the higher altitude means less power, less boost, but it still wont stop a car knocking when its tuned for 93.

at 3000ft on stage 2 with 91 octane, my car knocked like a jehovas witness.

 

I never agreed that it was a good idea that they do this.



I think it is stooped...but I guess the locals are used it

 

Before I tell you where I'm coming from, I want to make sure you know I have no gauge that monitors KR, but I can tell you I hear no pinging, even at WOT.

I've never seen more than 91 at any of the stations in CO. I have S2 installed, and I regularly fill up with 91 since that's all we have (pumps are usually 85/87/91 up here). DA's up here are around 5k-8k feet, with some around nearly 9k on a really hot day. I'll admit though, I didn't hear it anywhere, it was something I read.

Seems plausible enough, less air = less pressure, less pressure = less bang, less bang = less heat, less heat = less knock. That's so dumbed-down I can see the flaw already, but hey, if I'm gonna get my brain re-wired, I might as well get it re-wired all the way

Maybe I'm up high enough that a 93 tune runs fine with 91. It also might be true that my car is pulling timing like a mother and I need to re-train my ear.

Thoughts? I've had this car a year and I'm learning new stuff about it every day. If there's something about the way tuning works that I'm missing (which is 99.9% possible), please do tell, I'm seriously all ears.

 

well its certainly possible that given enough altitude, the engine will be making so much less power that it could reduce the chance of knock....
so i guess we can compromise somewhat on it.
In the same way you cant confirm or deny knock at your alt, neither can I assume it is there.
but all I can say, is that ive been at DAs as high as 6300ft in calgary, and with 91 octane, the stage 2 tune had plenty of knock.

also, knock has to be pretty bad for it to be audible in my experience...
you could easily get a few degrees and pull timing without hearing or feeling anything.

Its just a pet peeve of mine that people spend thousands of dollars on higher power trim levels, hundreds of dollars on mods, and then try and convince people that cheap ass fuel is fine.

(not that it was you suggesting it, as you obviously are putting in the best that you have available to you, but that was the main reason for my comments in the thread)

 

I would knock with the Stage 2 tune and 91 octane. Couldn't hear it, but could see it on the interceptor gauge.

 

Most appreciated. I may need to go grab an interceptor and see if my car's pulling timing. You've got me curious now.

And I agree on the cheapo gas, why anyone would even try to put 87 in this car is beyond me. Why GM would say that 87 is do-able is even more beyond me.

 

I just found this thread and I find it personally hilarious.... because I've been using 87 since I bought my 2006 2.4, only because I had no idea it recommended 91.

This is my second Cobalt. My first was a 2005 2.2, which got ruined when a woman slammed into me head on. I was happy, in a way, to get rid of my Northstar powered 300 horse Cadillac Eldorado, my previous (used) car, because it was always a pain to mix 89 and 93 octane to get to the recommended 91 at each fillup for that car. It had a lot of problems also. You can't just buy 91 in all places here. Some stations only have 87, 89, and 93. Most Sunoco's have 91.

Now I know a LOT of people had these Northstar powered Cadillacs and always put 87 in them, including a good friend of mine. Nothing bad ever happened. You think gramma and grampa, who bought all those Cadillacs, worried about octane ratings at the pump on all those Sevilles they bought? Mine came from some woman in West Virginia. It ran geat and I drove it hard for over 20 k miles. I of course have no idea what octane was used it in before I got it at 63k. But I digress.

So I go out and buy a new Cobalt since I liked the first one. Now they have this 2.4. I like performance, so I get it. My salesman never told me it recommended 91. I never thought to check the book. If they got 145 ponies out of a 2.2, 171 out of a 2.4 sounds just about right, so who would think it needs extra octane, right? I checked the book last night, and sure enough, it's in there.

So now I have 29,400 miles of 87 octane. No known problems, obviously. I'm a bit below half a tank, so if I go throw in 93 that should start to balance it out.

I'm thinking maybe I should unplug the battery to reset the computer so it can recalibrate to the better octane once I drive around.

My mileage has also always been iffy. If I go high 70 mph, like 75-80, on the highway, I top out at 25 mpg. If I go slower, I can creep over 25 to maybe 25.5. We'll see what this does for it.

 

^^^ Anything higher than 91 is fine for the LSJ/LNF and 2.4's, it's even encouraged. Octane is your friend against knock.

Just don't go lower than 91. I can only speak for the LSJ though, since that's all I have. 87 on these engines makes them take a ****. The 2.4's might pull less timing, less often, but I hear they still do with 87 in the tank.

 

I swear its peppier with the higher octane gas compared to the 87. Since I read this thread I have been sticking to the good stuff myself. I seems to do fine with the 87 but i could swear it wasn't getting as good as gas mileage.

 

You drive a 2.0 or 2.4?

Either way, there's a power difference with the 87 stuff. Like An0m said earlier, you might not be able to hear a few degrees of KR, but hooking up an Interceptor or other gauge will show you if you're knocking on 87. Gas mileage change? Dunno. You'd have to run a few tanks of each to see for sure.

 

its a 2.4, i definitely can tell it runs a little weaker with the 87, can't hear anything though like you guys were saying.

 

It's not your imagination.....it IS peppier with the proper fuel in the tank. The computer will pull timing with 87 octane and you will not get the rated hp of the engine.

 

u are also "wrong" the 2.4's also recommend 91 ..again like you said about the ss/sc u CAN run 87 on the 2.4 also but it will run just as **** as the ss/sc would on 87

 

i tuned my G5 GT with the 93 octane and its pretty beast but now i have to run 93 oct and its about 4.50 a gal now. I get good mileage though so im not worried about it.

 

Make this a poll!

I used to run 91, but the gas prices are astronomical. I did not see any performance loss or change in mpg. my engine, 07 2.4 manual, is very sensitive to humidity and temperature though... it seems to like mid 70's and very little humidity which is very rare here during the summer months.

 

Definitely unplug the battery for an hour so the computer resets or it can take some time to fully calibrate the fuel curves.

You are getting way low mileage for you car. I get 25-26 over a tank in town and my last trip I got 32.6mpg on my best tank with the cruise set to 71mph.

I might just run a partial tank of 87 and monitor with my Dashhawk just kicks.

 

you need to run 91+ in the cobalt ss/sc and ss/tc at all time or it will screw something up in the the engine and it will start troughing lights if you go to a lower octane

 

Yes the manual does recommend a 91 octane fuel..but alittle higher octane is fine, the higher the octane the hotter and more efficient combustion.
I ran one full tank of 87 through my 2.4 auto and then I ran a full tank of 93. I got better fuel mileage and the car seemed to have better throttle response with 93..

 

please please for the love of God, just buy premium.

there are so many of these threads.

the gasoline that i have to put in my suburban here in Baghdad has such a low octane that its got a CEL. trust me, the lower the octane, just gets worse for your motor.

 

correct me if im wrong but octane directly coincides with an engines CR, higher the CR higher the octane

octane changes the combustion pressure, for example diesels have 25:1 CR ratio so im assuming their fuel is loaded with octane.

so obvisously using the wrong grade could directly effect performance, but not to any great extent, and using a lower octane could in the long run cause dammage, hence engine knock == predetonation?

 

Octane doesn't change the combustion chamber pressure. That is indicated by combustion chamber size in relation to the piston measurements. I believe what happens with lower octane fuels in high compression engines is that the fuel actually explodes prematurely, instead of the most efficient point in the power cycle.....I could be wrong on that though.

 

No, I think you are correct. because the lower octane is easier to ignite, therefor it burns prematurely and like you said not at the ideal stroke. I can understand people trying to get away with putting 87 or 89 in their 2.2s and 2.4s but anyone putting 87 in their SS SC should be hit in the head with something heavy. Why buy a supercharged performance car if you cant afford to put an extra dollar or two in the tank each time? Get serious. Plus, with my stage 2, CAI, catback, Gm tune I get 34mpg highway and 26 city. It would be way lower on 87-89. Put in 93, spend extra 3 dollars on a full tank, but gain 5 mpg, better performance, and longer engine life (supposedly)

 

The octane rating of a spark ignition engine fuel is the knock resistance (anti-knock rating) compared to a mixture of iso-octane and n-heptane. By definition, iso-octane is assigned an octane rating of 100 and heptane is assigned an octane rating of zero. An 87-octane gasoline, for example, possesses the same anti-knock rating of a mixture of 87% (by volume) iso-octane and 13% (by volume) n-heptane. This does not mean, however, that the gasoline actually contains these hydrocarbons in these proportions. It simply means that it has the same autoignition resistance as the described mixture.

A high tendency to autoignite, or low octane rating, is undesirable in a spark ignition engine but desirable in a diesel engine. The standard for the combustion quality of diesel fuel is the cetane number. A diesel fuel with a high cetane number has a high tendency to autoignite, as is preferred.

It should be noted that octane rating does not relate to the energy content of the fuel, nor the speed at which the flame initiated by the spark plug propagates across the cylinder. It is only a measure of the fuel's resistance to autoignition.

 

Here is a more indepth summary of what InfinityzeN just stated...


http://en.wikipedia.org/wiki/Octane_rating

 

So which fuel do people suggest for my girlfriends stock AUTO 07 Cobalt SS 2.4L

(*NOT*) SS/SC? <- - It isn't super/turbo charged. All STOCK! Everything is this ******* forum that people post and argue about is confusing and doesn't make any sense to me nor anyone else. Someone help.

Ive been using the cheapest gas in her car. She says its slow too. Oh well she is a girl and doesn't need to be driving at high speeds as us men! Ive been using 91-93 in mine.