Just finished running 2 tanks of 91 octane Shell gas after 22 months of using 87 octane Shell. I have the RB exhaust and REVi intake. Performance-wise the car was not as responsive, especially when accelerating. My mileage was: first tank of 91 (after resetting the NVRAM) 234 miles on 13.4 gallons. Second tank 240 miles on 13.5 gallons. This averages out to 17.62 mpg with a combo of about 50/50 street and freeway driving. A/C is on all the time with the temps being consistently high 90's low 100's during this time. My average mileage using 87 octane is low 19's. Yesterday I filled up with 87 regular again, reset the NVRAM and I could tell the difference by the time I got to the freeway and ran it to 8k rpm in 3rd gear getting on the freeway. The car definitely has more power and response. With 91 there was a very slight (microsecond) hesitation between pushing the gas pedal and the car accelerating; with 87 there's no hesitation.

Don't know why my car likes 87 so much but there's no mistake about the results. And it's a good thing too since gas prices here are climbing to ridiculous levels. It's $2.75 a gallon for 87 and $2.95 for 91.

I went down to RB and picked up the CAI and a set of the screens (which are on sale; 10% off) for the oil coolers and radiator. While talking to the guy at the counter he said that when they were testing the CAI on the freeway, they put an air pressure meter inside the duct and it actually measured positive pressure at speeds above 65 mph. This is a very good thing. He said Jim is still working on the ECU mod but the performance gains aren't as much as they'd hoped so far. They're also concerned about cost. How much would people be willing to pay for an ECU mod? I told him probably quite a bit if it worked and provided usable power gains across the board.

I'm looking forward to installing the CAI and see what kind of difference it makes on gas mileage... I'll be using Shell 87 regular for the test. I'll report the results after a couple of tanks.

 

Interesting! I've been using Shell 87 octane for my car's entire 22 k miles. I had planned on doing a test with 91 octane, but fuel prices kept going higher and higher and my test never happened. I usually average around 19 mpg in 60hiway/40city driving.

 

This may be a "stupid" question, sorry, but I have to ask. Isn't the car design to run with 91 octanes? I was under the impression that not running the car with 91 could do something to it.

 

If detonation is detected the ECU will retard ignition timing. This will reduce performance slightly. If no detonation is detected then nothing changes

 

too bad you didn't reset the computer

 

This sounds much like the butt dyno talking. There are many reasons you could be feeling the things you are, this isn't a valid double blind test either. Can you also say that you drove the car exactly the same for the tank of 87 vs the tank of 91?

Also, and anyone can chime in on this? Are there any long term issues with running dirtier fuel?

 

Octane rating has NOTHING to do with fuel being dirty or clean. It a measurement of how the additives alter the way the fuel burns. Higher octane fuel burns slower, and therefore is less likely to detonate early, before the spark fires.

 

Choose your descriptor I guess. My point is I am not really believing based on 2 tanks of gas and no data that 87 octane heeds better performance than 91.

I'm going to research this nonetheless. I do believe that 87 octane leaves behind "dirt" or more "dirt" after a burn. Maybe this is just because it doesn't burn as cleanly.

 

I drive my car everyday, and have over 32k miles on it. I know it very well; any new sound or change in anything I can detect immediately. I can't speak for any other car other than my own, but there's no mistake about the change in performance. I drive the same route everyday to work. I always run the rpm's up to 8k in 3rd gear when I get on the freeway...it's a short on-ramp with very bad choppy pavement on the turn to get on it because all the big-rigs that use it mess up the road surface. The ruts in the pavement are at least 6 inches or so deep, so you have to drive through them almost like going over a speed bump...very slow....followed by a short ramp into freeway traffic that's usually moving at least 70mph. I usually hit 3rd by the time I'm at the end of the ramp and I keep it there to 8k....it get's me up to speed quickly, it's fun, sounds great with my RB exhaust and intake, and it helps to keep carbon from building up in the engine. I do this every day and have been for 2 years with my RX8. I know the "feel" of the performance of the car through all this.

I'm about 3/4 of the way through the first tank of 87 after the 2 tanks of 91 and I can already tell I'm going to get better mileage than I did with the tanks of 91. With the tanks of 91 at the halfway mark on the fuel gauge I had approx 120 miles on the odometer. With this first tank of 87 I was at 139 miles at the halfway point. All of this while driving exactly the same way I always do, and it's still hot, I still leave the A/C on just like with the other 2 tanks of 91. I'm going to finish out this tank to get a mileage reading, then install the RB CAI and see what difference that makes on the next tank of 87.

As for what you're saying about 87 burning "dirty" that's simply not true. In fact it's just the opposite. 87 actually burns cleaner because it ignites earlier and burns more completely than 91, thereby leaving LESS carbon deposits inside the engine. The only reason an engine needs 91 is if due to high compression the gas might ignite too early and cause pinging or knocking in the engine as a result. 91 doesn't ignite as easily as 87 and therefore "burns" for less time than 87 does, thereby increasing the possibility that it will leave MORE carbon deposits in the engine.

If you want to see what running 91 in a Renesis does, do a search for a thread about porting an RX8...the name of the guy who's engine was ported is Snoochie. The guy doing the port attached some pix that showed the carbon deposits on the rotor when he took apart the engine to port it.

 

Have You Seen The Gas Prices Lately??

with that said... many have had better mileage with 87, no doubt. And some have had loss of performance and pinging with 87. It varies...

 

It's "recommended" for best performance, however a rotary isn't quite the same as a piston engine in this regard. Rotaries can use regular octane effectively, although this seems to depend on how close to spec the engine is. IF the RX8's ECU detects ping or early detonation, it will "lower" the performance a slight amount to compensate. If it doesn't, then the engine will run same as it does with 91. The only advantage 91 might have is that it doesn't ignite quite as easily as 87 so if there's a hotspot in the chamber (such as a carbon buildup point that gets hot enough to ignite a compressed mix of gas) it won't ignite as quickly which usually gives the piston/rotor time to get where it's supposed to be so the sparkplug can ignite the mix. The higher the compression in the chamber, the more critical this issue becomes. Race cars run high octane because they run the highest compressions possible.

Due to production variances there seems to be some RX8's that can't run 87; they do in fact ping or knock and have to run 91. These same engines also probably get worse gas mileage than normal. The rotary doesn't seem to be very forgiving on tolerances. If you have one of those engines, you have to run 91 (or 93 if you're back east). If you try a tank of 87 and the car seems fine, then you have one that can run 87....my recommendation is try it and see if it works. If it does, then you can save enough on one tank to get a free gallon every fillup; over the long haul, this is going to add up, especially now that gas prices are climbing around the $3 per gallon mark. My car actually runs better with 87 than 91 but I don't know how typical my engine is....I may have one of the lucky ones that's very close to blueprint specs, although I don't think it is since others on the forum talk about getting better mileage than I do...I normally average in the 19's. For whatever the reason, my car likes 87 and runs very well with it. I can tell you that gas brand makes more of a difference than octane; I use Shell 87 exclusively. In a pinch I'll use Chevron or Texaco 3rd choice. All other brands I tried were nowhere near as good no matter what octane rating.

 

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previous naturally aspirated rotary engines have been proven to run better with lower octane due to the faster burn. The long and flat combustion chamber is not ideal for good combustion and the faster burning fuel suits better for this application.

So why Mazda recommends premium for the Renesis? Higher compression is somewhat more prone to detonation plus the PCM will advance timing a lot to try and get a better combustion. With lower octane the timing doesn't have to be so advanced and too much advance can actually decrease performance.

When the PCM detects preignition or detonation it will retard the timing to a safer level but since lower octane burns faster the performance change may even be unnoticeable.

About cleanliness of gas, octane rating has nothing to do with it. Different brands, different set of additives, etc will make a difference but not octane.

 

I mainly run 93 but haven't noticed a difference in performance between 87-93. Oddly, my car gets better gas mileage with 87.

A local place has 100 octane. With the 8 it sounds like it wouldn't offer any advantage.

 

... switching to 87 Petro Canada, will let y'all know how it fares...

 

Somebody dynoed 197whp with 100 octane, it will allow even more timing advance

 

Is this what you're offering? Or was it an attempt to claim what I was saying is this? On what do you base this grand offering of yours?

 

I am also calling BS

 

I'm always tempted to try a tank of 87, but I always chicken out when I realize it only takes one ping or knock to potentially damage the rotary engine severely. Plus it gets pretty hot and humid where I live. Gas prices are rising by 3 cents practically every week now...it's up to $2.91 for Shell premium.

Speaking of Shell, I notice at the Shell stations here that it seems like only the premium now has V-power, whereas I think before, all grades of their gas had the V-power label across it. Now only the premium has the label, in the form of a big red piece of foam tied to the pump hose. This is opposed to Chevron which still plainly has Techron across all their grades of gas. Is this happening with Shells elsewhere too?

 

Here's my BS for those that choose to call it such --
I ran 91 Shell for the 1st 10K miles. The last 21K miles have been either 87 Shell or Texaco.

That even includes a weekend tracking the car at Road Atlanta, when I was forced to use 87 Chevron, because of availability. I do not think others running at the track would say my car was slow.

My (MY) RX-8 runs wonderful on 87 octane, and gets 1.5 mpg better mileage. Performance difference is not noticable.

Put the more expensive gas in, if your car needs it, but mine does not.

Thanks Ole Spiff for the thread.

 

Yeah I've noticed that here too, it seems only 91 has the V-Power additive. I'm guessing on this but it may be because only 91 needs the extra detergent to remove the carbon deposits that can build up due to unburned fuel. If you checked for the thread on Snoochie's RX8 port and saw the amount of carbon on the rotors, you'd be amazed at how much can build up in a relatively short time. The V-Power additive probably helps to remove/prevent that build-up.

To those that say "bs" fine...throw your money into your gas tank. I have no personal interest or anything to gain by posting this info other than providing it to those who are interested and wish to see for themselves. I'm not the only one saying this about 87 regular; I'm just confirming that it's true for me too. Try it or don't try it. Getting an extra gallon for the same money each time I fill up with 87 instead of 91 suits me just fine.

 

You're welcome and I noted your car with interest....I too have a blue GT picked up on 7/21/03. Must be the blue paint that makes our cars so special :D

 

I wonder if the fuel you are using contains Ethanol. My understanding is that Ethanol is used to reduce pollution, however, it is diluting the pure gasoline. Would the RX8 ECU recognize this regardless of the octane number, and add more fuel? How many of you know whether or not you there is Ethanol use in your area? I know in Michigan, we do...I have hunted for some stations that don't use it, but all I could find is a station that just didn't have a sticker saying 10% ethanol. No guarantee.

Lastly, I have run several different brands of gas, and I claimed to drive the same way. I realized that when driving in the city, there is no way to say I drive the same. The number of times you get caught by the stoplight, slow down for a granny driver, or simply hold in a lower gear longer than you usually do without even noticing it. I made my best measurements on long drives on the interstate, and found that my PCM Flash version made more difference in my mileage than most things.

M flash: 15/19 Average: 15.52
N flash: 16/24 Average: 17.26
P flash: 18/27 Average: 18.94

Averages were over a two month period with each flash.

When I tried 87 in the summer, I had very poor mileage, never better that 170 per tank or 13.5 gallons. On 92/93, it was always 200-205(M flash). I have changed to use 92/93 in the summer, and 87/89 in the winter to improve starting performance in the cold.

 

Ethanol is common in Bama -- with the second tier gas stations. The pumps by law have to be marked stating ethanol use.

The top tier (Shell / Texaco / Chevron) gas stations don't use ethanol.

 

Interesting observations - so many can use 87 and be fine, no detonation. To me it sounds like those engines have lower compression - hence no detonation that would retard ignition and give power losses...
In contrast, there are cars that ping using 87, under high load and RPM... definately sounds like higher compression issue.