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Petrol - Leaded Or Unleaded?

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#31 Guess-Works.com


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Posted 06 July 2008 - 06:09 AM

Where is the sanity here. "Will it run on unleaded?" Absolutely!!!!! The fear (and myth) is that running a "leaded" engine on unleaded fuel with result in damage to the valve seats. The other oncern is lead, or higher octane fuel, is needed with high compression, high performance engines to prevent pinging (pre-ignition) that can cause problems with the pistons. Being a 998, unless you have it built to the hilt, it doesn't fall in the latter category of concern. The difference between a "leaded" and "unleaded" head to combat valve seat recession is hardened valve seats. But I have to say that in 30 years of mini owning and driving (all pre 68 minis over here in the god ole US of A), I have never had a cylinder head upgraded and I have never had a failure. A 998 mini, even if driven hard, doesn't tax the materials in the standard valve seats. My opinion is that you don't have anything to worry about. Just put unleaded gas in, drive it and have fun. I am continually miffed by the hype over lead addatives.

Yes you can just dump unleaded in an engine and run it, but firstly there will need to be a change in ignition timing. Mini's pre 89 are designed to run on 97+ ron, or at least UK spec ones are... Unleaded is only 95 ron in the UK.

Seat recession in the head does not necessarilly stop the engine working, but it will stop it breathing as efficiently as possibly, this is the effect which is experienced in the UK, when using unleaded fuel in a head without hardened exhaust valve seats....

I cannot comment on the quality of fuels in the US

but my advice is Don't.... unless you're intending on replacing the head anyway, then if it does go there's nothing lost... make sure you get the car tuned when you swap to using unleaded...

#32 samwell


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Posted 06 July 2008 - 07:02 PM

to add a bit of personal experience, i bolted a 1275 leaded metro head to my unleaded 998 and carried on runnig unleaded with the occaisional tank of valvemaster plus when i could be bothered.

Before i installed the head, i lapped the valves in and they lapped perfectly with a brilliant finish on the seats but having just removed that to check it over after roughly 3k miles, pitting in the exhaust valve seats was evident.

As guessworks said, running the wrong fuel will not directly stop your engine working but over time it can have a corroding effect ime. judging by the rate of damage to the seat i would suggest it would need another 10times those 3000miles to start properly effecting the sealing ability of the valve making a noticble change to the cars performance.


#33 Brams96


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Posted 22 September 2008 - 08:31 PM

This is all good advice!

If anyone has experience of swapping out the old leaded head or knows what is actually done when converting to unleaded perhaps they could write a guide with all the info like what valves to use with what guides, seats etc. I'm sure it would be a great help for everyone and maybe the admin could sticky it.

Just my 2p worth :wub:

#34 GrahamF


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Posted 02 October 2008 - 08:45 PM

Ok, heres one for you, i recently bought this engine off a bloke on here, hadn't really thought about it untill reading this but i'm unsure whether it is leaded or not.
the head is a MED Stage 3 1275 11 stud head, according to MED it was built in 1990, i bought the engine with low compression on all 4 cylinders, whipped the head off and basically its knakered, everything attatched to it needs replacing.
this is a pic of 2 of the ports, all 8 are the same, what would you suggest has been the problem and what fuel would you suggest to run when i finally get it back together?
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