Bottom line - use the lowest octane fuel you can get away with.

 

Actually that is a myth
All gasoline grades contain the same amount of energy.
You can not change the flashpoint or burn rate with out changing the amount of stored energy.

When you advance the timing you are compressing the air/fuel more before you send spark. If your peak compression exceeds the fuel rating, then you will need to drop down to lower heptane rating, (which is a higher octane rating.) Otherwise it might ignite from being compressed. causing the "ping" which is actually predetonation, (aka dieseling) from the fuel being ignited by being compressed, instead of being ignited by the spark.


My 5.4L had 8.4:1 peak compression ratio when it was new, (I'm sure it's even less now that it has 106K miles on it) When it was new and had top compression I could not have and could not now.... compress 87 octane (aka 13 neptane) hard enough to make it ignite. I could advance 18º past TDC and still run 87 octane, and beable to control when it ignites by the spark plug, with out fear of predetonation.

 

Thats the seceret in a nut shell.

 

The best gas mileage I've seen yet in my truck was on 85 octane mountain gas. No pings either.

 

I'm missing something in the explaination - old engine or new, we can argue the point on whether it's compression or timing. Bottom line is I get better mileage and performance by running the maximum timing without detonation.

What I was explaining was that using the tuner to advance the timing curve (again, whether that causes higher "compression" we could argue) to take advantage of the higher octane fuel - and get more than enough improvement in mileage to offset the cost of buying the more expensive fuel.

The reverse is also true, if you run E85, your computer retards timing (along with a series of other things it changes) to compensate. In doing so, you get around 30% worse mileage and performance. Therefore, the cost of E85 needs to be at least 30% cheaper to justify running it (not even going into the environmental impact discussion).

This is all real world data, compiled by myself and quite a few friends.

So, once again, stock will see little to no advantage with Premium. Hit it with a tuner and I can justify it in both mileage and performance. IF you are going to run Premium, get a programmer. IF you don't want to use a programmer,

We can argue engine management systems all day long, but the answer to the man's question is in the previous paragraph.

 

Fair enough, I am out of ideas for way to explain it.

Carry on.

 

The combustion process has changed?????

Man I am getting old, caus this is news to me, I am apparently behind the times!

 

No, E85 does not cause the timing to be retarded. E85 is over 100 octane and the timing will ADVANCE. The reason it gets bad mileage is because stoichiometric gasoline air/fuel ratio is about 14:1 and E85 is about 9:1. The O2 sensors see this and cause a lot more fuel to be dumped in. The performance on E85 is actually a tad better than on 87 octane gasoline.

Compression is compression - engines do not have a variable compression ratio. All the varying ignition timing does is determine exactly where in the piston cycle the spark plug fires. The more advanced, the earlier it fires before the piston reaches top dead center. The slower burning the mixture is (higher octane) the earlier you can fire it without ping - which is essentially the mixture cooking off before the piston reaches TDC. Maximum power is achieved if the mixture cooks off exactly at TDC. If it's after TDC, that's retarded and you lose efficiency.

 

You are correct, I misstated E85.

For PSS, what do you think octane is? It's the burn rate, but that's another can of worms that puts us into chemistry class. What I missed in the explaination was in your explaination, there is no point in anything other than 87 octane. Why don't we discuss why performance vehicles require higher octane? That's the discussion I wasn't going into, but PSS wants to push the point. There isn't enough room on the forum to discuss all the fine details, so as with virtually every post you choose how deep you want to go into the details. The cylinder pressure is higher with advanced timing, but not the compression ratio. You actually fire the mixture at a lower compression level (farther from TDC, lower compression level).

You need maximum pressure as the piston crosses TDC. You have to fire ahead of it or as you said, you are retarded. The higher octane burns slower and more predictably and so you advance the timing to maximum the fire.

In every form of racing, they run the maximum timing and octane for maximum power. Has been that way virtually forever, not likely to change unless the internal combustion engine changes dramatically.

I was trying to opt out of ten page posts to explain the internal combustion engine and get back to the original question: Will running Premium help on a stock engine. And I've answered that a couple times - so, one more time, no, no value on a stock set up, add a programmer and there is huge value.

 

You are soo correct, I've had my timing backwards. :o

Same actions just opposite.

An engine doesn't have variable compression but it has different compressions at differnt points in the stroke, at TDC it is at maximum compression, when it's at the bottom it's zero compression. On its way past TDC as it goes down you have vaccum, wich is the opposite of compression.

As an aviation mechanic told me....
A 4 stroke engine is simple
1. Suck
2. Squeeze
3. bang
4. push

thats all 4 strokes.

Compression increases as it comes up. The further before TDC you fire the plug the less compression you have on your A/F Mixture at the point of ignition, so obviously the less compression you are putting on the air/fuel mixture before you ignite it the more neptane you can have, so the lower octane.


Which makes using higher octane with a tuner even more silly.

 

Yes I am doing a DITY move. I am already at my station, I just need to get furniture I didn't plan on bringing before. They pay for your gas whether you use 87,91, or a billion octane. From FT.Riley KS to Georgia and back to Ft. Riley.

 

AHA!!!
Thats where you are missing it.

No it's not the burn rate at all.
That is 100% myth.

As I said all grades are idnetical in burn, temps everything...... EXCEPT... how much pressure they can with stand with out igniting.

 

With that train of thought then with a better tuner you could run alcohol and make power like a top fuel dragster!!!

I sooo almost commented on this myth earlier but deleted it before I submitted.... because it had nt been brought up....
This is exactly the train of thought that fuels the octane myths and confuses everyone.....

This is a myth and is not based on science, but based on the assumption that most high performance cars use high octane fuel, so therefore if you use high octane fuel you will increase performance. It simply does not work that way. An engines performance with certain fuels is by design, not the fuel used.

Edited for spelling and to add:

The reason they use the fuels they do is to control the ignition in their high compression engines.

 

PSS, you are making the wrong argument. They can advance the timing with higher octane, you seem to have an issue with programmers improving performance. It is not smoke and mirrors, nor is it made up by the programmer people. It works, whether you believe or don't. None of this applies to the man's question and we are hijacking the thread with the discussion.

And on it's way past TDC, it is not on the vacuum stroke, that's power, then exhaust, then vacuum (filling the cylinder), then compression again. But you knew that.

Several things come into play with performance, compression and octane are just 2 variable. To counter your argument about a good enough programmer could give you the performance of a top fuel dragster - you should be able to run on Diesel or even water, octane doesn't have any effect. They are both ridiculous arguments. If spark timing didn't matter, it wouldn't be adjustable. Same with fuel charge size, fuel charge timing, or or any other variables.

I picked this little gem off Wikipedia "Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause knocking. *****(Note that it is the absolute pressure (compression) in the combustion chamber which is important - not the compression ratio. The compression ratio only governs the maximum compression that can be achieved)****... However, premium grades of petrol often contain more energy per litre [citation needed] due to the composition of the fuel as well as increased octane." (asterisks added for clarity) So, we are both right to some degree.

You say things are myths, but offer nothing to substantiate your claim, just the your opinion is they are myths - sorry, I've been racing drag, mud, sports car, etc for years. Pitted with plenty of racers, turned wrenches with them. Guess they are all wrong and you are right.

Now you can post your reply saying it's all myths and every racer I know is dead wrong and doesn't know anything about the subject - everybody, have a great weekend, I'm outta here for now!

 

First off your totally taking this personally and getting emotional.

I never said tuners didn't work..... they do!!
However, they do not work by letting you use a different fuel. That is the silly part.

I'm also not the only one not producing anything to back my claims....

I do belive that I am the only one that has been able to provide a link to any info to back my intel......


My intel comes from the fuel distribution plant where my wife works... They might know a thing or two about each fuel... And it is all backed up in numerous sources if you care to do a little research....

I'll help you get started...

what is the difference in octane ratings

Higher octane makes more power as much as vinyl stickers add torque.

Yes high perfomance engine uses higher octane but it's not the gas making the power.