Running a fresh tank of 87 right now, so far so good.

 

It's pretty wild that this thread is so long. Yet varying observations.

When I picked up my car from the dealer with the full tank of gas, I noticed knocking during the 300+ mile drive home. I filled up with 87 and the knocking continued.

My next tank I got 89. Seems better. No knocking. Yet, I've noticed at idle it doesn't run as smooth and seems to miss once in a while. My old Rx-7 that had a 12a in it -- didn't seem to care what fuel you put in it ... it loved whatever you put in.

I'm gonna try a few tanks of 89 octane and see what happens. I'm curious also to see what impact all of this has on MPG. My drive back from the dealer on 87 octane was right on 24MPG. I don't know that I'm getting that now.

 

Great thread! Hope all 8 owners get a chance to read it so they can save some $$$ by using regular since the general impression here is that 87 octane isn't so bad after all.

About the MPG calculations, maybe you guys should agree on some sort of standardized method of calculating it to avoid discrepancies/disputes. I've found the most consistent way for me to calculate fuel efficiency was to fill it up and find out how much fuel I used since my last full tank. From my experience most fuel gauges aren't linear.

Excuse me if I'm being knit-picky :D

 

Even though most of you have already surpassed the 600mi. break-in, the engine could still be going through a break-in cycle and as such, compression will continue to improve over the next few thousand miles.

I just did a performance rebuild of a Cosmo 13B RE for my FD and gas mileage for the first 1000 miles was horrible. I wasn't expecting anything less given the fact it's a single turbo setup. I purposely left the primary runners small to help with low to mid rpm range and fuel consumption. The secondaries are as big as you can go without breaking through the water jacket. Oh wait, I did do that. Gotta love epoxy.

Anyway...at about 1800-2000 miles, I took a trip over to Ft. Lauderdale from Naples. Amazingly, I averaged 24mpg both ways with an average speed of 85mph. Not too shabby! Very little boost of course.

Another thing, the "butt dyno" (depending on how well yours is insulated) needs about a 10hp increase/decrease to notice a difference. If you fill her up with fuel and feel a difference, part of the difference can come from the fact that the fuel is cold, much colder than the fuel that's been circulating in the car for some time. Many circle track racers run "cool cans" which help lower the fuel's temp. The ECU in the FD (third gen RX7) and I'm sure the RX8, takes fuel temps into account.

We all may be in very different climates. I can attest (as can RodsterinFL) that the temps and humidity levels in SW Florida are a bit more extreme than Ohio. Assuming the RX8 has a knock sensor, cold, dense air (relative) would require a bit more octane than the stagnant stuff we breathe down here in the summer. Also, if the RX8 runs very little timing split, it won't benefit much from slower burning gasolines (higher octane) if it doesn't need it. Assuming best performance, the leading timing would be fairly advanced and necessitate the higher octane.

 

Okay another report -

87 octane is runnin fine.

I checked my mileage again this week and got 16.5 mpg with regular. I have also been reading the Wankel Rotary Engine Book by John Hege and The RX7 by Brian Long. - to get some background on this engine and car

There is also a recently posted thread about disabling the TCS and SCS system and a report of better gas mileage.

I am now convinced that the 13B REW engine we have is fine with regular. --less soot says a lot.

We may end up with a big surprise in MPG!!!!!

 

I concure, this engine is essentially the same as any other 13B except for the ports being on the side, and having larger ones.

I've been running regular for awhile now (1000Km almost) and no problems, NO sputtering EVER, on reving or on morning startup!

Less soot and MPG is the same or slightly better.

 

This is not entirely true...

The compression ratio on N/A 13B's (FC RX7) is 9.7:1
on the Turbo FC's and the FD it's 9.0:1 (FC for this case refers to 89-91 model, I don't have data on the previous version FC's)

The compression ratio for the Renesis in the '8 is 10.0:1

So it's got higher compression rotors than previous 13B's - intake port location is *not* the only difference.

It could be that they recommend higher octane gas as a bit of insurance against the higher compression rotors.

Although 10:1 vs. 9.7:1 isn't a whole heap more compression, and the NA FC's run just fine on 87 octane (and is what is recommended in the manual).

Simon.

 

With a 12A Rotary, years ago I was driving through Nebraska on I-80 and filled up with 87 octane. I recall that the pump advertised a higher amount of Ethanol then I later saw in the Colorado winter blend. I ended up getting 30 mpg on that Nebraska tank. Never saw that kind of mpg on a Rotary again. It always stuck with me due to the 30 mpg. I don't remember if it made it sound any different. Give a rotary gas and let it go!

 

So, what does this all mean? Are the gas companies paying Mazda money to tell us all to use 91 octane? Are we being scammed?

 

N/A rotary's have allways run better with a lower octane gasoline. Many racers have special low octane gasoline just for racing.

The reason is becuase of the rotarys natural thin combustion chamber. As a result that fuel needs to burn as quick as possible. And lower octane fuel burns faster than higher octane fuel.

The only reason why you would really want to use high octane fuel in a rotary is if you are running a turbo or a super charger. Higher octane fuels also have a higher ignition point, so that way the hot compressed air from the forced induction will not lead to detenation.

 

Or if the chamber compression was high enough to cause detonation. High compression motors (be it piston or rotary) are another reason why you might need to run higher octane gas - turbo/supercharging isn't the only reason.

Perhaps someone at Mazda thought the higher compression rotors in the Renesis might warrant it. Sounds like it's not neccessary though and even though the compression in the Rx-7 is higher than previous 13B motors, it's not enough to require premium gas.

Simon.

 

Well, after reading about this for awhile, I decided to put in my first tank of 87 octane. Only driven 2 miles on it so far, but so far there hasn't been any unusual noises or feeling from the car.

 

This may sound crazy, but what you could do is go to a gas station that has parking right next to the pumps. Fill it up, then push it into a parking spot. Go take a walk, do some shopping, come back, start it up, let it idol for a couple minutes. Then try and put some more gas in it. Then I guess you'll see if it eats a lot of gas on startup.

With my two 1st gen RX-7's, they loved gas. I got one down to 10 MPG one time on the way to get new tires (I had to make sure that there wasn't any tred left on the old ones!).

I really enjoy reading your comments. Very interesting!

 

I have only put 91 in car and will stay that way untill someone provides better proof that 87 is really beneficial

 

Right - despite my blatherings on the topic, I also continue run the recommended grade in the owners manual. I'm not willing to take the risk.

Simon.

 

The way I look at it, the owners manual says 87 is fine, but you may suffer from 'decreased performance'. Well, if I plan on going racing anytime, then I'll fill up on 93. Until then though, I'll be chugging on 87 octane and saving my money for something good

 

The benefit would be the price on the pump, what further proof do you need?

 

I tried the 87 octane right at the 2000 mile mark.

I have not experienced any hesitation or stuttering or anything that feels like detonation. Having once owned a '57 chevy pickup powered by a thrashy 10:1 383, I know what it feels like.

The power doesn't seem like its any more or less than 93 octane. But--it DOES seem "snappier". Slightly better throttle response, although the same power. And the owners manual does in fact say that you can use 87, it's just not "ideal" or whatever.

This is in Houston TX, by the way...plenty hot and humid; if there was a problem I think something would have happened by now.

And yes, for the record, I'm hoping that this means there's plenty of room left for super/turbo charging. Yeah, 10:1 shouldn't leave much room, but if it's running on 87....who knows.

 

all I'm saying is I know 87 is cheaper but the most important thing to me is maintaining my engine and I don't want to use 87 unless their is proof it wont hurt my car.I'll pay for 91 just for the piece of mind

 

I’m not sure what "proof" would consist of for you or even for me so I decided to experiment based on the fact that 87 was "allowed" in the owner’s manual even though it might not be the best. My car has about 2200 miles and I drive about 1/2 highway and 1/2 country roads where my average speed would be about 50mph . Let me give you the data on my most recent fill ups.

Octane -> MPG
91 ........ 21.12
91 ........ 20.28
93 ........ 19.67
93 ........ 20.37
91 ........ 21.84
87 ........ 20.57
87 ........ 21.12

Right now I have a tank of 93 in again just to verify that my similar mileage isn’t just because the engine’s continuing to break in. At this point I have a little more than a half tank to go and then I’m going back to 87.

I’d have to agree that with 87 the performance seems a bit snappier - definitely no problems at all with performance being worse – I think it’s better!

At this point I’m convinced that 87’s better so that’s what I’m going to be using from now on – especially with the prices the way they are I feel like I’m throwing money away!

 

My MPG is definitely improving. While I've only got 1K miles on my RX8, it's getting better and better.

Just filled up last night. Still using nothing but Premium. I'm not brand loyal when it comes to gas since I've never been able to tell the difference in brands with any car I've ever owned over the last 15 years (did find some differences before then, though). So, I always go to the places I can find the cheapest (usually Costco).

All city driving, I got 17.8 MPG....just about where the MSRP says I should be.

For comparison sake....I had a PT Cruiser GT Turbo. I never got over 15 MPG in it using premium...driving all highway. In city driving, it got about 13 MPG....THAT'S IN A 4CYL TURBO CAR. I would bet the SR-T owners are getting about the same.

 

So do any of you guys trying 87 octane see any difference in the amount of "soot" left on the tailpipes when you use it? Seems like that should improve at least a little, too.

 

I know that using 87 octane seemed to help Rodster but I haven't noticed much of a difference yet. There's definately still soot but as to whether there's less I can't tell yet. I'll have to wait until I finish my tank of 93 and then I'll go back to using 87 and clean the tips and keep an eye on it.

 

Since I'm taking the repurchase option, I've got nothing to lose so I started using 87 octane. No differences as far as I can tell. Still getting my 16.9 MPG for 80/20 highway/city driving. I have 2700 miles on the car, btw.

 

I hadn't thought to clean the tips and see if 87 octane reduces the soot...next time I wash my car, I'll give it a shot and report back. Actually, the thought that all the RX8's seem to universally have this soot on the tailpipes reaffirms my suspicion that the car is just running rich to begin with. Can't wait to hear from companies that start playing around with the fuel system/computer...