F355 coil-on-plug conversion

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Thanks for finding and sharing that. For those that don't know, Mark Glenwood now has his own shop in Arlington, WA called Seven Lakes Motorsports.
He is the absolute best in the NW https://7lakesmotorsports.com/

 

When comparing something where every mm makes a difference, I wouldn’t say these measure “exactly” the same. Why not just go off manufacturer specs to determine if the lengths are “exactly” the same? They made it, they would know.


In this case, what’s worth a thousand words is actual measurements, not a pic.

 

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The gap in a spark plug refers to the distance between the center electrode and the ground electrode at the tip of the plug. This gap is critical because it determines how the spark is generated, which in turn affects engine performance, fuel efficiency, and emissions.


Why Spark Plug Gap Matters:


1. Proper Ignition – The correct gap ensures that the spark is strong enough to ignite the air-fuel mixture in the combustion chamber.

2. Engine Performance – If the gap is too wide, the spark may be weak or inconsistent, leading to misfires and poor acceleration. If it’s too narrow, the spark may be too weak to ignite the mixture efficiently.

3. Fuel Efficiency – An incorrect gap can lead to incomplete combustion, which wastes fuel and reduces efficiency.

4. Emissions Control – Proper combustion reduces unburned fuel emissions, which helps meet environmental regulations.


Common Spark Plug Gap Issues:


• Too Wide → Weak or no spark, engine misfires, hard starts.

• Too Narrow → Weak combustion, poor fuel efficiency, higher emissions.


• Some Iridium or Platinum plugs are non-adjustable – They come pre-gapped and should not be altered.


Adjusting the spark plug gap correctly ensures proper ignition, fuel efficiency, and engine performance.









Yes, there can be a difference in performance between a .031” gap and a .028” gap, though the impact depends on your engine, ignition system, and driving conditions.


Key Differences:


1. Larger Gap (.031”)

• Produces a larger spark, which can improve combustion efficiency.

• May lead to better throttle response and fuel burn, potentially increasing power and efficiency.

• Requires a stronger ignition system—if your coil or ignition system is weak, it could cause misfires.

2. Smaller Gap (.028”)

• Easier for the ignition system to fire, making it more reliable, especially under high compression or forced induction (turbo/supercharged) conditions.

• Reduces the likelihood of misfires in engines with weak ignition coils or worn components.

• May lead to a slightly weaker combustion due to a smaller spark.


Which One Is Better?


• If your ignition system is strong (modern coil packs, high-performance ignition), a .031” gap may give slightly better performance.

• If your engine is high-compression, turbocharged, or older, a .028” gap might be more reliable.

• The difference is small, but in high-performance applications, proper gap tuning can optimize efficiency and power.


It’s always best to follow your manufacturer’s recommended spark plug gap, as modern engines are designed with specific tolerances. If experimenting, small adjustments should be tested carefully to avoid misfires or poor performance.




Thank God for AI .

 

Where are you finding the projected tip length specs for non projected tips?

 

To add to the length discussion, I cut the gasket off and run them without. Otherwise they are a bit short for the plug bore.

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Which plugs?

 

I understand this is a COP thread, but perhaps running something like an HKS Twin Power with the stock ignition system along with stock plugs (pmr7a or pmr8a), which we know are spec’d correctly could give satisfactory results. At least for now just to simplify things.


Certainly would eliminate the need to locate Densos, which posters have mentioned aren’t easy to find new (I haven’t looked) and creating spacers for the cover.


HKS Twin Power + factory ignition system on my RX-7 was satisfactory improvement for me.


HKS Twin Power units are smaller than MSD units, so should hide better as well.

 

I’d imagine no one knows this and runs them with gaskets. Nice discovery!

 

NGK customer service gave me this info.

 

Big mistake using AI...

I used AI to search for an answer to a Ferrari question.... and it quoted a rather old post of mine on FChat. I was wrong at the time. Thankfully, I now know better.

AI is well on the way to "Idiocracy"

https://en.wikipedia.org/wiki/Idiocracy

 

Only they don’t. The specs are posted.

Who conned you into believing your spark plugs were gapped at .028”?




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You said yourself you experienced misfire with the iridiums (which were new) compared to the PMR7’s that were previously on the car with the stock ignition system. That should already tell you something is amiss (if they were exactly the same) as new iridium plugs should perform better than old platinum PMR7’s. Perhaps insignificant details such as the gap or non-projected length weren’t matched as you believed.


Your iridiums only improved after you augmented the system with COPs, then later with an HKS Twin Power, which the OP already proved has worked fine for the past 18 years (using MSD instead of HKS). Your setup suffices.



These details are for new readers who are contemplating improving their ignition system from scratch and my hunch is one can possibly achieve 50-75% of the improvement of COP+Iridiums (off spec)+HKS with simply an HKS Twin Power on the stock ignition and correctly spec’d NGK platinum plugs for the car simply because the HKS Twin power is that good AND you’re actually using the correct plug specs for the 355. At least experiment with it first because it’s the easier route. We just need to get Cribj to produce 355 harnesses.


The HKS Twin Power can be a bandaid because it’s that good. It can mask underlying problems with malfunctioning/underperforming coils/plugs otherwise not discovered until the HKS is removed. Has happened often enough.

 

The engine happened to misfire in "very cold" weather with the platinum plugs too.

 

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I couldn't for the life of me figure out why an ignition box was needed, should just be power and trigger from the motronic, same as the factory remote coils. I would do this in a second if there was a cop that fit without needing a spacer for the cover.

 

An ignition box may allow you to modify the spark duration and timing. Or maybe COPs need a stronger trigger signal (beyond the capabilities of the Motronic ECU transistors).

 

The purpose of the ignition box is to increase the voltage to the coils which are paired in series, which is usually not recommended, and allows getting closer to their rated output. The engine will already run cleaner without an ignition box as I have noted (at least when it was not misfiring like mine did on a few occasions in very cold weather regardless of the ignition and coils used).
Running without the cover helps ventilating the coils as it is not like the whole area is particularly fresh.

 

It depends on the CoP, right? If it's a 6v CoP you'd have to wire them in pairs, if 12v all in parallel, if 3v then all in series. Either way as long as the 12V supply to each side has enough amperage it shouldn't need an ignition box to do anything, unless I'm missing something (which is possible).

The lack of trigger strength could be one but I thought that was a ground signal (which shouldn't have any strength or voltage to worry about).

I'd worry about the plug wells filling up in the wet without the covers. If you never drive in rain it would probably be ok but I don't really like worrying about that.

 

I've procured a set of hyabusa CoP's and have looked at how they're assembled. I think the height of them can be reduced a few mm to fit under the stock covers as both the boot and plug attachment appear to be removable and can be modified, I'll let folks know the results one way or the other. I was able to buy them in a set of 4 for $80, so not too expensive. Not guarantees on timeline, I have half a dozen (fun) things to do to the car first and it isn't having any problems so this is more of a 'next time I change the plugs' thing to try.