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Confussion about spark plug gap on 74 engines with HEI

Started by collector, February 03, 2010, 04:26:39 AM

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collector

As we all know the decal on the header shows information only too the standard ignition system. There was never any mention about the HEI ignition system plug gap on the decal or in the factory manuals. The isssue Im concerned with is that since the spark is almost 3 times stronger on HEI systems than standard ignitions, isnt the factory gap a little too tight. I know in 75 the gap increased a vast amount. Should I gap the plugs to that specification since HEI became standard in 75 and up? Is there someone who has a service bulletin about this matter? I find that too close of a gap can cause damage to the plug and a rough idle in most cases. What should I do and or what has anyone else, who has HEI ignitions done?

The Tassie Devil(le)

G'day John,

With the Factory HEI, the Plug Gap should be something like .060".

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

TJ Hopland

Somewhere I have a supplement manual for the HEI but its been a while since I have seen it.  Its one of those things that came with a car I bought and went into a pile and over a couple of moves Im not sure which box its in.

I cant think of any reason it would be any different for 74 than 75 or 76.  The things that usually have an effect on plug gap dont appear to have changed over those years like the compression ratio and head design.  Biggest change I can think of is adding the cat converter in 75. 

1975 manual says gap 0.060 inches.  R-45NSX plug.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

"Cadillac Kid" Greg Surfas 15364

Jphn,
What are your existing plugs gapped at now? Tune up/plug changes  frequencies are scheduled at 25K and I believe you said you car had less than that so there would be no reason anyone would have changed them. I recently found a Coupe de Elegance with a bit over 50K on it and the original plugs had not been changed. It ran well, so under less than severe duty I would expect your car to have the original plugs.
The HEI was incorporated, basically to assure the combustion of the leaner fuel/air mixtures that were mandated by the (then) tighter emmissions mandates.  With a higher voltage the spark would be capable of jumping the greater gap and the providing a greater spark presence, helping to ensure positive ignition.
If the motor has the correct 120CC combustion chanmer cylinder heads then the R-45NSX (or variants for heat range and usage differences) will fit without obstructions and they were intended for the 0.060 electrode gap.
Greg
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

RobW

My Chilton manual from 1976 includes all models from 69-76.
For 1974 eldorado 500 cid R-45NS spark plugs gap .035 Timing 10BTDC
For 74 eldorado 500 cid electronic R-45NS spark plugs gap .035 Timing 10 BTDC
For 75-76 500 cid Spark plug R-45NSX gap .060 Timing 6BTDC
For 75  500 cid EFI R-45NSX .060 Timing 2BTDC
For 76 500 cid EFI R-45NSX .060 Timing 12BTDC

So you can see both the plugs and timing changed for 75-76.
Maybe there were changes to the HEI in 75-6? Maybe kv increased?
Rob Wirsing

TJ Hopland

Anyone know what the difference between a NS and NSX plug is/was?   

I have had a couple of Cadillac HEI distributors that had a different mechanical setup for the advance.  I dont know for sure what year they came off of but I did also have other parts that appeared to be from a 74 so I assumed they were 74's.   I also have a fairly low mileage apparently unaltered 75 thats distributor innards look like every other GM one I have ever seen so I am thinking that the odd ones were 74's. The odd design seemed to work fine but looked more complicated and perhaps more expensive to produce? So thats maybe why there was a change?   I will go look but I think the last one I had ended up in the 78 I sold last year.  Other than the mechanical bit I dont recall anything else looking unusual but that does not mean there was no difference.

The supplement manual I have came with a 73 shop manual and some other supplements that if I remember correctly had 73 copyright dates on them.  I think one was for the Frigidaire AC system and if I am remembering correctly the HEI one had the same cover design.  Anyone else remember seeing this manual?  I looked through a few boxes and did not see it so it may be in one of my storage places which I can no access till the spring thaw.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

collector

So Ill regap the plugs from 60 back too 35 but that is such a small gap. The plugs should wear out much much faster. Too much spark for such a small gap. The smaller gap was for the lower voltage so it didnt need much space to jump but with HEI with such a massive amount of power I would have thought the gap would be larger