Some guy posted this question on another board:

So my dad is just about finished up designing our new cylinder heads for sprint and midget engines and the last piece of the puzzle was figuring out where to put the spark plug. We put the plug at the very highest point of the combustion chamber because we believe that way all the fuel will burn in a constant downward direction, more efficiently pushing down the piston.

Last weekend he took our new head to his booth at the PRI show (Performance Racing Industry) and almost everyone there said its in the wrong spot and should be as close to the exhaust port as possible. My dad talked to a few very smart friends and they said thats an old engine design rule from back before very strong ignitions. The theory is the heat from the exhaust port helps ignite the fuel/air charge. My problem with this is it would have to be below the highest point, causing the fuel to burn up towards the top of the combustion chamber resulting in a power loss.

What are your guys thoughts on this?

His reasoning is:

I know, thats what im saying, but if the plug is below the highest point of the chamber, the combustion of the mixture will have to expand up into the top of the chamber, instead of down on to the top of the piston.

Hmm, thats a good way of looking at it, but I still would think that it would be better for all the combustion to be directed downward. You are right about it being sealed, it will still go pop wherever the plug is, but if its on the side, the main force of the initial combustion will be on the opposed side of the combustion chamber, not on the top of the piston.

My replies:
(cut from a few different posts)

It's not the plug placement that does the pushing on the piston, it's the combustion of the air/fuel mixture (expanding and pushing the piston down). Imagine it this way, no matter whether the spark plug is as low or as high as it can go, when it ignites the mixture there will still be the same expansion of gas and pushing force taking place.

What I'm saying is the same force is acting on the piston no matter where the plug is. A combustion chamber is a sealed area (minus blowby), with the only moveable part being the piston.

Imagine a sealed container filled with an explosive gas. It doesn't matter whether you light it from the bottom or top, the container is going to feel the same force either way.

I posted more but don't feel like doing all of it. Thoughts?

 

Feuling reinvented BBC heads a few years ago. They almost became a hemi style head with the plug near the top of the cylinder. BBC heads already have canted valves so plug position wasn't so critical but then the combustion chamber was also redesigned. It was more peanut shaped. Other than it bolted onto a BBC block, nothing about the head was interchangable with any other BBC head components. Special valves, rockers etc. Although the design idea was good, having to use specific components didn't make them a big seller.

Your best bet is to have some engineering software run combustion flow tests over the piston and chamber design to best determine where the plug should be. You don't think companys such as Dart, AFR etc just make a head on paper without a lot of simulations and expect it to be perfect.

SBC, BBC and just about everyone else uses a wedge head design. The fuel mixture is ignited on one side of the cylinder and the combustion is forced across the piston mainly due to the combustion chamber design. Dome pistons force the energy over the dome so in theory, they're not as good but my 13:1 BBC engine has huge domes with no problems.

On the other hand, Dodge plus a bunch of import engines use a hemi design. This places the plug at the top of the cylinder and has the valves on both sides of it. The theory was that the ignition of the fuel pushes down over the piston dome (very few hemis have flat top pistons) which can get a better burn from the fuel and produce more power.

Still lots of 426 hemis at the drag strip making lots of power.

 

Yep, the key here (at least in my opinion) is combustion efficiency. Placing the plug equidistant from the edges of the cylinder walls gives the flame front an equal amount of distance to cover and ensures (or should ensure) a more consistent and complete burn of the fuel in the chamber. Starting combustion on one side is probably a less efficient way of getting the job done, but nonetheless effective in "less than maximum effort" engines.

The rotary engine is so grossly fuel inefficient because of the long, narrow combustion chamber shape, and the solution is to add a second "trailing" spark plug to burn more of the fuel while it's still in the engine. Some (ignorant) rotary fans may argue with this, but given that Mazda's LeMans winning (don't even get me started on that supposed proof of rotary engine superiority...) 26B 4-rotor actually used three plugs per rotor, I think the assumption is extremely valid.

Because the flame front has to travel so far (relatively speaking) during each combustion event, a lot of unburned fuel is left in the combustion "chamber", and exits into the superheated exhaust... which is why rotary engines A) have such high EGTs, B) are so damn loud, and C) blow lovely fireballs with an unrestricted exhaust path.

Top Fuel engines also have three plugs per cylinder unless I'm mistaken, so the problems associated with lighting off nitromethane at extremely high pressure and in volumes bordering on hydraulic lock aside, the goal is obviously to burn as much fuel in the chamber as possible while it can still do you some good.

 

Top fuel engines have 2 plugs per cylinder and each set of 8 is ignited by a seperate magneto. The amount of energy jumping across each plug is like an arc welder. They burn up 16 plugs in one 1/4 mile pass.

Gasoline need about 14.7:1 air/fuel for a proper mixture. Alcohol needs about 6:1. Nitro needs 1:1. A nitro engine is very close to hydraulicing the engine there's so much fuel dumped into each cylinder. With such a high fuel mixture they need dual magnetos and plugs just to ignite the fuel.

Ignition coils all produce roughly the same amount of voltage. Normally between 30,000 and 50,000 volts but the amount of amps varys. I use a top of the line MSD coil which produces 2 amps. The MSD magneto produces 44 amps at roughly the same voltage as a normal ignition system.

You put a big enough spark in the cylinder and it doesn't matter where the plug is located.

 

haha So I guess I just need to tell him to run a magneto and forget about plug placement.

All jokes aside, good info.

 

http://www.jonkaaseracingengines.com...CJ%20Story.htm

That might be a good read for you.



This is a good example of moving a plug (along with the valves and chamber) to the right location rather than the as designed location in the 60's.

When Jon modified the heads for the EMC he moved the spark plug even more...



I've seen the same sorts of things on heads where BES has moved the spark plug as well.

Generally you want the plug as close to the center of the bore as possible and aimed at the exhaust valve.

REMEMBER IT'S ONLY AND EXPLOSION IN A NITRO MOTOR, IN A GAS MOTOR IT'S A BURN AND THE FASTER YOU CAN DO THAT THE BETTER.

Bret

 

I could have sworn that I've seen pictures of a Top Fuel cylinder head with a plug in the center of the roof and one on each side. Must be imagining things.

 

I think reason most hemi's run a dome piston is because of compression. To me it looks like the hemi chambers have a lot of volume so they need a large dome to get that compression back. Just my observation.

 

It's hard NOT to put some thought to this subject when you watch the change in head design of factory motors over the last 25 years (my effective weekend wrenching career, plus experience with motors that are now as much as 40 years old). For what it's worth, here's my thoughts...

The facory is interested in EFFICIENT combustion. That means power and economy, but it also means better emissions performance. The key seems to be getting it as close to the center of the chanber as possible. Couple of examples...

4 valve import motors: The little dead space between the 4 valves in the clover leaf is an ideal spot for a plug and very close to dead center. These little motors can, obviously, return some VERY impressive emissions numbers and this is probably ONE contributing factor.

LS-1: They put the plug about as close to center as they could without having the valves bang into it when they open. Putting it up "on top" of the chamber helped without having it stick WAY in from the side.

Vortec: Still over on the side but with a plug boss that protrudes WAY into the chamber, putting the (extended) plug tip fairly close to center. It's moved slightly to the exhaust side, despite being a "straight plug" head. I think this was done more so they could squeeze the plug as tight and far towards center between the intake and exhaust valve as possible, not so much just for the sake of getting it closer to the exhaust valve. AFR and other aftermarket heads do this too, even in their straight plug versions.

L-98 (TPI): GM even way back in the 80s started pushing the plug away from the chamber wall and closer to the center of the combustion chamber via a fairly large plug boss that sticks into the chamber from the side.

70s era stuff: Plug flush with the wall of the combustion chamber. Bottom of the barrel.

With respect to putting the plug closer to the exhaust valve..... I don't think it's got as much to do with helping the spark ignite the mix as it does with detonation. Detonation typically starts where things are hottest (near the exhaust valve). But if you burn the mix closest to the exhaust valve FIRST, you leave less chance that detonation can start there.

 

jimlab - I might be wrong but last I saw it seems like the top fuel engines have two distributors (might be the mags though). I would assume that if they had three plugs per cylinder, they would have one distributor going to three plugs per cylinder, or three distributors. Top fuel engines are kinda mystical to me though, so please correct me if I'm wrong.

Seems like the original poster to loadre on the other message board might be kind of confused as to what causes the greatest force on the piston, the speed and direction of the flame front or the expansion of the gas itself. I have been wondering about this myself for a bit...does the speed and direction of the flame front even account for a significant proportion (>10%) of the force on the piston, or is it mostly the expansion of the gases from combustion?