Ole Spiff

Just finished my first tank with 87 regular after running 2 tanks of 91 premium. Mileage was 242.4 on 13.2 gallons, which equals 18.36 mpg which is a slight improvement over the 17.62 mpg I got with the 2 tanks of 91. This of course includes whatever amount of 91 was still in the tank after filling up with 87.

I just installed the RB CAI (I already have the REVi and RB exhaust) and the cooler screens this morning, and took it out for a spin. Car seems a bit quieter on acceleration but the pull all the way to 8k is noticeably better. It's now very strong all the way up the revs. At normal speeds below freeway speeds it seems about the same in performance. I was already a quarter of a tank into my second tank of 87 when I installed this so the mpg reading will not be entirely accurate for this second tank. I'll take the reading anyways and then take one with a full tank to see how the new CAI affects things. I used to average in the low 19's on gas mileage before when I was running 87, and the last reading was slightly less than 1 mpg worse than I usually got so I attribute that to the amount of 91 that was still in the tank when I filled up. The new reading will be partially affected by the CAI being installed, but even without it there's no question that with my particular car, it does run better and gets better mileage with 87 octane gas as opposed to 91 octane.

Oh just as a side note: the cooler screens look really cool! You can't see the one inside that protects the main radiator, but the oil cooler screens are nicely visible and give the car that extra, expensive "finished" look. I don't know why Mazda doesn't just include something like this to begin with; the cost would be negligible considering the overall cost of the car, they protect the coolers and look really nice.

TeamRX8

iTrader: (25)

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Kawi

For the BS folks:
http://en.wikipedia.org/wiki/Octane_rating

r0tor

iTrader: (1)

i run 89 octane because I also have found a good 1-2mpg difference in fuel economy over 93 octane. Performance changes are minimal unless i put 93 octane in and load a different map into my cz...

Sigma

Cars are smart these days. They learn to deal with what they're given. Give a car 87-octane forever and it'll like it and, in most cases, do just fine. Switch it up and give it 91 and it'll go "WTF!" pretty readily. And that may be what happened here. Resetting the NVRAM alleviates some of those issues, but a factory fresh default could have its' own problems with one fuel over another until it smooths itself out.

And the Renesis is hand-built, leaving quite a bit of room for tolerance. What compression ratio one might be running at is not necessarily the same that another is running at. And the quality of gas that one person is using is not as good as another person. So, while for many people using 87 might be fine, for others it might not be. And then there's the question of whether for those that it's working fine for, if it's really working fine, or what the potential long-term issues might be. It's alright to run a non-recommended fuel occassionally,but generally not the greatest idea to run one all the time.

Really it's all a crapshoot. Personally if I thought 87 was working fine, I'd just split the difference and use mid-grade. Give myself that extra little cushion.

Photic

The Shells here use Ethanol. Might be different per region.

When it's hot out I'll use 91, when it's not too bad I'll use 89 and when it's cold I use 87. I have also noticed better mileage running lower octane.
Personally I don't notice any power difference.

Nubo

Hold off on the BS calls. It's entirely possible for an 87 octane gas to have a higher specific energy density than a 91 octane. The energy content of the fuel does not necessarily correlate with the octane rating. While the power observations of the poster are subjective, it is very common for posters to report better mileage with 87 octane. This suggests to me a higher energy density for the fuel. The relationship between octane and power in the RX-8 comes into play IF knock is impending. But, IF the 87 fuel has more energy AND it is not knocking, then I can believe that you would feel better power with it. Sure, you might get more power by using very high octane and modifying the ignition timing, but on an unmodified engine it seems reasonable that the power could go either way.

Fanman

Sorry, I think this is more a case of wishful thinking. You want to keep using 87 in your car, so mentally you "notice a difference." It's like somebody puts on a cat back and claims that acceleration is better. We can't notice a .1-.2 sec. difference in acceleration. The human body just isn't that sensitive. This whole thread is pure conjecture. There have been several threads on this. Some come from the viewpoint that 91 does indeed give more hp (rather the engine will not retard the timing as it does on 87). Nobody has done a back to back dyno run with both levels of gasoline. Closest thing is an article I posted on a previous thread from Car & Driver where they tested cars rated from the manufacturers to run on 87, only gained a few hp from going to 91, but the cars rated for 91 lost between 10-12% of their power (it was a Saab turbo & BMW M3). On highly tuned engines (ala M3) the timing did in fact drop off. it really depends on how highly tuned you think our engines are.

Like they said it could possibly be that the engine has retarded the timing to run 87. The slight hesitation may be alleviated & also acceleration increase could possibly occur if you reset the ECU.

ECU's (especially ours) these days are sensitive enough that the car won't knock for 30 sec. before it retards timing. It may very well be unnoticeable by us if that happens. We literally accelerate for a sec., the ECU senses detonation & retards the timing before many of us even notice it. ECU's these days constantly advance/retard timing based on several factors, not like cars of yesteryear where you have to sit there and adjust timing yourself.

alcimedes

Just for kicks, I filled up on 87 this morning and reset the ECU when I'd parked. I'll run it through its warmup after work, and see if gas milage improves.

I have noticed that if I'm using gas with ethonal in it, I always get at least 2+ mpg worse than non ethonal fuel. Fortunately there's a list of gas stations that sell non oxy fuels, so I'm just careful where I fill up now.

On 91 I'd be lucky to break 18mpg. We'll see if two or three tanks of 87 offer different results. Won't be doing any unusual driving for the next few weeks, so although it's not scientific, I should at least get decent MPG numbers. Power on the other hand is a bit too subjective, so I may just leave that part out.

Psylence

If you are cursed as some of us are, then there is no way to get non-oxy fuels. It used to just be "winterized" fuel, but now we get it year round. You could always tell when they used to switch, because mileage would drop off noticeably on the oxyfuels over the winter.
This is pretty much all thats keeping me from running 89 or 87 in my car.. MTBE/Ethanol additives SUCK for mileage and power, and I don't wanna gamble on knock resistance with my '8.
Yeah.. these boutique blend fuels are yet another piece in the high gas price puzzle.

Umbra

There are to many factors to put any weight on the observations. Temperature variance alone could account for the differences. I HIGHLY doubt that you would ever get worse performance from 91 octane vs. 87 or 89.

Nubo

Not completely true. I pointed out the common experience of getting better mileage with 87 octane fuel. I believe that's significant and not conjecture since it is relatively simple to get an accurate mileage reading and there are enough posters who know how to do that.

Sure, it's conjecture to suggest that improved mileage might correlate with better power when only the fuel has changed, but I think it's a good educated guess.

I think it's also clear that a higher octane will result in better power only under certain circumstances. The difficulty lies in defining those circumstances and how often you'll encounter them, if at all.

Fanman

I was talking about power moreso than mileage. Sure if you really don't care about max power, or are just doing around town driving than stay with 87. Even a loss of 10% probably wouldn't be noticeable in those kind of coditions.

In terms of gas mileage, it is tough to measure, because of varying conditions. All of sudden that day did you hit traffic ? Was it stop & go ? Did you hit particular stoplights ? How was the weather ? Did you have the AC on vs. not ? etc. etc. This would hardly be considered as "normalized" or " "scientific." So in a way it is conjecture.

RotaryIT

It just seems like there are so many factors to consider that it makes it very difficult to narrow down what the causes are... My 04 with now 32k miles on P flash is getting me consistent 18-19mpg with mixed driving, as in my earlier post, this is up from previous flashes. I have never heard or felt any knock from my engine, even though there were instances when I fed it 87 or 89, instead of the usual 92-94. I did notice that it starts easier in winter with 87 or 89, at least moreso than with 92-94.

Considering the heat we have had this summer, I chose not to take any chances. The engine bay seems sooo hot after a relatively short freeway drive on a 90 degree day that I can imagine that the ECU wouldn't dial things down a bit as a result. The last thing I would do is put 87 into it if it might make it worse. I think the variances we see with mileage have more to do with the AFR's and driving style. I personally beat the hell out my car, and I will redline several times a day...I can't help it. I still use Castrol GTX 5w-20, and typically 92-94 octane fuel. I still notice every now and then that my car feels flat on hot days from 6-8k rpm (no A/C). My car runs the best when it is between 55-70 degrees...it idles smoother, and revs cleaner without hesitation or that occasional upshift hiccup. When it is very cold, it seems a little high strung, and in the high heat it feels starved. My car is all stock, but an intermittent CEL, my broken CD changer, and what I believe could be a dowl pin seal leak are really spoiling my fun.

I am running 89 in my car right now, and I will report any change in mileage. 92 was 2.99 a gallon, so it felt a tad better to pump the 89 for 2.89.

Ole Spiff

My "test" was not based on one day's driving. I generally fill up every 5-6 days so the driving conditions are for a whole week. As far as your allegation that I'm suffering from 'wishful thinking'... how many miles and how long have you been driving your car? Do you not "know" it? Every sound, every action/reaction to driving it gives you? I drive my car every day to work and back and have been driving it for over 2 years...I know my car well and how it "feels". I can tell immediately if something is "off" or there's a change in sound in ANYthing. I can tell if a tire has lost some air...the car feels different. I know when I press down on the gas pedal how the car reacts; I've done it thousands of times over the last 2 years. Maybe you don't drive your car that much to know what I'm talking about but I'll wager there are many drivers who do.

I'm telling you there's a difference in performance with my particular car. When running 91, there's a very slight hesitation when I press the gas pedal before the car accelerates. The mileage difference may be that I'm pushing farther on the pedal to try and get the same feel of acceleration I get with 87; that I could accept. When accelerating at full throttle the car feels "heavier" with 91, like it suddenly weighs another couple hundred pounds...it just doesn't pull as strong or as readily as it does with 87. I'm not "imagining" this. I know how my car feels when I accelerate onto the freeway; I do it everyday going to work and coming home.

I'm not a believer that dyno numbers are from god and are the indisputable indicators of things. They can give a relative indicator of results after making changes compared to starting results, but with today's sophisticated computerized cars the ECU's are probably detecting the difference between all wheels moving and only the rear wheels moving. The DSC system is constantly comparing the relative speeds of all 4 wheels and makes adjustments if one or more are too different in speed compared to the others. The real story is told in the real world with real acceleration on real road surfaces. I'm not all that far from California Motor Speedway so I may drop by and see if and when they allow street cars to do timed runs on the track. Now that I have everything installed (RB exhaust, REVi and CAI) I'm going to run a couple of tanks of 87 to clear out any 91 and then see if I can get some timed runs at the track. Once I have that I'll switch over to a few tanks of 91 to clear out any 87 and do some runs with 91 to see what the difference is if any. I'll make sure my car is setup the same for both runs; half a tank of gas and tire pressure equal all the way around. I have no control over weather but I'll note what the temps and conditions are for each set of runs.

kmg1186

Ole Spliff, you seem very knowledgable on this. I've been running 93 (no 91 here) since I got the car just because it is what it recommends. After reading everything you had to say, I'm interested in testing out a few tanks of 87. Say that it ends up pinging and knocking when I do test it out....will that have lasting damage to the engine like one poster before said it would?? I am curious to try 87 but if there is a chance for damage then I don't think I will. Your thoughts?

Btw, my car is also winning blue built in summer '03

Fanman

I've been driving my car, just about as long as you. The car isn't like it has been out for 20 years. I've had mine for the same 2 years you have. I drive it everyday, I notice little differences too.

I say "wishful thinking" because you see how there are people that comment on how they "feel" more power when they put on a cat back exhaust, or they feel more power when they add a air intake. Truth is the human body isn't sensitive enough to feel .1 or.2 sec. increase, it can't feel 3-5 hp on this car, yet here you have people that claim they know their cars. It may idle a bit smoother, but in terms of feeling things. The only time I ever felt a substantial difference is when I put on my RB suspension package & I put on my turbo. When I put on my air intake, hi-flow cat & pullies I wanted to feel a big difference (as I just spent a $1000) but truthfully I didn't. Has nothing to do with "not knowing" your car. has everything to do with the fact that the human body & our senses are just not that sensitive. Would you feel a few degrees of retardation of your timing, probably not. The dyno would, but you won't.

RPIRX-8

<rant>

There is no way in the world you can say with any scientific certainty that you can feel a degredation in power from switching fuel. That loss/gain of power would have to be in the 10-15hp range for you to feel it, just not possible. And before you say that I must not know my car or something, I've driven 48,000 miles in 20 months in my 8, i know my car too. The ONLY time I felt a difference in power is when I put in my CZ and tuned it properly. I guarantee your exhaust gave you more of a power gain than you say you get from switching from 91-87 octane.

I have no problem with you posting impressions just as long as you offer them as impressions and not state them as fact without numbers data to back them up. This is no different that aftermarket companies saying you will get 15hp from a cat-back exhaust, not.....gonna.....happen.

As for your mistrust of the dyno, are you saying your "impressions" are more accurate than a computer? Also, the DSC has no part in messing with a dyno run as you shut it off completely. The only factor that will affect it is if the coolant temp reaches 190 degrees F. When that happens the ECU basically shuts things down.

</rant>

Ole Spiff

I never said "with scientific certainty" anything. I always stated that with MY particular car I noticed the difference. As far as you and somebody else claiming as though it's a "scientific certainty" that there's "no way" I can feel any difference...on what do you base that? I'm only reporting what I CAN feel in my car; if you and somebody else can't feel that kind of difference, I have no explanation for that nor do I condemn you because you say you can't. So what. I'm reporting it because I DID notice the difference, otherwise I'd never have posted anything. It got my attention to the point that I made a few "do it yourself" tests to make sure it was NOT my imagination. I was not "looking" for these results when I discovered them. The first time I ran 87 was when I was on empty and pulled in to gas up, and the station was having trouble with their pumps and only had 87 available. I didn't think I had enough gas to go to another station at that time so I put a few gallons in so I could drive to another station. I noticed the difference in the idle; it was smoother....and in acceleration; it seemed more responsive. This was a surprise and I was not expecting it. I checked the manual and it said premium was "recommended" for best performance but it said nothing about not running 87. I checked the forum at that time and some were saying the car ran fine on 87 and that rotaries in particular seem to like 87; some even raced with it. So I started using 87 and have been ever since.

Since then I got the RB exhaust and REVi intake. After reading a few more current threads on octane, I decided after all this time to try a few tanks of 91 again to see if it made any difference. IT DOES and it's not my imagination. Is that scientific? Obviously not, nor do I or have I ever presented it as such. When I put a tank of 91 in I noticed that first off...idle was the same; didn't make any difference compared to 87. This is different than the first time I used 87 but since then I've had several ECU updates and of course the RB mods. Around town driving was about the same; where I noticed it was when flooring it to get on the freeway. There was that slight hesitation and the car felt like it was heavier; just not pulling like it normally does. I went through 2 tankfuls to make sure, then switched back to 87. The engine response came back and the car felt like its old self.

You can call that what you like but to declare it's "impossible" for me to notice any difference is not scientific either and just as much your "wishful thinking" as you claim my statements are. Supposedly humans can't hear anything above 18khz yet I know someone who is in the military and was tested and verified who can hear up to 20khz. He was complaining about high-pitched noises coming from some machinery which no one else could hear. He kept complaining so somebody brought out some equipment to "prove" to him he was wrong. They discovered sonic peaks just above 19k; they then tested him to see if he could actually hear frequencies that high and he could.

If you can't tell any difference in that small an amount of performance, then by all means go ahead and use 87 because you may, and it just might be scientifically provable... you just MAY notice a difference in your wallet. If 87 feels the same as 91 why throw away the extra money for 91? If I couldn't feel any difference I'd do it just for that reason alone. $40 fill-ups are getting old real quick.

Ole Spiff

Pinging and knocking would be a problem; however go easy the first time just to try it out. Let the car get down to the orange empty light, then put in 5 gallons and see what happens. If it starts to ping and knock, go right to the station and fill the rest up with 93. If it runs okay, fill up with 87 and test it out yourself. See if it performs any better; if it doesn't see if it performs any worse. If the performance seems about the same then start using 87 and save some money. If you're just not comfortable with doing this then stick with 93, or maybe try the next grade lower; out here it's 89. With gas prices the way they are, the savings start to add up. Here there's a 20 cent difference per gallon between 87 and 91. For 15 gallons that's $3.00 per tank more to use 91. If you can't feel any difference in performance, why throw away 3 bucks every tank? In my case my car actually likes 87 better but I have no idea why; it just does.

Excellent choice on color too :D

r0tor

iTrader: (1)

I've conducted experiments on this subject and read other reports from experiments - they always come back to a human can not feel anything less then a 5% difference in power without given some kind information to fire a placebo effect.

So in other words, your not going to truthfully feel anything less then ~10hp swings.

Nubo

The 91 runs may be influenced by the advantage of experience with your car at that track, which could easily make more of a difference than any changes in vehicle performance.


On the issue of not being able to "notice" power changes, keep in mind that there's more to this perception than peak hp, or hp at a particular rpm. In Ole Spiff's case he's noticing a difference in tip-in throttle response. If there is a lag with 91, it could certainly exceed that magical 5% difference in hp, for that fraction of a second, compared to the 87 with which he doesn't notice the lag. Steady-state power is another issue, but that initial throttle response (or lack of), plays a significant role in our perception of power and in the enjoyment of the driving experience.

Ole Spiff

I've never driven on this track before. Are you saying just doing a couple of runs with 87 would make that much of a difference a week later or more (don't know when they allow runs at the track) doing the 91 runs?

Ole Spiff

What I can feel is the difference in throttle response; I don't know how much actual power difference there is. The engine responds quicker with 87; just a fraction of a second so it isn't dramatic but it was enough to get my attention the first time it happened, and I noticed the difference again when I filled up with 91 because the car felt like it does when I have a passenger in it. Just not quite as responsive. I don't know how else to describe it.

Fanman

Then you either have to reset your PCM (it is set at the retarded timing level) or you may have an issue with the PCM. You have ran on 87 for a while now the PCM is set on a certain level of negative degrees of timing, so when you put in 91 it can now adjust the timing, but in your car there may be an issue as the PCM is telling the engine to adjust the timing (up), but it is not & that is why you are getting the throttle hesitation. I do not have a throttle hesitation issue that you have mentioned. Though my car is far from stock now. Never had the throttle hesitation. Honestly, I have never, never heard of a car running worse with higher octane fuel. People buy lower octane fuel for the price (and sometimes for the cold weather resistance) but in the 10+ track events that I have been to & meeting enthusiasts with all kinds of performance cars from Mustangs, to Corvettes, to Vipers, to Porsches, to Ferraris, to S2000's, NSX's & of course RX8's never have I heard from anybody that lower octane fuel helped performance. I have heard several times higher octane is better (some like the Mustang & Camaro there were no differences between 87 & higher octane)...performance increases because of it (highly tuned engines ala Porsche, NSX, S2000, etc.)...never go below recommended specs.