| [GMCnet] Spark plug gap? [message #76120] |
Tue, 09 March 2010 15:04  |
Don Baker
Messages: 19
Registered: October 2006
Karma: 0
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Junior Member |
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I have a 1977 Kingsley. Just bought spark plugs at Autozone. The books according to the counter man says to set at .080". The 1975& 1976 maintenance manual says .040". The 1977& 1978 Supplement says .080. What does the Gmclist say?? That seems wide to me.
thanks, Don Baker '77 Kingsley, Burleson, Texas
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| Re: [GMCnet] Spark plug gap? [message #76145 is a reply to message #76120] |
Tue, 09 March 2010 17:56  |
John Wright
Messages: 118
Registered: September 2008
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Senior Member |
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Don,
As stated in the GMC motorhome info if you can use many plug gaps. I still
use the AC#7 in the coach 403 eng and they are gapped at .036 as they come
out of the box, same for the 455. GMC stated the .080 gap for the HEI
system and it was hot, but it also caused the coil to fail in some coaches
and later changed it to .060 to reduce the load on the coil. Dick Patterson
did alot of dino testing and found no difference in the small gap as
compared to the larger gap when it came to engine performance. I personally
run a smaller gap.
J.R. Wright
GMC Great Laker
GMCMHI
TZE Zone Restorations
77 Eleganza II (For Sale)
75 Avion (Under going Frame up Restoration)
On Location at Florida Space Coast
On Tue, Mar 9, 2010 at 4:04 PM, Don Baker <baker-don@sbcglobal.net> wrote:
> I have a 1977 Kingsley. Just bought spark plugs at Autozone. The books
> according to the counter man says to set at .080". The 1975& 1976
> maintenance manual says .040". The 1977& 1978 Supplement says .080. What
> does the Gmclist say?? That seems wide to me.
> thanks, Don Baker '77 Kingsley, Burleson, Texas
>
>
>
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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| Re: [GMCnet] Spark plug gap? [message #76147 is a reply to message #76120] |
Tue, 09 March 2010 18:14 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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| Don Baker wrote on Tue, 09 March 2010 16:04 |
I have a 1977 Kingsley. Just bought spark plugs at Autozone. The books according to the counter man says to set at .080". The 1975& 1976 maintenance manual says .040". The 1977& 1978 Supplement says .080. What does the Gmclist say?? That seems wide to me.
thanks, Don Baker '77 Kingsley, Burleson, Texas
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Don,
The link Gene passed along is good, but not very informative.
If there was a catalog listing for spark plugs for GMC motorhomes '73 to '78 with 455, it would probably call for a 0.035~0.040 spark plug gap.
As many of us know, it may be an Oldsmobile 455 built in Lansing, but it sure is not your daddy's Oldsmobile. There are many variations from the passcar version. Some don't matter a lot, but intakes, valve gear covers and some other tell the story.
Back to the story:
The problem stems from '70's emissions efforts.
GM discovered that a lot of spark cuts HC by reducing the number of misfires. The easy way to get more spark is with more spark plug gap.
There is a problem with this plan, more gap takes more power to fire. They addressed this problem with HIE. It almost worked. There were a lot of people out there that were real happy with spark plugs going 50~70k miles.
But, the things that routinely run high cylinder pressures (like trucks and motorhomes) were having problems with coils, wires and ignition modules that just could not manage the sustained load of the heavier vehicles. Big Warranty Time. Very quietly GMC reset the specification for truck and such. But the 455 never had a truck application, so most everybody else just sort of forgot about it.
You can not see the difference in WOT performance between 0.040 and 0.080 spark plug gap on a development dyno (the most sensitive that exist).
Emissions testing was and still is largely done at road load regions of operation. With the lower cylinder pressures, the mixture may be easier to fire a spark through, but it is harder to light and the bigger spark does help. At coach loads, you won't ever see the help that the larger gap might be, but you will fail coils and modules (particularly common - I told - I don't have any) with distressing regularity.
Now you know with the tip of the fox's tail is white. (An excerpt from "Just So Stories).
Matt (the refugee from the engine labs) Colie
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: [GMCnet] Spark plug gap? [message #76201 is a reply to message #76120] |
Tue, 09 March 2010 22:32 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Well told Matt. I remember the 'shrinking gap' days as my friend was a GM service manager and I had a 76 Grand LeMans with HEI. Proof of what works on paper and in real world may vary.
John Lebetski
Woodstock, IL
77 Eleganza II
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