Tayninh

yep! I use 91 Octane only and that 's the end of it.

quack_p

Towing something? What might you be towing?

Now that is a violation of the manual.

PoorCollegeKid

Haha, I guess that would be :p

I've always wondered how much low octane gas you're getting when you use one of those pumps that flows but 3 (or 4) different grades of gas through one hose/nozzle. It'd be pretty bad if you were paying an extra $0.20/gallon for few gallons of 87 just because the guy that went in front of you didn't need high test.

bowman

I'm just playing devil's advocate---If you follow the owner's manual to the letter, the only approved octane levels are 87-91 (91-96 RON). Using 93 octane is not allowed according to the owner's manual, and I'm sure using unapproved fuel would be grounds for denying warranty service.

MazdaspeedFeras,
It's not a simple comparison of the price of the fuel. I've read a lot of posts where people claim to get much better mpg with the lower octane fuel.

Feras

iTrader: (1)

fair enough i did a cost analysis on this in another thread
i doubt such a slgiht difference in combustion point in fuel is giving morethan 2mpg so tack on an additional 13-19 bucks a month in savings.

bowman

Great idea, although it's comparing only mpg.


12,000 miles
14mpg @ $2.01/gl---1722.86
18mpg @ $1.81/gl---1200

Difference---522.86-(43.57/month)

I freely admit that I pulled the mpg numbers out of the air.

DreRX8

All I know is that I had a horrible sound when I used 87 octane one time.

Grabitquick

Here's what the FTC says about octane ratings:

http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

Tayninh

Yes and the writers of the FTC writeup don't own RX-8s. I truly think the owners manual is correct. 91 Oct. I asked my dealer several times when I bought the car about the octane, and everyone at different times and not with any of the others said to only use the 91 and no lower. This came from the service dept manager as well as the sales staff. Yeah I know you can't trust the sales staff but I think they are telling the truth here. My mother-in-law owns a caddy. The manual says 91 oct and she uses 87 or lower. The engine knocks too and I can't convince her she is hurting the engine. She said the salesman told them they could run on lower octs!! I would not want to risk the rotary with a lower Oct but its your car and so you can find out what is what with the Octs.

quack_p

If there's one thing my buying experience proved to me, it's that the sales staff don't know squat about the technical aspects of the RX-8. Anyone who has spent 10 minutes browsing this forum knows more about the RX-8 than the average Mazda salesman. So no, I wouldn't trust a salesman's advice on how to take care of your car. My dealer put 87 octane in the car while I was standing next to him. I told him to stop, I'll forgo the free tank of gas.

Tayninh

Your dealer put in 87 oct in your RX? Wow! Yeah I known he sales staff don't know squat about these cars. I seen it first hand from three of them. I did believe the service manager and I only bought the car a few weeks ago so they know a little about these cars by now I hope. LOL

Grabitquick

Tayninh, I'm not opining on whether the FTC is right or not. I posted it only FYI. You'll get basically the same information from virtually every source you check. Bottom line is, if the engine doesn't knock or ping during normal driving with 87 octane, then you're not hurting it. If it does knock, then switch to a higher octane, but the engine still may need a tune-up at least as much as it might need premium fuel. If you use at least 87, then you're within the Mazda specs and you haven't raised any warranty issues.

rx7and8

My dealer said they put 87 octane in all their rx-8's (and I too watched him fill mine up with it). I didn't mind, since I didn't rev it past 4K on the first tank anyway. That was the last time she tasted 87 octane, however...

bryrx804

Well I fill up with 93 every tank..cant find 91 no such number it goes 87, 89, 92 or 93 depends on the station.. Cheaper stations like speeway have 92.. which I dont go to.. only fill up a MOBIL or Marthon, and last SHELL.

Last tank.. went 280miles on 11.435gal.. Topped it off and then went 308 miles to 12.631

Wooot.... have 11,004 miles on the car. Smooth as ever..

rx7and8

I can only find 91 max octane around here (California) even Shell V power is just advertising hype...

ptiemann

In the beginning I drove a few tanks of 91 octane, then always 87.

Never had a problem until on a road trip when I was forced to buy COSTCO gas. I took 87 Octane and I heard a pinging sound at speeds over 80 mph.

Lower speed - no ping.

Immediately filled up with 91 Chevron, no ping. After that I returned to 87 gas, but no Costco again.

Well, as I said, I was *forced* to fill Costco there. 15 miles on yellow light, at night, unknown area, no gas station in sight except that Costco one.

I don't think that good brand 87 octane rated gas damages the RX-8.

Fanman

Our gas here in Cali. is especially diluted an attempt to reduce emissions. That being said all the engine is doing is retarding the timing. It can run on 87 octane, but you are losing hp. Just because you are not hearing detonation doesn't mean it is not harming your engine. What you heard when you dropped the 87 octane in from costco was the engine basically retarded the timing as far as it could go, but the gas quality was still below that program so you got pinging. If you don't drive the car with any performance in mind then by all means 87 is fine, but then again I would question why you got this car in the first place. Maybe looks ?

ptiemann

@fanman:

Thanks for the explanation. That makes sense. I'd like to hear a quantification how much I lose. Because nobody here has said that yet. I personally could not feel a difference over several tank fills.

Thinking it through, I suppose I bought the car for the looks. Certainly not to race others. And I enjoy the feeling of control in curves at rather high speed. Would that be an 'extreme' situation requiring higher octane? I didn't think so.
I don't do 7000 rpm clutch drops. Maybe doing that one would get a 5.9s 0-60 run versus 6.4 or save a 0.2s on a quarter mile. I'm willing to learn though. So:

My question is: how much difference does 91 versus 87 really make? Quantify it in hp, or average difference in 0-60 or 1/4 mile runs.

I'm tired of hearing "you sacrifice hp", because I couldn't notice the difference in every-day driving. For the record, I can feel the difference between a 200 hp Firebird, a 228 hp Boxster and a 238 (?) hp RX-8.

Anyway, thanks for your good explanation but numbers please.

Fanman

ptiemann,

Does that mean you are offering up your car to measure the difference ? I for one would not want to put my car on a dyno, rev it to high heaven (what is usually done on a dyno), and get detonation, or drive it down a 1/4 mi. track as fast as I can revving the engine up, and getting detonation. I was at the Racing Beat even and watched as Jim Mederer ran the test bench through the revs. The PCM was constantly monitoring the revs and changing the timing. At high revs the PCM was retarding the timing already. With 87 octane it would just be more so to protect the engine. This is not strictly a Mazda thing. All hi-performance engines pretty much have this feature. On my 911 it had this. In this case I'm not sure if anybody would offer up their car to test for this. Any takers ? About the closest test I ever saw done was in a Car & Driver once.

Fanman

I found the test :

http://www.caranddriver.com/article....&page_number=1

Enjoy. Notice the Honda didn't gain like I had thought, but the Saab & BMW M3 loss quite a bit of hp. And the Mustang 7 Dodge Ram gained a few hp (That gain in the 1/4 mi. by the Mustang is unbeliavable !!!

bryrx804

well filled up again drove 323miles on 13.237gl..

Low Fly'n 8

When you have an engine with 10:1 compression that freely spins to 9000+ rpm, why would you ever think of NOT putting the highest octane you can find in it To save 2 bucks per tank To get an extra 10 miles out of a tank? Read the decal inside of the fuel door...Premium Recommended. I run nothing but 93 every tankful. Consistant 19+ mpg runnin it hard with A/C on around town. No problems whatsoever with 10K miles. Nuff said...

TJLack

[QUOTE=bryrx804]Well I fill up with 93 every tank..cant find 91 no such number it goes 87, 89, 92 or 93 depends on the station.. Cheaper stations like speeway have 92.. which I dont go to.. only fill up a MOBIL or Marthon, and last SHELL.


Sunoco is one that you seem to find 91 at consistently. Most others jump from 89 to 93...

ptiemann

@Fanman:

that C&D article was a good link, thx. The article mixes absolute numbers (hp gains) with % numbers there (Accord lost 2.6% of hp) The 2.6% that the Accord lost, correspond to ~6hp.


3 questions about that.

1) Terms. Detonation = pinging = knocking? Is that all the same?

2) Frequent detonation harms (= damage) the engine, I can see that. But if the engine timing is adjusted, then it cannot harm the engine - only lose 5 or 10 hp. Did I get that right? I don't consider losing hp 'harming' the engine.

3) How do we know that the RX-8 is calibrated for high-octane gas? Because of the sticker? If it's calibrated for regular gas, there's a good chance that it cannot advance timing beyond their nominal ideal setting when burning premium.

Nope, I'm not providing mine for a test :-)

-Peter

Fanman

1. Yes, pinging & knocking are the audible signs of detonation. However, you can have slight detonation without hearing knocking. If you experience a bog, or hesitation that could be detonation.

2. The engine timing can only go so far. Like you said with a batch of Costco 87 you ran into issues. 87 is the minimum standard of gasoline that you can put in the card. In essence you are at the far end of tolerance with where the PCM can retard your engine. Any lower, or a lower batch of 87 and you will possibly harm your engine. With 91, even if it is a slightly lower quality you will be OK.

3. According to the Owner's Manual the RX8 functions optimally with 91 octane fuel. From section 4-2 :

Your Mazda will perform best with fuel listed in the table :

Premium unleaded fuel - 91 or above octane rating

*You may use a regular unleaded fuel with an Octane Rating from 87 to 90 but this will slightly reduce performance.
* Fuel with a lower rating than 87 octance could cause the emission control system to lose effectiveness. It could also cause engine knocking and serious engine damage.

In this case our cars would function closer to the Saab & M3 as it is set to function with 91 Octane. It's PCM is set to function optimally with 91 (not just what is on the sticker, but the owner's manual as well). It's not a question of not being able to use 91 octane, as this is considered the nominal/regular/optimal setting of the PCM (If you use above 91 you will not see much improvement), not 87. The Mustang or Dodge Ram were set to function with 87 Octane as the normal standard for that vehicle (larger, lower compression engines)