance." (OM, pg 4-2)

It also says that Fuel with a rating lower than 87 octane (91 RON) could cause the emission control system to lose effectiveness. It could also cause engine knocking and serious engine damage. (OM, pg 4-2)

So, are you going to trust Mom and Pop Gas Shop to be honest and use their el-cheapo 87 gas which may in fact be of lower octane rating? Go ahead. Seems prudent to me.

Here is a fact. I have used mid-grade 89 octane from Texaco, once. My car was sluggish and knocked horribly. A knocking rotary engine will cause rotor damage long term. I have seen the effects to the combustion face of a rotor that experienced pre-detonation long term. Looked like it had been hit repeatedly with a ball peen hammer. Experiment over. Those who say they have no knocking are either not listening or just dont want to hear it.

Good advice is hard to come by but when it does dont call it bad.

 

Exactly...

There is a reason gas stations are required to post octane rating. It's called a LAW. If it says 87 octane it better be 87octane. If you're so concern about gas stations cheating you on their octane rating then what's stopping your reasoning from going further? How about cheat on the premium and give you 85 or even less octane? Then JoeSchmuck Gas Station will really make a killing on their gas pumps! Seriously though, on the outside chance that something like this happen (and I know they do... In Mexico) then just get your gas from a reputable gas station. There are Shell, Chevron, BP, Texaco, ad nauseaum at every corner. I make it a point however, to use a higher grade fuel when I find myself at El Cheapo gas station (it does happen) not because the gasoline grade maybe less than advertised but mainly because higher octane carries more detergents.

Nobody's putting a gun to your head... If you like premium then use it. Somebody asked about everybody elses opinion on the matter so that's what he'll get. People here have reiterated it and I will say it again: If you hear (or felt) possible predetonation or the car have gotten sluggish then switch to a higher octane, unfortunately your RX-8 is not among those that can safely run lower octane. However, if you feel confident that there is no knocking and performance is unaffected at all then running premium grade is not giving you any more advantage NOR will it give your engine less reliability.

 

Exactly. It lists no other contra-indications for using 87

So, what's to stop Mom or Pop from mis-labeling their 93? You can get a bad batch of gas anywhere. If my car starts knocking I'll add half a tank of '93 until it gets sorted out. So far I've had no problems with 87 octane from wherever I've gotten it. Typically Shell or Chevron. According to the owners' manual all I'm losing is MAYBE some performance. The performance is just fine for me, so I'm happy.

 

Ah, the famous "guy at the dealer" who has inside Mazda information that isn't published for normal customers. Sorry for 'dissin' but I have heard so much BS over the years from similar "guys"...

 

I've used regular from the first day I bought the car. I have 12k on it now and never had a problem. It runs fine. The latest flashes have more positive affect on my car's performance than anything else.

 

And noone ever breaks the law and you believe everything you read. Ok, then. If gas as advertised is lower by a few octane, no big deal when premium is say 90 or 91 vs. 93 and midgrade is say 87 vs 89. But what happens when the 87 is say 85 or 84? Long term trouble.

Ok, dont believe that gas varies from station to station or dealer to dealer, read this:

http://www.cars.com/carsapp/national...odaysgas1.tmpl

None of them are.

You dont "feel" the knocking because your engine is smarter than you are and is compensating for you using less than recommended gas.

Pure nonsense.

 

Google Cars.com and Knocking.

The knock-sensor provision is a mixed blessing. Without it, drivers would know something is wrong. With it, ignorance may be bliss. It’s perhaps more important than ever for motorists to respect the vehicle’s octane requirement.

 

I'll say it again... If you don't feel confident about your local gas station then go find another one. Worse case scenario is that you'll find a decent gas in 1:10. :p

I guess you are the uberexpert when it comes to knocking. There are a vast number of rotary enthusiast that will tell you they can "hear" knocking. A predetonating engine will also give you a drastic change in performance and therefore you will be able to "feel" it.

On your post about it being more of an imperative to use high octane gas because modern cars uses knock-sensors, it just not make sense... knock sensors allow engines to be more forgiving in running a wider range of octane rating compared to older engines not the other way around.


BTW, your link does not work.

 

Just go read the articles, wedge.

 

I got your link figured out... Here's a quote:

Many conditions in the engine itself can cause or promote combustion anomalies, but when all other factors are equal, gasoline with a higher octane rating is more resistant to knocking and pinging than a lower-octane fuel. So why doesn’t everyone just use high-octane gasoline? Because it is more expensive, and there is no advantage whatsoever in using premium gasoline in a car that doesn’t specifically require it. At one time, only premium gasoline included additives that prevent fuel-injector clogging; now, all grades are basically the same in this regard.

The above lends to my argument. But lets be objective and look at the next quote:

If your car requires high-octane gasoline and you habitually use regular gas because the engine exhibits no sign of knock, you’re outsmarting yourself. Most modern, computer-controlled engines include a knock sensor that detects knock and retards the ignition timing, causing the spark plugs to fire slightly later in the cycle. This typically prevents abnormal combustion and knock, which allows vehicles specified for premium fuel to run on lower-grade gasoline if it is all that’s available. While this removes the immediate hazard, it’s a bad idea to make a habit of running a vehicle on gasoline of lower-than-recommended octane. Retarding the spark causes a richer fuel/air mixture, which decreases fuel economy, increases emissions, causes the engine to run hotter, and reduces the longevity of both the engine itself and the catalytic converter. The money you save by pumping low-grade fuel into a car that demands higher octane is lost anyway, in decreased fuel economy and possibly gradual damage.

As you can see, the above statement talks about a scenario of running lower octane gasoline and causing predetonation in the engine, thereby the engine responds by retarding timing and supposedly cause damage to your engine. What is not mentioned though is the fact that the article talks about a scenario as it occurs on a piston-type engine and not a rotary. Although some of the effects are the same: timing retardation, lower performance, hotter exhaust are not disputed here, the richer AFR result in in lower economy, increased emission and lower engine life are just not true.

Before the flamewar starts, let me again say this: I do not condone usage of lower octane than what the engine will run optimally with. My argument is you use the lowest octane without causing engine knocking.

 

I voted "regular" because I figured the cheapest available gas would be classified as such. In Nebraska the cheapest is gasahol, because of tax incentives, (10% ethenol) but has an octane of 89 (midgrade?). Never a hint of ping and it's what the salesman put in after the sale.

 

The principles of internal combustion engines are the same whether the piston be round or triangular.


What backs up your claim here?


Ignorance truly is bliss.

 

What Causes Knocking and Pinging Knocking sounds like someone repeatedly rapping the engine with a hammer, and the quieter pinging resembles marbles being shaken inside a tin can. Knocking and pinging are actually symptoms of abnormal combustion in an engine, mostly heard when it is under heavier load, during acceleration as opposed to cruising. What causes abnormal combustion? Many things — some of them avoidable and some of them not, but the common solution to almost all of them is gasoline with a higher octane rating (see Demystifying Octane Ratings).

To understand what constitutes the abnormal combustion evidenced by knocking or pinging, you must comprehend normal combustion as it occurs in the cylinders of an internal-combustion engine (see How Vehicles Use Gas).

Knock, Knock . . . What’s There? That knocking or pinging sound that you hear under the hood is a common symptom of abnormal combustion, possibly caused by gasoline with too low an octane rating for the engine. The following illustrations show how the fuel/air mixture should properly combust in a cylinder (Figure 1) and how the mixture may self-ignite to cause knocking or pinging (Figure 2). Figure 1: Normal Fuel Combustion In normal combustion, the spark plug fires when the piston nears the top of its compression stroke, igniting the fuel/air mixture. Combustion progresses down through the fuel, exerting pressure on the moving piston. Figure 2: Abnormal Fuel Combustion Certain conditions, such as gasoline with an octane rating too low for the engine, cause the fuel/air mixture to self-ignite away from the spark plug. The two “flame fronts” literally collide and cause a knock or ping. During each cylinder’s compression stroke, the piston compresses the fuel/air mixture, and then the spark plug fires, inducing combustion (see Figure 1). The plug actually may spark before the piston is at the top of its travel in the compression stroke, but the objective is for the combustion to produce maximum pressure in the cylinder after the piston has begun moving back down during the power stroke. The amount of time it takes for the fuel/air mixture to combust completely is critical to efficient operation. In a four-cylinder engine that reaches speeds of 6,000 rpm, combustion occurs 12.5 times per second in each cylinder. With this frenetic pace, it may be difficult to think of combustion taking time at all, but the fuel/air mixture burns progressively from the spark plug to the surface of the piston. Combustion is a relatively slow process compared to an uncontrolled explosion, the latter of which occurs as much as 50 times faster.

Knocking and pinging result when the fuel/air mixture self-ignites somewhere else in the cylinder, away from the spark plug (see Figure 2). In these cases, the combustion from each source burns toward the other until their leading edges — called the flame fronts — literally collide, causing the ping or knock sound. Severe cases, termed detonation, result in a knock. Pinging is just a quieter symptom from a lesser instance of the same phenomenon.

This detonation is a little more like an explosion than controlled combustion. It occurs faster, burns hotter and is, at an instant, more forceful. The timing is all wrong, so what force the detonation does create is not converted into as much power in the engine’s crankshaft as would be generated by normal combustion. This is why engines that knock and ping during acceleration seem to bog down.

Detonation’s force is hardly benign. Its increased heat and power can crack spark plugs, melt or blow holes in pistons, crack cylinder heads and blow head gaskets — at least over time. So what causes the fuel/air mixture to self-ignite and lead to detonation? Either the gasoline is of too low an octane rating or some condition in the engine is creating excessive heat, or a combination of the two.

When the fuel/air mixture is compressed during the compression stroke, its temperature rises. To simplify a bit, a gasoline’s octane rating quantifies the temperature and pressure at which it will self-ignite. Higher octanes represent higher ignition points. The greater the engine’s compression ratio, the hotter the mixture in the cylinders gets during the compression stroke, thus, the higher the fuel octane required to prevent premature ignition.

The compression ratio reflects the engine cylinder’s volume with its piston at the bottom versus the top of its stroke. Current vehicle engines range from 8:1 to 10:1 and above. Higher compression ratios deliver higher output, all other things being equal. Ratios around 9:1 (10:1 in our cars) and above typically call for premium fuel of octane 91 to 94, though the engine’s compression ratio isn’t the only factor. Turbochargers and superchargers pre-compress the air that gets mixed with fuel, so the net pressure in the cylinders may be higher than the cylinder ratio alone would suggest.

High heat and compression within the cylinder is one potential source of self-ignition. That said, fuel seldom self-ignites before the spark plug fires. When the spark ignites the mixture, the resulting combustion increases pressure and heat within the cylinder — in some cases, enough to cause the fuel to self-ignite elsewhere, away from the plug, causing the rapid and uncontrolled burn from two fronts that causes knocking or pinging.

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Self-ignition also may occur if the engine’s ignition timing is over-advanced, meaning the spark plug fires too early in the compression stroke. Retarding the spark, making it fire later in the cycle, may prevent the knocking or pinging. Though this can solve the problem, it’s not always the best way to do so.

For example, many modern, computer-controlled engines automatically retard the spark if their knock sensor detects trouble. Engines that call for premium fuel can then run reasonably well on lower-octane fuel. Drivers need not worry about the damage associated with abnormal combustion if high-octane fuel isn’t available or if they get the occasional “bad” tank of gas. But retarded timing makes the fuel/air mixture richer; combustion is less complete, which lowers fuel economy, allows unburned hydrocarbons to pollute the air, fouls the catalytic converter allowing further pollution and causes the engine to run hotter, reducing its longevity. The knock-sensor provision is a mixed blessing. Without it, drivers would know something is wrong. With it, ignorance may be bliss. It’s perhaps more important than ever for motorists to respect the vehicle’s octane requirement.

Fuel also can self-ignite in an engine’s cylinders before the spark plug fires, a phenomenon called pre-ignition. The cause is some type of hot spot within the cylinder. For example, carbon deposits that may form on a piston head tend to retain heat, and they easily pre-ignite the fuel. The same is true if an exhaust valve builds up too much heat, if the whole engine overheats or if one installs the wrong kind of spark plug — a type that doesn’t dissipate heat efficiently enough.

Pre-ignition mostly causes pinging, but it can result in detonation and knock.

In addition to the above-mentioned causes, other conditions that can promote abnormal combustion include:

• too-lean fuel/air mixture (not enough gas for the air present)
• incorrect valve timing
• engine cooling system inadequacy
• faulty exhaust gas recirculation (EGR) valve
• high intake-air temperature
• driving in too high a gear
• excessive carbon deposits
• poor combustion chamber design/shape

To put it mildly, abnormal combustion should be avoided at all costs. Clearly, there are many potential causes. Higher-octane gasoline is an obvious solution, but the wiser and more economical approach is to stick to the vehicle’s maintenance schedule, see a mechanic if your car knocks, pings or suddenly loses fuel economy, and always use gasoline of the recommended octane.

 

bump (for the poll)

 

Not to sound like a dick but if you can't afford premium fuel for a sports car then you can't really afford the car...

 

BTW all these posts are a little dumb. Ethanol added at 10 and even 15% quantities in some places REDUCES engine knock.

Ethanol INCREASES octane rating - that is why companies use it. It's a cheap way to dilute down gasoline and keep the octane rating high. You can use more of the heavy stuff - mix in a little light stuff and get the desired octane ratings.

It's the same reason diesel (all be it - not clean diesel) is cheaper. It's a heavier less refined petroleum product. It also has a higher heating value as such and therefore contains more "energy" per defined mass.

Bottom line is that ethanol isn't bad for a rotary but 87 octane could be.

 

Wow...7 years thread revival!!!

Anyway, I use 89 octane Chevron with Techron. Never had a serious problem yet...yes, not yet.

 

Only 93 chevron in my tank...