Model A & B

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Brumfield 7/8-18 Inch Spark Plug Listings

Brumfield Heat Range Test Method

In many postings on internet message boards, Larry Brumfield has shared a lot of his knowledge about engine operation, combustion, intake and exhaust theory, and cylinder head design, construction, and manufacture.

His cylinder heads were produced and sold for many years with nearly all customers reporting great satisfaction and trouble-free service.

Many thanks to Larry for providing the 7/8-18 inch spark plug heat range information below, and allowing it to be presented on Ford Garage!

Here are Larry's latest 7/8-18 inch spark plug listings:

Spark Plug Heat Ranges:

The following list of 7/8-18 inch spark plugs fit the original Model A head. However, the results of the use of some of them could be detrimental to your engine or at the very least, foul out. In other words, just because a plug has the correct thread and length to screw into your Model A head does not mean it is the one you should use.

A heat range test (discussed below) is the best way to determine the correct spark plug for a particular engine setup and a person’s driving habits. Champion plugs appear to be the brand most frequently used but that does not detract from the quality results of other brands.

Champion recommends the W-18 to be the modern replacement for the 3X as their heat ranges are fairly close.

Concerning the W16Y, according to Champion, the W16Y, being a projected nose plug, is hotter than the W18 or even a W20 at low speed but cooler at high speeds over 5500 RPM, which according to Champion, the high RPM is necessary to cool the projected nose with a rush of incoming cooler air even though there is greater combustion. Obviously, such high RPM will never apply to most Model A’ers. Regardless, however, most people report that the W16Y runs cooler than the W18 at ANY speed in a Model A.

The performance of the W16Y in a Model A, as compared to Champion’s general description of its performance as a projected nose plug, appears to be an area of controversy for some Model A’ers. However, the Champion 7/8 plugs, including the W16Y, fit a variety of industrial and agricultural engines that run under all kinds of running conditions and loads and obviously Champion does not feel the need to make statements about how the W16Y specifically performs in many Model A’s.

Consequently, if some Model A’ers make blanket statements based on the results of the W16Y’s cooler performance in many Model A’s it should be considered inaccurate as a blanket statement because the W16Y may not perform the same in other engine brands as well as some Model A’s. Plus, the results of Champion’s work would no doubt be determined under controlled conditions where the air/fuel ratio, spark intensity, etc., would be near perfect which is not the case for most Model A’s.

So it's still best to use the following list as a general guide, but also do your own heat range tests and reach your own conclusions and opinions when running any of these plugs in a Model A engine.

Champion (hotter down to colder):
W16Y (For Model A engines it ought to be somewhere between W18 and W14)
W18 (About equal to the original 3X per Champion)

Also the original 3X which Champion lists as normal along with the W18.

Motorcraft (hotter down to colder):
TT15 (the same as Champion W89D)
TT10 (the same as Champion W14)
TT8 (the same as Champion W14)
TT4 (the same as Champion W10)

3076 (the same as Champion W14 and Motorcraft TT10)

Mopar (hotter down to colder):

Prestolite (hotter down to colder):

Stitt (hotter down to colder):
137 BSP
147 B

Larry Brumfield
August 2016

Spark Plug Heat Range Test:

A heat range test is the best way to determine the correct spark plug for a particular engine set up and a person's driving habits. When in doubt always choose a colder plug first until you determine what works best. The worst thing that can happen is the plug can foul out whereas too hot a plug can cause pre-ignition and possible piston damage, i.e., a hole melted right through the piston! Some people may think it's baloney but it's the truth.

Conduct a heat range test as follows:

Drive out someplace where you can run the car at a sustained speed for a few miles without having to slow down or stop; very important. This allows the heat to build.

Run the car up to the maximum speed in high gear that you would typically drive if you decided to drive fast, and I don't mean dangerously high speed. Hold the car at that sustained speed and without letting off the throttle, reach over and turn off the ignition. Let off the throttle, push in the clutch right quickly and coast over to the side of the road. Open the hood and allow the engine to cool off a bit. Remove the spark plugs and examine the color and condition of the electrodes and insulators. The color will be a true indication of how hot or cold the plugs are running.

Now remember this .... if you run the engine and then let it slow down or idle and then check the plugs, you will not get a true indication of the heat, at speed that is. I've seen many a person judge a spark plug after an engine has been idling and use a hotter plug to get the reading they want, only to find out that at speed, the plug was too hot!

Of course this is not all set in stone. A person who putts around at say 35 or 40 MPH and never faster may need a hotter plug. The lower the combustion chamber temperature, the hotter the plug should be. On the other hand, as conditions cause the combustion chamber temperature to rise, a colder plug may be necessary. Also, a fuel mixture that is too rich or too lean can confuse the readings; plus a poor ignition system, poor compression, an oil burner, etc., etc.; so the accuracy of the above depends on an engine with proper function.

In conclusion, the plugs should run hot enough to keep the deposits burned off and that's it.... This means the color should range anywhere from light brown to grayish tan. Sooty black is too cold and chalky white is too hot.

Larry Brumfield
July 2019

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