What are the best plugs for the twincam with unleaded petrol , n9y ?
I don't think the petrol has much to do with it but N9Y are the wrong ones; they are meant for the OHV engines and later single cams.
I believe the correct spark plugs for the twincams are:
Champion: N6Y (better N6YC best N6YCC). Not easy to find these days so some people suggest changing to N7YC or N7YCC.
NGK: BP8ES which are very difficult to get hold of. BP6ES (or BPR6ES, the 'R' is for 'Resistor' which acts as a suppressor for radio interference) are equivalent to N9Y so I'd say not the right ones for a Twin cam. Having said that, I have seen them listed as suitable for Fiat Twin cams. By the way, if you have good quality suppressed HT leads then it's better to go for non-suppressed spark plugs (without "R" in the name)
Bosh: W200T30 These are meant to be equivalent to Champion N6Y but I found them to be useless in my 2000 Twin cam lasting only about 3000 miles so I'd strongly recommend not going for these. But I am now using Bosch Super-4 W56 spark plugs in my Fiat with good results. W56 plugs are also getting hard to find these days but the suppressed equivalent (WR56) are still readily available.
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