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lt1silverhawk 07-16-2012 07:04 PM

Spark Plugs: Projected vs Non-Projected Tip
Hi All,

I have a general question regarding spark plugs. What is the difference between projected and non-projected tip spark plugs? The only info I've been able to find is that people use the non-projected in the forced induction applications. So for normal daily drivers, should I just use projected? (a bone-stock 350 in a truck in this case).

Thanks in advance :thumbup:

66GMC 07-16-2012 07:29 PM

1 Attachment(s)
NGK Spark Plugs (my personal preference, BTW) has a bit of a blurb on spark plug design.


Spark Plug Basics

The spark plug has two primary functions:
  • To ignite the air/fuel mixture
  • To remove heat from the combustion chamber
It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.

The rate of heat transfer is determined by:
The insulator nose length
Gas volume around the insulator nose
The materials/construction of the centre electrode and porcelain insulator


A projected style spark plug firing tip temperature is increased by 10C to 20C.
So ... to answer your question, I would go with the recommended plug right fom the catalog for a "bone stock" engine. If the engine has been modified, the recommendation is to go to a "colder" plug.

bracketeer 07-16-2012 08:29 PM

high compression engines
My dealing with spark plugs revolve around whether or not the piston hits them and mashes them closed. In high compression (racing engines), the piston dome can hit the plug. So two ways of stopping this is indexing the plugs or go to non protruding.

1971BB427 07-17-2012 10:02 AM

Older hi performance engines with higher compression ratios ran hotter plugs with exposed or extended tips. They were designed to run those plugs and work well with them. You can't get more hp or better performance by just tossing in a plug with a certain length electrode. You need to stick with the plug that works with your engine stock, or with whatever mods were done to that engine.

lt1silverhawk 07-17-2012 10:29 AM

Thank you everyone for the very helpful information. I was shopping for spark plugs on Summit and the website gave the option of projected or non-projected. spark plugs. Having never been asked that before, I thought I'd play it safe and ask first.

Thanks again! :thumbup:

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