RON recommendation (Pi)

[frenchiemk2]

About my "french" 72 Pi, the onboard documentation says "RON 97 or supérior".

Here we only have had 98 RON for a long time. Were the continental engine head modified to run 98 or do i need to put lead substitute?

Thanks.

[glang]

Hi, RON is to do with the resistance the fuel has to ignition under compression not protection of the valve seats which is something lead offered. If the octane (RON number) is too low the fuel will ignite too soon leading to pinking (rattling noise under load) and, if excessive, damage to the engine. As you have access to 98 you shouldnt have this problem and can even try 95 which I find works perfectly in my Vitesse although on long fast runs I do add a combined octane booster/lead substitute (Millers VSPe)... 

[JohnD]

The Pi cars were intended to run on "Five Star" fuel - 100 octane!

Part of lead's action as a fuel additive was for anti-knock, or pinking, and without it 98 is the best that is available, unless you live near a race circuit or aerodrome where AvGas at 100 octane may be sold.     My Vitesse has Pi, and a CR of 10.5, which is at the limit for low octane fuel, but I find it runs fine, no pinking, on 98 Octane.   I favour Shell's V-Plus.

BUT, to get the octane rating up there, modern fuels have additives that are volatile, when tetra-ethyl lead was not volatile at all.   Today, fuel that has sat in the tank for more than a month or so may no longer be at 98 octane.    Run your tank down to the minimum before winter storage, or else add a 'fuel stabiliser'.

John

PS Europe uses the "RON" (Research Octane Number) to classify fuel.     In the US they use MON (Motor Octane Number). which gives a lower number for the same fuel.  (98 RON is the same as 90 MON)  J.

[glang]

If pinking is a problem because you cant get a high enough RON fuel another possibility is to reduce the ignition timing advance progressively until the rattling noise is within the correct limits. This will of course reduce engine power though...

[thescrapman]

My PI ran OK on 95, but preferred 98.

My TR hates 95. 

You can Aral in Germany that is over 100, (101 or 104 I think) and like rocket fuel. 

The Shell Vpower is also higher RON in Germany than the UK version.

In France the 98 has been fine for my PI cars, usually E5 same as UK.

[glang]

Please dont forget theres the same energy available in all of these RON fuels so a higher rating doesnt necessarily mean more power. If your car runs well (pinking as specified) with the correct ignition timing on 95 then theres no benefit going to a higher RON. Some people even consider using a higher RON and increasing advance more than the recommended setting which is even more ridiculous...

[JohnD]

14 hours ago, thescrapman said:

You can Aral in Germany that is over 100, (101 or 104 I think) and like rocket fuel. 

So many believe this!     There is no more energy in 100 octane fuel than in fuels of lesser octane rating.   The octane number ONLY refers to its resistance to knock, pre-ignition.    This does mean that you can run much higher compression which will generate more power, and modern engines that have engine management and knock sensors to adjust ignition automatically will produce less power with low octane fuel, but those are due to engine tune and timing, not the 'power' of the petrol!

John 

[drofgum]

On 13/09/2023 at 08:33, JohnD said:

The Pi cars were intended to run on "Five Star" fuel - 100 octane!

Part of lead's action as a fuel additive was for anti-knock, or pinking, and without it 98 is the best that is available, unless you live near a race circuit or aerodrome where AvGas at 100 octane may be sold.     My Vitesse has Pi, and a CR of 10.5, which is at the limit for low octane fuel, but I find it runs fine, no pinking, on 98 Octane.   I favour Shell's V-Plus.

BUT, to get the octane rating up there, modern fuels have additives that are volatile, when tetra-ethyl lead was not volatile at all.   Today, fuel that has sat in the tank for more than a month or so may no longer be at 98 octane.    Run your tank down to the minimum before winter storage, or else add a 'fuel stabiliser'.

John

PS Europe uses the "RON" (Research Octane Number) to classify fuel.     In the US they use MON (Motor Octane Number). which gives a lower number for the same fuel.  (98 RON is the same as 90 MON)  J.

John,

We don't use MON here in the USA. Nothing that simple. We use "pump octane" This is found by adding RON to MON and dividing by two.

I just filled my Spitfire with 93 "pump octane" fuel. The RON and MON ratings are not displayed at the pump. 93 is the highest octane available in my part of the States. Other sections have only 91.

 Regards,

Paul

[glang]

So Paul I suppose your 93 equates roughly to our RON 98 and RON 95 to your 91? How does your Spitfire handle the fuel or is it a US spec model designed for it? 

[JohnD]

1 hour ago, drofgum said:

John,

We don't use MON here in the USA. Nothing that simple. We use "pump octane" This is found by adding RON to MON and dividing by two.

I just filled my Spitfire with 93 "pump octane" fuel. The RON and MON ratings are not displayed at the pump. 93 is the highest octane available in my part of the States. Other sections have only 91.

 Regards,

Paul

Well, you do use MON rating in the US, but make it even more complicated by taking the mean between the two!    " 93" is a  bit lower than the mean of 98 and 90, so bad luck!

JOhn

[glang]

Didnt the US versions have lower compression anyway so dont need higher octane fuels?

[JohnD]

Yes, but that was to reduce oxides of nitrogen in the emissions, which contribute to smog.

On fuel and alcohol, it's worth noting that pure alcohol is a superb knock suppressant!      While a CR of 10.5 is about the limit with 98 octane fuel, cars to run on ethanol can be built to have a CR of 14:1 !!!

[thescrapman]

On 14/09/2023 at 09:56, JohnD said:

So many believe this!     There is no more energy in 100 octane fuel than in fuels of lesser octane rating.   The octane number ONLY refers to its resistance to knock, pre-ignition.    This does mean that you can run much higher compression which will generate more power, and modern engines that have engine management and knock sensors to adjust ignition automatically will produce less power with low octane fuel, but those are due to engine tune and timing, not the 'power' of the petrol!

John 

This was in an early PI car with timing tweaked to most advantageous and a TR4 with higher compression than usual and again tweaked ignition.

Cars even sounded different to normal.

Guess they were just making better use of the energy within the fuel, especially as both were designed for 5-star.

 

[glang]

Certainly higher compression is likely to need an increase in fuel octane and I also wonder if there can be differences between standard engines of the same type. Some owners report they need one fuel while others are happy with something different so could engine wear or settings or combustion chamber carbon build up be factors?