which plug is correct

[pj the dj]

I have a '29 engine in my '30 coupe. It has a 5.5-1 head, a b cam, modern distributor, alternator and was computer balanced. I have Motorcraft plugs no.TT-10 in it now. A friend told me to use Champion W-18 plugs. Today I took out the plugs to see if it warranted changing. They were not dirty, but were gapped at 30. I re-gapped them to 35 since that's what the book says. Should I change plugs to the Champion or stay with what I have and go back to the 30 gap? Thanks for all your help..Phil

[Tom Wesenberg]

Nothing wrong with the .030" gap. As far as plugs, just keep an eye on the color of the center electrode. If it stays a light tan, then you are in the right heat range.

[pat in Santa Cruz]

if what you have is working, stick with it. Plug gap on a higher compression head is usually smaller than stock, so .030 is probably OK. TT10s are the cheaper driver plugs. They work fine, but the metal spark leads sometimes seem a bit too short with them. The equivalent replacement for the original 3x is the W16Y, a taller plug that fits to the metal leads better. A W18 would be hotter, not a good idea with a HC head. Often with HC heads the W14 works better. Keep your eye on the color of the insulator. If its too white, the plug is too hot, black its too cold ( or too rich). A nice tan is the right color.

[Purdy Swoft]

The original spark plug was the Champion 3X.My favorite spark plug for a driver car is the Champion W 18 . I use the W 18 with 5.9 heads with very good results. The compression ratio really makes no difference. You just need a spark plug that is hot enough to run clean at all speeds.. I found that the W16 Y doesn't run clean at low speed and the Motor craft TT 10 is too cold at any speed. None of these plugs are really the replacement for the heat range of the original 3X plug but is what the venders chose to sell. The Champion W 14 is WAY too cold and would run sooty and give poor performance , worse than the Motor craft TT 10. I had to remove a set of W 14 plugs from a friends truck because they ran sooty, didn't crank good and gave poor performance. If the plugs that you are using run clean and perform well, there is no need to replace them.

[pat in Santa Cruz]

Originally Posted by Purdy Swoft

. The Champion W 14 is WAY too cold and would run sooty and give poor performance , worse than the Motor craft TT 10. .

it all depends on the engine. After all these years and all the various modifications, every engine has its own personality. The one I referred to was a new AER engine, with bigger valves, Stipe 340 cam, stock Zenith and Snyder head. W16s ran white and crispy even after a 1000 miles with marvel oil additive. Switching to W14... and it ran perfect. My engine with bigger intake, winfield B downdraft carb, Stipe 330, Brumfield head, Ross pistons runs great with W16s. My stock, .040 over, babbitt rods, std head engine with Zenith carb runs best on 3X and very well on tt10s.

I start with a 3X equivalent (W16 according to Champion) then determine the final choice by reading the insulators in conjunction with engine performance.

[29er]

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Here's a link to a useful dissertation on plugs for the Model A:

http://rockymountainmodelaclub.org/S...k_Plug_101.pdf

... with thanks to Andy Wiedeman

My '29 special coupe has a 6:1 head and the TT10's always were a little oily, indicating they are maybe a little cool as Purdy says. I replaced them with Champion W18's and they run cleaner. I gap them at 0.035 as per Les Andrews suggestion for high compression heads.

Mike