How to determine proper spark plug heat range [Datsun 240Z L28 stroker]

[@kyushaspeed]

What plug heat range do you use? Check out my blog post on this as well: builtonpurpose.co/how-to-determine-spark-plug-heat-range/

[@alienpoker]

Thanks for doing this Aaron. As I probably mentioned to you, I use one step colder from stock NGK-BR7ES on my NA 3+ litre stroker L6. I don’t have experience with and ECU and fuel injection, and I run MSD plus a electronic distributor where you can do some crude timing and vac advance/retard tuning. The plug choice was after discussion with a Datsun L6 racing engine builder, and running on an engine dyno. In practice, I only can look at the fouling on the electrodes and the colours of the insulator as you described -my shop tools are typical for a ‘guy in his shed”. From the stock NGK BR6ES (heat range 6) the BR5E would be Hotter. BR7E would be Colder. Some racing plugs do go in .5 increments but never messed with that. Method should be to rev to say 7K on the road (under load) and then put in the clutch, shift to Neutral and coast to a stop switching off the engine. As you said driving it or idling would contaminate the ‘readings’ somewhat. In all, a fantastic video and I’m happy to see you go down this rabbit hole and share your findings.

[@simonilett998]

Hi Aaron, Interesting result, the coldest heat range plugs showed the best looking colour, and correct position of the heat band on the earth electrode, which I guess indicates you must be running a fairly lean tune. Although, I think you said in the video, where your heat banding sits correctly was towards the 'higher heat range plug' but it's actually labelled in your pics as (BPR8ES) which is the coldest plug of the 3 pics. The NGK chart you showed near the start of the video is an important one people should take note of carefully, showing BKR5E-11 hottest, BKR7E-11 coldest. It's easy for the inexperienced to get confused, since NGK heat range numbers work the opposite to every other brand of plug, which is kinda counter intuitive to what makes better sense🤷🤣👍🇦🇺

[@kyushaspeed]

Sorry that is my bad when I described the plugs. I thought I’d be smart and try to describe left to right from the audience view point but in post production I was wrong 🤦‍♂️. The labelling is correct in the video. NGK don’t make it easy using a reverse scale to the majority of other plug manufacturers.

[@fulcrum811]

Great video. Helped alot. By the way. Is the airbox you were printing finished? How does it look, how does it work?

[@hansosl]

Just right....I messed around with that a lot. Take also in consideration the fuel. Here in europe we have 98 or 102 octane...thats why we advance our Timing more than in the USA...which also means the need for colder plugs....

[@Texassince1836]

98 octane European fuel is only 93 American Octanes. We rate on a different scale.

[@Aeropooch]

Curious if any adjustments were made on your spark plug gaps?

[@kyushaspeed]

No changes so far however I have not had much tuning time to further investigate.

[@keithmckibben8774]

I just pulled a spark plug, and I was surprised to find a nice white, clean plug after about 1000 miles of mixed driving, though mostly highway cruising at light throttle in top gear about 3Krpm. I recently switched to projected tip NGKs like you run, in the standard 6 heat range. There were three distinct bands on the ground strap, one each at the three positions you describe, which seems about right for the variety of loads the engine has seen. So I am a bit confused. If the reason to run a hotter plug is to keep it clean, why would I test at wide open throttle? Don't I want it to stay clean when it is running at lower combustion temperatures? Of course, race cars run colder plugs because they are almost always running at higher combustion temperatures. But not many street driven cars can maintain those higher combustion temperatures, so the heat range for the spark plug is higher. Maybe you are confusing the combustion temperature the car is capable of with that it sees under "normal" driving? I would like to see your BPR8s after 1000 miles of normal use.

[@kyushaspeed]

Hi Keith, I don’t think the temperature of combustion inherently changes for a given fuel when burned at a consistent air fuel ratio. The amount of heat energy (different from temperature) produced will alter however. The reason I chose heat range 8 is to make certain the earth strap and electrode would not erode or decay from heat build up under the expected maximum heat producing situation ie. wide open throttle. I am also able to run stoichiometric conditions for most of my driving with the EFI and I suspect this will produce more heat energy. Self cleaning plugs are a bonus I guess. Maybe I should make a follow up video if I ever chalk up 1000km more to see the condition of the plugs?

[@keithmckibben8774]

@@kyushaspeed Oops, very poor use of terms on my part. Yes the temperature of combustion is constant and yours is relatively high if you are around stoichiometric, though the range is probably less than 10% at other air/fuel ratios. I should have said the heat energy retained from combustion, which is what the spark plug heat range is meant to address I think. And yes, I very much encourage you to drive 1000km (maybe take a weekend and really enjoy driving your well-dialed machine!) and check the plugs again. That will show whether they run hot enough under normal conditions to stay clean. I am sure they will be fine for 10000km.

[@michaely6665]

Increasing ignition timing initial advance DOES increase cylinder/plug temperatures (see NGK advices). and may be a reason why NGK 8's work well as you said you already optimised (maxxed advanced?) ignition before this. Modern fuels are slower burning but hotter than old leaded fuel of 1970/80's apart from octane changes!

[@flokiloki6845]

Do you need to retune the ECU when changing spark plug heat range ?

[@kyushaspeed]

Not normally in a domestic situation.

[@robwhite240z]

That is a big can of worms. carby v EFI v EGR . But rule of thumb for carby ,no EGR is if you cruise short trips to the shops NGK 5 heat range , all round driving 6 heat range and for WOT racing 7,8,or 9 heat range

[@alienpoker]

@robwhite240z So for twin carbs (dome top SUs) would you stick to stock 6ES ? 🤔 On my ‘daily’ 240Z for cruising around it has Z-Therapy rebuilds with SM needles. The stock plugs look more fouled than they should after driving around. But I drive her fairly gently. I’m considering 5ES and/or changing metering needles and retuning. Thoughts?

[@robwhite240z]

@@alienpoker Depends om your cam specs and comp ratio. EG If your cam works from 2800rpm and you cruise at 2500 rpm then your plugs will foul up

[@kyushaspeed]

@robwhite240z I figure a large part of selecting spark plug heat range is the actual in cylinder temperatures which is why I undertook the experiment. I am able to run 13.1:1 at WOT and 14.7:1 for idle and cruise with EFI. It is better fuel economy but means in cylinder temperatures are higher. I suspect if I were to richer my AFRs (like when I was using round top SUs) I would require different heat range plugs.

[@kyushaspeed]

@alienpoker I found SM needles and Ztherapy nozzles were too rich on my stroker and changed to SB needles. My BP6ES were fouling with SM needles but cleared up a bit with SB needles and some tuning. Do you know your AFRs?