legacycentral bbs

Can anyone explain to me why the turbo Legacy requires premium fuel. My understanding is that higher octane gas can be compressed further without prematurely igniting from the pressure before the spark goes off and is thus needed on cars with high compression. The turbo's compression is only 8.0:1, however, so why is it that it needs premium fuel while the N/A Legacy at 9.5:1 (I believe) does not?

The turbo pushes more air into the cylinder and raises the CR well above 9.5, basically what any forced induction system does is raise the CR, thats where all the extra power comes from...

Plus the turbo heats up the air before it goes into the engine. So you are stuffing more hot air that gets really compressed into the chamber. You'd never want to run 87 in the engine (even though some people do )

And for those two reasons, a turbo makes an engine more prone to knock. The octane number is the anti-knock index, which indicates a fuels resistance to knock.

You WILL lose a lot off power if you use 87RON fuel due to the ECU retarding your timing so far its not worth speaking about lol, stick the best in and it shall reap the rewards. Just as an aside, over here our standard fuel is 95 RON and the supers 98 RON (what mine runs on), pretty sure that the japs run 102RON!

87 RON would be ridiculously low even for most naturally aspirated engines.

When Americans refer to octane they're talking about AKI, which is the average of MON and RON. RON tends to be higher than MON for any given petrol fuel. Also, the higher the octane is, the larger the difference tends to be.

So, your 95 RON is close to our 89 AKI. Our 87 AKI is similar to 91 RON. Your 98 RON super fuel is similar to the 93 or 94 AKI fuel us non-Californian Americans get.

Of course, these are approximate, since we're never really told the MON or RON for our fuels.

(MON + RON) / 2 = AKI?

I think i remember seeing that on pumps. Is the higher MON or RON # better? For either, is there a way to tell what combination different gas stations use?

Yes, Anti Knock Index = (Research Octane Number + Motor Octane Number) / 2.

You could try asking, but I don't think there's any way to find out what the RON/MON breakdown is. If you can get them to tell you the "sensitivity" of the fuel, you're good. Sensitivity is the difference between RON and MON. So, RON=AKI+Sensitivity/2 and MON=AKI-Sensitivity/2.

It's not a combination, though; the fuel is just tested two different ways. They both involve a kind of reference engine design, but the MON testing process is higher stress than the RON process. So, the MON number is lower and kind of represents severe conditions and the RON number is higher and kind of represents more mild conditions. The sensitivity indicates how sensitive the fuel is to varying conditions.

Higher is better (in terms of resisting knock) in any case. I think the AKI is a better indicator than RON alone, but it'd be better to have both AKI and sensitivity.

I just wanted to add that the amount of pressure coming from the turbo greatly effects the "final" CR generated. If you look at some of the drag only built cars the compression ratio is pretty low for the amount of power they run, 91White-T and Brat4by4 nailed it.

Greg Donovan posted an octane question on special stage a few weeks ago. It had some pretty good responses and I learned alot from it. I now run 100 AKI octane from Sunoco in my Colt and it runs much better than it did on 91. The 4.20/gallon is kinda rough but I hardly drive it. Anyhow, here is the link to the discussion on Special Stage
http://www.specialstage.com/forum/cgi-b ... /1062.html

and the link to info on boosting octane via toluene (like Rod is doing if I remeber correctly) and some basic octane info
http://www.gnttype.org/techarea/misc/oc ... ained.html

and finally the link to VP Racing Fuels website with info on Octane
http://www.vpracingfuels.com

I am currently running a tankful of ~97 aki via toluene in my N/A, and the difference is remarkable. The car no longer starts on the first click of the engine, but midrange and topend power is quite noticibly stronger. Idle is better too. There is no hesitation when you tap the throttle at idle. I attribute it to the ecu advancing the timing. Oh, i put a couple ounces of oil in the tank as well, in hopes of preserving my rubber bits.

I doubt i would run like this regularly (it's an NA, after all) But i will start buying 91ron. and probably use the rest of my paint thinner next time i go on a road trip, for the milege.

Scottzg,

What ratio do you mix the toluene with your fuel?

uhh, i put in a gallon, but it wasn't a full tank. Just figure the toluene is 114, your gas is 91 (if you're me, anyway) and if you add Z gallons of gas and Y gallons of toluene, then (Z x 91 + Y x 114) / Y+Z = octane in tank.