plug gap explanation
Thread Starter
Racer
Joined: Jul 2004
Posts: 450
Likes: 0
From: Fredericksburg VA
plug gap explanation
Can someone please explain to me plug gaps? I realized that beyond knowing what my plug gap should be I know nothing about it. What does increasing and decreasing plug gap do? I would assume something with how hot the spark is? Am I even close?
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
Very very large gaps are best. It starts a large flame kernal to begin the fuel charge ignition. It also is good for emissions and fuel economy. These I believe are in the .60 range. Also believe around the late 1970s to 80s. An easy fix for these cars was to reduce the gap back to sanity.
The problem is if you have any kind of whats the term
peak pressure? the spark just can't jump that large of a gap. You don't get a spark or its so weak it barely ignites the fuel. Your back to where you started. a small flame kernal. So you reduce the gap down to where the spark still contains a huge amount of energy and does easily ignite the fuel.
A worst case would be like 12.5:1 compression with the engine also adding more from the cam. Its not just compression but the active pressure in the compression stroke. That damned term I forgot.
An extremely excessive way of seeing it is if the spark gap was from one side of the cylinder to the other. The entire head volume would ignite evenly and spread very nice right down the cylinder, everytime.
Ignition advance could be reduced.
A tiny itsy bitsy gap. The spark plug itself can block the flame kernal from spreading, slow it down. It also might provide so little energy that it doesn't light. Or it does light but look how far it has to spread just to reach a normal .35" gap.
Ideally you want the flame to work exactly like those models you see of a 4-cycle. But in reality the flame is being started apposed to the piston rising. Your fighting it. It takes time for the flame to expand enough to cause a rise in pressure. Thats why there is so much timing advance. Its to allow this flame kernal enough time to grow.
This is also why they tried microwave ignitions but I guess they never got it to work.
The problem is if you have any kind of whats the term
peak pressure? the spark just can't jump that large of a gap. You don't get a spark or its so weak it barely ignites the fuel. Your back to where you started. a small flame kernal. So you reduce the gap down to where the spark still contains a huge amount of energy and does easily ignite the fuel. A worst case would be like 12.5:1 compression with the engine also adding more from the cam. Its not just compression but the active pressure in the compression stroke. That damned term I forgot.
An extremely excessive way of seeing it is if the spark gap was from one side of the cylinder to the other. The entire head volume would ignite evenly and spread very nice right down the cylinder, everytime.
Ignition advance could be reduced.
A tiny itsy bitsy gap. The spark plug itself can block the flame kernal from spreading, slow it down. It also might provide so little energy that it doesn't light. Or it does light but look how far it has to spread just to reach a normal .35" gap.
Ideally you want the flame to work exactly like those models you see of a 4-cycle. But in reality the flame is being started apposed to the piston rising. Your fighting it. It takes time for the flame to expand enough to cause a rise in pressure. Thats why there is so much timing advance. Its to allow this flame kernal enough time to grow.
This is also why they tried microwave ignitions but I guess they never got it to work.
