Spark Plug Heat Range

[Patrick Westphal]

I found a diagram that illustrates the main factor in determining a spark plug heat range that I was trying to describe to you during the chapter get together.

Here it is:

 

The main factor is the length of the ceramic insulator from the middle of the spark plug to the tip (where the spark happens)  

The insulator length of the plug on the left is much longer than the insulator length of the plug on the right.

That insulator length is the path for the heat on the tip of the plug to flow to the cylinder head.  

The balance that we are always working on is to make the plug 'hot' enough to burn off the carbon that builds up on the ceramic 

(especially during a cold start and warm up when the choke makes the mixture rich) but not so hot that you get pre-ignition

in the cylinder when everything is hot.  We have specific tests to determine how susceptible a plug is to getting fouled and a different

test to determine how close it is to pre-ignition.  You always design to stay further away from the pre-ignition point because 

that can damage an engine much faster than being fouled.  

Pat Westphal

[Jason Edge]

Thanks Patrick! Added to our Help Page.

[Tony and Ginny 429]

Rejetting a carb to be getting your plug color correct might be necessary 

When installing a  new Edelbrock 1406 on my 64-429 I had to go two setting richer on my metering rods and primary jets than it came with 

The Edelbrock tec’s told me what adjustments to make when you give them the current numbers stamped on the metering rods and the jets that you currently have 
My own thing is to be having my engine tuned to perfection as it bring me a lot of personal JOY 

Baaaarrrrrrrrrroooommmmmm ! !

Enjoy 

[Tony and Ginny 429]

Patrick 

The important thing with spark plugs is that they are burning the correct color when you know all other indicators are on spec (new  points - cap and rotor  timing- vacuum issues addressed - carb adjusted only after a fresh tune on a tac and dwell condensed RPM meter  (  usually just once ) - and new ignition wires if it is time 

The correct color is a brown paper bag color —whitish is lean - darker is rich 

The plugs heat range is adjusted up or down  to get to the correct  plug color 

I have been using Autolight 85 plug in my 429 for 50 years which is the equivalent of the stock Champion plugs heat range but a very much more long range plug regarding its durability ( established from my drag race racing days on the track ) 

The plug color is the very best indication as to how well the cylinders are performing internally 

Lean produces excessive heat  in the cylinders which is never good for the engine or its oil  ( heat breaks down oils ability to perform as it should ) 

Our year Cadillacs require a tune up ( ignition refresh if points and grease job) every 6,000 miles which is basic maintenance which could be overlooked due to the cars today requiring much less  maintenance than our pride and joys do 

That should help to get you engine running on a perfect tune up which will get you the best possible gas mileage with the higher cost of gas we are all subject to these days 

I do remember when gas was 35 cents a gallon myself 

Enjoy