I'd like to put some thoughts/opinion out there for consideration, and since I'm bored at work right now :)

Please correct me if I'm wrong. This applies to a PCM-controlled engines only.

1.) If you have octane booster in your fuel, it won't reduce the amount of fuel in the combustion chamber, but it will increase the volume of "liquid" injected into the combustion chamber; the computer modulates the injected fuel volume to maximize the A/F ratio per constraints programmed into the PCM. 14.7:1 being stochiometric and 13:1, I think, is best for max power (damn, should have written it down :) ).

2.) If you retard your timing, as will occur if the PCM detects knock, you'll get more low-end torque but will lose total power as a whole (you can get an idea by measuring the area below the power curve, HP or torque, on a dyno sheet).

3.) Pre-ignition causes wear in the engine. Not as big a problem with the PCM-controlled vehicles, but I don't like extra wear. To check for pre-ignition, one of the indicators is aluminum deposits on your spark plugs. Where do you think the aluminum comes from??? I prefer my metals to stay where they were designed to be.

4.) Stock PCMs are programmed with a timing curve and a set timing advance max. If you re-program the PCM, you can take advantage of greater timing advance which would require higher octane fuel. Unless you have ping on 89 at maximum advance, you don't need higher octane fuel....but what's your a/f mix? If your running rich, you'll tend to reduce detonation but you're sacrificing power at the same time. If you're running the engine too cold, same thing. So many factors are involved.

5.) You need higher octane to make more power through compression ratio increases, either by head changes or power adders (turbos or s/c's). Higher octane fuels won't help you make power without the compression ratio increase, though.

6.) Winter fuels are different than summer fuels. Different air temps require different volatility. If you have summer fuel in your car and it drops to 30F, you might see some rough engine performance while the engine is cold.

7.) My tires spin a LOT more when it's cold :D It's not only due to >HP, it's more due to less traction.....the tire rubber is harder. I notice it's harder to lay rubber in the cold months, too; plenty of spin, too cold to melt rubber.

8.) Octane requirements reduce when you increase altitude since there's less oxygen in the less dense air.

9.) You're unlikely to hear knock on your LS-1 or LS-6 before the PCM pulls timing. You need something like AutoTap to see if timing is being pulled or not.

10.) Using octane above what's necessary is a waste, but since operating conditions can change so much, I prefer to have the extra cushion so I won't pull timing due to detonation and have every little bit of power available for when a rice burner or 'stang gets ants in their pants on the interstate and I have to give 'em an attitude adjustment. :cool:

11.) Don't forget fuel additives for cleaning and corrosion prevention. Fuel pump replacement is very expensive and water/dirt will help foul the filter sock on the pump inlet, reducing flow and leading to premature pump failure. Not to mention injector cleaning, combustion chamber cleaning, etc. A few more cents at the pump or Redline fuel treatment on a regular basis (or, for me, both) will go a long way in preventing premature failures. Also, in case you don't read the article below, it's not good to get gas right after a truck has filled the station's tank. You should wait overnight to let all the water in the station tank settle back to the bottom of the tank, otherwise you end up pumping a fair share of water into your tank.

12.) Alcohol absorbs water. Alcohol used as a fuel modifier is a no-no as a result (unless you're getting water out of your tank since your girlfriend did you a favor and filled your tank at the "El-cheapo" down the street). Don't buy fuel from a retailer that uses methanol (aka gasahol).

There's a good, albeit long, article about fuel on this site: "http://www.idavette.net/hib/fuel/index.htm"


Hope this helps those that are interested.

TC

 

I would agree with all your points except a couple.

The only thing that makes power in the combustion chamber is the proper ratio/mixture of air and fuel. Adding anything to the fuel (like octane), reduces the amount of explosive mixture in the chamber. The fuel is diluted if you will (with octane), and is only taking up space in the chamber that could be used for more mixture. The PCM only corrects for non-wot operation with respect to 02 feedback for stoich.

The only thing money is buying at the pump for premium is the octane additive. The fuel is no different or better than the least expensive 87 octane. In my experience higher octane fuel will deposit more garbage in your chamber.

With that said however, higher octane fuel is the solution for higher compression engines, especially in hot environments to solve pre-ignition, but just enough octane is optimal. So for most who dont want to tinker with diagnosing thier car, or seasonally squeeze all the power out of it, the premium octane at the pumps will make the vast majority happy over time for a stock ls1/ls6.


[Modified by kewlbrz, 8:41 AM 11/17/2003]

 

I have to disagree with your assumption. "more mixture" means more combustible material, which pertains directly to the A/F mixture. If you had more mixture, you'd be running richer. If you were to add water vapor, such as after a rain, you would be adding water to the mixture, taking up more space within the combustion chamber, but that doesn't mean less fuel will be added to the combustion chamber. The O2 sensor measures the level of carbon in the exhaust and determines how much unburned fuel is present and adjusts the fuel input (injector pulse) accordingly based on the "map" programmed into the PCM.

Now, if more oxygen is available, the PCM will give you more fuel which will give you more power. Conversely, if you have less oxygen, say if you had Nitrogen-rich air mixing in the combustion chamber, i.e. more stuff in the air, then yes, you would have less power because you would have less oxygen per cubic foot of air entering the engine. However, the contention that more stuff in the fuel will produce a noticeable change in power is not true until you're talking about such high volumes of liquid that it actually will affect the temperature of combustion thereby reducing the thermal efficiency of the engine.

The PCM "map" is programmed into the PCM for WOT, so you're right in one way, the PCM won't dynamically adjust for the changes in the exhaust, but will choose the "WOT run-rich" mapping.

However, if this were too lean for the engine, e.g. aftermarket components were added, such as headers and intake components, and the PCM wasn't re-programmed with LS1 Edit, Predator, etc., you would benefit from a higher octane fuel since it would reduce pre-ignition and the damage that comes with it. To optimize, you'd want your WOT map to give you a 13:1 A/F ratio instead of (in my case with stock PCM programming) an 11:1 A/F ratio. Too rich can also cause the oil to be washed from the cylinders reducing lubrication, increasing friction, causing oil to mix with the combustion mixture (hurting overall octane) damaging the engine and reducing the performance all at once (oil entering the mix, e.g. a poor PCV vent, will lower the effective octane entering the combustion chamber).


This may be true, but octane can fluctuate and the R+M/2 octane might actually be 86.7 instead of a true 87. I believe the Vette owners manual recommends a minimum of 89, but 87 can be used if that's the only octane available. I believe, but can't substantiate, that higher grade fuels may also contain more cleaning agents, which is a plus for those reasons already identified.

Again, I'd check the owner's manual and make sure that 89 isn't the minimum recommended octane.

Happy driving!!

:cheers:


[Modified by TopCat, 10:30 AM 11/17/2003]

 

I disagree again...

You would not be running richer if you have more explosive combustible mixture. That's simply not possible. The explosive combustible mixture is the same whether (for example) it's 14.7:1 or 147:10. One would not be running richer with this order of magnitude increase in mixture.

O2 sensors are oxygen sensors. Not carbon sensors. They measure the amount of oxygen in the exhaust stream, relative to stoich. For example, more oxygen means a leaner burn, less oxygen means a richer burn.

Without considering the effect of knock retard. If I fill the combustion chamber with fuel that has octane additive, that octane additive is taking up space in the chamber that could be a more combustible mixture. It doesnt matter if its water, sodapop or any other non or less combustible liquid. Let's say that knock is not an issue going from 93 octane to 89 octane. That means I have reduced the finite space in the chamber that could have held more explosive mixture, and thus reduced its efficiency to produce power.


There are several mappings that will dynamically change depending on WOT load.

I was just using 87 octane before to show a point. didnt say anwhere that that was the min recommended.

I also would encourage you to also read something I put together a few years ago. (more specifically the part on air/fuel ratio.)
http://www.vetteguru.com/mods/howto/




[Modified by kewlbrz, 11:36 PM 11/17/2003]

 

WOW.... I read the paper you wrote... So.. what I hear is... with stock 9.5:1 compression... I will have more low end power on 89 octane, and as long as I am not at WOT the timming will not pull back? Also if the 89 octane has "enough" octane to retard the flash point to where the car does not "ping" or KR does not kick in... all is not only good... but better, as octane does not "make" power... it does not burn, it is a "filler" to raise the flash point to when the mixture will ignite... and when the lower octant stuff does ignite.. it "can" have more power as it has less filler in the same volume.??? Am I close? Also the car to me is faster off the line, and I have not missed any top end... This is with my SOTP meter... :cool: :seeya :chevy :auto: :steering: :skep: :leaving:


[Modified by AtHomeSoda, 12:40 AM 11/18/2003]

 

My '00 coupe got 34 mpg average on the highway on 91 octane so I don't see anything there that would drive me to use 89 octane.

As far as performance and KR, it all depends on the fuel you buy. I know some owners who buy just enough of the major oil companies fuel to keep their sign out front and the rest is from spot market or discount suppliers.

The quality of the fuel will dictate whether you experience KR, better or worse economy or more or less power.

There are gas stations I know to stay away from even though they have a big name sign out front because their gas makes my car knock. Same octane rating as other stations.

Funny thing is, no one ever tests the fuel in the stations tanks to see if it really is what the sign says it is.

cc

 

Absolutley, I could not agree more. I also know of stations that do the same thing. I have confronted them about it but they just shuck and jive. They have the tanker dump 87 in the 93 octane tank and keep the extra bucks. :iagree:


[Modified by boomer 33, 8:50 PM 11/17/2003]

 

Ok...So I buy Chevron...and here I have seen trucks with tanks on them that test the volume of gas, that a gallon is a gallon...but you mean to tell me nobody tests the octane, to see if it is right?.... The one I go to so far seems not to cheat... man... that's not right...
:banghead: :smash: :confused: :nono: :nono: :nono: :nono: :rant: :banghead: :auto:

 

You then ahve a "good" one and should always buy your gas there if possible. There are other stations that will have the displays of a major oil company. But, they contract with samller, regional gas delivery trucks to bring the gas to the station from the main gas pipline for the region. They won't have the major oil company logo on it, usually will have the name of a local (regional) tanker company on it. Same size trucks, but they might contract to deliver to several stations in the area, regardless of brand. In other words, the station instructs them how much of each grade to drop in what particular tank. There is where the shenanigans come in to play. Shell station may say Shell, but who knows what went in to the tank. Complained about this to the regional manager of a large Shellstation in Little Rock after passsing through and getting a full tank of 93 octance that made my engine sound like it was producing gravel. Happened at another Shell station off of I-40 , different trip, same results.


[Modified by boomer 33, 11:44 PM 11/17/2003]

 

Very good points, kewlbrz..... I love a good debate....but I need to drink more coffee or read more carefully, or both, before posting. I misunderstood what was intended by "more mixture" and interpreted it to mean more fuel. Brain fart on the O2 sensors....touche'. I knew that, but went off on a tangent and didn't re-focus my thoughts, etc., etc. - more coffee was required :)

I still disagree with the higher octane than necessary = less power, though, except when MTBE or alcohol is added to the gas. MTBE does take up considerable space WILL lower the power output in this case. Similar with alcohol (up to 10% by volume), but it also lowers the BTU value of the fuel; 'nuff said.

Sidenote: Cold weather (winter) fuels have lower BTU values than summer fuels, but that's another discussion.

As far as quality octane adders decreasing the combustion efficiency or total power output due to using up a finite amount of space in the combustion chamber that could have been fuel/air mix instead, this is technically correct (on a parts per billion scale) and could only be measured in a bomb calorimeter. SOTP increases are not realistic.

Upshot - using octane ratings higher than you need will hurt your wallet, but not your performance.

Using lower octane fuels, e.g. 89 instead of 91, will not increase your performance UNLESS octane adders that introduce oxygen (e.g. MTBE) or reduce BTU's (e.g. ethanol or methanol) have been used to increase the octane to 91.

This being said, let me reiterate that higher quality gasoline IS a must for the best performance.....stay away from Mom and Pop's and the discount gas dealers for many, many reasons if you love your car.


:cheers:

 

Well I'm in Texas and tried one tank of 89 instead of 93 (that we have here).
89 is usually .10 gal less than 93, but mine pinged like :crazy: :crazy: under the slightest load !! :eek: It was either Exxon or Citgo gas (don't remember now) but I haven't tried 89 since. Now that it is cooling off (a little - suppose to be 80 tomorrow !) I might try 89 again when it's down in the 50s or so and see what happens ! :jester