Spark Plugs
 

Might seem like a dumb question -- but what spark plugs should I use?

Track engine
3.8L 11.4:1 Compression, probably low-mid 300hp at wheels.

Motec - LSx CoilPaks for each plug.

Mike

The best plugs to use are ones you can readily buy.

The choice is based upon what the engine wants not what I may say or anyone else. I suggest you run what you have already and do a plug check. Then read the plug and it will tell you what plug to use. If you have not run the engine yet start off with a cold plug and work up, hotter from there. Any of the main plug manufactures can help here.

From the description of the other plug suggested, that seems a little to warm for a track engine and it has a firing end not the best for a race engine. Air cooled race engines run hot head temps and over the the length of a race, the plug will have issues trying to dissipate the heat back into a hot head. You have to think about the heat over an entire race length. This often requires a plug temp range cooler than what you think and what the plug reads. Multi ground electrodes are for emissions reasons making sure the plug fires in all directions to burn the most or more complete mixture. Remember the plug only fires on one electrode at a time. Not all three at once. Within the cylinder the plug will fire in the direction of the less pressure or least Resistance.

That plug also has a thick center electrode which requires more energy to fire. You state you have Inductive coils so the energy released is all about battery voltage, coil winding's and the dwell settings. A fine wire plug will require far less energy to fire. Battery voltage and or Alternator condition are all helped by a fine wire plug. The precious metal electrodes, Iridium are better for wear as well. They will last longer than the old fashioned copper or Platinum plugs. Go with a single ground and you open up the flame front and will gain faster combustion. The more electrodes the more you plug up the flame travel.

I'd suggest a colder plug and if the piston clearance is there, go with a projected type to start. I only know NGK so a 9 heat range would be a good place to start. Go with a fine wire Iridium and a projected firing end, and choose one that has a wide gap. They are already gaped so don't touch them. And do not use any anti seize on the plug. Then do a plug check and read the plugs. If you are unsure of what to do, there is a good read about spark plugs on performancedevelopments.com – Engine Design web site in their tech papers.

To run a plug that is too warm can cause all sorts of problems. It will cause pre-ignition type detonation and will rattle the rings first. As the piston comes up to TDC under compression and the firing event happens the top ring is twisted in the groove. It turns down under pressure to help the sealing against the liner walls. If the pressure between the fire band and the top ring becomes larger than overall cylinder pressure from pre-ignition detonation, it bounces the ring back and forth within the groove under very high frequency. This is what breaks the ring. Often the engine still runs or seems to for average owner, and appears to use a little more oil and breathes a more heavier. Broken rings cause damage to the Cylinder walls and pound out the ring grooves.

Choose carefully and let the engine tell you what it wants, not me.

Absolutely solid gold advice.

Wow, feel like I just finished plug grad school,
Where do I PayPal the Prof a beer :-)

A lot to read, but I think I get it.

Thank you everyone

Mike

You are welcome.

Racing is supposed to be fun not hard. Fixing engines is hard.

Was reading this post and couldn't figure out where to start... I've been running fr5-dtc's in my car for several years and was wondering what to go with for a real race plug. Track only motor, similar to motor the thread starter noted, however, using two msd's through stock coils, with 993 twin distributor.

It was suggested you look at the paper I wrote about spark plugs on our web site. Read that and learn how to read a plug. This will be a huge help in choosing a plug. If you cannot read a plug, how do you know what to do.

Without knowing your engine and its parts, its hard to suggest what you should use.

First question is, are the coils you are using designed for CDI Ignition? if so, you are Ok but if they are for Inductive, you should look to change.

Race engines need colder plugs as the engine temps are always high with so much WOT running. Add in air cooled and the colder you can use the better. Warmer plugs cannot remove the heat, from the heads, quick enough. As long as there is no fouling, a cold plug will not hurt you. You will always start the engine and warm it up to running temp before any track use, so it will not be an issue.

The further you can place the firing end into the chamber the better. Piston clearance is something to check. The more the firing end is projected the "hotter" the same cold plug will run. Pre ignition detonation can happen with a plug exposed in a hot chamber

Most race plugs are "colder" and will have a retracted firing end anyways. Try to find a non resistor plus too. But today, just finding 14.00mm race plugs is getting harder. You can remove the resistor. Thin center electrodes of precious metals will last longer and require less ignition energy to fire.

Find a plug that will give you the piston clearance, pick a colder heat range and test. Do some hard running and perform a hard ignition cut, coast into the pits and remove a plug and read it. The plug will tell you what the engine wants. For racing, the only real change will be how projected you can run the plug into the chamber. I cannot think you will have any fouling issues if the fueling is correct.

Regarding 14mm plugs, is there any reason not to weld up the head and use a smaller diameter plug, other than the cost and trouble of doing so?

Could get the tip slightly nearer the center of the combustion chamber, if that's worth anything.

Dan

Sent from my Nokia 7.1 using Tapatalk

thanks... I was looking for the spark plug article in the 'technical articles' area and didn't see the 'tech forum' section. Yes, I have cdi. Not the most powerful; msd 6A's going through stock coils, but cdi none the less.

Navigate a bit further to Tech Corner you will find the Spark Plug article there among others worth your time. I have tremendous respect for Neil and Performance Developments.

Maybe I am blind, but I can't find the sparkplug article or the Tech Corner either. If someone who knows where it is can post a link, that would be great. Thank you!

Thanks. I was searching here on the Pelican site. Duh.

GREAT SUBJECT....!!
I will read the article on reading plugs, but, a stock sc (78-83) and stock carrera (84-89) engines, should these engines be using different than original spec plugs?
If so, brand, plug number and what metal plating on electrode ?
Thx for any input...))
Now to read article and await any input posted here...
Thx... Bob

Any comments to my question...?

Why no anti-seize?

I'm curious about the comment here and in other places related to Porsche engines and the recommendation to not use anti-seize.

I've regularly used anti-seize in all my oil cooled engines, aircraft, NA, and turbo 911's for 30 years. In the past I used the silver paste type, although recently I've gone to a high quality aircraft copper based anti-seize.

I've never had a problem with ignition due to poor grounding of the spark plug, I've never had a properly torqued plug back out, and I've also never damaged any threads from galling or other issues because I use anti-seize.

I'm careful to not get it on electrodes or on the first few threads on the nose of the plug.

Why the recommendation for no anti-seize?

Because anti seize is more often used in excess. It changes the heat range of the plug as it acts as an insulator. Not to mention, when used in excess, it can get onto the insulator showing as if the plug is overheating.

[QUOTE=911 tweaks;10386525]GREAT SUBJECT....!!
I will read the article on reading plugs, but, a stock sc (78-83) and stock carrera (84-89) engines, should these engines be using different than original spec plugs?
If so, brand, plug number and what metal plating on electrode ?
Thx for any input...))
Now to read article and await any input posted here...
Thx... Bob[/QUOTE

I say yes.

But the stock plugs will work just fine too. Plug design has come along way since these engines were designed and a plug selected for them.

The 2, 3 or 4 ground electrode plugs are old fashioned too. My advice is to move over to a fine wire type as these require less energy to fire and open up the kernel area generating higher combustion pressure.

In no disrespect to Pelican parts, I do not agree that plugs should be sold as race plugs. They list plugs as race plugs. Plugs are either "hot" or "cold" with plugs in between, based on their configuration.

An engine may run a hotter or colder plug depending upon its design use. Yes, typically race plugs are cooler and less projected. I think it dangerous to sell plugs as "race plugs". I advise that any plug used needs to be selected by checking what the engine wants and when used in long distance races and hotter climates, a cooler version is often the better choice.

If you are running an inductive Ignition, plugs with the thicker electrodes and resistors are not your friend. The finer wire plugs will help a lot here. As I have suggested before many times, running Inductive Ignition on these hemi chambered 2V engines is done for cost reasons and the long duration spark helps for emission reasons. Cylinder pressure is always lower, so anything you can do to increase the cylinder pressure is a good thing.

I hope this is understood. This is a long on going argument with those that suggest CDI over Inductive. I can tell you from years of doing this, I have tested (cylinder pressure measurements) just about every coil sold and none have produced cylinder pressure levels as much as CDI has. As an engine person, I could care less which one is used as long as it produces the most. Spark plugs are an important part of the Ignition system and plugs will help here too.

Good info. The one time I used the thin wire plugs, I experienced pre-ignition. (not in a Porsche).

So the question is, how do you prevent this? Lower the heat range more?

Pre-ignition is disasterous in these expensive builds. I’d start with something in the W2DPO or W3DPO range. One advantage I recall from Bosch training is the side gapped electrode will actually burn away first with preignition, killing combustion in that cylinder sort of like a fuse. Not that I’d rely on that as a substitute for good tuning. The side gap also has zero shrouding effect.