Originally Posted by Foghorn

Here'* a write up done by Will Rishworth (Overkill) of the Ontario Quebec Club Grand Prix. He brings up some interesting points. Last time Will dynoed his car he was at 313 whp.

Which Spark Plugs Should I Run?

Give me a spark plug 101

A spark plug has two main tasks: to ignite the air/fuel mixture properly, and to remove heat from the combustion chamber to maintain an optimal temperature.

Platinum, some hear that word and think BAM these are premium plugs. What makes platinum different? Well the answer is in the design of a platinum plug. So lets start there.

A spark plug'* firing task is to transfer electrical current or "spark" between a power and a ground electrode to ignite the air and fuel mixture. We want this spark to be powerful, to promote a nice clean burn of the mixture, as this will promote the best efficiency and therefore the most power. Now imagine two spheres electrically charged, hold them closely together and you'll see spark jump across it. Now try and separate them and you'll see the spark disappear. NOW try this with two sharp pointed electrical rods and you'll see you can hold them further apart and the spark will still jump between them. What does this mean? Sharp points will transfer the spark the most effectively. Ever notice the tip of the spark plug and how most platinum plugs are sharp tipped center electrodes?

Sharp tips are all well and fine, but thats a hot environment down there. Infact the biggest problem is withstanding the heat. A sharp center electrode will get very hot, this promotes easy ignition (less charge to spark the plug) and a clean running plug free of deposits, but try this with a standard copper plug and it'll melt! Platinum has a much higher melting point than copper which is used on the classic style spark plug, thus the tip can be sharpened like it is. Iridium is the new guy on the block; same thing, it has an even higher melting point, thus the tip can be sharp. Now a copper plug needs to have a wide body to disperse the heat over a larger area so its tip can run cooler. Now did that set off any light bulb on where that would be beneficial? If you said forced induction or nitrous applications, pat on the back! In applications where you want to minimize existing heat in the combustion chamber, platinum may not be a good thing.

Here is another stick in the spokes of the platinum bandwagon wheel: Platinum does not conduct electricity very well. Platinum was chosen due to its resistance to heat and erosion, however it conducts electricity about a third as well as copper. The new iridiums are an attempt to find a metal with similar heat and erosion resistance as platinum but better conductivity of electricity; however, iridium is still only about half as good a conductor as copper, iridium plugs are one of the best marketing tools for spark plugs today and nothing more in the authors opinion. Now this disadvantage of platinum is offset to an extent by the ease of firing that platinum plug design offers, with its sharp pointed tip. Whether a copper plug can produce as much spark energy with a smaller gap however, than a platinum counterpart with a wider gap, is a good point of debate. The author’* viewpoint is yes.


Before I read further, what plug do you recommend for my application?

My recommendation for GTPs is to use a copper Autolite plug whenever possible. Copper runs cool, will perform better than exotic metals in my experience, should last its user about 2 years, and best of all are inexpensive.

For a stock pullied car, if you are unmodified and looking for a maintenance recommendation for plugs rather than a performance one, look for Autolite platinums which are part number coded AP for platinum and APP for double platinum at the very beginning of the part number.

Non-intercooled:
3.8” factory: Autolite 605 @ .055-.057” gap (or AP605/APP605 platinums @.060”)
3.4-3.25” pulley: Autolite 104 @ .052-.054” gap
3.1-3.0” pulley: Autolite 103 @ .047-.050” gap
2.9-2.7” pulley: Autolite AR94 (4 range colder race plug) @ .044-.046”

Intercooled
3.25-3.0” pulley: Autolite 104 @ .050-.052”
2.9-2.7” pulley: Autolite 103 @ .046-.048”
2.6-smaller: Autolite AR94 @ .044-.046”

My recommendation for GTs is to again use a copper plug for performance since I have found them to perform as well or better than most exotic metal plugs, however platinum/iridiums can be used for longevity without the risks associated with GTPs.

Maintenance/Mileage:
Stock heat range platinum/Iridium @.060” gap

Performance:
NGK TR55 V-Power copper @.055” gap


Why does the author not recommend platinums in a GTP, aren’t they better?

Better? No, not in my book. And there is a good reason I recommend not using platinums in a supercharged application, and that’* because they run HOT. The small platinum tipped plugs are designed to run very hot to promote easy firing for mileage and emissions at idle and low vehicle speeds, along with erosion resistance so the owner of the car can neglect its maintenance for 100,000miles (yes, neglect is IMHO the appropriate word here!). These reasons are why they are considered better, but now that you know the whys, you know enough to make a judgement call for your own application.


Why do we step down heat ranges?

With a supercharged motor, the process of adding boost also adds heat to the incoming air. This is a normal byproduct of compressing the air. By running 10psi of boost from an M90 on a 3800, with pulley sizes anywhere from 3.8” to 2.7”, we are also adding 170-320F temperatures to the inlet air.

One of the spark plug’* functions is to remove heat from the combustion chamber, contrary to whatever you may have heard about spark plugs adding heat. We can use this function to remove or counter balance the heat which we are adding to the combustion chamber from the supercharger. By fine tuning the heat range of the spark plug we use, we can ensure an optimal temperature in the cylinders for proper combustion without pre-igniting the mixture from excessive heat.

Techie Note: The heat range of a spark plug is varied by the amount of the center body exposed in the plug; think of it as a mountain and the center electrode as its peak, the higher the mountain the hotter the plug. The shorter the mountain, the shorter the distance for heat to travel through the spark plug body and disperse into the cylinder head, the colder the plug.


How many heat ranges should I step down with my L67/L32 supercharged motor?

This will vary greatly on what pulley size you run. By going smaller and smaller to keep boost levels up, we are adding more and more heat by spinning the supercharger faster, even though we’re running the same amount of boost. As such, pulley size is a good basis for choosing a spark plug.

Stock pullied cars may find it beneficial to step down up to 1 heat range in plug, even without any mods. But many can also stick with stock heat range, since we assume if you’re running such a large pulley still that you are not entirely interested in any additional horsepower

3.4-3.25 pulleyed cars should step down 1-2 heat ranges, 3.1-3.0 2-3 heat ranges, and any 2.x” pulley sizes should really be running a race-engine-bread 4 range colder plug as offered by NGK and Autolite.

If you have an intercooler, you should step down one heat range less than normal. So if you were running a 3.0 before the intercooler on a 3 range colder plug, step up to a 2 after the IC. If you were running a 2 range colder plug on a 3.25 before and are moving to a 3.0 after the IC, you should be able to run the same plug. Reason being is that the intercooler drops the inlet air temperatures, thus less is needed to be removed from the combustion chamber with your plug selection.


Should I step down in heat ranges with my L36 non supercharged motor?

Under most circumstances, you should not be forced to step down heat ranges in your GT. As power is added onto a naturally aspirated motor, there are very few modifications that tend to add combustion heat (advancing timing through a PCM reprogram comes to mind though) and many which tend to help lower combustion temperatures (cold air intakes, free flowing exhaust systems). Once you have the right balance of modifications, your combustion temperatures should be relatively unaltered from factory. At most I would step down 1 heat range but only as needed.

Note: If you are having knock problems in your GT, especially after a DHP, you absolutely must ensure you are running a high octane fuel. Premium fuel is the FIRST step to combating cylinder heat, which is added from increased ignition timing. It should also be assured you are not running too lean, and that your engine is free of carbon deposits by running fuel injector cleaner regularly, using clean fuel from Sunoco or Pioneer, or best yet from a Motovac injector flush. Also, running colder spark plugs in an attempt to run lower octane gas with your DHP is a practice the author whole heartedly opposes, it is NOT a sound, safe or smart tuning practice and more trouble than its worth.


Why does the gap decrease as the plug heat range goes down, whats the link there?

The reason for the decrease in plug gap is in relation to the pulley size being run, not necessarily the plug itself.

First off, I think its unwise to run a copper plug at anything over .055” without a very powerful aftermarket ignition control box, so all copper plugs I recommend that gap as a maximum.

As you step down in pulley sizes, more air is being moved into the cylinders as more power is being made. This results in an increase in pressure in the cylinders on the compression stroke. It’* the author’* belief that more power is lost from incomplete combustion from a wider gapped plug than most realize. Due to the rise in pressure, it makes sense to decrease the plug gap slightly to correspond, to ensure good combustion for more power, and even less knock retard in some instances in my experience. Wider is not always better.

Keep in mind that if you want to run a wider gap, upgraded ignition wires can ensure that you can run as close to .055” as possible. With an MSD Digital+4 ignition box, gaps as wide as .070” have been run successfully on a GTP.


Is there any way to increase performance, using spark plugs?

Unlike the kid who rushes out to buy “race” plugs cause he assumes there will be a power increase, there are ways to increase your performance with spark plugs BTW race plugs are usually just a very cold heat range, nothing more.

First is to “index” your spark plugs. What this means is you want to install the spark plug so that the direction of the open gap faces towards the center of the combustion chamber or towards the exhaust valve. Due to quality control with the threads tapped into the cylinder heads and the threads on a spark plug, it is not always a guarantee that the plug will be properly oriented to achieve this when installed. We can achieve this by using indexing washers from companies like Moroso; you want tapered seat washers, they will come in a pack of 30 (10 each of 3 different thicknesses), approx cost of $30CDN whether you buy locally or from the states.
Tape or mark the spark plug socket where the ground electrode sits and screw it into the head, note the position of the mark. If it is between 5 and 7 oclock, torque it down to 11-18ft-lbs. If it is not, try another plug to be sure it is the head casting and not the plug causing it, and if the results are the same, follow the instructions on the package to install the appropriate washer to rotate the plug’* orientation counter clockwise (will usually be 90-180-270degrees). Once achieved, torque the plug down. Repeat on the rest of the cylinders. Note that it is usually the cylinder head casting that causes plug misalignment, thus that cylinder will require the same indexing washer each time you install a new one. If only 2-3 washers are required of the 6, keep the others handy as they can be used for multiple plug changes in the future.

Another trick is to “backcut” the ground electrode. On a copper plug, you want to position the tip of the ground electrode so that it is over the first 1/3 of the center electrode body and no more, and it can be as far out as the edge of the center body if longevity is unimportant (Plug should still last 10,000kms or more). This will normally require shortening the ground stap by filing or cutting it; be decreasing its length you are also decreasing the amount of time it takes for heat to transfer from it to the head and reduce its temperature as that ground electrode can get red hot and be a source of pre-ignition. If you buy Autolite race plugs or NGk V-Power TR5 (not TR55s) or TR6s, you will notice its already got shorter ground staps, and in the case of the Autolites a wider ground electrode for better transfer of heat.

A final trick is to file smooth the sharp ends of the ground electrode to make a smooth U shape, and also the sharp edges ontop of it. These sharp edges are the greatest sources of heat; by smoothing them you create a more uniform dispersal of the heat in the stap, making for a cooler operating temperature. This is again important, since they get so hot and can be a source of pre-ignition. Do not smooth the underside however, you want to retain the sharp points there since they are what the spark energy wants to jump towards.

Keep the rest of your ignition system healthy. Good quality ignition wires will ensure the energy from the coils reaches the plugs. And the coil terminals should be kept in good shape, aluminum will oxidize and brass will tarnish with time so inspect them regularly to ensure they retainer their shiney finish. A brilo pad will clean them up if tarnished, but ask the wife first or better yet spend the $2 and get one for yourself!

Finally, change your plugs often. You would be amazed at the performance gained by changing copper plugs once a year, it’* a harsh environment in there and dull edges and deposits can greatly diminish spark energy over time.


Disclaimer: This information is to be used as reference only. Its author is not responsible for any negative effects that may occur as a result of using the methods provided in this guide. He is however hopeful that it has been helpful in selecting the right spark plug for your 3800.

Cheers,