AC spark plugs
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
AC spark plugs
I see that originally my '74 called for the AC R44T and then the next year they went with R45TS because of the HEI ignition. This is a hotter plug? And is to be gapped to .045 instead of the .035. I imagine these plugs are also recommended when including MSD boxes to the pre-electronic cars. thanks if you know more about that.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
The hotter plug was more about emissions. The wider gap was possible do to a hotter ignition. Going to a hotter plug is not advisable unless you are having a plug fouling problem. Too hot of a plug can cause pre ignition which can be extremely destructive.
Mike
Mike
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
The engine is newly rebuilt so I should go back to the R44T I suspect even with the MSD box. I was running a hotter plug with a fouling problem previously.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
R44 is pretty cool 45 not much hotter.
I run a 5 heat range in the ngk which is hotter than both those plugs. Normal Street driving stuff, not racing.
If the plug is too cool your plugs will run dirtier, the right heat range allows for self cleaning.
.045 works good with msd imo as well.
I run a 5 heat range in the ngk which is hotter than both those plugs. Normal Street driving stuff, not racing.
If the plug is too cool your plugs will run dirtier, the right heat range allows for self cleaning.
.045 works good with msd imo as well.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
.035" gap is for typical well tuned engines. The wider gaps were used to prevent misfire on "leaner" tuned emissions applications. If you no longer run a lean tuned emissions application you do not need to use the wide gaps anymore.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Starting with the HEI distributor installed in Vetts in mid '74 they called out for a .045 gap which seems to be the callout for electronic ignition systems that run with higher voltage coils. If you are running points with a normal coil the .035 gap is correct.
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Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
Better to say - the bigger gaps required the HEI.
Last edited by stingr69; Mar 14, 2019 at 12:09 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Bigger gap means more spark 'length' to maximize probability of firing properly. BUT, if the ignition system isn't capable of providing adequate voltage to jump that gap....EVERY TIME...then you would set to a smaller plug gap. HEI system has higher spark voltage than point system; MSD system will also IF the coil is hot enough to provide spark voltage required.
I went to NGK iridium plugs years ago, because the tips won't erode from use and they prevent fouling (unless you have an oil-burn problem). Changing plugs on a stock-dress small block is a total PITA. NGK iridiums will last 100K miles. I'll be dead before they need to be changed again. Well worth the difference in price. What's another $30 to assure I never have to change plugs again?
I went to NGK iridium plugs years ago, because the tips won't erode from use and they prevent fouling (unless you have an oil-burn problem). Changing plugs on a stock-dress small block is a total PITA. NGK iridiums will last 100K miles. I'll be dead before they need to be changed again. Well worth the difference in price. What's another $30 to assure I never have to change plugs again?
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Remember when the HEI first came out and the gap spec was .060?! Boy GM fixed that in a hurry..........
Closer gap requires less energy to jump. Sharp edges also require less energy because the edge is where the current likes to "jump off"......this is why a worn out plug hurts performance.....it can't "jump off" as easy because of rounded edges.
I like the NGK V-Power R5671 and R5672 depending on application.....4 heat ranges from 7 to 10 for whatever you may be doing. The iridium stuff is gaining traction in the community due to good performance over a period of time and resistance to fouling......but pricey.
Jebby
Closer gap requires less energy to jump. Sharp edges also require less energy because the edge is where the current likes to "jump off"......this is why a worn out plug hurts performance.....it can't "jump off" as easy because of rounded edges.
I like the NGK V-Power R5671 and R5672 depending on application.....4 heat ranges from 7 to 10 for whatever you may be doing. The iridium stuff is gaining traction in the community due to good performance over a period of time and resistance to fouling......but pricey.
Jebby
