Mustang Club of Maryland

Looking to get an answer on this issue.

Short story is purchased the car last year, recently checked to see what plugs were currently in the engine. Turns out to be Autolite 764's, I then start looking up the application for those plugs and find out those are for GT40 heads. Well I looked my heads over quite a few times and do not think these are GT40 heads, there are no indicators to be seen on the head.

So if these are truely the wrong plugs, as these plugs have a shorter reach then the correct ones. (E7 head stock--stock 25 (5) [14m, .460 Reach, 5/8" Hex] verse GT40 stock--765 (Heat Range - 5) [14mm, .708 Reach, 5/8" Hex]

So what does that difference in reach effect in performance terms or in any other terms?

honestly, I don't know. Casting info would be under the valve cover. Find the engineering prefix and suffix, and call FRP for info as to what the corect plug for that head would be.

Regarding your question about reach; a projected tip on a spark plug moves the spark kernel closer to the center of the combustion chamber pocket. This places the spark kernel into an area of better homogeneousation (sp?) of fuel and air, promoting easier and more complete combustion. Two different plugs can have the same heat range, yet have different reach. As for your concerns about piston interference; consider that cam lift is typically between .300 and .400, multiplied by the rocker ratio of 1.6:1, minus lash of 0.050, gives you a valve lift of .430 to .590. Now, I point all this out because spark plugs don't normally project into the chamber nearly as far as the valves. So, by adding an additional .300 of reach to plug, more than likely, will not cause any interference issues. However, the only way to be sure is to install one of the longer plugs and tun the engine over slowly by hand, feeling for any contact. I use a projector tip Champion plug RN12YC in place of the original NGK BP7RE in one of my motorcycles. The Champ plug projects about .400 more than the NGK. I get much better throttle response (keep in mind its a 650cc twin), and the plugs don't foul anymore.

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Jim DeAngelis