This way we can keep all the related info together for future reference.
The book I have been reading refers to what they call Octane Requirement Increase (ORI). Logically, as carbon builds up in the combustion chamber (from petrol remnants, oil past rings, etc), it marginally increases compression ratio. With every increase of ratio by 1, they state octane requirement increases by a minimum of 4-6 points.
However, they also state that these carbon and other deposits also do not carry heat as well as the regular steel does - and that this is in fact the main reason for ORI. Hot spots cause the preignition/knock.
From new, the deposits build up and progressively increase over several thousand miles. Levels then plateau when they can't build up any thicker without being burnt off. Overall they stated that there is normally around a 3-5 octane point increase requirement for most cars within 10,000 miles from new.
Some engines tend to have the deposits form near the intake - making ignition from these possibly red hot deposits even more likely.
Fuels high in aromatics, along with city stop start driving, will cause the worse deposits.
The lead also made it worse - presumably leading to having to add the lead scavenging chemicals that often rotted the immediate exhaust system.
Fortunately, there are anti ORI additives - ranging from boron, a 'polyetheramine detergent' commercially known as 'Techron' was noted to reduce ORI by 30-40%. They also mention a 'PDP5400' additive that was launched in 1985.
Caltex used to distribute techron, however I emailed them 2 months ago and they no longer stock it.
With the above information, it makes sense that using 98 octane in a car designed for 91 will probably only result in a power restoration to near (but below) the manufacturers specified levels. The knock sensor automatically retards timing to prevent the more prevalent knock caused by these deposits. With 98 octane, it will be able to advance back to BTDC degree closer to that of a new engine without the deposits - hence return to original power.
This is just my lamans understanding and I am really interested to know other peoples views on this interesting topic
The book is 'Motor Gasoline' by E.L. Marshall and K. Owen, 1995 ISBN 0-85404-409-4
If you have a newer style car with EFI and a knock sensor than at least if you run a lower octane petrol the computer can compensate and retard the timing as to prevent the engine from pinging.
But on all older carby powered engines they just run at whatever it is set to and there is no automatic tuning for different octane levels.
I used to have an old tired Mitsubishi Colt with the factory 1.4 litre inline 4 cylinder engine. I tried to retard the timing a little bit but all this done was to make it have even less power than it already had..lol So I thought well i'll just set all the timing to the factory specs which was 5 degrees BTDC and I thought i'll just drive to the servo and try some of that premium 98 stuff in it. This thing was bad and was pinging quite a fair bit even at middle throttle opening. I am assuming there is a ton of carbon build up in the combustion chambers is was an original 1982 engine and never had been rebuilt or repaired. Anyway it pinged a bit all the way to the local BP servo then I filled it up with Ultimate 98, after I drove out of the servo and down the road a few minutes the pinging completely stopped. Not a peep out of it ever again, at all throttle openings I couldn't get it to ping again! lol
I think I once tried just using the premium 95 stuff and I noticed a slight return to the pinging every now and then, so I just stuck with the 98 octane after that.
But yes you are right most of the Australian made old school engines with cast iron heads like in the older Holdens and Fords from back in the day were the one's that needed the lead additive. Many imported engines like from Japan and many that have alloy heads already had hardened valve seats and these could be run on unleaded with no problems. Well so they say anyway...lol.
I have found and attached the old BP fuel guide and another BP fuel information guide that I downloaded years ago about the change over to using unleaded petrol in older cars from the old back up hard drive. I tried searching the net and I couldn't find these anywhere now.
Hi Tyler I checked the car list on this guide and it doesn't mention the Holden Camira as being able to run on just straight unleaded, is your Camira built after 1986? Because I think this guide if for pre-1986 vehicles only and i'm assuming anything made after that can just run on straight unleaded.
Cheers!
Last edited by Converse; 17/08/1902:32 AM.
Zip a dee doo dah, zip a dee ay, another trash picked Victa, hip hip Hooray!
Hi Converse, I do find that my 1965 Enfield Interceptor does run better on the BP high octane. It is impossible to start without a spray of start ya bastard in the manifold. Many years ago I fitted a can in the tool box and it is direct injected into the manifold after the carby. I don't worry about anybody stealing it because they will never get it started
I actually had to google search what your "1965 Enfield Interceptor" was as I don't have a clue what it is. Now I have seen some pictures of them and It's a great looking old motorcycle you have there mate! I have never heard of these one's before.
Cheers!
Zip a dee doo dah, zip a dee ay, another trash picked Victa, hip hip Hooray!
Hi Converse, I had the exact same effect when I switched - only took a few minutes because it was effectively a new full tank of 98 - noticeably less pinging. Then I put in new plugs, put the timing back a bit more, then added a new fuel filter a couple tanks later - lots of sludgy muck in the filter when I cut it open. Much better now, but I still need to put it back slightly.
Knock sensors have probably prevented a huge amount of engine damage over the last decades they have been used - most people wouldn't even give the noise a second thought now.
My mates sister blew the engine in her car a couple years ago - ran it out of oil.
Her father said she surely must have heard the knocking/flicking/ticking noise that an engine running low on oil makes, she said that the noise was annoying her so much she put on the radio full bore.
Apparently he went off big time and by the time he had to stop because he was turning purple, she was in tears.
The other thing to consider, especially when storing fuel, is that the higher the octane the faster it loses the octane and goes off. I remember a few years ago at a stihl service school they had a letter from bp themselves stating that average 91 octane fuel begins to degrade substantially after about 10 days depending on conditions.
E10 is a whole other bag of worms, the ethanol is great, environmentally friendly etc, but it is plant based, and actually pulls water from the air. I don't know how many carburettors we used to clean that were full of water & had all the orings perish.
Fuel stabiliser is a fantastic product, never sold or promoted in a big way, but it definitely helps make fuel last longer
Years ago when I had a VS Commodore (T4 Lexcen actually) it would ping on 91, 95 and 98. On E10 or LPG it would not.
Hi I have heard that LPG is actually very good at handling higher compression before it may self ignite. It can probably even handle higher compression than even premium 98 unleaded can.
But I don't know about that E10 stuff though. I would have thought that would be similar to regular unleaded 91 in how much compression it can handle before it self ignites.
Hi Nath, As for storing fuel I am pretty bad I think, I have sometimes had 2 stroke mix stored in the shed for over one year and then I still chuck it in the old 2 stroke and mow the lawns with it. I always give the fuel mix a good shake around first in the container before pouring it into the mower. I know I should use fresh fuel but it just runs and mows the lawns. I have been doing this for years. Probably because I used to stock up on around 15 litres of 2 stroke mix some years ago. Now I only use 5 litres at a time so as to not let it sit in the shed for a very long time. I don't use any fuel stabiliser products and I just used regular unleaded 91 in the mix with Castrol 2 stroke oil.
Hi Tyler, maybe he should install a race car style loud buzzer and huge red light on the dash warning the driver that the oil pressure is critically low! lol Also with your Camira and the slight pinging, you could probably try a bottle of octane booster in the tank to see if a slight increase in octane will stop the pinging. The thing is I think if you add say Nulon Pro Strength octane booster to 98 petrol, I think it only raises the octane by around 2 points, so it will make the fuel around 100 octane. I used some of these things years ago when I was messing around with older engine's and pinging etc. It raises the octane more on 91 and 95 fuels. Not much on the 98 fuel.
Cheers!
Last edited by Converse; 18/08/1911:24 PM.
Zip a dee doo dah, zip a dee ay, another trash picked Victa, hip hip Hooray!
The other thing to consider, especially when storing fuel, is that the higher the octane the faster it loses the octane and goes off.
Umm, no. If it's in an unsealed tank, the octane number can actually increase slightly, as the more volatile components [aka 'light ends'] evaporate! Loss of these light ends will however make starting more difficult, and the engine will run richer. See attached BP info doc.
High octane number fuels contain less 'light ends' to start with, and will change less quickly with evaporation.
'Going off' of fuels is a separate process, and involves chemical reactions between some of the chemical compounds in the fuel with atmospheric oxygen. 'Unsaturated' alkyl hydrocarbons [ olefins] are the components prone to this.
Quote
I remember a few years ago at a stihl service school they had a letter from bp themselves stating that average 91 octane fuel begins to degrade substantially after about 10 days depending on conditions.
Yup, it's been my experience that 91 'goes off' much faster than the higher-octane grades, so I don't buy it for small engine use. 98 is really formulated for fuel-injection systems [and is often higher density, and may run rich], so 95 is the best option for carburetted engines IMHO. Any E10 petrol is just bad news for small engines, and is to be avoided.
At least in Oz, we're not yet in the situation that exists in many areas of the USA and UK, where the only quality small engine fuel [that won't result in exy maintenance issues] available is very expensive prepacked stuff like this; https://aspenfuel.co.uk/products/aspen-fuel/#a4
Cheers, Gadge
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