octane rating and engine performance

patperry2766 wrote:

Turtle n Peeps wrote:
News flash: Why are you people talking about "boosted" engines? The OP has a 5.0 Ford.

A 5.0 Ford. Not an ecoboost, not an Indy engine or even a top fuel engine. He owns a 5.0 Ford.

A 5.0 will not detonate on an average day in Denver on the cheap stuff they sell there. End of story. There is a reason Denver fuel has less octane. :R

OK, so after great debate, it seems that the most commonly taken stance is that since the altitude is higher and there is less air and pressure in the cylinders, then there is no need to worry about the 85 vs 87 rating on my 5.0.

Now, as I understand it, the octane has all to do with preventing detonation, which my logic believes to be usable power in the engine.

If at 85 and under load in the mountains, the engine will R E T A R D ignition to reduce knocks & pings (correct?) then to me this means that it is reducing engine performance further that just based strictly on altitude alone.

So....if I run the 87 or higher under load, then the computer might not be so inclined/required to R E T A R D ignition thus giving me more usable hp/tq??

FYI, I guess R E T A R D spelled any other way is considered offensive and deleted out like it was a cuss word or not PC.

Ok, without writing a bible book about the subject here's the deal.

No, you engine won't ****** the timing. The reason is air...……...or lack there of; but it gets a little more complex than that.

If you pack a lot of "air" into an engine you have a lot of o2 in there also because air is around 20% O2.

As Joe Dirt would say: O2 makes things splode. This is why you should run a high octane fuel when one uses nitrous "oxide". Nitrous oxide contains a lot more O2 than air does. Exploding breaks things, like rings and piston lands and rods and things like that. That's what detonation does...…...it breaks things.

What we want is a nice even burn in the cylinder. Not too fast, not too slow but a nice even burn. Not an explosion, but a burn.

The reason engines make more power as sea level is because the air is more dense there. The tracks I race at usually have a density altitude of around 200 to 250 feet or so. My home track is something like 20' above sea level.

My engine is designed to burn premium at sea level. No problem with detonation at all. Now if I went to Denver and raced there I could burn the cheap stuff because of the density altitude there. I would guess the DA there on a hot day would be like 6 to 7,000 feet. The air is just too thin to pack the cylinders full of air and detonate.

Take note. My engine has no puter mumbo jumbo that can pull timing out. It's a set it and forget it system.

Your engine has knock sensors on it. If they detect knock they will tell the puter to pull timing out of the base map. (base timing) Since the air is so thin up there and so little O2 there is no need for a higher octane fuel. This is why Denver has lower octane fuel than lets say San Francisco. Your engine will not pull timing because there just is not enough air to pack into the cylinders to detonate.

All that being said, my answer would be different if you had a turbo engine. All a turbo is, is a pump. It pumps air. That's all it does. When you are at altitude and the air density drops, the turbo (pump) can make up for this by pumping more air into the engine. This keeps the engine at pretty much the same power that as sea level. (slightly less for you nit pickers) When that pump, pumps all of that air into the engine you will need the same octane fuel as when at sea level so the puter won't pull any timing.

My chart for a N/A engine would be:
Sea level on a hot day= Run good octane fuel.
High altitude on a cool or cold day= Run the cheap stuff.
Anything in-between your going to have to figure it out.