93 octane in Denver? Options

[35th_Anniversary...]

seems like stupid question, but Topeka sure as heck didn't have any 93.

[Rob Hood]

I didn't think they had that high of octane at that elevation. Maybe 89 is the highest?

Check this story out though - http://www.koaa.com/news/view.asp?ID=251

[Sam Strano]

Lat time I was there 91 was it because of the altitude. I'm sure Mike Minear can explain further as he lives there. But apparently with less oxygen the detonation problem is reduced and you really don't need the extra octane.

If you really want it, I think you have to bring it. (IMG:http://www.frrax.com/rrforum/style_emoticons/default/smile.gif)

[trackbird]

Thinner air reduces the filling of the cylinders, therefore if the cylinder only contains 50% of the air molecules that it would see at sea level (for example, it's not truly that low) and you compress it 10:1, it's only a true 5:1. So, the altitude lowers cylinder pressure and reduces detonation problems.

In a nutshell......

[35th_Anniversary...]

Yeah, but with a "mass" airflow sensor doesn't the stupid computer detect less mass per unit volume and reduce the amount of fuel going into the cylinders?

[trackbird]

Yeah, but with a "mass" airflow sensor doesn't the stupid computer detect less mass per unit volume and reduce the amount of fuel going into the cylinders?

Yes, but compression causes detonation. It's a function of peak pressure created when the piston is headed for top dead center. As you compress air, it heats up. If it is compressed enough, it can get hot enough to cause fuel (low octane) to ignite. This can be helped by hot spots in the chamber, carbon deposits, etc.

However, if you only fill the cylinder half full, you will not see nearly the peak cylinder pressure during compression. Therefore, it doesn't matter how much fuel you've put in there (as long as the mixture is right). Less air = less pressure, less pressure = less temperature rise. Less temperature rise keeps you out of "trouble".

Geez....why am I explaining this to engineers..... (IMG:http://www.frrax.com/rrforum/style_emoticons/default/rotf.gif)

[35th_Anniversary...]

the lower compression ratio is really a misnomer. The valves always open and close at the same time thanks to our camshaft. Because atmospheric pressure is lower the pressure inside the cylinder when the sparkplug ignites is also lower. Octane isn't needed for "higher compression ratio" engines, but those with a certain internal cylinder pressure. The compression ratio on most GM vehicles with superchargers is very low, but they still require premium gasoline to prevent detonation / early combustion.

right?

[trackbird]

Right. We are actually discussing dynamic compression and not truly static compression ratios. I was trying to keep this light, so I over generalized it.

You've touched on one reason that cars with larger cams (longer duration) can use more compression on pump gas. The extra valve overlap allows the engine to push some air back out of the cylinders at lower RPM and not truly fill the cylinders. So, to correct for the loss of pressure (and therefore torque), we raise the compression ratio to help bring the cylinder pressure back up and restore some of that torque. Engines are less sensitive to detonation at high RPM. The mixture motion of the air and other factors contribute to reducing the tendency to detonate (as RPM increases) and at high RPM that longer duration cam can take advantage of the tuning effect of the intake runners and the high velocity air flow through those runners (which has considerable energy and is often just short of being super sonic at high RPM, air moving that fast has a lot of energy and it is used to pack air into the cylinder even though the piston is already coming up towards top dead center). But, we are now headed into a far more in depth discussion about engine dynamics (instead of just answering your question about altitude and octane).

If you have a cylinder in a vacuum. It has nothing inside. When we make that cylinder 1/10 its original size, we still have no pressure. It would have a mechanical compression ratio of 10:1, but the effective pressure is 0. Because 10 X 0 = 0.

If we super charge that cylinder and pack 20 psi in it (at bottom dead center, so it's truly full) and then compress it to 1/10th its original volume, we'll see 200 PSI. At 200 PSI, air can get hot (think of a 25 PSI turbo and the exit temps those can see, which is why we intercool). If it gets hot enough, BOOM. It detonates. Now, if we have sea level (roughly 15 PSI) and squeeze it 10:1, we are at 150 PSI (off the top of my head, I believe it's generically a linear relationship between starting and finish pressure...if not, our engineers will correct me, but this will illustrate the point). Again, at 150 PSI, things can get hot (which is how a diesel works and why they have no throttle plates, they need the airflow to make the compression to make the heat to detonate the fuel).

Now, go to 6,000 feet. Say we drop to 10 PSI atmospheric pressure (I don't think it's anywhere near that low, but I'm too lazy to look up the exact change). 10:1 only produces 100 PSI, far less than the 150 PSI we saw at sea level. So, the temperature rise is signifigantly less and our requirement for octane is also far less.

Again, if the pressure relationship is not truly linear, I'm sure someone will correct me. I'm trying to illustrate the big picture (as effectively as possible....without verifying my math 100%).

Better?

[00 SS]

I've never gone through all the thermochemistry, but generally our premium is 91 octane. There are a few places around town where you can get 93 and 100 octane unleaded. I've tried them in my car, but I saw no difference. My engine is pretty much stock. If you have a higher compression ratio you may be able to make use of the higher octane fuel even up here. Some people with 11:1 or higher say they do get some benefit from it. The biggest problem is the cost. 100 octane is between $4-5/gal.

Maybe you guys can help me out with something. It seems to me that if you use too much octane, you can actually reduce your power output. Octane makes the fuel burn slower, so if you don't have the compression or the timing to take advantage of it, it can hurt power right?

BTW, absolut atmosheric pressure in Denver is 12.6 to 12.8 psi depending on the weather.

This post has been edited by 00 SS: Dec 14 2005, 04:29 PM

[trackbird]

Maybe you guys can help me out with something. It seems to me that if you use too much octane, you can actually reduce your power output. Octane makes the fuel burn slower, so if you don't have the compression or the timing to take advantage of it, it can hurt power right?

BTW, absolut atmosheric pressure in Denver is 12.6 to 12.8 psi depending on the weather.

I believe that higher octane makes fuel more resistant to ignition (like detonation), but not slower to burn. Fuel contains the same amount of energy per gallon regardless of octane. It's harder to light off, but burns at virtually the same speed.

[00 SS]

Interesting. I thought the vapor pressure determined how easily it would ignite. Maybe the octane effects the vapor pressure?

This post has been edited by 00 SS: Dec 14 2005, 06:12 PM

[trackbird]

Interesting. I thought the vapor pressure determined how easily it would ignite. Maybe the octane effects the vapor pressure?

It's been years since I used any of this info and I'll have to start with the research if I'm going to go much deeper into this one....I just forget the rest of it..... (IMG:http://www.frrax.com/rrforum/style_emoticons/default/banghead.gif)

[00 SS]

I didn't think they had that high of octane at that elevation. Maybe 89 is the highest?

Check this story out though - http://www.koaa.com/news/view.asp?ID=251

Thats an interesting article, but as usual the news has failed to back up their own claims. It's funny that the oil and gas folks couldn't back their assertions up either. I would think it would be pretty easy to baffle the reporter with some of the very info in this thread and make him think there is a very sound reason for lower octane at altitude even if the automakers don't seem to agree.

The bottom line is if your engine is not knocking or pinging and you have no knock retard, you don't need more octane.

[trackbird]

The bottom line is if your engine is not knocking or pinging and you have no knock retard, you don't need more octane.

Exactly.

Yea, that article smelled of "the world is out to get us, everyone call your congress people". Typical news stuff...borders on sensationalist journalism.

[35th_Anniversary...]

I'll run whatever Danny Popp says is okay.... If I have to cart 10-15 gallons of 93 into Denver I'm not going to complain. I'll save enough money to repay the cost of the fuel containers with just a few trips through Ohio. I just wanted to be prepared for the NT/Pro Solo.

How often you you ever get knocking in modern engines? they all have knock sensors and retard the timing appropriately.

The news story is what happens when you have reporters talking to marketeers and not engineers. Misinformation is what alot of the news contains and the rest of the news is propaganda.

[00 SS]

I'll run whatever Danny Popp says is okay.... If I have to cart 10-15 gallons of 93 into Denver I'm not going to complain. I'll save enough money to repay the cost of the fuel containers with just a few trips through Ohio. I just wanted to be prepared for the NT/Pro Solo.

How often you you ever get knocking in modern engines? they all have knock sensors and retard the timing appropriately.

The news story is what happens when you have reporters talking to marketeers and not engineers. Misinformation is what alot of the news contains and the rest of the news is propaganda.

Correct, but if the knock sensors are causing your timing to be retarded, you're loosing power. You would need a scan tool to know if you have knock retard or not.

[Spectator]

I thought higher octane fuel actually had slightly less combustion energy... not enough to be significant in real life, but it's there.

That statement has nothing to back it up except my vague memory of thermo lab in school, but perhaps it might jog someone else's memory. And if that statement gets rebutted with facts, I refuse to tell you the name of my school. (IMG:http://www.frrax.com/rrforum/style_emoticons/default/unsure.gif)

[pknowles]

How often you you ever get knocking in modern engines? they all have knock sensors and retard the timing appropriately.

I only get knock when I pull the clutch out a little too fast and lug the engine at a traffic light. I have an aggresive timing table in the Camaro too; up to 10 degrees more then stock in some spots. I would say 90% of the time that I do knock it is below 2200 rpm. Before I modded my "timming vs. air temp" table I would knock more when it was hot outside ~90+.

[pknowles]

I thought higher octane fuel actually had slightly less combustion energy... not enough to be significant in real life, but it's there.

I know higher octane is more stable, meaning it's harder to ignite. But you gain efficiency because you can operate at higher cylinder pressures.

[Spectator]

I understand that. I'm talking about the actual energy that you get from combusting a given unit of 87 octane fuel vs. 93 octane fuel, in an engine tuned for 87 octane, and without including factors like delta T.

Again, my memory's pretty fuzzy on this, so I can't say this with much certainty.