The RDX, Fuel Prices and Lower Octane
Thread Starter
haole kama'a-ina
Joined: Jan 2007
Posts: 593
Likes: 125
From: south of here
The RDX, Fuel Prices and Lower Octane
Although we can use lower than 91 octane in the RDX, the apparent savings at the pump may generate higher expenses later.
The lower octane, lower flash point fuel is still lit by the spark, but heat from rising compression causes it to explode in uneven pockets in the chamber, rather than the desired, smooth spreading, even burn. When the knock sensor detects this, the computer retards ignition -- lighting the spark later, when compression is stabilized -- to obtain a more even burn.
Honda sets the ignition timing in the RDX at 14 degrees before-top-dead-center (+/-2). Retarding the timing to account for low octane fuel, means the combustion event occurs later (12 or less degrees BTDC) The fuel mixture isn't done burning and less pressure is available to drive the piston down on the power stroke. Fuel economy suffers and there is a greater tendancy to foul the spark plugs. Also, more unburned fuel is likely to escape on the exhaust stroke, burning in and possibly damaging the catalytic converter.
It's easy to get confused with the terminology and sometimes we see convoluted terms such as "pre-detonation" which increases misunderstanding.
The link below is a good, straight-forward explanation of abnormal combustion.
From: Engine Basics, Detonation and Pre-Ignition
The lower octane, lower flash point fuel is still lit by the spark, but heat from rising compression causes it to explode in uneven pockets in the chamber, rather than the desired, smooth spreading, even burn. When the knock sensor detects this, the computer retards ignition -- lighting the spark later, when compression is stabilized -- to obtain a more even burn.
Honda sets the ignition timing in the RDX at 14 degrees before-top-dead-center (+/-2). Retarding the timing to account for low octane fuel, means the combustion event occurs later (12 or less degrees BTDC) The fuel mixture isn't done burning and less pressure is available to drive the piston down on the power stroke. Fuel economy suffers and there is a greater tendancy to foul the spark plugs. Also, more unburned fuel is likely to escape on the exhaust stroke, burning in and possibly damaging the catalytic converter.
It's easy to get confused with the terminology and sometimes we see convoluted terms such as "pre-detonation" which increases misunderstanding.
The link below is a good, straight-forward explanation of abnormal combustion.
From: Engine Basics, Detonation and Pre-Ignition
Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal combustion. Pre-ignition is another phenomenon that is abnormal combustion.
Detonation is the spontaneous combustion of the remaining fuel/air mixture in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn.....The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.
Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.
Last edited by 737 Jock; Mar 8, 2011 at 09:16 AM.
Intermediate
Joined: Dec 2010
Posts: 30
Likes: 1
From: Calgary, AB
I think he means the possibility of higher expenses down the road, due to fouled plugs, damaged cat converter. Also, it's implied that by retarding the ignition, a complete burn isn't happening, therefore producing less power, therefore losing the economic advantage of buying cheaper (lower octane) fuel in the first place.
Suzuka Master
Joined: Apr 2009
Posts: 8,490
Likes: 609
I think he means the possibility of higher expenses down the road, due to fouled plugs, damaged cat converter. Also, it's implied that by retarding the ignition, a complete burn isn't happening, therefore producing less power, therefore losing the economic advantage of buying cheaper (lower octane) fuel in the first place.
Advanced
Joined: Nov 2010
Posts: 60
Likes: 0
From: Slippery Rock, PA
I think this is funny b/c my Acura dealer told everyone at their open house that you can run 87 octane most of the time and throw in some premium once in a while.
I honestly rotate between 2-3 tanks of 89 and then a tank of 91-93. I average 2mpg over the advertised mileage and have no trouble with performance.
I honestly rotate between 2-3 tanks of 89 and then a tank of 91-93. I average 2mpg over the advertised mileage and have no trouble with performance.
Drifting
Joined: Feb 2001
Posts: 2,637
Likes: 118
From: Ontario, Canada
I think this is funny b/c my Acura dealer told everyone at their open house that you can run 87 octane most of the time and throw in some premium once in a while.
I honestly rotate between 2-3 tanks of 89 and then a tank of 91-93. I average 2mpg over the advertised mileage and have no trouble with performance.
I honestly rotate between 2-3 tanks of 89 and then a tank of 91-93. I average 2mpg over the advertised mileage and have no trouble with performance.
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