Regular Gasoline

[wwest]

Why increase the amount of fuel to reduce pinging, and increase emissions, when you can change ignition timing. These days it's about lowering emissions, not keeping power/torque at it's peak. WWest you should read a live data list from a Lexus ECU and you'll see how much more timing changes than injector pulse width, especially during detonation.

Boy, do I really, REALLY know how to miss-state, miss-represent, my own case, argument.

Let's start out using today's more likely scenerio wherein the vehicle engine was designed primarily for the use of premium, 92 octane, fuel.

Premium fuel is less subject to predetonation, dieseling, so we can begin by raising the compression ratio above that which would seemingly be required for using a lower grade fuel. Let's also throw in the most ignition advance, statically and dynamically, we possibly can in order to achieve the highest level of HP/Torque possible from each ounce of that EXPENSIVE high octane fuel.

Okay so far..?

But now we want the owner to have the capability of fueling with 87 octane if the need arises.

Can't lower the compression ratio....??

Lower grade fuel, 87 octane, under the high compression ratio of "this" engine would undoubtedly be more subject to predetonation, dieseling.

Timing....??

Retarding the timing will do little good if the knock/ping is the result of dieseling, self-preignition of the A/F mixture.

Advance the timing...to a certain extent, yes.

Pre-ignition due to dieseling often results in an uneven, "ratty", flame-front progression and "tweaking" the timing forward a slight bit might provide a more widespread flamefront and thereby less stress on the engine. Not recommending this as a practice in any way, just thinking out loud.

So, advance the timing..??

NOT...!!

Sorry, I used a poor example, a really poor example, using my 78 Porsche.

So, how do we prevent dieseling of a low grade fuel when used in an engine with a compression ratio tuned/designed for the use of premium?

Enrich the mixture.

[wwest]

Read this> http://en.wikipedia.org/wiki/Air-fuel_ratio

Than you can change it to suit your hairbrained ideas like you did before. :)

Sorry, the referenced Wiki statement regarding proper/correct A/F ratio is already perfectly aligned with my own.

To quote:

"A stoichiometric mixture is the working point that modern engine management systems employing fuel injection systems attempt to achieve in light load cruise situations."

Operative term being...

LIGHT LOAD CRUISE SITUATIONS.

My 2001 C4 would not likely be able to produce its rated 300HP were the mixture left at the stoichiometric ratio as the throttle is opened farther and farther from the otherwise meager light engine loading cruise requirements.

At high throttle openings, high engine loading, the "front" oxygen sensor(s) otherwise used to monitor for proper stoichiometric ratio (no unburned oxygen in the exhaust gasses) are ignored. Instead the MAF/IAT sensor outputs are used to control the mixture at a typical 12:1 ratio or beyond. A seond point, somewhat important, is that during light loading operations the MAF/IAT sensors are "calibrated" via the front oxygen sensor output monitoring. If the MAF/IAT signals should for some reason drift from the previous calibration point the ECU simply "learns" the new one.

That's why those ebay gadgets that modify the IAT signal output, seemingly to result in a higher mixture ratio and thereby more HP from a given engine design, simply do NOT work.

On the other a more sophisticated design, one that only modifies the IAT signal upon/with high throttle openings, might well work.

Haven't yet noted that any of those Ebay device vendors want, or even know, to go to that extra effort.

[LeodLion]

I believe the recommended octane rating to use is 91. I assumed that the Lexus engineers did a lab study and concluded among other things that 91 is the optimum (RON + MON)/2 octane to use. I accept this irrespective of all the wonderful discussion on octane relationship with engine ping which I find very helpful and informative.

Now here in Texas we have three choices: 87, 89 and 93. If I want to use a gasoline with octane of 91, I would think that the best way to achieve this is to blend 89 and 93. The question is what is the correct volume mix? I am a chemical engineer and I can tell you that there are complicated formulas to achieve a certain blend of gasoline based upon the properties of each blending component. In a refinery there are several tanks of components which are mix to finally arrive at the desired specification of the gasoline product. Octane number is only one of the determining factors but the most important.

In a practical point of view I think that a 50-50 mix of 89 and 93 will be approximately close to achieving a desired octane of 91. Therefore if I fill up and needed 10 gallons, then I would start with 5 gallons of 93 then followed by 5 gallons of 89. Yes it will take more time. I am not sure if you have to swipe your credit card twice for this. But I will do this next fillup.

The main purpose is to try to use the recommended octane of 91. There will be some savings but this is not my main objective. I dont want to waste resources.