Quick_lude

I've read your other post explaining why you will do this and I guess for your situation 87 will do. But I don't understand the money saving motivation. Unless you drive 1000 miles a week, with the cheap cost of gas in the US, how much $ can you really save? $10 a week? Pittance. I just don't see the point of buying a sports car and detuning the engine right away with lower octane gas. Also don't forget the Renesis is a high compression engine and I'm sure the premium will work much better than the regular. To each his own I guess...

Efini 8

does the rx8 have a knock sensor?

wakeech

yes, IIRC of course.

8_wannabe

Let's take the emotion out of it and look at the facts. One, 87 octane (R+M)/2 falls within the recommended fuel by the manufacturer. Two, it does no harm to the car. Three, at most I will give up 2-3% of performance. Let's say I'm down to 240 HP. My question remains: So what? I just unloaded a Honda minivan with 140 HP and more curb weight than the 8. I am so far ahead of the power equation it isn't funny. Call me frugal, I don't buy stuff I don't want or need. I want the 8, but I don't need the extra gas bill and I will gain nothing from it. Why should I spend $30-40 more per month to satisfy a non-requirement? If you got $30-40 per month you'd like to fritter away I'll give you my PayPal address and you can set up a monthly deposit to my account. I'll add that to the money I'm saving on gas and I can do something with an extra $70-80 per month. I'm enjoying the car as-is and I don't need to spend extra money to demonstrate my macho. I didn't buy the appearance kit for the same reason. One could argue, why buy a $30,000 and not spend a little more on the appearance kit? My answer: It's all window dressing and I'm just not into that. The car, even with this minor reduction in performance, far exceeds my expectations in just about every way. I'm a happy camper.

elusiv

well said

blizz81

From that paragraph in the manual it sounds like the 8 is equipped with a knock sensor.

However I don't know if challenging the knock sensor to work its hardest is what you want to do all the time. First off, it's a reactionary device which will retard timing after it detects vibration from pre-detonation. Of course any actual long-term damage threat of constantly using 87 would depend on things like how sensitive the KS is, and how exactly the ECU remembers and changes ignition timing over a period of car shut-offs, etc.

Second off, my somewhat limited experience with researching knock sensors is that over time they can become a finicky sort, where it's hard to tell if they're actually failing in conjunction w/other sensors in the car...and can be hard to test due to location...so they can wrongfully retard performance sneakily with the driver (at that time with mileage on the car) having a lot of possible causes for the performance degredation. Of course, you can by-pass the sensor with a resistor mod, etc, but my point here is, making it work more often isn't going to help out on longevity.

But if you're always running 87 you won't notice anyway Personally I'm w/quicklude, I dunno why you'd buy a smooth, high-revving machine just to retard it to 85% or something.
Even if you're not driving around at 9k all day every day, that still seems like just as much a relative 'waste' as the $ spent on premium.

Sputnik

Bliz81 is right. You don't rely on the knock sensor. It wasn't designed as a tuning device, it was designed to help protect the engine in the event that something is wrong.

---jps

Quick_lude

Well said.

Quick_lude

Well said x 2.

Besides how do you know the loss of power is 3% only? Once again, if it was my high compression engine I would feed it the 92 octane and not have to worry about the knock sensor compensating for the lower octane.

neit_jnf

I don't remember the link but I posted this copy/paste in another thread:

commentator

Wow, all of these high powerd discussions of fuel octane make me giddy. I have a Honda Van that recommends premium and I will run whatever suits my fancy at the time I fill up. Nearly 70 thousand miles and no problems yet.

I must say that with the recent reports on this car being tested out on a dyno at a horsepower loss of 29% at the wheel and then they are recommending premium fuel I am thinking real hard about waiting for something a little bit better. If I am going to use premium, I should be getting better performance than this engine has so far demonstrated. I know it is early but I take delivery in late August and if the news doesn't get better I might spend 30 grand on something with a little more uuumph than this thing has. The difference in cost is not the issue, it is a car that as we find out more and more, just is not cutting it!!!

Jury is still out.

mantisflie

As you said commentator; "the jury is still out". It all depends on what you are looking for. I think this car is cutting it and exceeds my every desire. No car is perfect, but if you want a tight looking car with fairly good performance, the 8 has it. If you want mad power, then get and Evo or something (though I can't stand there looks). As for wannabe, hey man, it's your car. You put whatever you darn well please in it. I seriously doubt running it from time to time on a lower grade fuel is going to end the world or your car. Maybe give it premium once a month as a treat?! :D

8_wannabe

Folks, I know most of you ain't agonna like this, and I was almost convinced from your discussion of knock sensors and delayed timing on the octane issue. For those of you who took the time to really explain the mechanics (as opposed to responding from sheer emotion) I thank you. I am certainly better educated now than I was. Not ready to concede yet, I sent an inquiry to Mazda over the weekend about fuel grade for the 8. They just sent the following reply:

"Thank you for contacting Mazda North American Operations. I appreciate the opportunity to respond to you.

Mazda does recommend 91 octane gasoline for the 2004 Mazda RX-8. Because the rotary engine is a high performance engine it has been determined 91 octane gasoline will provide the best performance for this vehicle. 87 or 89 octane gasoline will not cause engine damage, but please keep in mind it will not provide the best performance and could cause engine noise. The type of gasoline you use should not affect the gas mileage the vehicle will get.

Again, thank you for contacting Mazda. It has been my pleasure to assist you. If for any reason this response has not completely
satisfied you, please feel free to reply to this message. You may also contact our Customer Assistance Center toll-free at 1-800-222-5500.

Please take a moment of your time to give us your opinion about our e-mail service. Click the link below to complete a brief, online
survey.

Regards,

Heather Drake
Specialist, Customer Assistance E-Business"

The subject line of this response had some kind of code: KMM113833V95443L0KM. So there you have it: No damage, no decrease in mileage, but yes a decrease in performance. I am gratified that they answered exactly what I asked. Second, I'm gonna take 'em up on it and post my results. After a another tank of 91, I will shift to 87 and let y'all know what I think. Of course, if I ever do hear "engine noise" I will go up a grade permanently.

My driving style is moderate. I am being told by local rotorheads to keep the RPMs low, around 4000, except of course when I want to kick butt. Shifting guidelines in the owners manual tend to confirm that. I've not yet had it above 7000, though some have said to redline it periodically (after break in) to clean out deposits. My point is, it's not in my nature to try to milk every last horsie from this engine so I really won't miss the power delta from lower octane. I know this is troubling to some of you, but it makes perfect sense to me. Happy cruising and I'll let you know how it all works out.

TJRX8

Well after affording the car I can't afford the gas...:p


The owner's manual says basically the same thing the above E-mail says.

8_wannabe

Thank you, TJRX8, for stating that so succinctly. I feel so validated! You're right, the owners manual does say that but if you read the sometimes emotional thread many people just don't want to believe. They got me concerned enough that I decided to go right to the source.

TJRX8

No problem. After going through 2 1/2 tanks of gas in less than 400 miles I'm thinking of going to Regular every now and then.

86rx7

If your knock sensor is ever used, i suspect you wont have a motor anymore. Rotarys greatly dislike detonation. I suspect mazda recommending 91 octane is to cover there backside. 89+ NA rx7's were 9.7 to 1 compression, and ran 87, i fail to see how a .3 increase in compression matters that much. Also its VERY hard to get a NA rotary to detonate. Such abuses as turning the crank angle sensor twards advance, untill the engine stumbles a little at idle, then backing it off a tad and leavign it like that have been documented without detonation in the second gens(not recommended).

Keshav

I see a thread like this on every car site I visit. Here's a good FAQ on Automotive Gasoline. Well worth the read.

Bottom line is, engines make the most power using the fuel for which they are tuned. My GSL-SE was tuned for 87 octane and it never made a single extra hp on 93 octane. Higher octanes do not magically carry extra power ... they simply make using a higher state of tune safer. This is one of the reasons why the JDM versions of many engines are more powerful: they are tuned for Japan's higher octane fuels.

Run your 8 on whatever you want. It won't blow up on 87 and it will get a bit more power from 91, because the ecu can take advantage of it. Good times.

Smoker

Hmmm... let me gather up the facts here and form up some sort of logic here:

-So if we choose to use lower grade fuel, the performance of the engine will decreases.

-Performance decreases because the ECU retards the engine timing to prevent knocks.

-The ECU retards the engine timing because it detects knocks in the engine through the engine knock sensor.

-The ECU would periodically go back and use the default optimal timing just in case you start using High Octane fuel once again for the engine.

So..... that means if we are using a lower grade of fuel, the ECU will use the Optimal Timing until it hear a *knock* in the engine, then it will slow down the timing. But then once in a while (don't know how often) it will just check to see if you are using its recommendated High Grade fuel again by setting the timing back its Optimal setting. Since we still using Lower grade fuel, engine will *knock* again, and the timing will be adjusted back to the slower setting again.

So what I'm seeing here is that, by using a lower grade fuel we are actually letting the engine continue to have *knocks* over and over again while the ECU periodically check to see if it can advance the engine to the most optimal timing ??!!!

That's nuts !!! We know knocking is bad for an engine and EXTREMELY bad for the Apex Seals in a Rotary Engine. No way in hell we should allow our engine to get "knock" around all the time and every time when we step on it.

I dunno man, but since Mazda acknowledge the fact that performance will decrease with a lower grade of fuel, and that means some knocking will occur when you drive it, that's good enough of a reason for us to stay away from lower grade fuel.

(if they say no difference in performace, then that's a different story)

Keshav

Allow me to quote the Mazda rep's email from earlier in the thread:

The threshold between optimum performance and pre-detonation is more than 4 octane points. How low do you have to go before you see knocking? We don't know. 87 is safe according to mazda, so don't be afraid of using it. Decide for yourself if the difference in performance is worth the $2.50 or so per fillup difference in price.

When the turbo kits arrive, even 91-93 won't be enough ... you'll see more threads on toluene and xylene and water injection. For now, enjoy the fact that your gf or wife could fill up your car with 87, against everything you've told her, and it wouldn't be the end of the world.

86rx7

I'd love to see a dyno of 87 vs 91, i bet there would be no differance. As far as im concerned the ecu has no way of telling what kind of gas you put in it. If it used knocks to tell, mazda would have LOTS of engines to replace real fast. Mazda woldnt do that again (learned from 3rd gen hopefully). I really think mazda is just covering there behind, knowing that if your putting 91 in it and you get a bad tank of gas, say 1-2 octane points lower then spec, the motor is still well withing its safty net, where as a bad tank of 87 could push it a little. They know that by saying 87 "reduces performance" that most people will use 91+, thus less chance of damage from a bad tank gas, thus less warrenties they have to make good on.

8_wannabe

Thanks, 86rx7. My feelings exactly. If you read back through this thread I've been the antagonist all along, but I don't have the technical background to put my feelings into words. You've captured what I suspected all along was the truth. Mazda is just covering their 6, as they say.

8_wannabe

Good story in USA Today entitled "Why use premium gas when regular will do?".

More fuel for the fire, so to speak.

Some extracts: "Engines designed for regular fuel don't improve on premium and sometimes run worse. And today's engines designed for premium run fine on regular, too, their makers say, though power declines slightly."

"'I personally use regular even though my owner's manual says you'll get better performance with premium,' says Lewis Gibbs, 45-year veteran at Chevron oil company. He's chairman of Technical Committee 7 on Fuels, part of the Society of Automotive Engineers (SAE) Fuels & Lubricants Council. Gibbs knows gas."

"The main advantage of premium is that it allows automakers to advertise a few more horsepower by designing and tuning engines to take advantage of premium's anti-knock properties. But auto engineers generally agree that if you use regular in a premium engine, the power loss is so slight, most drivers can't tell."

"Burning regular when the owner's manual specifies premium won't void the warranty, nor damage the engine, even the most finicky automakers say. "You're giving up perhaps just a little bit of performance that a customer wouldn't really even notice, it's so slight," says Furey. Some auto engineers estimate that power declines roughly 5%."

neit_jnf

So by using regular I'm losing aproximately 12 hp (5% of 247). To me that's a HUGE loss.

8_wannabe

That's why it's such a personal choice. To me, the 12 HP is utterly inconsequential.