Octane question

[77 Z]

The only other question I have is that in the owners manual it states..........

Your Datsun (non California model) is designed to operate on either unleaded or low - lead gasoline with a research octane rating of at least 91.

Incorrect fuel whose octane rating is too low will result in knocking , after run or overheating.

So ....now what ..most of you are telling me that you run 87 and the manual says 91...not that I think you guys dont know what you are talking about but whats the deal....

As I said the compression ratio is 8.3 to 1 which is now probably lower due to the mileage on the original engine..which is 83,000 miles...

OK ...what gives...

Thanks for the patients guys

77 Z

[mdbrandy]

I believe that the way Octane rating is calculated changed some years ago. Something about RON (Research Octane Number) versus MON (Motor Octance Number). I think the current value is (RON + MON)/2 . There's a bunch of info at :

http://www.repairfaq.org/filipg/AUTO/F_Gasoline6.html

RON appears to give higher numbers, and was used some years ago. MON gives lower numbers, so octane numbers are lower now.

I'm still doing some searching, and am not quite sure about this yet....

[Bambikiller240]

At the time the maual was printed regular gas was pretty close to 91 octane.

You can pay more if you want, but it really isn't going to buy you anything except a warm and fuzzy feeling that you are "helping out" the poor oil industry which is struggling to break even on the product they sell.

If everyone is telling you that they use regular and it works fine in their car, why do you not believe us? I'm curious.

Yes we know all about knocking, after run and overheating. Been there, done that. We would not run our cars on fuel that caused that to ocur in our engines, would we?

If you are asking for a scientific "reason" why we have no problem w/ 87 octane , I don't think you'll find one here.

[mdbrandy]

OK. A little more research, from http://www.synlube.com/glossary.htm#octane%20number :

expression of the antiknock properties of a gasoline, relative to that of a standard reference fuel. There are two distinct types of octane number measured in the laboratory: Research Octane Number (RON) and Motor Octane Number (MON), determined in accordance with ASTM D 2699 and D 2700, respectively. Both the RON and MON tests are conducted in the same laboratory engine, but RON is determined under less severe conditions, and is therefore numerically greater than MON for the same fuel. The average of the two numbers — (RON + MON)/2 — is commonly used as the indicator of a gasoline’s road antiknock performance. The gasoline being tested is run in a special single-cylinder engine, whose compression ratio can be varied (the higher the compression ratio, the higher the octane requirement). The knock intensity of the test fuel, as measured by a knockmeter, is compared with the knock intensities of blends of isooctane (assigned a knock rating of 100) and heptane (with a knock rating of zero), measured under the same conditions as the test fuel. The percentage, by volume, of the isooctane in the blend that matches the characteristics of the test fuel is designated as the octane number of the fuel. For example, if the matching blend contained 90% isooctane, the octane number of the test fuel would be 90. In addition to the laboratory tests for RON and MON, there is a third method, Road Octane Number, which is conducted in a specially equipped test car by individuals trained to hear trace levels of engine knock.

I don't have a conversion, but it appears to me that what used to be, say 91 Octane on the RON scale is now more like 87 on the (RON + MON)/2 scale. Thus, octane ratings may not have REALLY dropped over the years, but knowledge of what is important, and the ways that it is measured have changed. Of course, since I don't have a conversion, it may be that 91 RON might be 89 (RON+MON)/2, so there may have been some degradation in Octane. I haven't seen any real conversion. What I have seen is that Octane rating is very complicated.

[77 Z]

Thanks to all that have responded to my question...and it has nothing to do with not trusting you guys especially considering some who have given an answer to my question...I BELIEVE YOU ALL.......seriously at one point I was running 87 and for some reason maybe reading something in the manual I switched to the higher octane..it all makes perfectly good sense to me as you guys have explained it...and again I believe you as too many of you really know what you are doing with these cars as far as the engines...Im good on about everything else but that area and am trying to become at least better than avg with fuel injection....thanks for all the scientific explanations as well.

Thanks again to all and believe me I do believe you guys...

77 Z

[MikeW]

Just to clarify a bit (or perhaps to confuse things) there are some other issues at play here. Temperature, altitude, and driving conditions affect things. A car may run just fine on 87 octane but might start knocking when driving uphill in hot weather. Driving at a high altitude actually lowers the octane requirement. Also, unless your tank is completely empty when you change grades you will have a mixture of octanes which might affect your evaluation.

Sepeaking of octane mixtures: mid-grade gasoline is often a mixture of regular and premium. If the 3 octane levels are 87, 89, and 93 the mid-grade is actually 2/3 regular and 1/3 premium. Around here the mid-grade price is often right in the middle (say $1.90, $2.00, and $2.10). If that's the case you'd be better off mixing it yourself.