Many engines do in fact run better on regular fuel than high octane fuel. In some engines, you can't tell the difference. In those cases, use the cheaper fuel and save a few bucks. What ever fuel works best in your application is the right one for you. One may make more power but less mileage, or the opposite. One may make more pollution, no matter how it runs. My old 66 Grand Prix barely ran on regular. My Celebrity does not care. The old Sentra ran better on regular.

Low octane is more voletile than high octane fuel.
But in fact Diesel fuel has much more BTUs than gasoline. The further down on the refining cracking scale, the heavier or thicker the fuel is, and the more BTUs the fuel has. It's harder to light, but will burn longer and produce more heat. Gear oil burns a very, very long time. And it produces more BTUs than any of the above, once you get it lit.

What is important is how much heat you are able to transfer into the air in the cylinder.

The combustion is not really what pushes the piston down. It does contribute, but it is not the main force. When the piston is between 10 and 25 degrees after top dead center, the flame is out, combustion is gone. From there what pushes the piston down?

We pull air into the cylinder which is about 21 percent O2. The rest is mostly Nitrogen and some other gasses. Nitrogen is inert. It does not react in the combustion process. But what does happen is the burn superheats the air. The superheated air expandes, and produces the pressure that pushes the piston down.

The more you heat the air, the more the pressure rises, and the more power you get out of the engine. Plus, the more air you get in the cylinder, the more expansion and more pressure. You control the air into the engine with the throttle connected to your right foot. Turbos and superchargers work becuase they put more air in the cylinder, so the expansion pressure is higher.

Low octane fuel does burn hotter. High octane burns slower and cooler. This does not mean the air gets hotter with low octane. Due to the longer burn time of high octane, the air is heated for a longer period of time. It's like holding a match under your hand. You can do it for a short time and not get burned. But holding it slighly further away so it's cooler, and holding it there for a long time will cause severe burns. More heat transfer.

So back to the question of which fuel.

And we go back to the engine design answer.
Some engines will not ignite high octane fuel very well. So the burn is not complete, and the air is not heated. Low octane fuel fully lights, and produces more power.
Other engines will benifit from the longer burn time of high octane, and can properly ignite the slower less voletile fuel.

There is no one correct answer that will cover everything. Typically, high compression engines will prefer high octane. Lower compression engines prefer low octane. But this is not always true. The valve timing and cam profile, head design, cylinder quench areas, piston design, and ignition system all play a role.

A quench area is a cooler spot in the cylinder caused by cooling system and/or metal design/shape that causes the fuel to burn slower in that area. This allows the fuel burn pattern to be controlled in the cylinder, and propogates a more controlled burn pattern. All engines have them. Some are intended, some are just results of other design. Different fuels respond differently to the quench areas.

I hope this serves to answer the question, and not confuse the issue. If I can clear anything up, ask away.

|UPDATED|10/6/2004 11:20:48 AM (AZT)|/UPDATED|

Ken,

Of course, everybody knows how an engine works,
just ask my wife, she'll tell you...

"You put the key into the thingy, and it runs."

You don't need to get into all this high-tech crap there, Ken.

Just make sure that the thingy works.



Alan

|UPDATED|10/5/2004 9:05:32 PM (AZT)|/UPDATED|

i have a 96 deville the dealer just did a top end carbon clean he said the adaptor cost him 1300 and cleaner was 70 a pint but what a difference it did help

OHH the thingy! My wife talks about that too!

Thanks Alan, I needed that.

Shyucks, durn, and gollliee.
That explains phasing the tail light push rod to the hemulator so the heramastat will operate the comboobulator and reemulate the pistons return springs!