spark plug location

Hello all,
Building a single cylinder 4 cycle engine. I want to know if I install the spark plug on the side of the head Vs on the top of the head makes a difference?

If is on top, the spark will happen directly in the combustion chamber but if the spark plug is installed on the side will not have direct contact. I'm not sure if I'm describing this correctly or not but wanted to make sure if it cause an issue with running?

Thanks
Borna

I built 5 single cylinder horizontal and vertical Upshur 4-stroke engines using both locations. Also the ends of the plugs are recessed quite a way inside the heads. I was concerned about all of these issues, but they didn't make any noticeable difference.

A more important factor is to locate the plug as far away as possible from parts like the rocker arm(s) and valve stem (s), to eliminate the possibility of the spark jumping to a convenient ground (arcing-over).

-MB

I think the spark-plug located center top is theoretically the best. Think of how the flame front propagates... when the compressed charge is ignited center/top, then the burn is even from center to perimeter.

The side location can be a bit trickier. I think it helps to consider where it is located relative to the intake and exhaust valves... you want the fuel/air mixture to be fresh where it contacts the spark terminal, with a minimum amount of waste gas.

But yes in the end, for a simple model engine, you can put it in a wide variety of places and it'll run. It's very easy to get overly concerned about IC things like spark & valve timing, cam geometry, etc, and in the end, much of that stuff has been researched to death so as to squeak the last 5% of power out of an engine. If your cam opens the valves at roughly the correct time, if the spark fires close to TDC, and you have compression, the engine will run!

The issue of sparkplug placement becomes important on larger displacement, high compression engines. The goal is to control the flame-front geometry, so as to not create detonation inducing hot-spots. Or more specifically – obtaining a proper combustion chamber design (which includes sparkplug location, and the design of cooling passages/fins) to maximize the engine’s compression ratio for a given fuel octane. When TEL (lead) was removed from fuel in the early 70s, some engines were found to be much more sensitive to low octane fuel than others of the same compression. Proper combustion chamber design is very important. In addition to removing lead, engineers also made changes to the combustion chamber designs.

Another aspect of plug location is obtaining a rapid pressure rise. Normally accomplished by placing the plug in a central position over the combustion charge – it minimizes the time of the combustion event, causing a rapid pressure rise for high-rpm power production. Unfortunately a centerlized plug location will make an engine more octane sensitive, not less. Again, this is much less important for lower rpm engines, like big V8s of the 70s. Modern rice-burner engines will have a Centralized plug location, as they normally operate at much higher rpm.

For this hobby, octane is not normally a limiting factor (folks on these boards typically run under 6:1 compression). There’s also not a big push to maximize power production at high rpm. The engines are small, so even at high rpm, the flame-front doesn't have a long way to move.

Thus I agree with Metal Butcher, in that you should place the plug in a convenient location of your choosing. Get the spark into your charge, and it will burn just fine. . . .

thank you all for the comments.

by the way I forgot to ask. Does the I.D of channel between the spark plug to the combustion camber matter? as long some of the gases can reach the spark, then I'm ok?

Thanks
Kamran

On my Upshur the 'L' shaped passage is the same size as the spark plug's tap drill. If I look down the hole under the head the electrode end of the spark plug can't even be seen. No need to worry.

-MB