Autolite 3924's or 3923's ?
 

I saw an article were Roush was using Autolite 3923 spark plugs on their strokers (aluminum heads of course). I've been using Autolite 3924's in my Vic Jr. headed 393. What's the real difference? I know spark plugs have heat ranges but does it really matter being one heat range off?
Thanks.
Scott, SPF#1230

From the Autolite web page:

Spark Plug: Heat Range Facts

Typically, engines which are stock or are only slightly modified, can use the original equipment (OE) recommended spark plug.

Racing Modified Engines- Generally, any engine modification that alters the overall compression ratio of the engine will involve spark plug selection issues. One thing to consider is heat range. The heat range of the spark plug determines how much heat the spark plug is capable of removing from the combustion chamber.

The heat range of the spark plug does not increase or decrease horsepower. It changes the temperature of the tip of the spark plug. If the heat range is too cold, the deposits will not burn off the tip and the spark plug will foul. If the spark plug heat range is too hot, pre-ignition can occur.

A switch in heat range of the spark plug will change the temperature of the spark plug tip by 70-120 degrees.

If you are unsure of the correct heat range to use, always start with a colder spark plug. If the plug is too cold, the plug will eventually foul and a misfire will occur. This will not hurt the engine. Unfortunately, though, if a spark plug that is too hot is inadvertently chosen, preignition and detonation can severely damage your engine. Don't go to a hotter heat range unless you've consulted your engine experts.

As there are many different ways to modify your engine for racing applications, we can't tell you here which specific spark plug to use. If you have made modifications to your engine, you should refer to your component manufacturer's specifications and/or contact the experts at Honeywell at www.autolite.com.

I'm still researching though.
Scott, SPF#1230

Usually one heat range makes little difference. Probably the only way to tell would be on back to back dyno tests. I'd try them and see if there is any change. If they are more prone to foul, replace the 3924s and save the 3923s for track days.

Thanks David.

Scott