Projected-tip spark plug benefits?

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Projected-tip spark plug benefits?

Post by BradH »

Anyone done testing on whether a projected-tip spark plug shows benefits over standard style? I'm thinking more in terms of chambers where the plug isn't as centrally located as I see in other designs. In general, is getting the spark closer to the middle of the chamber "better"? Thx
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Re: Projected-tip spark plug benefits?

Post by Walter R. Malik »

BradH wrote: Fri Jul 30, 2021 1:14 pm Anyone done testing on whether a projected-tip spark plug shows benefits over standard style? I'm thinking more in terms of chambers where the plug isn't as centrally located as I see in other designs. In general, is getting the spark closer to the middle of the chamber "better"? Thx
It helps the combustion process on some, hurts on others and doesn't make any difference in most; other than benefiting the plug itself.
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Re: Projected-tip spark plug benefits?

Post by BlitzA64 »

Way back I was racing a class with a 265 Chevy under "stock" rules. I was reading one of the Hot Rod type magazines and there was an article with a 331" Chevy build that was using surface gap plugs that came in Mercury Outboards. The engine picked up power from the usual plugs by moving the spark closer to the center of the chamber. I was getting ready to go to the dyno with my 265 and happened to run across a box of 10 L63y plugs, 1/2" reach rather than 3/8" the same length as the surface gap plugs. I was not allowed to run any ignition but stock points and coil so surface gap may not have worked but these would. Put them in a lathe and turned off the end threads and took them with me. We tuned the engine and had it really good, now was the time. Replaced the plugs and did a pull, a couple hp. started working with timing and got it to 9hp gain! now we are talking about 60 year old chamber shape but 9hp on a 260hp engine was something for a set of plugs.

I guess the moral to the story is there is no real way to tell without testing but that deal sure worked for that engine
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Re: Projected-tip spark plug benefits?

Post by MadBill »

Champion invented the projected--tip spark plug in the early sixties. Apart from its other virtues, they were able to show apples-to-apples power gains on almost all totally stock engines because most cars were specced with timing somewhat retarded from MBT (Minimum spark advance for Best Torque) timing to cover off various worst-case scenarios. A projected tip effectively advances the timing by several degrees, so optimum advance should be reestablished after a change from one to the other.
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Re: Projected-tip spark plug benefits?

Post by BOOT »

MadBill wrote: Fri Jul 30, 2021 11:08 pm A projected tip effectively advances the timing by several degrees, so optimum advance should be reestablished after a change from one to the other.
I was just reading an article bout the effect of projected tips & timing the other day, same with plug position in the chamber.

edit: Second recent plug thread. Need to get my junk done, been planing a plug video since winter!
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Re: Projected-tip spark plug benefits?

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MadBill wrote: Fri Jul 30, 2021 11:08 pm A projected tip effectively advances the timing by several degrees, so optimum advance should be reestablished after a change from one to the other.
Or would it be more like moving to a chamber that requires less advance? Probably some combination.

Anyway I noted the Mercedes M139 engine (that appears to be a design exercise in detonation resistance) has projected plugs and a boss that they stick out of, so projected even further. I'd love to have a clearer picture, but the boss looks to be 3mm or so thick.

For whatever it's worth *I* think it will slightly reduce timing required and improve detonation resistance, but I have zero related practical experience
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Re: Projected-tip spark plug benefits?

Post by BradH »

There would definitely be some testing of ignition timing to optimize any change in plug configuration. I'd expect a heat range change might also be appropriate.

The Autolite AR3911 I have now are listed as a "medium projection" style. I noticed some different plug brands & types that include "extra projection" done in slightly different ways.

FWIW, what got me thinking about this (again?) was a chamber pic of a Vortec SBC. The general shape is similar, but there is a significant difference in relative plug placement vs the bore center & plug-side cylinder wall.
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Re: Projected-tip spark plug benefits?

Post by BLSTIC »

Slightly Off Topic, have these been shown to have heat shedding problems?
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Re: Projected-tip spark plug benefits?

Post by splinter »

"A projected core-nose acts as a 'warm' plug at low speed and cools off to act as a 'cold' plug as RPM is increased."
-Bill Fisher's How to Hotrod Volkswagens

I have found they sometimes contribute to 'crisper' driveability and decreased HC versus non-projected variants.
The longer insulator length of projected plugs tend to run 'warmer' by virtue of longer heat transfer path.
Fine (and oft-reccomended) for typical OE highway use but when pressure and heat are increased significantly cooler plug construction is often required.
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Re: Projected-tip spark plug benefits?

Post by MadBill »

splinter wrote: Sat Jul 31, 2021 5:56 pm..The longer insulator length of projected plugs tend to run 'warmer' by virtue of longer heat transfer path.
Fine (and oft-recommended) for typical OE highway use but when pressure and heat are increased significantly cooler plug construction is often required.
I believe that's taken into consideration in the manufacturer's designation, i.e., to get the same heat range as a 'standard' plug, a projected tip one would likely have the same length heat path down the insulator, thus reaching the outer shell closer to the firing end.
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Re: Projected-tip spark plug benefits?

Post by NewbVetteGuy »

BlitzA64 wrote: Fri Jul 30, 2021 2:56 pm Way back I was racing a class with a 265 Chevy under "stock" rules. I was reading one of the Hot Rod type magazines and there was an article with a 331" Chevy build that was using surface gap plugs that came in Mercury Outboards. The engine picked up power from the usual plugs by moving the spark closer to the center of the chamber. I was getting ready to go to the dyno with my 265 and happened to run across a box of 10 L63y plugs, 1/2" reach rather than 3/8" the same length as the surface gap plugs. I was not allowed to run any ignition but stock points and coil so surface gap may not have worked but these would. Put them in a lathe and turned off the end threads and took them with me. We tuned the engine and had it really good, now was the time. Replaced the plugs and did a pull, a couple hp. started working with timing and got it to 9hp gain! now we are talking about 60 year old chamber shape but 9hp on a 260hp engine was something for a set of plugs.

I guess the moral to the story is there is no real way to tell without testing but that deal sure worked for that engine
And just to clarify on machining off the first couple of threads on the spark plug, the purpose was to avoid the exposed threads from becoming a hot spot AND to push the spark plug further towards the center of the combustion chamber? (Whole plug extended further into the chamber to go along with the extended tip?)


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Last edited by NewbVetteGuy on Mon Aug 02, 2021 1:47 pm, edited 1 time in total.
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Re: Projected-tip spark plug benefits?

Post by NewbVetteGuy »

Walter R. Malik wrote: Fri Jul 30, 2021 2:05 pm
BradH wrote: Fri Jul 30, 2021 1:14 pm Anyone done testing on whether a projected-tip spark plug shows benefits over standard style? I'm thinking more in terms of chambers where the plug isn't as centrally located as I see in other designs. In general, is getting the spark closer to the middle of the chamber "better"? Thx
It helps the combustion process on some, hurts on others and doesn't make any difference in most; other than benefiting the plug itself.
-I think this thread is missing the discussion on WHY projected-tip spark plugs often see improvements.

My understanding is that if the spark plug is centrally located in the combustion chamber 3 dimensionally, the flame front has less DISTANCE to travel to reach all of the distant walls and therefore you burn all the fuel+air faster even with the same flame front / burn speed. (Which means less spark advance created and theoretically minor increases in torque and efficiency, I think.)

---This also makes me wonder why a more centrally located plug wouldn't see any improvement or would potentially go backwards. (Already good chamber / piston design means "diminishing returns"?)


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Re: Projected-tip spark plug benefits?

Post by Geoff2 »

The projected tip is more exposed which helps burn off deposits to prevent misfires.
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Re: Projected-tip spark plug benefits?

Post by BradH »

BradH wrote: Sat Jul 31, 2021 1:09 pm ...
FWIW, what got me thinking about this (again?) was a chamber pic of a Vortec SBC. The general shape is similar, but there is a significant difference in relative plug placement vs the bore center & plug-side cylinder wall.
Here's a SRH Vortec chamber which reminded me of how different the plug location is vs my original pic above of one of my chambers
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Re: Projected-tip spark plug benefits?

Post by BradH »

NewbVetteGuy wrote: Mon Aug 02, 2021 1:33 pm And just to clarify on machining off the first couple of threads on the spark plug, the purpose was to avoid the exposed threads from becoming a hot spot AND to push the spark plug further towards the center of the combustion chamber? (Whole plug extended further into the chamber to go along with the extended tip?)
That's what I would have done for the reason you stated. Here's a pic of an Autolite AR3911 and a Brisk "extra projected" iridium-tip plug. Note the Brisk extends the whole tip and keeps the ground strap length basically standard, rather than extend porcelain, center electrode & ground strap way past the standard config where you end up w/ a very long ground strap.
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