Manual calls for 87 Octane

[toms89]

I

Your vehicle is equipped with knock sensors and it uses them to stay on the verge of knocking, not by detonation, but by igniting the fuel to early. It is a self adjusting system.


Just stick with 87

Detonation is commonly caused by too much ignition advance. The extreme pressure wave created by igniting the fuel too early in the combustion cycle causes the fuel in another area of the cylinder to ignite spontaneously. Much like a diesel would but creating multiple pressure waves in this case. Higher compression, boost, combustion temps all increase the likelyhood of this happening. This is bad but it has much more serious consequences in boosted vehicles.

Higher octane fuel resist this but as said burns slower. So no power generally is to be gained running higher octane unless the motor is designed for it or tune is adjusted accordingly.

I would venture to say most vehicles since the late 80's have included knock sensors.

The common theory seems to be that the pcm runs the engine on the edge of detonation utilizing the knock sensors.

My belief through my own experience via tuning forums, text and learning to tune my own expedition is this not to be true in many cases. There are a couple of reasons. One is that maximum brake torque (power) does not always come with the highest timing possible before the detonation threshold believe it or not. Another is that in order for the knock sensor to work it would have to sense some degree of detonation no matter how mild. When detonation occurs you must retard the ignition timing beyond what would be safe if it did not occur in the first place. More power should be had with less timing before any degree of detonation would occur.

At least with the 2000 expedition pcm software it appears to me the knock sensor is set up more as a safety or preventative measure. Not really as a tuning aid. It allows the pcm to retard timing if you get some bad fuel, poor tune or improperly running engine. Some examples would be lean air fuel ratios, hot spots in the combustion chamber, clogged cats creating excessive combustion chamber temps, too hot a spark plug, etc....


In the end I agree. I would run the recommended fuel unless your having issues. Higher octane will not hurt anything but doubtful you will see any benefit. The tune can be adjusted for higher octane fuel but the gains will be minimal unless you have a motor designed for it.

 

[metaldrgn]

Can you explain this please?

I can't remember the exact figures, but you get like 80% of your power in the first 20% of your power stroke. If the fuel is ignited too late, you lose power since you lose that max possible compression ignition. If it's ignited too early it causes a knock because the piston is still traveling upward and forces it to slap against the wall (I believe) and reduces power (also will destroy an engine) because it's still traveling upward.

The PCM reads the output of the knock sensor and can tell when it's too advanced by the signal (also nowadays this is what's telling you if you have a misfire too if I remember right). If you reset the PCM, it starts with default values and then starts adjusting the spark curve based off these inputs and strives to achieve a safe but efficient timing. From my OBD-I programming days, the spark curve table is defined under RPM vs load (airflow in grams/sec). There are a lot of other variables too, but they are under different tables. The PCM has on-board memory that stores these new values so it doesn't have to relearn every time.

It's not as easy as just igniting it at TDC (top dead center) because fuel doesn't burn instantaneously. It's fast, but in engines that's still a lot of time and only starts at one point. Old school standard timing is 6 degrees before TDC because of this and as the RPMs go up you need to ignite it sooner (more advance) because the pistons are moving faster, but the fuel doesn't burn faster. After the mid to late 90's you don't even set the initial timing anymore. I don't know how much you already knew, but I thought this would also be good for others that don't know this as well.

 

[metaldrgn]

Detonation is commonly caused by too much ignition advance. The extreme pressure wave created by igniting the fuel too early in the combustion cycle causes the fuel in another area of the cylinder to ignite spontaneously. Much like a diesel would but creating multiple pressure waves in this case. Higher compression, boost, combustion temps all increase the likelyhood of this happening. This is bad but it has much more serious consequences in boosted vehicles.

I think you are mixing the 2 types of knocking. Detonation is when the fuel you are using spontaneously ignites without the use of the spark plug because of a lower than recommended octane rating or just some bad gas (it's an RMS average rating of 2 test standards so it can differ from station to station) or other variables like environmental. Knocking/pinging from too much advance doesn't cause detonation from anything I've ever read.

I would venture to say most vehicles since the late 80's have included knock sensors.

Yes, anything with a digital fuel injection or digital carburetor probably had one. My '85 Camaro had one (first year of fuel injection!).

At least with the 2000 expedition pcm software it appears to me the knock sensor is set up more as a safety or preventative measure. Not really as a tuning aid. It allows the pcm to retard timing if you get some bad fuel, poor tune or improperly running engine. Some examples would be lean air fuel ratios, hot spots in the combustion chamber, clogged cats creating excessive combustion chamber temps, too hot a spark plug, etc....

All cars use knock sensors in the same manner. The software dictates how far to retard the timing from knocking. Yes those examples are other good reasons why they use knock sensors because if they weren't there, that knocking would destroy an engine.

 

[toms89]

I think you are mixing the 2 types of knocking. Detonation is when the fuel you are using spontaneously ignites without the use of the spark plug because of a lower than recommended octane rating or just some bad gas (it's an RMS average rating of 2 test standards so it can differ from station to station) or other variables like environmental. Knocking/pinging from too much advance doesn't cause detonation from anything I've ever read.

.

Detonation Explained


http://www.onestopauto.com/What-causes-spark-knock.html


http://www.contactmagazine.com/Issue54/EngineBasics.html


In the world of boosted engines its well understood that detonation is most likely caused by too much ignition timing, too low octane of fuel or high intake air temps. Obviously there are other factors as well or a combination thereof, but the point is ignition timing too advanced does cause detonation.

 

[FordandPolaris]

It is my understanding that premium fuels simply have additives to increase octane, but do not have any benefit by itself in performance terms. It simply allows you to use things such as turbos and high compression to make more power.

 

[toms89]

All cars use knock sensors in the same manner. The software dictates how far to retard the timing from knocking. Yes those examples are other good reasons why they use knock sensors because if they weren't there, that knocking would destroy an engine.

My point there is the pcm does not constantly advance ignition timing till it senses detonation to find the threshold of ignition timing as I was led to believe.

It's set up more as a safety should conditions outside the norm be encountered.

This is a large reason I believe aftermarket tuners are able to easily increase power. The factory generally leaves at least a few degrees of timing on the table for safety which the tuners can quickly add back in. After all the factory has to warranty the vehicle where the tuners do not.

I have starred at my timing tables, adders and modifiers for hours on end while tuning my setup and nothing seems to indicate otherwise.

 

[metaldrgn]

Detonation Explained


What causes spark knock and how do you get rid of it


Engine Basics: Detonation and Pre-Ignition by Allen W. Cline


In the world of boosted engines its well understood that detonation is most likely caused by too much ignition timing, too low octane of fuel or high intake air temps. Obviously there are other factors as well or a combination thereof, but the point is ignition timing too advanced does cause detonation.

You're right, that makes sense.

 

[metaldrgn]

My point there is the pcm does not constantly advance ignition timing till it senses detonation to find the threshold of ignition timing as I was led to believe.

It's set up more as a safety should conditions outside the norm be encountered.

This is a large reason I believe aftermarket tuners are able to easily increase power. The factory generally leaves at least a few degrees of timing on the table for safety which the tuners can quickly add back in. After all the factory has to warranty the vehicle where the tuners do not.

I have starred at my timing tables, adders and modifiers for hours on end while tuning my setup and nothing seems to indicate otherwise.

That rings a bell . That they just retard starting from the values in the spark table, but not advance past it. It's been awhile. I believe it will constantly try and advance it back to the defined settings though so that way if you had some bad gas or whatever you wouldn't have to reset your PCM periodically.

I totally agree. There is a safety zone because there are so many small variations in all the engines and it has to work for all of them.