Spark plugs after cat bypass

[foot2firewall]

I've installed cat bypass pipes on my S3C - what is the advice on spark plugs; should I use the ones specifically for a non cat engine now? Is there any recommendation on part number and brand to chose?

[phillpot]

According to NGK "cat and "non-cat" engines have different design of plug, taper seat in the cat engines and a flat seat/gasketed plug in non-cats

Cat = BPR6EF

Non Cat = BCPR6E


this chart explains the codes, the heat range, 6, is the same so I would stick with same plugs.

[tvrgit]

phillpot said:

According to NGK "cat and "non-cat" engines have different design of plug, taper seat in the cat engines and a flat seat/gasketed plug in non-cats

I'm not entirely sure that's always the case - years ago I had a non-cat 2.9 Granada with taper seat plugs (although admittedly it might have had the wrong ones when I got it, and I kept putting the wrong ones in for the next 150,000 miles). There is a "K" or an "F" cast into the cylinder head above the front left exhaust port (and above the rear right one although that's harder to see). AFAIK, the "K" means taper seats, "F" means flat seats.

Otherwise, as you say, the plugs are exactly the same, so no need to change whatever is in there.

[v8s4me]

I've been led to believe that the two main considerations with plugs are the reach and heat range.

Reach - if the plug is too short ignition will take place away from the optimum point; too long and it will punch a hole in the piston.

Heat range - too "hard" and the engine will be hard to start in cold weather and may not run properly until it is good and hot; too "soft" and the worst case scenario is that it will burn a hole in the piston.

So, as advised above, stick with what you've got.

[foot2firewall]

Thanks all for the feedback, it took a lot of plus gas and a spark plug wrench upgrade to get the first plug out! The head is marked F and the seat for the spark plug is flat, but the plug I removed is tapered, I guess someone overtightened it to compensate for being the wrong type. It's also not the correct temperature rating (5 not 6.) All being replaced with NGKPPR6ES tomorrow.