Premium vs. Regular and the catalytic converter?

[roycehof]

Defiant,
I believe you are correct. - I know that gas station stock regular and premium. Their mid-grade is a mix of the two at the pump itself. Usually they are posted at 87, 89 and 93 octane. If the math equalled the chemistry, I think you would expect the midgrade to be 90. Next time I talk to a petroleum chemist, I'll inquire and post back. With that in mind, it makes me more hesitant to try the "cocktail" approach (at least until I inquire further).

[mrnickleye]

Originally Posted by bigwiki

Engine with high compression ratio requires high octane gas. I'm not sure if 300e's engine has high compression ratio.

BTW, Higher octane gas is not better than lower octane gas. It's just higher octane gas explodes in higher compression. My friend use 91 octane gas in his 91 Honda Civic which is completely waste of money. He insists his car runs better with 91 gas. Actually, what he need is a tune up and adjust the timing, etc.

Those 'explosion' words don't make sense.

The higher octane fuel is for higher compression engines because it slows down "pre-detonation".

When the fuel/air charge enters the combustion chamber, it encounters many hot spots (some actually glowing redhot), from the previous burn.

Low octane fuel (regular gas) can easily ignite from those hot spots before the spark plug fires. Premium fuel (high octane) has additives in it to help prevent this from occuring, thus allowing the fuel charge to more evenly spread across the entire chamber, and fire when spark is entroduced at the spark plug.

This helps prevent "pinging" (pre-detonation" under load. Also, pinging can occur in a very 'lean' (more air, less fuel) mixture by raising the combustion temps.

This is one GOOD reason to have the EGR system working properly, to help prevent pinging.

[petegammon]

Understand that I'm cheap....and paying for premium HURTS. BUT, on a recent road trip 800 miles each way, I used regular going and premium coming back. My gas mileage with premium was 14% better, yet the cost difference for premium was only 8%...........the net is that the Premium cost me less to use than regular.......PLUS, the car runs better overall. I am NOW a believer!!

[White_Knuckles]

I see some applied theory here. Defiant and mrnickleye are homing in on the magic of high octane.

A better "explosion" is a common statement. Actually, it's somewhat accurate. Premium will yield a slower burn envelope when ignited. This cooler, longer duration burn does indeed prevent pre-ignition. Low octane produces a rapid burn which is perfect for lower compression.

The Honda guy running premium is wack, it does nothing for performance. But don't tell him, they never believe it. After all, high-octane just sounds better. Should run it in our lawn mowers to impress the neighbors.

[Glide Ride]

Originally Posted by Defiant

Um... unfortunately, chemistry does not equal "math"; your concoction isn't yielding 91.

Unburned fuel will overheat a cataclysmic diverter. How much, over how long will "depend". What kills them is leaded gasoline: the lead coats the catalyst, and insulates it from operating. Often, reverting to unleaded will clean them back to functioning over time.

I don't understand the references to unburned fuel, cataclysmic diverter or leaded gasoline? My suggestion is to only blend equal parts of 89 and 93 octane gas (unleaded implied since leaded gas was eliminated in the US in the early '80s). I don't pretend to be a chemist, but I do know math and that the average of 89 and 93 is 91.

Below is a reference from Wikipedia.com about octane rating. Note the method in the US to average the two different ratings.

In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States, Canada and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US (R+M)/2, and even deliver 98 (RON) or 100 (RON).

[Defiant]

Glide, you're getting yourself into an odd vortex.
Octane ratings, as posted on the pumps, is RON + MON / 2 . What that means is the Research Octane Number -the octane rating as determined by chemical evaluation, and the Motor Octane Number -the actual rating of the fuel's octane as determined by being run in a Research Engine (a variable-compression, single-cylinder engine outfitted with more gizmos and sensors than you can wag a tree at) and having its knock resistance determined. Since the numbers are almost never the same, but are the methods used which the industry members agree on, the two are added up and averaged. I believe the Research number is usually higher, but that may be information from the previous century.
However, mixing various octane gasolines isn't a matter of adding up and dividing. There are chemical events which occur both in the mixing of the fuels and during what goes on in the contained hell of a varying-displacement cylinder (which in itself is one of the internal combustion engine's Achille's heels as regards emissions: by the time the flame front gets to the bitter end of the available fuel, its composition has been so altered by the heat and pressures involved that it's no longer anything like gasoline) that simple math does nothing to determine what your fuel's octane rating actually is. Whether it winds up being higher than the lower, lower than the higher, or even higher than the higher will depend on the nuance of variation in the two mixes' composition.

[mrnickleye]

Well said Defiant.

Leave the mid grade alone.

Here's a little test you can perform yourself to see if your engine will 'ping' under a heavy load.

Only do this for a moment, as the tranny oil will heat up quickly if you keep doing it.

With a fully warmed up engine, Put the brakes on very firmly, so the car won't move. Now place the car in reverse (make sure you have some room behind the car).

Now press the gas pedal way down (all the way is not necessary).

This puts a big load on the engine, and if you hear some 'pinging' in the engine, release the gas right away.

If you don't hear 'pinging', then let off gas too.

Now put car into park position and let the engine idle for a few minutes to let it cool down.

If you heard pinging, then you need to run higher octane fuel. If you are running premium, then maybe some octane booster will help.

Of course we are assuming the timing is set correctly, and the computer system is functioning normally, to retard the timing to prevent the pinging.

Retest with better fuel, (or booster), and if pinging still occurs, have a professional inspect the engine timing and such.