wow.. i never knew that! i always thought the opposite was true. If you engine is designed to run on 87 octane.. then you get no performance benefit of running 89 or 91 octane gas. the only benefit is the engine's resilience to knocking.

Engines in vehicles built for sale in North America are designed to a specified octane requirement to make sure they don't knock or ping (engine knocking reduces the amount of power it can deliver to turn the wheels). Once that octane level has been met, in normal instances your car will not experience more power or better mileage if you use a higher octane fuel.

In other words, if your Canadian vehicle owner's manual specifies an octane rating of 87, running on gasoline with an octane rating of 89 or 91 or more will normally not make a difference.

this is directly from Shell itself. http://www.shell.ca/code/motoring/encyclop...nes/octane.html

Yea, I thought that was common knowledge among car enthusiasts...

My tC takes 87 octane and that is what I fill 'er up with. This is why people use higher octane fuel on older cars (prevent knocking)...

I didnt know that! Im actually quite supprised.

I knew that along time ago , higher octane fuel means nothing to regular mid-sized/sport compact cars. Unless of course they're Porsches or what not. I've known people who hought that it made a difference Higher compression ratio= higher octane fuel

I wish I could use 87.

It means a lot on turbocharged cars. Especially ones with built in knock sensors.

It's sad how many people think that it's better for their cars or they get more power by running high octane fuel. I've met a *lot* of people who buy 89/91/93 for their Hondas and I often have a hard time convincing them that it's a waste, even though they know that I live for cars.

I have an '89 civic... after 3 years experimenting with 87/89/91/94(sunoco), I'm convinced that I get better mileage outta the higher stuff. The only thing left to do is calculate if the extra charge is worth the extra miles.

I find I get better mileage when I use chevron...but that's all anecdotal and has nothing to do with octane.

I've never fiddled with octane because my car takes 91 minimum!

What about that "race" fuel you can buy at Canadian Tire? does it melt away your pistons and blow your engine?

No, race fuel will just burn slower than lower octanes, the same way premium burns slower than regular, with the corresponding loss in performance (for engines that weren't designed to run on higher grade fuel)

Yeh, I just remember some commercial on SPEED marketing this kind of Race Fuel at Canadian Tire. The commercial went on to show a guy pouring it into a riced Civic and then shifting to 40-50 year olds who claim how much performance gain and better fuel mileage they've gained with it. The commercial only lasted a week. I wonder how the product is....

Don't most supercars demand 91? I'm pretty sure the Vette does... Don't know about others.

The only time that a higher octane fuel provides any benefit is if you're past the knock threshhold of the fuel, which is achieved through high cylinder pressures. High cylinder pressures can be achieved both/either by high compression ratios, or forced induction.

-Jesse

Ok so if i'm reading this right, putting high octane fuel in a regular car can slow the burning process of the fuel? And if your car is running on a turbo or super you need to use higher octane fuel?

This is one of the things that makes US cars pale in comparison when compared to their Japanese counterparts. Look at the USDM (United States Domestic Market) WRX. EJ20t turbocharged motor, 227 horses. Nice, fast, fun, great bang for buck, right? Yeah.

Except you do some research and you find that the JDM WRX makes 280~ depending on the revision. Yay.

The difference really comes when you start tuning them. The US WRX tops out at 280 realistically, whereas the JDM WRX can absolutely spank it at 320+ properly tuned. That's STi territory, with less displacement!

Why, you ask? In Japan, gas stations all over carry 95 octane. When you're slapping a MASSIVE turbocharger to a tiny engine, detonation matters. A couple of good dets and you need new valves. Also, JDM parts are just in general better than export parts but quite a bit of the massive power differences comes from the fact that the Japanese can get super-ultra-mega octane at every pump. Yeah it costs five bucks a gallon, but do you know how far I'd have to drive to get 95 octane?

Then there's the USDM @#$%ing transmissions which are geared for idiots, but that's another rant.

That's interesting about the octane, I didn't know that. I stand corrected on that as well as the 250hp base, I was going from memory. Replace Octane with Emissions laws in my post and my point stands.

The problem is that the USDM cars aren't tuneable anywhere near as high as the JDM versions. Stock, yeah there's about 25-some-odd hp difference, its when you start tweaking that the differences become obvious. I understand that you want to keep the good stuff at home, but its frustrating to those of us who live in Not Japan.

Its kind of like international Chevys being Daewoos in disguise. If you bought a Chevy in Europe, and then drove in a US Chevy, you'd be astonished at how much better it is. Not that that's saying much, but still.

As much as I understand it, it annoys me. The thing that drives me the most batty is the transmissions. The USDM Subaru transmissions absolutely suck. Bottom line. They're two-piece casings with too-tall gearing and piddly clutches. JDM stuff is just stronger, full stop. I guess I'm not really talking about octane anymore though, am I...

On my user's manuel, it says 93 octane or higher only. But we don't have that here in CA!!! Only crappy 91... that's why when I got my car, which is the first batch, knock like crazy. Then Subaru took the ecu and reflashed it, but it still knock occasionally.

Yeah, I have been telling my friends and family this for Years!!

Then I send them articles...

so how exactly do you know if you car is knocking? i know i'm dumb

it quite literally "Knocks."

Imagine the sound of marbles in a tin can.

It is that bad

I'm pretty sure the Corvette requires 91 octane. Almost any turbocharged car and most cars with a compression ratio greater than 9.8:1 need 91+ octane.

In order to increase your compression ratio you have to install a turbo or super right? Anything else you can do?

Forced induction increases your RELATIVE compression ratio clark

But not your measured compression ratio.

OMG!

Yes but relative compression ratio is a term that's often used so wasn't 100% wrong.

Bah.

The point is that a turbocharger doesn't change the compression ratio that's listed in the spec sheet. My engine has a compression of 9.5:1 whether or not the turbo's there.

I always use 91 octane....

out of all the knowledge that exists in this forum i am surprised you guys didn't know that about octane. i believe car and driver did a test a few years ago with a dynamometer and several cars. on average, there was no gain nor loss. i think it was the mustang that actually gained a little power and the M3 lost a little, or maybe the other way around. but that could be varying changes in weather, temperature, etc. octane numbers are a measure of the fuel's resistance to ignite. heptane is the ignited portion of the fuel. the higher the ratio of octane to heptane, the more resistance the fuel has to ignite. so higher octane means it needs a higher compression to explode.

so i'm guessing diesel has a really low octane (if at all?) since it can explode without any spark, right?


as for the tall gearing of US cars, it's simply b/c americans do more freeway driving than other countries. with higher speeds and longer distances, taller gearing just makes sense.

forced induction engines actually have lower compression than most N/A engines to compensate for the compressed air, if porschedream is interested.

wow, never noticed that till now...

checked my truck's stats and requires 87 octane when ive been doing 89 the whole time.. wasting all that money for nothing when i could pay less for 87

Ok, so higher octane gas needs a higher compression to ignite, i knew that. But like wouldn't some cars not be able to produce such a compression? I mean lets say you put 91 octane in a Geo Metro. Would it be able to produce enough compression to ignite? And one more thing, clark said his engine has a compression of 9:5:1, what do the numbers stand for. i'm dumb

Yes, I'll admit I know nothing about compression ratios also.

Yes you are dentist but I'm here to help.

Octane doesnt REQUIRE a compression ratio to ignite, it just simply helps the process. When a piston is compressing air the temprature of the air goes up because the volume is decreasing. This is actually the only reason why motors work as well as they do because the higher temp of the compressed air fuel mixture lends to better ignition. But anyways what happens is the more you compress air, the higher the temprature gets. Once you're up above 10 to one on a naturally asperated car the air get SO hot that it pre-ignites the mixture before the spark plug fires and basically you have a motor with a cylinder trying to spin backwards. Break. That's what knock is.

Higher octane fuel burns hotter so it can be compressed more. That's what the different octane ratings are for.

Compression ratio is basically a ratio between a cylinder's max volume and min volume.

Thanks Mr. Dakian , now tell me, 9:5:1 means?

9.5 basically means if you take the max volume, divide it by the min, you get 9.5.

oh, so it's like the displacement of a car sort of?

The best way to describe it is the size of the combustion chamber at the top of a piston. The higher the number the smaller the combustion chamber.

My dirtbike has a 12.5:1 comp ratio. If I dont have gas, and have to stop out in the country I'll get 87.. as the "premium" has probably been sitting for a long ass time out in hicksville. Runs fine. Doesnt knock.

But I will be running nothing but race gas this year. Not for the octane, but for cleaner fuel that will leave less deposits and shit. It's a 4 stroke specific fuel, only 94 octane. I'd pay about the same getting that in the states as I would buying 91 octane locally.