Technical Help &/or Questions > Engines - Carbies - Fuel Systems - Exhausts - Radiators

425 Ignition Issues

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My 79 coupe DeVille has AC Delco R45 XLS6 plugs fitted - which appear to be no longer made. I'd like advice please on the best replacement available in Australia. Of course, the originals are available on E-bay, for about $50 delivered (for 8), but I would prefer something in Oz.

The VEI tells me the gap should be 0.060" - a little larger than I am used to, but should handle it with electronic ignition. BUT when I removed one for a look, the gap was 0.120" - twice that allowed - and it was still starting and running fine. I assume the timing would be retarded slightly because of that? I'll adjust the timing once I have new plugs - the old ones look fine, but will not have been replaced in 38 years.

Timing: has anyone advice on whether a little bit of extra advance is good for this engine? And, by how much?

Bruce Reynolds:
The HEI specs call for .060 gap.

Not sure what the plugs are in my own 500 running an HEI, but I will check in the next few days.

The amount of advance will depend on what octane rated fuel you are using.   Too much advance will cause destructive pinking.

Bruce. :devil:

Thanks Bruce - I'm using 95, and no pinking/pinging.
US don't use RON, so comparisons are difficult, but they recommend 88.

Bruce Reynolds:
I use 91 Octane in my 500, and it would have a higher compression ratio to your 425.   If your 425 is stock, then basic Aussie Unleaded will do fine.   Don't use the cheaper E10 stuff, or you will be asking for trouble.

Bruce. :devil:

PS.   91 Octane is better in Tasmania than Melbourne.   When I went to Wangaratta, I found that the 91 in Melbourne made the car ping its' head off.

Back to replacement spark plugs.
In the AC Delco catalogue for Cadillacs, the original spark plug has disappeared, and the only alternative given as an AC4 (platinum) with 1.0mm gap - this plug includes a resistor.
In the Champion catalogue, an AC4 is replaced by an N9YC - but just in case, I checked for Cadillac: and Champion recommends RN12YC with 1.1mm. The R means it has a resistor, which I assume I need.
I can't find an NGK catalogue for comparison.
So, unless anyone has a better idea, I'm going to fit Champion RN12YC plugs.


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