C1 Fuel Octane Ratings and Timing

[Paul Bishop]

Hi, I wondered if anyone could give me some advice on octane ratings and ignition timing. I live in England UK and have a 1956 Corvette with the original 265 cu. in. with dual Carter 4 barrels, the octane ratings here are regular unleaded 95 Roz/Ron or super unleaded 98 Roz/Ron you can at a price still get 100 Roz/Ron fuel. I've been using 95 and 98 and occasionaly adding an octane booster, fuel in the UK is expensive and the car seems OK on either but is not used hard so if there is a problem maybe I not realising it. I wanted though to set up the car correctly and get the timing right for different fuel octanes, what timing settings should be used for 95 or 98 Roz/Ron octane fuels. I think you may use PON ratings in the US, roughly 95 Roz/Ron is 91 PON and 98 Roz/Ron is 94 PON although it can fluctuate by brand of fuel.

Thanks as always,
Paul.

[Duke W.]

Re: C1 Fuel Octane Ratings and Timing

The advertised CR of your 265 is only 9.25:1. As built, most OE engines are lower than advertised and most "rebuilt" engines are even lower because a thick aftermarket head gasket is substituted for the thin steel shim OE gasket.

Most engines are probably a half to a full point lower than their owners think, and many owners use higher octane fuel than is necessary. There are really very few detonation complaints on this board.

The best approach is to set the timing at the OE specification and make sure the timing map is at OE spec.

Then let the fuel get very low and add a couple of gallons of the lowest octane available. Drive the car and listen for detonation. The worst case is usually high load at low revs.

If no detonation is noticed add a few more gallons and continue the test until the older fuel is fully diluted and you are sure there is no detonation.

If it does detonate on low octane fuel, use higher octane or continue experimenting to find the minimum blend of high octane that results in little or no detonation.

Slight transient detonation is usually harmless, but continuous detonation under load must be avoided.

If the engine doesn't detonate on low octane fuel, increase the initial timing or quicken the centrfigual curve to find the limit. OE spark advance maps are usually fairly conservative to cover worst case conditions, which would be hot, dry air on a high pressure day for your altitude.

I don't know what you mean by "Roz", but European octane ratings are "Research Octane Number" (RON), which is what was used in the USA back in the sixties. We now use "PON" (Pump or Posted Octane Number), which is the arithmetic average of Motor Octane Number (MON) and RON, which is call the "Sensitivity" and is usually in the range of 8-10.

I know of at least five different ways to measure/compute "octane number". Whenever the subject comes up it is rare that the specific "octane number" is referenced, so such discussions are essentially worthless and often compare apples and oranges.

I commend you for understanding and mentioning the difference in Euroland and USA octane numbers.

For reference here are the five octane measurement methods I am aware of.

1. Research Octane method - used in the USA up to about the mid seventies, and also currently used in Europe; considered appropriate for "low load" operation, which is typical of passenger cars.

2. Motor Octane method - uses a higher air inlet and cooling jacket temperature so it results in lower numbers than RON; considered appropriate for high load applications such as trucks. The difference in RON and MON is usually about 8-10 points and this is known as the "sensitivity".

3. Pump (or posted) Octane Motor - a bureaucratic invention of the US government that is merely the arithmetic average of RON and PON.

4. Aviation method - a different test procedure than RON or MON, but the result is usually very close to MON; used currently for aviation gasoline.

5. Performance number - an older aviation method used in the age of supercharged reciprocationg aircraft engines, which included the old 100/130 and 115/145 ratings. The dual ratings are based on "lean" (near stoichiometric) and "rich" (as high as 50 percent excess fuel) mixtures.

The octane number is the percent iso-octane in a mixture of iso-octane and normal heptane that exhibits the same detonation as the test fuel. So "100 octane" means the test fuel has the same detonation resistance as 100 percent iso-octane, and 90 octane is the same as a blend of 90 percent iso-octane and normal heptane.

Octane numbers over 100 are determined by comparing detonation of the test fuel to a reference fuel of 100 percent iso-octane with a specfied amount of TEL measure in grams or cc per gallon (I forget which), so 103 octane is the same as 100 percent iso octane with 3 gr (or maybe cc) of TEL per gallon.

Octane of production gasoline is measured by continuously sampling the straight run gasoline in a Waukesha CFR (Combustion and Fuels Research) engine, the basic design of which goes back to the 1920s. It's a flathead design with a variable compression ratio. Back in the early days detonation was detected by experienced technicians. Nowadays, detonation sensors are used.

Duke

[Duke W.]

Re: C1 Fuel Octane Ratings and Timing

The advertised CR of your 265 is only 9.25:1. As built, most OE engines are lower than advertised and most "rebuilt" engines are even lower because a thick aftermarket head gasket is substituted for the thin steel shim OE gasket.

Most engines are probably a half to a full point lower than their owners think, and many owners use higher octane fuel than is necessary. There are really very few detonation complaints on this board.

The best approach is to set the timing at the OE specification and make sure the timing map is at OE spec.

Then let the fuel get very low and add a couple of gallons of the lowest octane available. Drive the car and listen for detonation. The worst case is usually high load at low revs.

If no detonation is noticed add a few more gallons and continue the test until the older fuel is fully diluted and you are sure there is no detonation.

If it does detonate on low octane fuel, use higher octane or continue experimenting to find the minimum blend of high octane that results in little or no detonation.

Slight transient detonation is usually harmless, but continuous detonation under load must be avoided.

If the engine doesn't detonate on low octane fuel, increase the initial timing or quicken the centrfigual curve to find the limit. OE spark advance maps are usually fairly conservative to cover worst case conditions, which would be hot, dry air on a high pressure day for your altitude.

I don't know what you mean by "Roz", but European octane ratings are "Research Octane Number" (RON), which is what was used in the USA back in the sixties. We now use "PON" (Pump or Posted Octane Number), which is the arithmetic average of Motor Octane Number (MON) and RON, which is call the "Sensitivity" and is usually in the range of 8-10.

I know of at least five different ways to measure/compute "octane number". Whenever the subject comes up it is rare that the specific "octane number" is referenced, so such discussions are essentially worthless and often compare apples and oranges.

I commend you for understanding and mentioning the difference in Euroland and USA octane numbers.

For reference here are the five octane measurement methods I am aware of.

1. Research Octane method - used in the USA up to about the mid seventies, and also currently used in Europe; considered appropriate for "low load" operation, which is typical of passenger cars.

2. Motor Octane method - uses a higher air inlet and cooling jacket temperature so it results in lower numbers than RON; considered appropriate for high load applications such as trucks. The difference in RON and MON is usually about 8-10 points and this is known as the "sensitivity".

3. Pump (or posted) Octane Motor - a bureaucratic invention of the US government that is merely the arithmetic average of RON and PON.

4. Aviation method - a different test procedure than RON or MON, but the result is usually very close to MON; used currently for aviation gasoline.

5. Performance number - an older aviation method used in the age of supercharged reciprocationg aircraft engines, which included the old 100/130 and 115/145 ratings. The dual ratings are based on "lean" (near stoichiometric) and "rich" (as high as 50 percent excess fuel) mixtures.

The octane number is the percent iso-octane in a mixture of iso-octane and normal heptane that exhibits the same detonation as the test fuel. So "100 octane" means the test fuel has the same detonation resistance as 100 percent iso-octane, and 90 octane is the same as a blend of 90 percent iso-octane and normal heptane.

Octane numbers over 100 are determined by comparing detonation of the test fuel to a reference fuel of 100 percent iso-octane with a specfied amount of TEL measure in grams or cc per gallon (I forget which), so 103 octane is the same as 100 percent iso octane with 3 gr (or maybe cc) of TEL per gallon.

Octane of production gasoline is measured by continuously sampling the straight run gasoline in a Waukesha CFR (Combustion and Fuels Research) engine, the basic design of which goes back to the 1920s. It's a flathead design with a variable compression ratio. Back in the early days detonation was detected by experienced technicians. Nowadays, detonation sensors are used.

Duke

[Duke W.]

Correction

CFR stands for "Co-operative Fuels Research", and it's an OHV design.

More info on this venerable engine:

http://files.asme.org/ASMEORG/Communities/History/Landmarks/5519.pdf

Duke

[Duke W.]

Correction

CFR stands for "Co-operative Fuels Research", and it's an OHV design.

More info on this venerable engine:

http://files.asme.org/ASMEORG/Communities/History/Landmarks/5519.pdf

Duke

[Jack H.]

Re: C1 Fuel Octane Ratings and Timing

Paul, I'm a US member of both the UK and Dutch NCRS chapters and I've been to your side of the 'pond' rather often including a couple of cross-Europe trips in these classic Corvettes to LeMans for the 24 Heurs du Mans endurance race. All of those with 'normal' engines (built to factory spec versus being 'souped up' for on-track competition) haven't had problems filling up and burning your super unleaded....

[Jack H.]

Re: C1 Fuel Octane Ratings and Timing

Paul, I'm a US member of both the UK and Dutch NCRS chapters and I've been to your side of the 'pond' rather often including a couple of cross-Europe trips in these classic Corvettes to LeMans for the 24 Heurs du Mans endurance race. All of those with 'normal' engines (built to factory spec versus being 'souped up' for on-track competition) haven't had problems filling up and burning your super unleaded....

[Paul Bishop]

Re: C1 Fuel Octane Ratings and Timing

Thank you both, very informative!

Kind Regards,
Paul.

[Paul Bishop]

Re: C1 Fuel Octane Ratings and Timing

Thank you both, very informative!

Kind Regards,
Paul.