Spark plug location

[Willit Stop]

Does anyone know why Ford placed the spark plugs above the valves instead of above the pistons? (And don't tell me"Because that's where the holes are.")
Just curious. Thanks.....have a good day.

[Bulligen]

Could it be that since the combustion would start in the valve area, the explosion then travels toward the piston and then pushes it, rather than the explosion over the piston and the expansion then can travel towards the valves and also down with the piston? Just thinking out loud!

[Ol' Ron]

The reason was quite simple. Back in the day the plugs, rings and a few other things, were not as good as they are now a days, so fouling was an issue. Placing the plug in the path of the exhaust flow was a simple way to keep it clean. The correct place is near the center of combustion to keep the flame front as short as possable,

[38 coupe]

Disclaimer: all the following is based on book reading. I respect Ron's experiences and advise. I will disagree with Ol' Ron about where the center of the combustion chamber is on our flathead V8s. He is correct about wanting the plug in the center of the combustion chamber so the flame front is as short as possible.

Consider the Ricardo combustion chamber. If you get the piston close enough to the head then the edge of the combustion chamber is effectively the chamber area cast into the cylinder head, not the far edge of the cylinder bore. The area between the piston and the head is so close that it doesn't play into the ignition event. This is discussed in detail by Mr. Ricardo himself in his book. I expect the Ford engineers who placed the plug originally read that book and followed his example along with some experimentation of their own. For anyone who want to read up on this, a pdf of the book is available for free here: http://www.mediafire.com/download/gn...cardo+1931.pdf

There are two places in the book on spark plug location. They start respectively on page 45 (page 49 of the pdf) and page 98 in the book (page 103 of the pdf). I'd love to have an in depth discussion of these theories along with practical experience (the kind that Ol' Ron has way more of than me).

[Ol' Ron]

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JWL relocated a plut in his book Flathead facts. I thought tht was such a good idea I did tne same. Unfortunately I haven't had an opertunity to try it out. The location is in the center of the transfer area. I too agree the piston takes it's self out of the equasion when the piston to head clearance is very tight. This forces all the AF in the cylinder into the transferarea and with a power tip plug aids in firing a very lean mixture. Thus increasing economp in cruise. The trick here is to have an ignition and fuel system that can comtrol the spark curve and AF at the same time. I think Richard came close to this. The only change I'd make to this is to install the plug at the same angle as the transfer area. Here again the angle is important as it's partof the intake port after the AF leaves the valve. And yes Recardo is one of my heros. I've read all his publications. What's important is to keep testing