ffas23

I got involved in a discussion on the C-3 forum about the potency of the octane in todays gasolene verses what was available to us years ago when Sunoco 260 had a octane rating of 104 and other brands at the time had a octane rating of at least 101 for the high test they sold. At the same time back then if you remember regular gas was 95 Octane.

A member who just purchased a 73' Vette questioned what grade of gasolene to use in his car. Another C-3 member wrote Quote:All '73s were designed to run on regular unleaded gas, irrespective of displacement or horsepower ratings.
I wrote back that: This quote is somewhat true but regular gas in 1973 was 95 octane and high test depending on where you purchased it was from 101-104 Octane. Sunoco was the only gas station to offer their Sunoco 260 blend which was 104 Octane. Unleaded gas although Amoco originally sold it wasn't mandatory until 1975 in the states when it was required in all 75' and up cars.
The other C-3 member answered me back saying:You're forgetting that the rating system has changed also. Yesterday's 95 octane rating is today's 87 rating. The old way of rating gas was with just The research number, todays method is the average of the research and motor numbers. 95=87, 100=93.

I wrote him back and told him I never heard of this before and asked him if he could direct me to where this was ever mentioned but he hasn't been able to show me this. I found this info hard to believe being in the auto parts business 27 years and never hearing of this before. I even questioned others in the business to see if they ever heard of this and no one did. This bothers me because if the gas sold today is as potent as it was years ago and now the only difference is a numbers change one wonders why manufacturers came out with all the different octane boosters over the years because of supposely low octane fuel out there. Also why are some of you fellows mixing airplane gas or buying Cam 2 fuel to mix with the gas offered today if what this C-3 member says is true. Maybe someone on the C-2 forum could weigh in on this and let me know if what has been told to me on the C-3 forum is true or not. The link to that thread is: http://forums.corvetteforum.com/show...2&page=1&pp=20

Fred

Joel 67

Your friend is part right--the octane rating system did change. If you search this forum, you will find more info on the topic than you probably want.

A '73 should run fine on 93 unleaded with out any problems.

ismithlj

This article does a good job of explaining octane ratings.

Octane rating
The most important characteristic of petrol is its Research Octane Number (RON) or octane rating, which is a measure of how resistant petrol is to premature detonation (knocking). It is measured relative to a mixture of isooctane (2,2,4-trimethylpentane) and n-heptane. So an 87-octane petrol has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane.

There is another type of Octane, called "Motor Octane Number" (MON), which is a better measure of how the fuel behaves when under load. Its definition is also based on the mixture of isooctane and n-heptane that has the same performance. Depending on the composition of the fuel, the MON of a modern petrol will be about 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.

In most countries (including all of Europe and Australia) the 'headline' octane that would be shown on the pump is the RON: but in the United States and some other countries the headline number is in fact the average of the RON and the MON, sometimes called the "Road Octane Number" or RON. Because of the 10 point difference noted above this means that the octane in the United States will be about 5 points lower than the same fuel elsewhere: 87 octane fuel in the United States would be 92 in Europe.

It is possible for a fuel to have a RON greater than 100. This reflects the fact that isooctane is not the most knock-resistant substance available. Racing fuels, Avgas and LPG typically have octane ratings of 110 or significantly higher.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane allows an engine to be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane petrol. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption. This is false - engines perform best when using fuel with the octane rating they were designed for.

The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, unmixed with other isomers of heptane or octane, distilled from the resin of Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.

SWCDuke

Everything you need to know about gasoline - no myths or misinformation, just science!

http://blizzard.rwic.und.edu/~nordli.../gasoline.html

Duke

Mike Ward

'Him' (AKA me) lost interest in the C3 thread when you showed reluctance to either accept what several people had offered as hard factual evidence, or the suggestion to do a bit of leg work on your own. Seems you've now appealed to a slightly different audience to redundantly ask the same question.

I gather that either you won't accept people's word that yes indeed the system did change or that you want the exact date of when the change over in the rating systems occurred. As I previously posted this was in the late 70's in Canada, but I cannot comment as to when it happened in the US. I trust that since you have now has corroborating posts from the C2 guys, your mind is at ease. You mention that you've been in the parts business for 27 years, ie 1978. I would suggest that you entered this field a good 10 -12 years after changeover occurred, so it's not surprising that you were not aware.

As to the original question on the C3 side, the 1973 owners manual clearly states that any gas of 91 research octane or greater is acceptable. Since retail pump gasoline is no longer rated simply by this criteria (and as clearly explained in the hyperlink) a 'conversion formula) is required to ensure that gas of the correct octane rating method is selected. Again, the web site I quoted has an interesting calculator built in to perform this. Duke has provided a link to one of the other sites easily found with Google to give you more info (except for the exact date)

I hope this now satisfies you .

ffas23

Mike,
I want to try to address you here as best as I can. First of all I didn't feel my question on the C-3 forum was answered by what you claim to be hard factual evidence. Why you lost interest I don't understand. You made the claim and I only asked if you could point me in the right direction so that I may read about this because I wasn't awhere of it. You wanted me to look up what you claimed by using a Google search yesterday and I didn't have anymore time to spare and needed to go back to work. Last night I took a look around on the net but didn't read anything that satisified what I was looking for although I do understand the rating system had changed. What bothers me being in the parts business so long is the fact that over the years different companys came out with octane boosters to boost up all this supposely low octane fuel being sold to us at the pumps. This was an item I was asked about alot over the 27 years I have been selling auto parts. Fellows with high performance cars would come to me and say that their older high performance cars from the 60's up thru the year 1970 wouldn't run good on the gas that was offered to them thus they were looking for a octane booster. Now if what you say is true the gasolene that was rated at say 100 octane back then that ran good in the 60's HP cars is basically the same as the fuel offered today minus the lead and is 93 octane today. If this is truely the case I would believe that someone with a 60's high performance car should run just as well on todays 93 Octane fuel as the car did 40 years ago using the 100-104 Octane gas that was offered then but from all that I have heard and also from what I have read this isn't the case. Because I couldn't find anything written that explained this to me I figured why not ask this question on the C-2 forum where I have read in the past that some of the members with these older high performance cars were purchasing fuel at their local airport to mix with the fuel they would purchase at the pump and others who go out of their way to find Cam 2 racing fuel and some who just look for the best Octane booster they could find out there to mix with pump gas.

SWCDuke

PON is the arithmetic average of MON and RON. The arithmetic DIFFERENCE is called "sensitivity", and traditional fuels are usually about eight, but some modern fuels are closer to ten.

Thus, to convert today's PON to RON add 4 to 5, so that places today's 93 PON in the range of 97 to 98 RON, which is not quite as good as sixties vintage premiums, which were usually in the range of 99 to 100 RON. This is why some sixties vintage engines need a bit more than current 93 PON premium to operate detonation free, but usually a sllight "detune" in the form of a less aggressive timing map or a thicker head gasket to reduce CR a quarter to a half point will do the trick.

The original unleaded regulars from the early seventies were rated at 91 RON, which is approximately equivalent to today's 87 PON. In that era leaded regulars were about 94 RON, which would be about 90 PON.

Duke

ffas23

Duke thanks for your calm input here. Mike took me the wrong way when he jumped all over me in the last post he made. All I basically wanted to know was if the gasolene sold today at the pump was as potent as the gas sold years ago and you explained to me right here that todays gasolene is Quote not quite as good as sixties vintage premiums) and I remember the rating to be the same until 75' when it was mandatory to sell unleaded fuel. I would guess this to be the same situation with yesterdays regular gasolene being better then todays regular gasolene. So if the 73' Corvette although equipped with a low compression engine called for what Mike says was regular gasolene in 1973 one would think they would need to use premium today in that same 73' Corvette. I use todays premium in my own 73'. Although there seems to be only a slight difference with the Ron and Pon ratings based on what you have shown it sure doesn't seem that way at the pumps where even I purchased gasolene from some stations that would ping in even todays cars only to fill up somewhere else after running the tank down to find a big difference with another brand of regular gas.

Fred

SWCDuke

Beginning in 1971 ALL GENERAL MOTORS PASSENGER CAR AND LIGHT TRUCK ENGINES - AND EVERY SINGLE ONE BUILT THEREAFTER AT LEAST THROUGH THE END OF THE SEVENTIES - were designed to operate on 91 RON unleaded fuel, which is equivalent to today's 87 PON.

The first subsequent GM engine after 1971 that required a modern fuel higher than 91RON/87PON did not come along until sometime well into the eighties.

Assuming your '73 engine is not modified in any way, it should operate satisfactorily on 87 PON. If it does so without detonation you do not need higher octane fuel.

I will also add that after the general adoption of catalysts beginning in 1975, OEMs were able to advance the timing in most of the operation regime, which improved fuel economy, but many of these vintage cars developed a reputation for detonation, especially at part throttle when accelerating moderately or climbing hills. This caused a lot of consumer concern, and I recall one industry wag referring to detonation as "the sound of fuel economy", but economy drops when the engine is detonating and sustained detonation can lead to cumulative damage and premature failure of a piston or valve.

The pressure was so high to increase fuel economy to meet CAFE standards that the OEs accepted detonation that would have been considered unacceptable in pre-emission days.

Pre-catalyst engines had to rely on relatively retarded timing in most of the operating regime to keep NOx low and keep the EGT high so oxidation would occur with injected air. Thus, pre-catalyst engines have less tendency to detonate on 91RON/87 fuel than the first generation of catalyst equipped engines with carburetors, which usually had air preheaters.

Fuel injection, which doesn't need air preheating and knock sensors have pretty much eliminated detonation, and that's why you can run a 10.9:1 CR LS2 on regular unleaded without detonation. The torque curve may be lower, particularly at the bottom end, because spark advance willl be reduced at low manifold vacuum, but there should be no significant detonation.

Duke

ffas23

Duke, Good explanation. You gave me a bit of a education today.

Fred