Spark Plug Modification
04-24-2004, 06:33 PM
#1
Racer
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Spark Plug Modification
Every so often you'll see a racing tip in a car magazine that says you can pick up a few horsepower by trimming back the negative electrode on a spark plug so that the end is even with the center of the positive electrode. The rationale is this is supposed to unshroud the spark and allow better flame front propagation. Is this real or wishful thinking?
While we're on the subject of spark plugs - on the old, big diameter plugs that come with the loose sheetmetal gasket, which way should the gasket go - small diameter lip againts the block or the other way around? Does it make any difference? I've asked this of many experienced car guys and nobody seems to know for sure. I've been changing plugs for about 40 years and have always put the small diameter towards the block but never knew why. Just seemed to look right.
While we're on the subject of spark plugs - on the old, big diameter plugs that come with the loose sheetmetal gasket, which way should the gasket go - small diameter lip againts the block or the other way around? Does it make any difference? I've asked this of many experienced car guys and nobody seems to know for sure. I've been changing plugs for about 40 years and have always put the small diameter towards the block but never knew why. Just seemed to look right.
04-24-2004, 06:42 PM
#2
Melting Slicks
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Re: Spark Plug Modification ('69CoupeDude)
Im not sure how much cutting the strap on the spark plug will help with hp, but I also have heard that it helps. Same thing with indexing the plugs so they all face in a certain direction. Doesnt seem like there would be much difference with a good ignition system such as an msd or similar products. I know that cutting the strap on sparkplugs is recommended when running nitrous or high amounts of boost though. I also install the sparkplug rings the same way as you. Not sure why though or if it is the right way.
04-25-2004, 06:22 PM
#3
Re: Spark Plug Modification ('69CoupeDude)
From Emanuel's book "Small Block Chevy Performance":
"Want to pick up five horsepower free? Cut back the ground electrode, so that it covers no more than 1/2 of the ground electrode. The plugs won't last as long, but dyno testing have shown definite powere increases."
The reasons are for exactly as you stated - unshrouding the electrode and exposing more of the spark. Also in the book - A MN speed shop saw cut back electrodes produce 5-8 HP increase in back to back tests. Some spark plug designs, "spit-fire" and "rapid-fire" plugs have this design built in, but you pay much more for them. You can easily cut back a regular cheapo spark plug for the same result.
"Want to pick up five horsepower free? Cut back the ground electrode, so that it covers no more than 1/2 of the ground electrode. The plugs won't last as long, but dyno testing have shown definite powere increases."
The reasons are for exactly as you stated - unshrouding the electrode and exposing more of the spark. Also in the book - A MN speed shop saw cut back electrodes produce 5-8 HP increase in back to back tests. Some spark plug designs, "spit-fire" and "rapid-fire" plugs have this design built in, but you pay much more for them. You can easily cut back a regular cheapo spark plug for the same result.