Racing spark plug tip preference
 

Is there a performance advantage among tip design such as extended tip over intermediate or flush configuration. Without considerable shimming or divots in the pistons, I have to find a plug with a shorter style tip generally. I have not run tests on a dyno but on track I can't say that I have found a performance difference. I would guess the further into the chamber, the better your charge exposure, but as the piston and head relationship tightens to nothing, the distance across the charge is less than paper thin. So the ground electrode is the distance separating spark from charge no matter what type of tip. What info do you guys have. Bob

This is kind of like an oil question. Lots of people have different opinions. From what I’ve read and seen from performance testing on YouTube videos by dyno shops, the brand or type of spark plug makes very little to no difference. I can’t say that i have ever noticed a difference either.

Bob,

What is the engine configuration that requires this shorter tip plug, and what are the ones you have tried so far, and the results from each?

Cheers

Compression ratio is what usually dictates spark plug intrusion into the combustion chamber.
Size of bore is also important , smaller wants smaller intrusion, 10mm peanut type plugs,
side pin type ground .

Ian

spark p-lug tips
 

3.0 911. 12.4-1.0 CR. top BKR8EIX, bottom DCPR8EIX. Ran these in my previous engine and the power felt great. The new motor has slightly larger dome so we have interference. I have already moved on and clearanced the plug tip area on the piston. The exhaust is the more problematic. In machining the spark plug hole, I went deeper than desired due to fin gaps. Could have I think not cut so much and still have had a good seal, but it is done. Waiting for NGK to get back to work to see if they have this plug with a shorter tip, but regardless, these plugs will work. Most will be shimmed anyway for indexing. Bob

Iridium plugs are designed for an emission based engines. They’re designed to run very hot, even if they have a cold number.

When it comes to racing spark plugs, a lot of folks prefer iridium or platinum tips because they offer better performance and durability. They can handle higher temperatures and provide a more consistent spark, which is super important for racing.

looks like someone is trying to up there post count

remember that name

Bot.

I have always used Brisk plugs in all my built vehicles. Running the Brisk Silver Racing L11SL in my 930. Brisk has a great website with spark plug tech and details about all their plugs. Here's their illustration about proper depth.

https://cdn.shopify.com/s/files/1/04...g?v=1591590353

I've seen some recommend extended tip plugs, the thinking being to avoid shrouding of the plug for better combustion. Makes sense, but if you need to clearance the pistons to accommodate this, are you just shrouding the plugs on the other end?

I called Brisk asked what plugs the recommended for our motors in race form and it was BRXXS. I'm currently running BR12S. I was running standard plugs on my prior race motor and put some Iridium ones in it. They came out a bit chewed up (from detonation?) so went back to regular ones.

Something that occured to me as well. On paper, to me the configuration on the right with the divot in the piston crown brings the plug deeper into the chamber and no sharp edges as you will have from the plug bore with a recessed plug. But as a coach stated, ''but the game isn't played on paper''. The plug ignites at TDC and the chamber is all but gone at this point and the disparity in speed of the f/a charge and the piston leaves me with many questions. I think I will take a head and dummy up a piston and cylinder at TDC to see how many CCs are available. Don't know what that will tell me, just curious. Bob
http://forums.pelicanparts.com/uploa...1735659608.jpg

Well, don't know where to where my brain wandered but I have that info already. In calculating your CR, you wind up with that info. -Cold the chamber on this motor is 44.6cc. Deduct the deck vollum of 7.0cc and you have 37.6cc hot. The majority of this is made up of the valve pockets I would think. You have the divots around the spark plugs and then the cylinder head dome and the piston dome are not a mirror of each other, so the space caused by that assymetry. So a fuel air charge of 37.6cc on a 3.0 liter engine at race pace. As you raise the compression that number goes down, but I would think the air becomes denser and consequently more fuel gets added. Extra power I would think from this progression. Bob

So, as I am about to begin assembling a 3.5L race engine, I figured I'd look again at the use of standard vs projected tip plugs for my engine. My engine has ported 3.2L heads, and the pistons are CP 12:1 compression. Previously I'd noticed that with standard tip plugs, the plug electrode is basically even with the contour of the combustion chamber. I'd assumed this would be desirable for high compression pistons, to avoid possible interference with the piston.

Well, on my workbench I laid one of the CP pistons onto the head with projected tip NGK plugs, installed, and then looked in through the ports to observe clearance from the plug electrode to the top of the piston. There is lots of clearance! Like 1/8" or more. I wonder if this argues for the use of projected tip plugs ...

If I could add something.

NA engine usually perform better with Spark plugs extended into the chamber. Only reason to pull them back is clearance and or to run "cooler" when heat range is not available. I have never run into this as heat ranges are normally offered.

The more the plug is extended, the warmer it will run. Not only are there extended tips, but there are wide gap plugs as well. Some are offered in extended tip plugs.

The drawing here shows the plug extended into the chamber but its completely wrong. The body of the plug should be flush with the chamber, only the tip is extended. If anyone is running a plug like the drawing, they have it all wrong.

High cylinder pressures change this. The higher the pressure the more the plug is retracted. Surface gap plugs are often used in Turbo engines as the pressures involved can effect the flame travel. Ignition type can help here as well. It all goes together to end with the best combustion.

Today, 14.00mm x 3/4" reach plugs are getting harder to find in as many choices as were available a few years ago.

Fine wire and precious metal tips are now common. Regardless of what plug you run, its up to you to read them to make sure your engine is healthy.

Neil - thank you as always for chiming in with your expertise. Your comment about an extended tip potentially running hotter is something I'd thought about. Would it suggest that it may not be advisable to run them on a high compression 911 race engine? Assuming a colder heat range like NGK 9 or Bosch 2 or 3.

I found that Brisk Silver Racing BR12S plugs worked better on my race engines than any of the plugs with more "modern" less conductive metals like iridium.

The Brisk BR12S is a Brisk heat rage of 12 which is the same as NGK 8 or Bosch 4.

Interested to know what you mean by "better". Is it a performance gain or do they seem to have less wear?

These are really the best sensors your engine has that tell you what is going on.

You’re on the right track thinking about the spark plug tip design in terms of charge exposure and how it interacts with piston/head clearances.