Fellow forum members of reason, I call on you to settle an argument. In this thread
http://forums.corvetteforum.com/zerothread?id=664104

There is a gentleman who is arguing with me that in a fuel injected car, higher octane is always better. I say he's wrong and that with either fuel injection or a carb you can have too high an octane fuel.

First some background which many of you already know. Octane is somewhat of a measure of a fuel's ability to resist detonation. A higher octane fuel is used in a car of a higher compression ratios to prevent detonation, allowing full timing advance for peak power. It does this by burning more slowly. The energy released for any given compression ratio is also lower, however the ability to run a higher compression ratio offsets this loss in high compression vehicles.

I have stated that if the engine is able to run at full spark advance with a low octane then going to a higher octane will only cost money. It will actually be harder to ignite, the results are harder starting and less power. I have also stated that no matter how the is introduced into the engine it doesn't have any effect on the basic principal of how the engine works. The same holds true for carbs and fuel injection, you want to run the LOWEST octane that you can with full spark advance for peak power.

Please choose from one of the choices below. Please feel free to add any coments for either side of the argument, and please if you have it list sources.



[Modified by Nathan Plemons, 5:34 PM 10/6/2003]

 

Nathan, I agree with you. Octane ratings like you said is how stable gasoline is. Hand in hand the more stable it is, the less power the fuel actually has. It becomes more and more of a slow controlled burn.


There is a point where more timing is a waste. Once you find that point with any engine I'd agree that you should run the lowest octane that allows you to run that timing as it will give you the most bang possible.

 

:iagree:

 

The bottom line is that you should give your engine the octane that it needs.

Too high an octane will burn too slowly to develope full power as the burn must be started too early in the compression cycle and too low will lead to detonation from burning too fast.

 

I lost 1mph and a tenth with 104 unleaded, instead of pump gas.

 

Nathan, You have it right you can also run lower octain in a high compression engine. ( the problem is you have to run very rich. Not good for emissions or the logitivity of the engine) Top fuel dragsters run the nitrometane so rich that the engine is on the verge of hydroloc. the reason for this is because nitro is so volitle (sp?) Let's look at the basic family nitroglycerin, nitrocelulose, nitromethaine See the nitro connection? anything with it will go BOOM! AV-GAS has a very high octain rating the Lycomming engines rev up to about 3400RPM max! So the flame front doesent have to move as fast across the piston as compaired to an automotive engine that revs at 6,00RPM (thats why you have to mix AV-GAS with regular to run it in your car) So, even though you can get AV-GAS in an octain rating of 100LL the specific gravity of the fuel is diffrent and that scewes the octain rating for planes a bit. (by the way the LL stands for low lead) dont run Lead in a fuel injected car for two reasons 1. lead will react with the platinum in the converter and cause it to get extremly hot and 2. If you dont have cat's the lead will coat the 02 sensor and render it useless in a hurry.

 

I saw a demonstration that had a clear cylinder and it had the amount of fuel that a top fuel car had at IDLE running through it and it was like a water hose. Then they hit it for WOT, it's amazing it can run at all.

Question:

A friend sent me this today. http://www.wku.edu/~nathan.plemons/video/firemelon.wmv

What octane and compression ratio was the mellon running?

 

Extra octane above what is needed for a specific engine is not beneficial to performance...

 

So guys in my stock L-98,,, is 87 octane better than 92????????

 

No, the car calls for premium fuel. If you run 87 it might not ping, but your knock sensor will retard timing to prevent detonation, then you'll lose power.

There is a little bit of spin put on the options in this poll.

 

My '88 owners manual recommends 89, but where I live now at a higher elevation I can run 87 all day long with no spark retard. :cheers:

 

One example is the dodge neon. If use anything other than the most cheapest 85/87 octance will 1) waste money, 2) burn up your spark plugs and 3) lower you mile per gallon

It is all about he design

 

Nathan, you have it right.

 

run the least octane you can get away with for best power higher octane will produce a slower burn :cheers:

 

this really helps explain some things for me. the last 2 times i filled with 93 octane, i noticed:

1. slight gas smell from exhaust, due to running a little rich
2. doesn't fire up immediately - took about 2 seconds.
3. seems to be a tad off in power

which begs the question....for my '90 L98, should i run 89 octane? 89 has seemed to work very well before :confused:

 

Not really, not if you read it fully. I said "run the lowest octane fuel that allows you to run full spark advance"

The key there being full spark advance, if your car is having to retard timing to prevent detonation then certainly by all means you are costing yourself power by running it. If however your car is not having to retard timing on lower octane then you have nothing to gain by going higher.

:cheers:

The poll may sound like there is a spin on it for my case, I can give you two explanations on that:

1. It's my poll so of course I want my choice to be the correct one.
2. Secondly though it's the correct answer, my answer is convincing because it is correct. My argument that the sky is blue would be more convincing then saying that it's green with nothing else said. Reason has a way of adding something to an argument.

 

Alright, this actually somewhat supports the other side of the argument but it has a logical explanation. The higher octane fuel might make the O2 sensor read richer, thus it will retard the injector pulse width. The problem though is that it will read richer because of an incomplete burn. If you cut down the injector pulse width further you are putting less fuel in, so you can have even less energy out.

So how does this affect performance and fuel economy. At first it might appear to help fuel economy since you are actually adding less fuel per cycle. The problem is that since each explosion has less fuel it will be less powerful. You can compensate by depressing the throttle a little farther, this will allow more air so you can add more fuel to make more power. Unfortunately we all know that pressing the throttle is the biggest thing that costs us gas mileage. It will also only work up to a point, because when you reach wide open throttle the engine can only pull in so much air, so it can only add so much fuel.

So, if this is true I will admit that the person who is arguing with me is quite possibly correct in that higher octane fuels will read richer on the O2 and that will decrease injector pulse width. However, that is no proof that it will increase gas mileage or power, in fact it is quite the contrary. I can remove a plug wire and that will effectively make the exhaust VERY rich, that doesn't mean I will make more power. A leaking injector can have the same effect.

Most engines like a slightly rich mixture for peak power. The richer mixture helps to cool the combustion process. Beyond a certain point though that unburnt fuel is just wasted power and increased emissions. We need to also understand that at WOT the engine reads off of set values and adds a given amount of fuel, ignoring input from the O2 sensors. If the car can run lower octane fuel at full advance at WOT it will see no benefit from higher octane. At WOT the car ignores the O2 sensor inputs so it is going to add the same volume of fuel, if this fuel burns too slowly it will rob the car power and the computer will do nothing about it.

 

Thanks Bud,,,, finally I got that question answered

 

nathan, you are correct...

Higher octane slows the flame kernal. Is less likely to prematurely explode do to hot spots, etc.

You always want to run the least amount of octane possible.

That is for FI and carb setups alike.

:cheers:

 

My owners manual says to use at least 85 octane on my '85. :lol:

After doing a compression check and getting 180-190 on all cylinders, I'll stick with 91. :yesnod: