They may also have other motives for there recomendations we don't know about you know engineer stuff .

 

Eh? I'm not an engineer (college drop-out, baby!), I just read and listen to those more knowledgeable than me, process the information, then regurgitate it when it's deemed needed.

 

Please explain...

 

If you have a STOCK S4 turbo you can safely run 87 octane (per the FSM). Higher than stock boost and I'd start to up the octane. Here's my best GUESS: up to 10 psi use 89 octane, up to 12 psi, 91 octane or better. Above 12 psi and your on you own.

Apparently the S5 turbo recommends higher octane so adjust accordingly--I have no experience with the S5 engine.

Reason I say GUESS is cause none of us have a proper testing facility to prove out all corners of the envelope--basically we're testing with our daily drivers. However, you get enough people developing and testing a standard and you have something that works--i.e. use other peoples experience.

I personally use 93 octane for 10-12 psi of boost.

 

The higher the octane the slower the fuel burns over all....which means that 87 octane burns faster than 91. Now, mix fuel with air at a 14:1 ratio, and then compress this to a 9.0:1 ratio, and it burns MUCH faster. Now, a turbocharger crams an extra amount of air into the engine, and injectors ona turbo engine dump an extra amount of fuel in the engine, this means that even MORE material is being crammed into the same space which means that the pressure is even HIGHER. You want a slower burning fuel in this enviroment to prevent the force of the explosion going back against the travel of the rotar (or piston)(ie; knock).

 

The factory calls for 87 in the series IV cars. But thats assuming stock boost level. I run 11psi and use 93 octane. I sometimes run 89 on a long trip for better gas mileage, but i cut the boost to about 7psi.

 

Of course I know that but you said "It's the equivelant of dumping more fuel in....but with less fuel...ummm...yeah...." I didn't understand what you meant by that. I guess I interpreted it wrong since I was referring to stock.

 

yeah I haven't had a whole night's sleep in like 2 weeks...

 

i thought the S5s had slightly more compression than the S4s, they do have different rotors and that is why i wonder if they recommend using higher than 87 minimum octane on the S5s. manufacters also revise their info after they do research, a few engine failures under warranty may have caused them to rethink and add in the higher octane numbers.

 

Same goes for when I lived in Colorado. But then I guess hi octane doesn't much matter when you drive at 7000 feet above sea level!

 

not really sure but don't turbo charged engines need low compression to run boost if you're turbo charged FC and doesn't 9:1 register as low compression?

 

I'm realy confused with this topic... I live in Portugal, and i have 95,98 and 100 octane... when i use 100 octane, it seems that the car have less power than 95 or 98 octane... in my case what is the right option?

 

let's get back to the part of the original question concerning using alcohol to pass emissions test. Using 87 or 89 octane for test will work but adding alcohol to the mixture, what kind and how much per gallon of gas?

As far as octane, use what the owner's manual states, the mazda engineers seemed to know what they were doing since y'all are driving 14+ year old cars adn the engines run at very high temps for 100K miles with the proper care and feeding. If you vary from the OM recommendations then expect shorter engine life or less than optimum performance.

 

A stock S4 is fine on 87 octane. I don't know about S5's. However, if you mod it and run higher boost (like almost everyone here) then I would recommend at least 91 octane.

-Joe

 

my car is an 89 gxl, my manual tells me to run 91, why i'm not sure,

86-88 n/a have 9.4:1 compression ratio
87-88 turbo have 9.0:1 compression ratio
89-91 n/a have 9.7:1 compression ratio
89-91 turbo have 9.0:1 compression ratio

i guess the difference between s5 n/a and s4 n/a make the difference for running higher octane

 

hmmm well. I was wonderin which type of alcohol and i was just thinkin like 2-3 bottles of it, drive gingerly to the emissions testing.... right after fill up.

anyone ever done this before?

 

I've never done it, but I believe that pure Ethanol will work fine.

-Joe

 

pure ethanol will work but you will get horible gas mileage, ethanol his less heat efficient