A while ago I read CarTalk’s analysis on premium vs. regular fuel; specifically, questioning whether or not one can simply rely on the knock sensor to run a vehicle that requires 91 on 87. According to CarTalk, you can run a premium required engine on 87, so long as it has a knock sensor and you don’t hear pinging:
But the general consensus among CarTalk, my dealer, and the internet at large is mixed. So I decided to dig deeper on the matter myself, and came across some sources of information:
The octane requirements of an engine are determined not only by its design and timing parameters, but also on the operating conditions and environments of the vehicle. For example, ambient temperature and altitude influence octane requirements.
For temperature, it seems that, roughly speaking, every 7C +/- results in a change of +/- 1 ON. For altitude, it seems to be about a reduction of 3-4 ON per increase of 1000m (or 10 kPa). Finally, spark advance/retard of 1 degree results in a change of 1 ON.
My hypothesis right now based on articles that I’ve read is that vehicle manufacturers also use the words “recommended” vs. “required” according to both engine design and assumed operating environments, and whether or not spark retarding can prevent knock under all possible operating conditions. For example, manufacturer might assume the vehicle to be operated between -20 F to 120 F. Altitudes -300 ft to 12,000 ft. Weight loads +80lbs to +1,000 lbs. (For typical consumer).
If a high compression engine design with a knock sensor permits the vehicle to operate in the full range of conditions by adjusting timing to avoid “significant” pre-detonation, then the manufacturer puts the label “91 recommended.”
If a high compression engine design with a knock sensor permits the vehicle to operate without knock in a subset of the full range of conditions by adjusting timing to avoid “significant” pre-detonation, then the manufacturer puts the label “91 required.”
As an observation, if you examine the new 2015 TLX, there are two engine types:
Compression 11.6:1: 4-cyl.
Compression 11.5:1: V-6 & SH-AWD
The 4 cylinder engine has a fuel label “91 recommended.” The V-6 has a fuel label “91 required,” even though its compression ratio is technically smaller. They both use the same mechanisms to compensate for varying octane number and operating environments: knock sensor with spark retarding. However, the V-6 model is also 300 lbs heavier than the 4 cylinder model. I also assume its operating temperatures may also be higher than the 4 cylinder, since it seems likely that 6 cylinders generate more heat than 4.
So obviously, compression ratio is not the only mechanism at play. If added weight and heat means the vehicle can operate without knock only between -20 F to 80 F and easy driving (reduced heat generation), instead of the full range of -20 F to 120 F and intense driving, the manufacturer would write “91 required” as opposed to “recommended”. I’m sure there are other factors at play here as well, but along the same lines of restricting the environments in which the engine can safely operate.
So I believe that if your engine’s operating conditions (easy driving, high altitude, or cold temperatures) are within the range that either knock is altogether avoided, or the engine can compensate through timing and knock sensor usage, no long term engine damage occurs, and your vehicle is fine running on regular 87 octane.
However, in an effort to understand how my own car (2005 Acura TL) would react to lower octane fuel, I set out to perform a few quantitative experiments, detailed in the next section.
Ideally, to understand under what circumstances the knock sensor is retarding ignition timing, you should read the “knock retard” parameter directly from your ECU using an OBD2 compliant scanning device. However, my car’s ECU does not report this value, so I had to measure it statistically.
To do so, I drove the same car twice: one trip on 100% 87 octane fuel, and another trip on 100% 93 octane fuel, in a 2005 Acura TL. The gas cap states this car “requires 91 octane” fuel. I recorded as many operating parameters as my ECU reports, and plotted ignition advance as a function of engine load for both trips:
In order to record engine parameters, I used the following Bluetooth OBD2 device:
And the “TorquePro” Android OS application.
Comparing ignition timing between 87 and 93 for the 2005 Acura TL:
- Does knock occur when using 87 octane in cold weather?
Yes. The knock sensor retards timing in cold weather, using both 87 and 93. The engine load required to trigger knock retard for 93 is about 70%, whereas for 87 it is sooner: about 50%.
- Does 87 retard timing more than 93?
Yes. By about 8-10 degrees on average after a load of 50%. Below 50% load is about the same.
- Does relying on the knock sensor cause engine damage?
Interestingly enough, 93 relies on the knock sensor under higher loads (>70%), as shown in the chart. So “relying on the knock sensor” to adjust timing does not, in itself, lead to engine damage, if the engineers are to be believed.
- Will using 87 cause long term damage my engine?
As shown in the chart, knock retard using 87 only leads to retards outside the realm of 93 when the engine load is above 70%. If the engine load stays below 70%, I think we can say conclusively that you’re not going to cause any damage, because the timing retard is still “in spec” for this vehicle.
Outside of spec, it’s hard to tell. During normal highway driving up a hill, I was hitting engine loads at high gear of around 70-90%. I drive with steptronic. The amount of time I spent above 70% load was roughly 17.6% of the time, or ~1/6. So that’s the amount of time I spent out of timing spec. But I could just as easily delay my shifting up in gears to prevent my engine from hitting those higher load values.
In conclusion, for my vehicle I tend to side with the CarTalk guys on this. So long as the heat & the load combined are not so great that the knock sensor cannot do its job, you are likely fine. You can also adjust your driving as well to prevent your vehicle from loading the engine too much. Clearly, the knock sensor is being used even when using 93 to keep the engine in proper operating order. As long as the engine is prevented from knocking, I do not believe engine damage will occur. Since environments, conditions, and operating states change based on geography, Acura cannot guarantee that in all ranges of expected situations the knock sensor will be able to do its job with 87 – but they do guarantee that with 91. So long as the knock sensor works with 87 in your environments, you’re probably fine.
Appendix: A Discussion of Fuel Detergents
Regarding fuel detergents, some top-tier gas stations put the same amount of engine cleaners in their 87 octane as in their 93, as well. Mobil is one such station:
Mobil even advertises that the only difference between their 87 octane fuel and 93 octane fuel is the octane level: