87 octane
06-28-2008, 11:17 AM
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87 octane
Has anyone used 87 octane in their car since gas prices have gone up??? What does it do. Witha normal motor it might make the valves rattle, but since we dont have that what does it do to it?????
06-28-2008, 12:50 PM
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I have run 87 before and didn't really notice much difference. Maybe it ran and idled a little rougher.
Regardless, the difference between a tank of 87 and a tank of premium is only a few bucks, so IMO why even bother just to save a few bucks when Mazda recommends premium, as do performance cars in general.
Regardless, the difference between a tank of 87 and a tank of premium is only a few bucks, so IMO why even bother just to save a few bucks when Mazda recommends premium, as do performance cars in general.
06-28-2008, 01:13 PM
#5
I've told people the same thing when they say, "You use 93 octane!!!"
If you fill your tank with 13 gallons of gas, and the difference in price between 87 and 93 octane is $0.20, then you'll end up paying an extra $2.60!
Come on... you've already put it over $50... whats a couple more?
06-28-2008, 01:32 PM
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^ because I get about 10% better gas mileage without 93 octane so thats another $5 on top of the $2.50 every week... so after a year I have enough to actually buy the car something nice...
06-28-2008, 01:34 PM
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06-28-2008, 01:54 PM
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I will admit I have never compared my own MPG between each octane, but in any existing octane threads I have read on this board I haven't been convinced that a lower octane might lead to problems.
I have never seen a convincing arguement that using a lower octane doesn't lead to repair issues down the road. In other words, I have never been convinced that the savings in gas and the savings in MPG (ie: $5 on top of the $2.50 every week) won't be negated by costly repairs like an engine replacement.
Course......I'll also admit I haven't seen a convincing arguement that lowered octane actually leads to repair issues. I know that the car computer regulates the fuel mix and octane and compensates accordingly, but I am not 100% convinced.
06-28-2008, 03:00 PM
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I switched to 89 months ago
I get 20-21 mpg in mixed city / hwy driving. On pure highway with windows up, I get 24-25 mpg. 25 is the highest I've ever gotten and 17 is the worst. I noticed my car has gotten better gas mileage after it broke in. I have 13k on it now. My oil has been RP 5w20 since first oil change.
I must admit, I am concerned about carbon running on a lower than recommended octane rating but seriously, I can't really justify the sticking with 93. I'm staying with 89 purely because of cost.
I must admit, I am concerned about carbon running on a lower than recommended octane rating but seriously, I can't really justify the sticking with 93. I'm staying with 89 purely because of cost.
06-29-2008, 12:06 AM
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okay i was wondering....i might try it soon. Has anyone used the 89 with 15 percent ethanol on a gas mileage test??? Has anyone been ballsy enough to use E85? haha
07-08-2008, 01:01 PM
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I got 2 gal fo e85 in the tank right now. (filled up from a 1/4 tank threw in 2 gal e85 topped of the rest with 89) I also got some south dakota gass on a road trip that was 92 octane with 10% alk. everything seems fine.
btw what "parts" of the engine internals will get damaged? please enlighten me
btw what "parts" of the engine internals will get damaged? please enlighten me
07-08-2008, 01:19 PM
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07-08-2008, 01:36 PM
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I got 2 gal fo e85 in the tank right now. (filled up from a 1/4 tank threw in 2 gal e85 topped of the rest with 89) I also got some south dakota gass on a road trip that was 92 octane with 10% alk. everything seems fine.
btw what "parts" of the engine internals will get damaged? please enlighten me
btw what "parts" of the engine internals will get damaged? please enlighten me
Your rotor housings (among other things) are made of aluminum.
So if you want your gasoline slowly eating away your rotor housings, stick with e85. I'm sure you'll have low compression and a new engine in no time.
It is generally believed that 10% ethanol is dilute enough not to cause damage, and many states like Massachusetts and California already have 10% ethanol in their gasoline.
I stick with 89 octane, and give it a tank of 91 octane Vpower once a month. It's a nice balance between cheap and safe for my engine.
Last edited by Socket7; 07-08-2008 at 01:39 PM.
07-08-2008, 01:54 PM
#19
*sigh*
Thank you.
Ethanol is corrosive to rubber and aluminum (among other things)
Your rotor housings (among other things) are made of aluminum.
So if you want your gasoline slowly eating away your rotor housings, stick with e85. I'm sure you'll have low compression and a new engine in no time.
It is generally believed that 10% ethanol is dilute enough not to cause damage, and many states like Massachusetts and California already have 10% ethanol in their gasoline.
I stick with 89 octane, and give it a tank of 91 octane Vpower once a month. It's a nice balance between cheap and safe for my engine.
Your rotor housings (among other things) are made of aluminum.
So if you want your gasoline slowly eating away your rotor housings, stick with e85. I'm sure you'll have low compression and a new engine in no time.
It is generally believed that 10% ethanol is dilute enough not to cause damage, and many states like Massachusetts and California already have 10% ethanol in their gasoline.
I stick with 89 octane, and give it a tank of 91 octane Vpower once a month. It's a nice balance between cheap and safe for my engine.
07-08-2008, 02:04 PM
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dont thank me! thank wikipedia. They have all kinds of PDF's from the department of energy discussing the corrosive nature of alcohol based fuels, and also the fact that when water contaminated ethanol is burned, it creates some kind of acid which can increase wear rates as well.
E85 is not good stuff.
E85 is not good stuff.
07-08-2008, 02:41 PM
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What does it do? Octane is a measure of resistance to detonation. In order to smoothly deliver power, the air/fuel charge has to burn at a steady rate. This allows the force to be transferred into the moving parts at a rate that can be tolerated. In detonation, the fuel/air charge burns all at once. This is more like an explosion. It puts similar energy into the parts, but it does it in a much shorter amount of time. Think of the needle when you jump onto a scale instead of lightly stepping on it. What you think of as "valves rattling" is really the sound of those explosions. You'll hear similar noise if you get detonation in the rotary. Detonation can cause damage to the apex seals, bearings, and even deform or rupture the rotor itself.
That being said, I've changed my mind on octane for my RX-8 a couple of times. 87 is acceptable per the owners manual so that's what I started with. Cheaper and better mileage to boot. When I changed plugs the first time I saw a bit of cracking on the insulator. To be safe I increased octane. Later I went back. I've probably gone another 20,000 miles since going back to 87. No problems and plugs are fine. On a recent trip to Las Vegas I ran some 89 octane due to temperatures in excess of 110F.
07-08-2008, 02:55 PM
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8000 posts like that does not make you much better than nOOb 
I may have a low post count but I do this thing called reading and I found this.
And this
I may have a low post count but I do this thing called reading and I found this.
E85 could be a good thing for a rotary. The operatve word is COULD.
First regarding corrosion;
Ethanol has no corrosive properties to aluminum. Ethanol likes to absorb water and after it reaches about 2% water it begins to become an electrolyte. In an aluminum carb that has brass jets ethanol methanol, isopropyl etc that has absorbed 2% or more water turns the carb into a battery and at that point it is corrosive. Fortunately modern automobiles including the RX8 won't suffer from that.
Ethanol like most alcohols (and MBTE) does accelerate aging on some lower density plastics. Not to an appreciable amount on higher density stuff like found in most of the fuel system - except the hoses.
Octane is between 100-105, and under evaporation (vaporization) cools much more then gas alone. So not only does it need recalibrating the mixture, timing can be advanced a bunch.
You can still run the motor leaner and put more heat in the combustion chamber rather then the exhaust. On a piston motor it will consume 15-20% more. Then again a piston motor only runs about 1200° EGT where a rotary runs about 1500° as it has so much detonation it has to be timed late.
Larger injectors and a good piggyback ECU would be required. While E85 has less BTU's per gallon the extra advance allows increased efficiency.
The only real drawback is E85 isn't quite as good of a lubricant as gas is.
First regarding corrosion;
Ethanol has no corrosive properties to aluminum. Ethanol likes to absorb water and after it reaches about 2% water it begins to become an electrolyte. In an aluminum carb that has brass jets ethanol methanol, isopropyl etc that has absorbed 2% or more water turns the carb into a battery and at that point it is corrosive. Fortunately modern automobiles including the RX8 won't suffer from that.
Ethanol like most alcohols (and MBTE) does accelerate aging on some lower density plastics. Not to an appreciable amount on higher density stuff like found in most of the fuel system - except the hoses.
Octane is between 100-105, and under evaporation (vaporization) cools much more then gas alone. So not only does it need recalibrating the mixture, timing can be advanced a bunch.
You can still run the motor leaner and put more heat in the combustion chamber rather then the exhaust. On a piston motor it will consume 15-20% more. Then again a piston motor only runs about 1200° EGT where a rotary runs about 1500° as it has so much detonation it has to be timed late.
Larger injectors and a good piggyback ECU would be required. While E85 has less BTU's per gallon the extra advance allows increased efficiency.
The only real drawback is E85 isn't quite as good of a lubricant as gas is.
Alcohol is not going to hurt your engine. It is not going to eat any parts inside the engine as nothing inside the engine is aluminum. The rotor housings are but their wear surface is a machined sheet metal insert and it's coated anyways. Even if it was aluminum, you'd still be OK. There are lots of people that have made their rotaries run on various alcohols. You "may" and I'm not even sure this still applies, have issues with O-rings swelling. I've seen this on older rotaries but that was only with Methanol and not Ethanol. I'm not sure what the differences are with the Renesis but with much of our fuel today containing at least some Ethanol, I'd be shocked if they hadn't anticipated this.
The main thing you'll need is a way to reprogram the ecu. Luckily a wideband O2 sensor reads in lambda so it doesn't know what fuel you are using. This makes tuning easier as all you need to do is figure out what target a/f ratio you need to aim for and just richen it up across the board. You should be able to run more timing too but there are always exceptions and I'm not getting into all of that here. Basically just find a way to retune for it.
The main thing you'll need is a way to reprogram the ecu. Luckily a wideband O2 sensor reads in lambda so it doesn't know what fuel you are using. This makes tuning easier as all you need to do is figure out what target a/f ratio you need to aim for and just richen it up across the board. You should be able to run more timing too but there are always exceptions and I'm not getting into all of that here. Basically just find a way to retune for it.
Last edited by C Ya L8r; 07-08-2008 at 03:00 PM.
07-08-2008, 03:11 PM
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Since octane is a measurement of how hard the fuel is to ignite, it doesn't tell you anything directly about energy content. Depending on additives and manufacturing of the higher grade, it can end up with less total energy content. I can attest that my car gets better mileage out of mid grade than premium.