How do you choose spark plug heat range?
11-05-12, 04:53 PM
#1
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How do you choose spark plug heat range?
I have been reading all day about what plugs people are using and can't find how you decided on them. Some are running 9s all around or 10/11 or 10/10 etc.
I have a 94 FD
New OEM engine (stock ports) from Ray @ Malloy
Precision 6266 T4 .84AR, 46mm gate set at 15psi spring pressure
Defined Auto vmount
PFC/datalogit
3" exhaust
ID725/2000 combo on FFE rails, supra pump
OMP deleted, premixing 1oz/gal
I am planning on tuning it to maybe 20psi... Depending on how well she does in the texas heat on 93+WI. Id like to land in the 400-450hp range if possible.
I don't have water injection on the car yet, but plan on adding it as soon as I decide what kit i want (probably activate it at 10psi)
I also plan to upgrade the ignition in the next 3-6 months, likely to the AEM setup.
This is a street car, but I often drive it like I stole it and I see a lot of 100F days here in south Texas.
How do you decide? Is there a balance between driveability and safety, etc? I am looking at the 7420s and I'd like to make the most educated guess the first time... expensive little bastards.
If you recommend a plug, can you say why you pick that plug?
I have a 94 FD
New OEM engine (stock ports) from Ray @ Malloy
Precision 6266 T4 .84AR, 46mm gate set at 15psi spring pressure
Defined Auto vmount
PFC/datalogit
3" exhaust
ID725/2000 combo on FFE rails, supra pump
OMP deleted, premixing 1oz/gal
I am planning on tuning it to maybe 20psi... Depending on how well she does in the texas heat on 93+WI. Id like to land in the 400-450hp range if possible.
I don't have water injection on the car yet, but plan on adding it as soon as I decide what kit i want (probably activate it at 10psi)
I also plan to upgrade the ignition in the next 3-6 months, likely to the AEM setup.
This is a street car, but I often drive it like I stole it and I see a lot of 100F days here in south Texas.
How do you decide? Is there a balance between driveability and safety, etc? I am looking at the 7420s and I'd like to make the most educated guess the first time... expensive little bastards.
If you recommend a plug, can you say why you pick that plug?
11-05-12, 06:34 PM
#2
talking head
you pick a bloody cold one once you push past 350 rwhp.. NGK 9's become 10s , become 10.5s
if the porting and or long idle periods causes fouling then you try one hotter
( but not if at the expense of detonation )
im in WA, oz.. we see 40 C regularly ( well past your 100 F )
the issue is usually with preignition and cracking of the sparkplug holes , especially around the trailing side
if there is a choice/ margin err the trailing one colder than leading
PS
note NGK heatrange is opposite direction to other brands,, high numbers are colder
if the porting and or long idle periods causes fouling then you try one hotter
( but not if at the expense of detonation )
im in WA, oz.. we see 40 C regularly ( well past your 100 F )
the issue is usually with preignition and cracking of the sparkplug holes , especially around the trailing side
if there is a choice/ margin err the trailing one colder than leading
PS
note NGK heatrange is opposite direction to other brands,, high numbers are colder
11-05-12, 10:24 PM
#3
Original Gangster/Rotary!
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At those boost and power levels you're past what the 'standard' NGK 9s can do for you..... look into 10s, either NGK or Greddy. The real ones, not the cheap snowmobile plugs. There are tons of threads floating around about this, many that I've posted in. Search and you'll find lots of good info 
11-05-12, 11:55 PM
#4
NGK R6725 platinum racing plugs and NGK Iridium Racing Spark Plugs R7420 are your best options, 4 of these plug are around $150 shipped to your door. If your going to track the car or run 20psi like you said, either of these plugs will work great.
You can get by on the street with 9's but when pushed hard they will wear the electrode down, opening up the gap to the point the plug will miss fire. If you keep the boost low 10-15 psi they are fine, much more than that and they will not last long, 3,000-5,000 miles the way I drive @ around 20psi on pump gas. At $7 buck a plug I don't mind changing them
You can get by on the street with 9's but when pushed hard they will wear the electrode down, opening up the gap to the point the plug will miss fire. If you keep the boost low 10-15 psi they are fine, much more than that and they will not last long, 3,000-5,000 miles the way I drive @ around 20psi on pump gas. At $7 buck a plug I don't mind changing them
11-06-12, 12:07 AM
#5
NGK B10EGV will be the best bang for your bucks without robbing your wallet! Their at a 10.5-11 heat range and costs less than $10 bucks a plug. You'll need the special sparkplug socket from Racing Beat though.
Gotta love them Hyabusa's for using em'
Seriously, I couldn't justify 25-30bucks + for a sparkplug......just not that loaded with bling. G/L
Gotta love them Hyabusa's for using em'
Seriously, I couldn't justify 25-30bucks + for a sparkplug......just not that loaded with bling. G/L
11-06-12, 08:16 AM
#6
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NGK B10EGV will be the best bang for your bucks without robbing your wallet! Their at a 10.5-11 heat range and costs less than $10 bucks a plug. You'll need the special sparkplug socket from Racing Beat though.
Gotta love them Hyabusa's for using em'
Seriously, I couldn't justify 25-30bucks + for a sparkplug......just not that loaded with bling. G/L
Gotta love them Hyabusa's for using em'
Seriously, I couldn't justify 25-30bucks + for a sparkplug......just not that loaded with bling. G/L
+1
Using the 19mm reach plugs for over 5 years now with no issues.
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11-06-12, 12:18 PM
#8
Original Gangster/Rotary!
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The disadvantages of the EGV and EIX spark plugs have been noted many times here on the forum..... for any kind of road race duty I'd avoid them like the plague for the sake of your rotor housings. For street driving they may be ok.
It's best not to talk about how much money you've saved with them until you tear down your engine and look at your rotor housings
It's best not to talk about how much money you've saved with them until you tear down your engine and look at your rotor housings
11-06-12, 12:39 PM
#9
ideally you work from the coldest plug down to the hottest where problems begin to cease.
colder plugs will foul relatively easily with even moderately rich mixtures but will yield the best spark energy output. a hotter plug can preignite if you push it too far, leading to engine failure.
colder plugs require more ignition power to drive them, like a CDI.
10-10.5 is the most common heat range for rotaries in the 350-600whp range.
and i still don't necessarily agree that 7420's are a suitable replacement for the 6725's.
colder plugs will foul relatively easily with even moderately rich mixtures but will yield the best spark energy output. a hotter plug can preignite if you push it too far, leading to engine failure.
colder plugs require more ignition power to drive them, like a CDI.
10-10.5 is the most common heat range for rotaries in the 350-600whp range.
and i still don't necessarily agree that 7420's are a suitable replacement for the 6725's.
Last edited by RotaryEvolution; 11-06-12 at 12:41 PM.
11-06-12, 04:29 PM
#12
Heat range has nothing to do with driveability.
The heat range of the plug simply is a measurement of the insulation around the tip which influences the amount of heat the tip is able to reject to the head. A cold plug does not retain as much heat as a hot plug. The purpose of the retained heat is to promote self cleaning of the plug or to prevent deposits from forming in the first place. A hot plug will be less likely to foul when exposed to rich mixtures or any circumstance that creates poor combustion, such as low compression, weak ignition, oil contamination, etc. The cold plug will retain less heat so that it is less likely to create a pre-ignition situation. That's where the hot plug will get you in trouble. To over simplify it, up the power and/or rpm range and a hot plug can turn into a glow plug and auto ignite the mixture before the spark event. Worst case, that leads to detonation and you have a badly broken engine in short order.
Reading plugs is the best way to determine if you have the right heat range. That's a huge subject that's badly misunderstood by most everyone. Cliff notes; if you're not doing plug chops with brand new plugs and using a magnifying glass to inspect the plug, you're not doing it right. In the end, you want a plug that's just hot enough to keep itself clean but cool enough to not cause pre-ignition. Without going through extensive testing, it's easy enough to rely on the experience of the community. My experience has been, if you're fouling any of the 6725 or 7420 plugs on gasoline, the tune is wrong, the ignition components are suspect, or the engine itself has issues.
11-06-12, 04:30 PM
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At those boost and power levels you're past what the 'standard' NGK 9s can do for you..... look into 10s, either NGK or Greddy. The real ones, not the cheap snowmobile plugs. There are tons of threads floating around about this, many that I've posted in. Search and you'll find lots of good info
ideally you work from the coldest plug down to the hottest where problems begin to cease.
colder plugs will foul relatively easily with even moderately rich mixtures but will yield the best spark energy output. a hotter plug can preignite if you push it too far, leading to engine failure.
colder plugs require more ignition power to drive them, like a CDI.
10-10.5 is the most common heat range for rotaries in the 350-600whp range.
and i still don't necessarily agree that 7420's are a suitable replacement for the 6725's.
colder plugs will foul relatively easily with even moderately rich mixtures but will yield the best spark energy output. a hotter plug can preignite if you push it too far, leading to engine failure.
colder plugs require more ignition power to drive them, like a CDI.
10-10.5 is the most common heat range for rotaries in the 350-600whp range.
and i still don't necessarily agree that 7420's are a suitable replacement for the 6725's.
Thanks, that was good info. I have already decided on NGK race plugs. With as much as they cost, I was hoping to make a very educated guess on which ones to order.
From what I understand, the only reason for 7s was for emissions. So is there any reason to keep the split if you don't care about emissions? like 9/10 or 10/10.5? Would you lean towards 10s or 10.5s to start? Will the OEM ignition do ok with those until I upgrade it or would I be better of with 9s until I upgrade the ignition?
Thanks for your time.
11-06-12, 04:39 PM
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Heat range has nothing to do with driveability.
The heat range of the plug simply is a measurement of the insulation around the tip which influences the amount of heat the tip is able to reject to the head. A cold plug does not retain as much heat as a hot plug. The purpose of the retained heat is to promote self cleaning of the plug or to prevent deposits from forming in the first place. A hot plug will be less likely to foul when exposed to rich mixtures or any circumstance that creates poor combustion, such as low compression, weak ignition, oil contamination, etc. The cold plug will retain less heat so that it is less likely to create a pre-ignition situation. That's where the hot plug will get you in trouble. To over simplify it, up the power and/or rpm range and a hot plug can turn into a glow plug and auto ignite the mixture before the spark event. Worst case, that leads to detonation and you have a badly broken engine in short order.
Reading plugs is the best way to determine if you have the right heat range. That's a huge subject that's badly misunderstood by most everyone. Cliff notes; if you're not doing plug chops with brand new plugs and using a magnifying glass to inspect the plug, you're not doing it right. In the end, you want a plug that's just hot enough to keep itself clean but cool enough to not cause pre-ignition. Without going through extensive testing, it's easy enough to rely on the experience of the community. My experience has been, if you're fouling any of the 6725 or 7420 plugs on gasoline, the tune is wrong, the ignition components are suspect, or the engine itself has issues.
The heat range of the plug simply is a measurement of the insulation around the tip which influences the amount of heat the tip is able to reject to the head. A cold plug does not retain as much heat as a hot plug. The purpose of the retained heat is to promote self cleaning of the plug or to prevent deposits from forming in the first place. A hot plug will be less likely to foul when exposed to rich mixtures or any circumstance that creates poor combustion, such as low compression, weak ignition, oil contamination, etc. The cold plug will retain less heat so that it is less likely to create a pre-ignition situation. That's where the hot plug will get you in trouble. To over simplify it, up the power and/or rpm range and a hot plug can turn into a glow plug and auto ignite the mixture before the spark event. Worst case, that leads to detonation and you have a badly broken engine in short order.
Reading plugs is the best way to determine if you have the right heat range. That's a huge subject that's badly misunderstood by most everyone. Cliff notes; if you're not doing plug chops with brand new plugs and using a magnifying glass to inspect the plug, you're not doing it right. In the end, you want a plug that's just hot enough to keep itself clean but cool enough to not cause pre-ignition. Without going through extensive testing, it's easy enough to rely on the experience of the community. My experience has been, if you're fouling any of the 6725 or 7420 plugs on gasoline, the tune is wrong, the ignition components are suspect, or the engine itself has issues.
11-06-12, 04:47 PM
#16
More good info thanks. I sent you an email recently about your ignition setup. I am still trying to decide if I am going to get a little PC battery up front or something different in the rear bin. I am going to do the ignition and WI probably at the same time. What do you suggest for my setup as a starting point?
Sorry if I didn't reply. Was out of town and some things probably got missed.
The AEM/Mercury coils, good wires, and 10.5 plugs all around.
11-06-12, 04:57 PM
#17
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I determine heat range by how many threads on the outside of the plug change color. I look for at least three threads that show signs of heat.
See how the first few thread show heat/dark in color.
See how the first few thread show heat/dark in color.
11-06-12, 04:57 PM
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No, you did reply. I just did not order yet because I don't know what harness to get. (trunk or engine bay mount). I still need to figure out what I can fit in my engine bay and what needs to be relocated (AI tank, battery, coils, catch tank, AST) .. Its all a mess right now.
11-06-12, 05:19 PM
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My current interest has been how to choose a plug that can handle what I am going to throw at it yet not give me issues for driving around town. Light load, putting around under vacuum at low RPM vs high teens/low 20s PSI @ 8k with water in the chamber. I realize a cold plug fouls easier and that the NGK 67xx or 74xx have a reputation of not fouling easy. Thats why I was leaning that way.
There are different opinions on what type of plug, but almost all of you agree on the heat range (10 to 11). I will likely go with 10.5x4 as a starting point.
11-06-12, 06:35 PM
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I did talk to him about it. In depth. But doesn't he only run 12-15psi? He was actually contemplating running slightly hotter since adding water injection, if I remember right.
11-07-12, 06:09 PM
#25
Your statement that a cold plug requires more energy to ionize the gap is simply false. Outside an engine, inside a properly tuned engine, or inside a turd, one has nothing to do with the other.
If you're trying to say a hotter ignition will fire a fouled plug more easily, there may be some merit to that. However, if you've fouled the plug in the first place you've already lost the battle. Proper tuning or fixing the mechanical issues that led to the fouled plug are the answer, not a hotter ignition.
Also, how can you claim the heat range of the plug dictates spark energy? Again, false.