Octane levels - engine pinging

[RobWarren]

O wise and learned ones...

67 BJ8
Engine rebuilt last year and now the car has done around 1500 miles.

On recommendation from a classic car mechanic, I stopped using 98 octane and started using 95. Within a few weeks I started getting pinging under load.

A different classic car chap I know has said he always uses 98 and even recommended an octane booster.

I'm a touch confused by the octane numbers for the USA that I've seen in threads here and how they relate to UK/ Australia - what's the difference?

What's the recommended type of fuel to use?
What does an octane booster do?

Your advice is, as always, greatly appreciated.

Rob

 

[TimK]

Wikipidia offers the following:
Measurement methods
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

There is another type of octane rating, called Motor Octane Number (MON) or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 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 rating, shown on the pump, is the RON, but in the United States, Canada and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, the octane rating shown in the United States is 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, is 91–92 in Europe. However most European pumps deliver 95 (RON) as "unleaded", equivalent to 90–91 US (R+M)/2, and some even deliver 98 (RON), 100 (RON), or 102 (RON).[2]

It is possible for a fuel to have a RON greater than 100 because iso-octane is not the most knock-resistant substance available. Racing fuels, AvGas, LPG, and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher — ethanol's RON is 129 (MON 102, AKI 116). Typical "octane booster" gasoline additives include tetra-ethyl lead, MTBE, and toluene. Tetra-ethyl lead (the additive used in leaded gasoline) is easily decomposed to its component radicals, which react with the radicals from the fuel and oxygen that start the combustion, thereby delaying ignition and leading to an increased octane number. However, tetra-ethyl lead and its byproducts are poisonous and the use of tetra-ethyl lead creates an environmental hazard. Since the 1970s, its use in the United States and most of the industrialized world has been restricted. Its use is now generally currently limited to being an additive to aviation gasoline


My BN7 runs fine on 89 octane, but if yours knocks on 95 RON, check your distributor advance. If it is correct, then use 98 RON. Knocking is deadly.

 

[GregW]

Hi Rob,
Tim's suggestion is good to make sure you aren't too advanced on the timing. When your engine was rebuilt, was the block or head machined to make it flat? That could cause the engine to have a higher compression ratio than that that mechanic thinks, unless he's really gotten into your engine. There is a slim chance you have a carbon buildup in the combustion chamber from running the carbs too rich which could lead to pinging, but I doubt it with only 1500 miles.

 

[Maccers]

Hi Rob,

It sounds like your freshly rebuilt engine simply needs tuning properly.

If it runs OK on 98 octane and has not been highly tuned, then this (and all other Healey engines) should run OK on lower octane fuels provided the timing is set correctly for that fuel.

I would recommend getting your car set up at this stage on a rolling road dynamometer. I did this on mine and the difference was fantastic even though there was no pinking before. On such a set-up, they set timing, dwell, mixture and can immediately diagnose any problems that will affect the running of the engine.

After a rebuild I would always recommend this and it is a small price to pay after expensive engine work - you will even get improved mpg.

As for which fuel to recommend, I would always go for the highest octane that I can find regularly - if that's 95, get it set up on 95 then use it all the time. I am not a great fan of additives, but know others use them to good effect.

Good luck

Mac

 

[vette]

Hello Rob, My BJ7 engine was rebuilt before I owned the car. It has BJ8,carbs and a non-standard cam, but I don't know anything about the cam specs. My timing can be set at about the same as stock specification but inspite of well sorted out carbs and a good tune up it is hard for me to get my engine to idle consistently below 800 rpms. I contribute that to the cam mostly. I can play with the timing alittle but if I slow it down too much I don't like the acceleration loss. So having said all that, I have found that I need a fairly high octane rating. Here in Pa. that means about 91 and up. That's usually what I get. 91 or 92. Sometimes I find 93. With out the higher octane, my engine exibits poor acceleration and then starts to diesel or run on when shut off.
Cheers,
DaveC.