Octane Ratings

[RX400h]

First, I'd like to quote my 2006 RX400h's owner's manual:

Select Octane Rating 87(Research Octane Number 91) or higher. For improved vehicle performance, the use of premium unleaded gasoline with an Octane rating of 91 (Research Octane Number 96) or higher is recommended

Okay, now to clarify nomenclature a bit, I found this bit of information in www.refiningonline.com:

The performance of an engine is dependent upon many factors, one of which is the severity of operation. Accordingly the performance of a fuel is also dependent upon engine severity. To account for differences in the performance quality of a fuel two engine octane numbers are routinely used. Thc Research Octane Number (RON, or F1) simulates fuel performance under low severity engine operation. The Motor Octane Number (MON, or F2) simulates more severe operation that might be incurred at high speed or high load. In practice the octane of a gasoline is reported as the average of RON and MON or (R+M)/2.

Classically, both numbers are measured with a standardized single cylinder, variable compression ratio engine. For both RON and MON, the engine is operated at a constant speed (RPM's) and the compression ratio is increased until the onset of knocking. For RON engine speed is set at 600 rpm and MON is at 900 rpm.

Look at the statement in bold print. This is significant because the gasoline we buy here in CA is labeled as having an octane rating of (R+M)/2. To me, this means that Lexus requires (87+91)/2 = 89 (midgrade), which is an average of RON and MON. If I am correct, this is the minimum octane of gas that is recommended. During Winter months, I have successfully used regular RON +MON average of 87 and have not experienced any knocking whatsoever. However it is my experience that this may be insufficient during the Summer months.

If anyone believes that they are buying pure Research Octane-rated fuel, please chime in and let us know, but I did check the pumps here and saw that our octane numbers are an average. Thoughts?

[Gryphon]

I have no quarrels with your octane data, but I was not aware that Lexus changed the engine specifications between 2006 and 2007. For that matter I'm not sure I have ever read any horsepower ratings for the RX400h.

I do know from hard experience and several conversations with Lexus engineers that I was probably unwise to use regular fuel in my 91 LS400 for several years.

Tom

[Jim Nazium]

Bottom Line...

If you put load on your engine, then to extract the maximum in terms of performance and economy use the highest possible grade.

If you drive in the winter, or colder conditions and you do not put any load on the engine, then use the minimum octane rating (recommended by the manufacturer). Lower octane fuel ignites more quickly or more efficiently than higher octane fuel (less detonation prone than regular or mid grade) and is more prone to detonating (hence causing the engine to !Removed! the timing and control detonation), thereby increasing fuel consumption.

If you drive for maximum fuel efficiency and it is not sweltering heat, then use regular grade (I wait for Thursday's, in my area to purchase SUPER at the price of MID grade).

In fall, winter and spring, I fill up with regular. In the HOT sweltering heat I go with Super. Heat is a major contributor to pinging, hence the higher octane additive which helps inhibit detonation.

Cheers,

MadloR

[RX400h]

Agreed. However, don't forget the middle ground - midgrade gas. Here where I live in San Diego, temps rarely exceed 80, so midgrade seems to work perfectly. Going to Las Vegas means making sure premium is in the tank, since temps can reach 114F at times.

Dave

[wwest]

First, I'd like to quote my 2006 RX400h's owner's manual:

Select Octane Rating 87(Research Octane Number 91) or higher. For improved vehicle performance, the use of premium unleaded gasoline with an Octane rating of 91 (Research Octane Number 96) or higher is recommended

Okay, now to clarify nomenclature a bit, I found this bit of information in www.refiningonline.com:

The performance of an engine is dependent upon many factors, one of which is the severity of operation. Accordingly the performance of a fuel is also dependent upon engine severity. To account for differences in the performance quality of a fuel two engine octane numbers are routinely used. Thc Research Octane Number (RON, or F1) simulates fuel performance under low severity engine operation. The Motor Octane Number (MON, or F2) simulates more severe operation that might be incurred at high speed or high load. In practice the octane of a gasoline is reported as the average of RON and MON or R+M/2.

Classically, both numbers are measured with a standardized single cylinder, variable compression ratio engine. For both RON and MON, the engine is operated at a constant speed (RPM's) and the compression ratio is increased until the onset of knocking. For RON engine speed is set at 600 rpm and MON is at 900 rpm.

Look at the statement in bold print. This is significant because the gasoline we buy here in CA is labeled as having an octane rating of R+M/2. To me, this means that Lexus requires 87+91/2 = 89, which is an average of RON and MON. If I am correct, this is the minimum octane of gas that is recommended. During Winter months, I have successfully used regular RON +MON average of 87 and have not experienced any knocking whatsoever. However it is my experience that this may be insufficient during the Summer months.

If anyone believes that they are buying pure Research Octane-rated fuel, please chime in and let us know, but I did check the pumps here and saw that our octane numbers are an average. Thoughts?

For normal non-defective gasoline modern day EFI engines there are two times, "phases" in the 4-cycles wherein octane rating of the fuel will be important. The first of these is actually during the late phase of the compression cycle where a too low octane rating may result in engine knock/ping due pre-ignition/dieseling.

The second is due to engine loading. Knock/ping due to "lugging" of the engine during the combustion cycle's downward stroke of the piston results from the piston not being able to move downward as fast or as quickly as the flamefront, EXPLOSION, is expanding.

With high octane fuels the progression of the flamefront is more "evenly" spread over the period of the downward travel of the piston. Whereas a lower octane fuel's flamefront will not be as "linear", detonation will in a more explosive, more defined PEAK, fashion.

So fueling a modern day high compression engine with an automatic transmission with a low octane will have no detrimental effects at all. The engine control ECU will simply enrich the mixture to prevent dieseling and the transmission will be downshifting to prevent any knock/ping from lugging.

But.

Anyone know of any advantages or detriments to fueling a modern day EFI (forget carburation) engine built for low octane fuel with a higher octane than suggested?

[RX400h]

As a general rule of thumb, if your owners manual specifies a minimum octane rating and mentions that improved performance may be achieved by using higher octane, then there is a benefit if one so desires. In essence, we have a choice between spending more money every time we fill up, or spending less and accepting the slightly reduced performance. My Mazda GT Turbo has similar nomenclature in its owners manual.

However, I suppose that a vehicle like the 4-cylinder Camry would not benefit from running premium fiuel unless the ambient temperture gets beyond a certain number (like 100 degrees F).

Thanks for the technical explaination, WWest.