Author Topic: 425 Ignition Issues  (Read 1979 times)

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Offline cad425

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425 Ignition Issues
« on: March 19, 2016, 03:49:41 PM »
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?
1979 Coupe DeVille 425 - project
2014 Subaru Tribeca - dd
2006 Fairmont V8 - the 'other' V8
1996 E220 Cabriolet - belong 'im spouse
1989 Capri Turbo - gone to Canberra

Offline Bruce Reynolds

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Re: 425 Ignition Issues
« Reply #1 on: March 19, 2016, 09:03:56 PM »
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:
Bruce Reynolds, Lindisfarne, TASMANIA
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
(Past President Modified Chapter)

Offline cad425

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  • Although optimistic - that fuel line is a worry!
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Re: 425 Ignition Issues
« Reply #2 on: March 20, 2016, 11:08:08 AM »
Thanks Bruce - I'm using 95, and no pinking/pinging.
US don't use RON, so comparisons are difficult, but they recommend 88.
1979 Coupe DeVille 425 - project
2014 Subaru Tribeca - dd
2006 Fairmont V8 - the 'other' V8
1996 E220 Cabriolet - belong 'im spouse
1989 Capri Turbo - gone to Canberra

Offline Bruce Reynolds

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Re: 425 Ignition Issues
« Reply #3 on: March 20, 2016, 09:32:10 PM »
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.
Bruce Reynolds, Lindisfarne, TASMANIA
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
(Past President Modified Chapter)

Offline cad425

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  • Posts: 23
  • Although optimistic - that fuel line is a worry!
  • Location: Sydney, NSW
  • Mood: Optimistic
  • My Name: John
Re: 425 Ignition Issues
« Reply #4 on: April 12, 2016, 11:50:28 AM »
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.
1979 Coupe DeVille 425 - project
2014 Subaru Tribeca - dd
2006 Fairmont V8 - the 'other' V8
1996 E220 Cabriolet - belong 'im spouse
1989 Capri Turbo - gone to Canberra

Offline cad425

  • 62 Series Member
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  • Posts: 23
  • Although optimistic - that fuel line is a worry!
  • Location: Sydney, NSW
  • Mood: Optimistic
  • My Name: John
Re: 425 Ignition Issues
« Reply #5 on: January 17, 2017, 02:11:14 PM »
A final message to close this one out.

A forum member provided a set of original plugs - thank you. However, I think I know where that huge gap came from: that is the original gap AC Delco provides in a new package, and a previous owner - can't be his mechanic - has fitted them straight from the packaging without checking the gap!

So a lesson for us all: check the gap.

Another lesson: the HEI in my 79 has no problems firing with that gap. But the timing did need amendment....
1979 Coupe DeVille 425 - project
2014 Subaru Tribeca - dd
2006 Fairmont V8 - the 'other' V8
1996 E220 Cabriolet - belong 'im spouse
1989 Capri Turbo - gone to Canberra

Offline Bruce Reynolds

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Re: 425 Ignition Issues
« Reply #6 on: January 17, 2017, 07:37:13 PM »
You did set the gap to .060?

Bruce. :devil:
Bruce Reynolds, Lindisfarne, TASMANIA
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
(Past President Modified Chapter)

Offline cad425

  • 62 Series Member
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  • Posts: 23
  • Although optimistic - that fuel line is a worry!
  • Location: Sydney, NSW
  • Mood: Optimistic
  • My Name: John
Re: 425 Ignition Issues
« Reply #7 on: January 18, 2017, 02:13:08 PM »
Yes of course. Maybe a little larger than that, because I know it can fire it, and the larger spark is good for leaner mixtures.
1979 Coupe DeVille 425 - project
2014 Subaru Tribeca - dd
2006 Fairmont V8 - the 'other' V8
1996 E220 Cabriolet - belong 'im spouse
1989 Capri Turbo - gone to Canberra