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4K views 28 replies 21 participants last post by  king723 
#1 ·
okay i know this is probably a stupid question, but the dealer really wanted me to use premium gas, 91 octane. is it necessary? i don't want to pay the extra for it if i can help it.
 
#2 ·
yes because of the compression- if you take the turbo out, it woulr have like 6:1- but with the turbo, its got like 10:1 or something (im not sure) so unless you like pinging and crappy performance, use the good stuff.
 
#3 ·
The reason why you need a high octane is because of its rate of burn. Higher compression engines (10:1) and Turbo engines require a slower burning fuel to counter-act the detination caused by cumbustion. The rate of compression is the same regardless of forced induction or NA, rather, the only thing that does change is the amount of air to fuel ratio. This ratio for proper cumbustion is known as stoikometric (14.7 to 1 I think?).
 
#8 ·
Hey all,

To add to DJ's comments:

Ideal stochiometric ratio is 14:1 (air to fuel), and modern ignition systems are not capable of achieving it. Although some race engines are close.

To quote from my old textbook: "The octane number requirements for a given compression ratio vary widely, but typically a compression ratio of 7.5 requires 85 octane fuel, while a compression ratio of 10.0 requires 100 octane fuel. There are even wide variations in octane number requirements between supposedly identical engines."

That octane # quoted for C. Ratio of 10.0 may not be current.... I will check.

Raising the octane level of your fuel will primarily help prevent knock.
Brian
 
#10 ·
I had 89 in when I put my boost gauge in...was running about 12 PSI max boost, when I put 94 in today I saw 13.5-14 PSI max boost all the way home 40+ miles...so that leads me to beleive the computer can tell the octane and retard timing to keep valves clused longer and therefore build more boost. I could be wrong on the reason, but I definately get more boost with higher octane.
 
#11 ·
you can run low octane fuel, regular, but the ecu will pull timing and you won't have anywhere near the HP that you bought the car for. the knock sensor is there for a reason and that's one of them.
if you want the HP that car came with, use 93 octane. it's only a few dollar difference over teh course of a fill up anyway, it won't kill your wallet.
 
#12 ·
Low octane fuel causes knock in turbos. Knock is excessive detonation, and gives you crappy performance. Dont buy a turbo if your not willing to spend and extra $2-3 when you fill up. Its not that big of a deal anyways.
 
#14 ·
Detonation can crack blocks, split heads, and blow holes in the piston and send the rod flying, and in our case into the other cylinders. You can use 85 but you would have to find some way to trick your ECU into delaying the timing to compensate for the slower burning fuel. The ECU wont fully delay timing, only to a point. And it CANNOT keep the valves closed longer or shorter, that would mean it would have to adjust the cam which is solid, only honda has that which is knows as V-TEC
 
#16 ·
I used to have a 98 Eclipse RS and ran regular in it. I put premium in it a couple times and noticed a 2MPG gain. RS was NOT turbo. I now have a turbo Eclipse and have never put anything under 92 octane in it. 91 octane is the minimum you would want to go. Try finding a station that has 93 octane.
 
#22 ·
You will for sure get better gas mileage with 93 or 87! you will get worse gas mileage with E85 but you can run 3-4 psi more all day long and its way cheaper! I figured it out and as long as it is 67 cents cheaper its a savings. I have an Evo tuned with e85 need about 27% bigger injectors and a better fuel pump to take full advanged of e85
 
#24 ·
This doesnt make any sense, why can you run more boost because of the E85? The mileage lost in burning E85 makes up for the premium gas you would use otherwise. 67 cents cheaper is only $6.70 (assuming a 10 gallon fill up which is about normal for my eclipse, could be different for a rex) Assuming you get around 300 miles per tank with premium and about 230-250 with E85. 50-70 miles is about 2.5-3 gallons of gas. 2.5 gallons x $2.70 a gallon for E85 = $6.75 So you see? you really arent saving any money by buying it. You are, however, saving the environment.
 
#26 ·
ok sorry 68 cents it has to be cheaper and there are alot of plus to its from the performance side as stated and it is normal about 75-80cents less. Also once wisconsin finish the 3 or so new e85 plants the price will go down ..kinda ..more like it will just stay the same while gas goes up for a little time.
 
#28 ·
around central new york only a few gas stations carry 91. i just run 93 partly because it's better than 91 and partly because it's everywhere. in the end it's only a couple dollars more per tank. and every oil change (3000 miles) i put chevron techron fuel system treatment in the tank when i fill up. i've done a lot of reading on the net and it seems to be widely accepted as one of the best treatments.
 
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