I know that in order to reap the benefits of a high octane fuel- you need a higher Mechanical Compression ratio.

High Octane fuel burns slower than reg gas.

Is there a performance gain by running say 91 Octane as opposed to 94 in our LT1's ?

Anyone actually know the min requirements for the octane ?

 

The owners manual says exactly this:I would use 87 if I were planning a long trip, but if I was just going to bomb around town all summer I would fill up on 91.

 

I agree with you Pandamonkey, there is actually no reason to use anything higher than 91. Time to start saving cash at the pump, thx.

 

Octane controls flame propagation. Your Z28 should have 10.4:1 Compression (maybe 10.6:1). That creates a very high temp in your combustion chamber.

The 91 Premium Gas (Cali Rating... not 93) slows down the flame propagation and reduces pre-ignition.

By running 87 Octane, the computer will detect pre-ignition via the Knock Sensor. This will cause the ignition timing to retard... as much as 10*... which will take a considerable amount of power away. Also, it likely could richen the mixture to aid against detonation and pre-ignition. That would result in less gas mileage due to the rich mixtures to save the engine by the ringing of the knock sensor. So, you can burn more gas for the pennies you save.

Will it be bad for the engine? No. The computer will retard ignition to save the engine It may also richen the mixture to save the engine. It will run.

 

I do not think the PCM will add fuel as a way to combat knock. It will definitely retard timing though, making your engine less efficient thus requiring more throttle to maintain the acceleration/cruise you require.

 

It will retard, but right, I dont see the computer adding fuel. That is something that boosted people will do, but "flooding" the engine is not a knock solution, especially not in an NA car.

I have heard of people running 87, its going to depend on the quality of the fuel as well. We have 91 in CA and I ran it with around 11.4:1 compression just fine, but I had a heads/cam car and of course worked the timing tables, so I see no problem with it on a stock car, that was MTBE gasoline as well

 

You are starting out with an incorrect assumption.

Octane has NOTHING to do with the flame speed of the fuel. Octane is determined by the compounds that form under the pressure and heat of compression and the initial spark from the plug. The resulting compounds, called "end-gases" determine whether you have detonation. If they have a low auto-ignition temperature, they will ignite and start a second flame front. If they have a auto-high ignition temperature, they will not ignite, and normal combustion will continue.

For factual information regarding "octane" and "detonation", check out this reference:

GASOLINE FAQ - Part 3 of 4

A couple of quotes:

.

 

Mid grade gas in these cars will be ok if you stay at part throttle. I tried some not too long ago, and I'll never do it again. From what I saw it looked like the ECM added fuel and retarded timing. This makes sense because the richer the mixture the cooler the combustion chamber.

If you don't mind driving like a grandma then midgrade is fine and will save you some money. But as soon as you step on the pedal you'll wish you had bought premium.

John

Edit: especially in the summer time when our engines run hotter anyway........ I'll use nothing but premium.

 

I have ran 87 in my car a couple times before and I didn't notice a whole lot of difference in power, of course i haven't had the car that long either. But now I always fill with 89 and the car seems to run great for me. And it's a whole lot cheaper than 91. I love the south dakota law too that 89 has to be cheaper than 87.

 

i use midgrade when i do long trips only. otherwise, its premium for me.

 

I have never filled either one of my T/A's with anything other than premium. In Nebraska the best premium I could get was 91 octane, and it seemed to run fine on that. Even on the 93 ocatane premium we have around here, I still get 1-2* of knock retard at higher RPMs (especially around the shifts, but thats probably false knock) I haven't tried running 100 octane to see if it goes away however.

 

I put in nothing but 93. It costs a little more but I'd rather not deal with a pinging problem with the 10.5:1 compression we have.

 

I rather not use 87 or 89 just to save $3 or $1.50 a tank over the 91 premium. I also noticed my gas mileage dropped when using 87 so I doubt there is any saving.

Now 94 is definetly not needed for stock.

Hal

 

Like Hal I also did the math, look at how many miles you drive the car per week and over the year if I was daily driving the car I'd save $15 a month between 87/91. Thats a fair bit of miles to put on the car (roughly 1200 a month), and maybe not everyone drives that much, so it would be even less.

www.gaspricewatch.com

 

I have seen fuel enrichening on vehicles where pre-ignition was detected...

Not to confuse everyone, Inguneer is correct. However, the fancy words of auto-ignite and pre-ignition are synonymous... the same. It is correct that it isn't the ENTIRE equation of flame propagation but rather a component of the equation. As stated earlier, it has a lot to do with compression ratio, combustion chamber temps, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.

If either of those components of the equation are reduced or eliminated, what is the end result... you will need to recalibrate timing, A/F ratio (stoichiometry). The idea of Octane is to reduce the potential of (auto-ignition) pre-ignition which will result in the ECM retarding timing.