Nubo

I know in general about speed of burn, anti-knock properties, etc. But to tell the truth I have no idea of what the scale represents. I've read about the test engine and the ratio of heptane to iso-octane.... etc. But what I'd really like to know is - just how different is 87 vs. 91 octane? I mean, they're only 4 points apart on a scale that seems to be calibrated to 100. That seems like a pretty tiny difference. How much of an increase in compression ratio can 91 octane support, everything else being equal?

I bought 65 octane gas once in the mountains (Montana, I think). My little 4-banger didn't have any problems with it.

zoom44

chart on this page for older cars http://www.cloverleaf-auto.com/fuel.htm

i like this "To get an idea of what octane to use for what engine here is a little chart, now remember engine design, driving habits, engine state all effect the octane requirements. And yes you may find you have no ping on one company's fuel and do on another of the same rating. how and what the additive package is makes a huge difference."

zoom44

nice chart on this pagethat is newer

http://images.google.com/imgres?imgu...ial_s%26sa%3DN

alnielsen

iTrader: (4)

Wow, that is interesting. I work at BP. I am a computer guy, not a chemist. But now I understand some of the terms and how they work in gasoline formulation.

army_rx8

interesting indeed :D

ZoomZoomH

iTrader: (1)

so, in effect, all grades of pump gas have approximately the same level of potential chemical energy, it is just that higher octane gasoline allows for higher compression, which allows more fuel used during combustion, which results in more energy released during combustion because there's more of air AND fuel?

TRZ750

Don't feel strange as there are many more variables than just straight 87-91 numbers. Actually the common number, like 91 is the average between two different engineering methods to determine how a fuel will react in an engine. R+m/2 is Reasearch octane + Motor octane number divided by 2 for the average as the engineers could not agree on one method. It happens that certain engine designs are more sensitive to one or the other. Like the 2 stroke piston motor is usually more sensitive to the motor number. So a 90 that is 85 motor+95 research would be less likely to detonate than a 90 that is 84+96. The difference between the two is refered to as the sensitivity of the fuel (I think that is the name used).

Both Research and Motor numbers are determined by actually running the sample fuel through a special test engine with a variable compression cylinder head. Just different conditions.

I do not know if the rotary is more or less sensitive to the motor number, like the 2 stroke. The point of detonation in a engine is very complex as there can be hot spots, localized high pressure areas (like the rear of the rotor), variations in the fuel mixture within the combustion chamber and between cylinders or rotors, etc.

Bottom line is for one RX8 with two samples of fuel the 91 may be best, but for another RX8 with different fuel the 87 could be best. Especially when you are comparing fuel in different countries or even west to east coast USA.

Razz1

Interesting value. Thanks for the chart.

Fanman

Do you believe our cars engine is closer to that of an M3 or a Mustang GT ? Article in Car and Driver that talked about gas octane effect on "regular" engines (Mustang & Accord), vs. highly tuned engine (BMW M3), vs. Turbo engine (Saab).

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

Mzdarx8

Wow, 10.0 compression renesis at 87 octane will not have some detonation?

Maybe Mazda should have put 87 oct or higher on the gas cap?

You want to race use 93 octane fuel or higher!

Just my 2 cents!!

bryrx804

also watch out for ethonal in gas. 87 has a higher % say 10 then 93 has 6%

2 years ago gas stations used to say if they had ethonal gas or not now they dont have to tell you.

bxb40

I think the Renesis is a sensitive engine. While breaking the engine in, I could use any grade I wanted - but preferred the 87. At about 4000 miles, I could not use 87 without ping at high RPM and load (and we HAVE to redline-it once per drive, the 4AT crowd, to prevent carbon build-up). I now use only 89 without any issues. Also, I always used only Shell, so that was not a variable in my "study."

PS no difference in between 89 and 91 that I can notice... and my spark plugs looked good at 16000 miles, as claimed by the dealer.

ddoroslovac

F.Y.I.
Here are some more facts for this disscussion. I am leaning on the terms that 87 vrs. 91 is virtually the same gas. Next time I go to the pump I know what grade to put in now.

These facts from a friend who found it on a site somwhere>>>>>>

The Rx-8 requires gasoline with an octane rating of 91 or higher as specified by Mazda. 89 octane may be used at the cost of engine performance for short periods but this should be considered a temporary measure only. Octane ratings higher than 91 will not offer any benefit to a stock engine/ECU.

The required octane level is determined by the ECU programming. When using gasoline with a rating lower than 91 the ECU will retard timing to prevent detonation. A rating higher than 91 will only cost more. The ECU will not advance timing to get an additional performance benefit from octane richer gas.

Gasoline is a mixture of 2 hydrocarbons (octane and heptane). The proportion of octane to heptane is indicated the the octane rating. It is not a percentage. An octane molecule contains 8 carbon atoms, heptane contains 7 carbon atoms.

All gas with the exception of Chevron is pumped from a refinery to a transfer station via a national petroleum pipeline system. Gas is fed into the system at one end and removed at the other. Shell's refinery in Corpus Christy, TX pumps 10000 gallons in and they take 10000 gallons out on Akron, Ohio. It's NOT THE SAME GAS! What they pump out may have been put in there by a different oil company. It's like a gas bank. You deposit $1000 and later withdraw $1000. You don't get the same bills you put in.

The pipeline system was established when Standard Oil was broken up and ensures there is a supply of gas wherever it is needed in the country.

At the transfer station each company adds their own additive mix (octane boosters, detergents, oxigenators) to meet local pollution standards and regulations. The Shell you buy in Los Angeles is not the same product as you would buy in Detroit.

All gas is basically the same. It comes out of the same hole (pipe) and has most of the same additives within the same locality. Chevron is the exception as they use their own system for distribution. That doesn't make it any better.”

NMShinka

So what octane is everyone using and with what result? I've been using 91 only because I felt it was worth it from a performance standpoint; but now I'm not so sure.

Oh, and for the record, ddorosloviac's statement about all gas being the same is not true. Giant is a refinery serving the southwest and it's process, like the Canadian process, is different. In fact, Detroit, most notably Ford, has now convinced Canada to alter their process due to long term detrimental affects on the Cats. I don't think Giant has bowed down as yet and Ford owners are advised not to use Giant.

Cheers!

auzoom

Well I originally used 98 RON fuel but changed to 91 RON (first 3000Kms were at 98 and have done 1000 on 91 now). The first few tanks of 91 were appaling performance, but eventually it feels more responsive than the 98. The other week I tried a tank of 98 again and lo and behold, it felt like I was driving a 1.3 litre Piston engine. It has taken a tank and a half of 91 before the car felt "Normal" again.

Overall I think the 91 feels better to drive, I get better milage than the 98 and its 10c/litre cheaper.

Andrew

ZZ8

Nice write up ddoroslovac.

What's not clear to me is how Shell (to name one) could be advertising "Innovation & Technology", "V-Power (Actively cleans for better performance)", etc. if all the gasoline (as you mentioned) comes out from the same pipe....

Raz0rama

Its all in the additives: there's a special blend of fuel additives that each distributor adds to their fuel to separate it from the crowd.

Ole Spiff

Interesting. I've been using Shell 87 regular exclusively for almost 2 years now. Car runs just fine with no pinging or knocking. I have the RB intake and exhaust for mods, otherwise it's stock. I'm here in southern California so maybe that makes a difference compared to other areas of the country as far as gas additives. Not too long ago we had an interesting thread in the forum from a member named Snoochie who had his engine ported as an experiment. When they opened the engine the rotor had a surprising amount of carbon deposits on it; he ran 91 and the deposits were the results of unburned fuel. The guy who did the porting said a lower octane gas burns quicker so it would produce less deposit build-up.

I've had nothing but positive results with 87 Shell in my car and the $.20 per gallon savings compared to 91 octane for the last 21 months has probably just about paid for my Racing Beat REVi intake mod. I fill up once a week with a few extra tanks for trips other than normal work/home/around town.

Vertigo-1

iTrader: (3)

Ole Spiff: what are the ambient temperatures in your area? Also do you drive aggressively and rev high often on 87?

Here in Hawaii it gets to around 85 degrees or so during the day, and that's humid, wet and nasty heat. With gas prices increasing practically every week though ($2.80 now for premium) I'm now seriously starting to debate taking a chance and trying 87. I did try a tank of 89 before during the winter here where temps drop to around the low 70s, and noticeably felt a lack of power after 5000 RPMs...so my 8 appears to be one that would be affected negatively by lower octane. Given that, with hotter weather, I'm not so sure I should dare try 87, even if I never revved it hard (and I do pop it to redline whenever I can).

Rick

Do not buy gas from Cheveron in North Bend, WA! Don't ask me why but today I filled up 3/4 tank of 87 from them and my 8 started knocking every time it down shifted. I noticed it was also knocking while just cruising down I 82 in the Yakima valley. I filled up about 1/2 tank in Prosser w/Shell 92 and the knock disappered within 5 min. I WILL NEVER FILL MY 8 W/ 87 AGAIN!
BTW, I have an 04 8A/T

Rotary Rasp

iTrader: (2)

arco gas rules!

87 octane no problem

Ole Spiff

Ambient temps here vary. In summer it's around 90's and can get over 100. Heat is generally dry heat. I have the MT 6 so maybe it's different with automatics. I rev mine to redline all the time and drive it to have fun. Give it a tank or two for the ECU to adjust to the different fuel grade and see if it runs okay. With gas prices climbing it's starting to get pricey to fill up with premium.

swoope

iTrader: (5)

if it helps any i am in orlando fl. have been running 87 since i got it. on occastion i have put 91 in it, yes it is a bit more peppy. but for driving on the street day in day out it is not worth the expense. now if i were doing a track day, auto x, or going to do a lot of 1/4mile runs i would use the 91.

28kmile only had a little knock once with i think the n flash.

beers

mattzen

is it possible to actually do any damage to the engine using 87? short term? long term? just curious....

Fanman

Knocking = detonation. Detonation can cause irrepairable damage to you car. Short term detonation MIGHT be OK, but depending on how bad it is could destroy your motor in short order.