My octane directly related to gas mileage debate!
 

Ok. Does anyone else notice the octane level directly effects your gas mileage. example. I have charted my gas mileage since day one when i bought my 8. I have had it for about 2 months now, and have put nearly 4000 miles on it thus far. I have never had a tank of gas( completely full ) yield over 211 miles....

When i use 87 octane i get roughly 200-210 miles to the tank and around 15 mpg. when i use 91 octance i get between 160-175 miles to the tank and no better then 13 mpg..

anyone else notice this trend?

AND DOES ANYONE KNOW HOW TO CLEAN THE AIR SENSOR?!?!

ive heard people talking about doing this d.i.y. can seriously increase your mpg.

If the car's new, I'd think the air sensor is already clean. That helps on old cars for sure.

I'd be afraid to use anything but premium, detonation is nobody's friend, even if it only happened a few times.

The benefits of premium gas is debateable, especially for the renesis. But from my experience regular is more efficient for fuel economy, but you lose some power at higher rpms. The Tech forum has some good discussions on gas, search for it.

HollywoodHall,

When was your 8 produced and what flash are you running? I just got 256 miles on my last tank (14 gallons premium). Virtually every shift was 5.5 - 6.5k and ~ 50% of the the time the motor was running over 3,750 rpm. Since "M" flash I have to virtually trash the car to get less than 17 mpg and if I baby it I can easily get into the 20s. My mileage has only improved over time. In 13,000 miles I have seen no difference in MPG between grades. My experience is premium has more power high in the revs and a slightly rougher idle.

If you change fuel grades, the computer should change mixture to avoid detonation, so that could explain an mpg difference. However, I think lower octane should give a richer fuel mixutre and lower mpg, if there is any noticable change.

--Dave.

I have the "m" flash. it did absolutely nothing for my gas mileage as far as an improvement. nor was it suppossed to improve gas consumption. i could not break 17 mpg if i drove 40 mph in 6th gear in a straight line, windows up, a/c/ off, coasting on a downhill for the entire tank.. its absolutely horrible.. it makes me want to cry sometimes because i spend $30.00 every 3 days in gas .....

Lower Octane will not give a richer mixture in normal driving. Lower Octane has more chemical energy in it so more potential for better milage, but more chance for detonation (ping) which reduces both milage and performance.

The rotary has no valves or aluminun piston to get damaged by detonation. In an extreem case the apex seal or even rotor housing or spark plug can get damaged, but not nearly as easily as a normal piston engine.

The rotary has a tendancy to detonate in the 5~7k rpm range. The knock sensor will pick up the vibration and reduce the timing (and-or richen the mixture) to stop this and performance will drop. Mazda even says in the owner's manual that you can use 87 to 93 octane fuel with reduced performance. It does not mention posible engine damage until you go below 87.

I use regular 89 and do notice a flat spot in power from 5~7.5K and then a surge in power above that. But I mainly drive below 5K so don't worry about it. If you like to race others use premiun. If you are concerned about milage use regular. I get 18 in town commuting and 20~24 on trips depending on how fast I cruise. If in steady trafic at 45mph+ I will shift to 6th. The rotory will pull OK for trafic even as low as 2500 rpm. If I need to accelerate faster or climb a hill I will jump down to at least 4th.

that's not true. under those conditions(actually you should keep it above 42mph in 6th) you would easily get 23-25 mpg. i and many other people have shown this.

There is so much conflicting information, it's hard to know what to believe.

TRZ, you claim that you will reliably knock with 87 over 5k-7k range causing a performance drop, and if you want to drive at those RPMs with good performance, you must use premium

However, I've seen 200 people post that 87 has more energy, a hotter flame front, and burns more efficiently in the rotary and actually can improve mpg in some cases. Count my dealer's entire service rep team into this list (See my new thread "Interesting Dealer Interaction" which is being discussed over now).

I'm not saying you're wrong, just that I'm confused with so much cross-information going around.

If my mpg was that bad my next investment would be a locking gas cap.

Its all relevant, I have not had less than 17.7 mpg since I Bought the car and I think I might get 19 this next tank. I am used to rotary motors so when I have 100 more hp than my 83 RX and still get better gas mileage, I don’t feel bad at all.

You're going to have to verify with rotarygod or one of the other gurus on here, but IIRC, NA rotaries produce more power with lower octane gas. This is because lower octane gas (by definition) sparks more easily, allowing the flame front to move more quickly through the combustion chamber--therefore, more of the injected fuel gets combusted, therefore, more power per cycle. Since the Renesis produces slightly more power at the same RPMs with low-octane gas, milage improves.

Mazda labels the RX-8 as "premium only" just to hedge its bets against detonation, for the sake of its warranty department. Speaking of the warranty department, you should NEVER run low-octane on a turbo or aggressively tuned NA rotary (a la CZ's mod). You will almost certainly get detonation, and that's BAD NEWS for a rotary engine.

Peace
policy

Unless you have forced induction, detonation is a NON-ISSUE on a rotary engine. If you encounter detonation at all with any octane, it is the result of VERY poor tuning. There is no excuse for this.

Nonturbo rotaries have always run better with lower octane fuel with no noticable difference in power between lower and higher octane fuels. There are people that claim they get more power out of higher octane. There are also people that claim they actually think their car is faster with a cone intake. We all know how I feel about that claim! If you enjoy spending more money on gas, go ahead you won't hurt anything. You also won't hurt anything with lower octane. It's your choice.

I'm not quite sure where the info about rotaries being more prone to detonation in the 5k-7K rpm range. That is pure fiction. The fact of the matter is that an engine (any engine!) is most likely to encounter detonation at it's torque peak. The Renesis torque peak happens to fall within this range centered at 5500 rpm. The RX-7's had a torque peak of 3500 rpm and this is where they were most prone to detonate if at all. This isn't exactly 5k-7k. The only time detonation is an issue is with forced induction or nitrous and even then only when poor tuning or extreme levels of gain are required. A higher octane does help resist detonation as well but that is it's biggest advantage. You don't have forced induction so what's the point? I have always used 87 octane in my naturally aspirated rotaries and always will.

-- i hear ya, i spend 30$ every 3 days or so also... it sucks
the car is a blast to drive but if i knew the gas milage was THIS BAD i prolly would have never bought it.

-- does anyone know why the renesis Sucks up SO much gas?? christ a 287 hp 350z gets better gas milage then us. kinda piss's me off.

it kinda makes me mad too man... i love the car. best car on the road in my bookx. wouldnt want to be behind the wheel of anything else.... the the gas is killing me. and without a doubt i feelno difference with the power on low octane gas. i hear no pinging either. and mileage is dfeinitly better. im on my second full tank of low octane and i am about to have the best tank ever. possibly 17 + mpg by the end of it!! im going for 240 miles on this tank.. compared to my typical 175-190 miles.

If you really want to cry, up here in my neck of the woods (weeds) gas prices are about $1 a litre. It run through about C$15 a day in gas. I just finished engine break-in ran through a tankful of premium right afterwards, and on a tankful of 87 octane and so far no perceptible diference in performance. Personally, I don't want to pay for premium gas to see a performance increase that I would never use.

I don't know for sure, cause I got the car yesterday, but the owners manual says any gas can be used, from 87 to 91 octane, without harming the engine. It says higher octane will give better performance, and you should not use lower than 87 or you could get emission control issues and engine knocking and serious engine damage.

Just looked it up and it says you can use 87-90 but recommends 91 or above.

OK Llathos and others. Detonation does usually occure at or near the torque peak, but not always. As air is compressed, either inside the engine or by a turbo charger, it gets hotter (ever hear about an intercooler?). Usually a engine is breathing the best and hense the highes pressure at or near the torque peak and that is why it is a peak. Inside a combustion chamber there can be local areas of higher pressure than the normal average. For the rotary the shape of the rotor and housing make higher pressures on the trailing side of the rotor and this difference will build with rotor speed. So the highest pressure can occur in a local area after the torque peak. Especially if the torque curve is flat like the rotory. Actually Racing Beat found: "there is a knock sensor for timing and two O2 sensors for fuel, but rotaries are typically insensitive to octane. There is some sensitivity to knock between 7500 and 8000 rpm, but that is about it." I only said 5 - 7K as I forgot the real number Jim Meeder gave me, but just looked it up and do notice a power difference in this range when I switch fuels. Usually I don't drive hard, so usually use regular.

As for fuel - A Otto Cycle engine's efficiency is determined by many things, but a key one is the COMPRESSION RATIO. As the compression ratio (actual pressure weather by turbo, ram, or normal intake) increases the potential power increases. But as the pressure increases the temperature also increases. So it can get hot enough to cause the fuel air to just explode. By using high octane fuel with less chemical energy you can run higher compression and the loss in power is less from the fuel than the gain from the compression!!!! So if a engine will run on low octain you should use it. I could go on with more engine combustion and power facts, but this should handle it for now.