Spark plug heat range for 360 Magnum

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MopaR&D

MopaR&D

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I'm building a 360 with Magnum heads and would like to know the best spark plug heat range I should get. I already bought a set of Bosch Platinum Plus #4002 plugs from O'Reilly and didn't think about the heat range at the time; they are spec'd replacements for stock Magnum engines in trucks. Looking online I found they are a heat rating of 8 which compares to 11 or 12 with Champions... is this too hot? I'm running 10.42 static, 8.71 dynamic compression with KB-107s, 0.040" quench give or take a couple thousandths, and stock iron 5.9L Magnum heads.
 
I have noticed very little gain in a colder plug, but more of a loss with a hotter plug. For street bound cars, mild to medium builds can use a stock plug to 1 step cooler. More aggressive builds can go 2 steps cooler.

I normally stick with Champs. I have no issues with them. A better plug can go outside the above ranges with little to no issue. A few runs at the track can help show you what works for your engine and car.

Experiment with plug gaps as well.
 
OK my main concern was with detonation which I've read can be caused by running too hot of a plug. I think I have to take these plugs back anyway as they seem to be more low-performance ones and are resistor-type which aren't compatible with my Mallory 6A CD box.
 
I run Autolite 3924s in mine.

Platinum plugs are typically a no-no for hot-rod apps.
 
I've never had a problem with resistor plugs.
 
So this is a really old thread but I brought it back because it's something I am running into an issue with now. I've been having mild pinging on warmer days here and there (especially sunny ones for some reason) running NGK ZFR6F plugs and while I know I still need to play with the advance curve some more I'm going to try dropping to the ZFR7F plugs (one step colder). I feel like the factory Magnum heat range is too hot for my 10.5:1 engine but do you think only one step down will make a difference? I wanted to go to an 8 but 7 is the coldest they make in that style of plug.

I got freaked out when I looked closely at my plugs after a WOT blast today where I heard some pinging and there were definitely specs on them... from what I can see of my pistons just looking down the spark plug hole they do seem to have small shiny spots in places like someone hit them with a tiny chisel, I know that's from detonation. I just hope my engine lasts it's been running like this since I built it 4 years ago, still going strong though no oil burning or blowby. Maybe the loose gaps I filed the rings to are saving the pistons from blowing apart lol
 
Khalid,what fuel are you running? ( And are you running into "summer blend ) fuel,yet? Finding out the alcohol percentage in your fuel,might tell you something.Looking at the actual plug profile,that's a pretty extended tip plug.. Running an MSD,I personally would run a 3924 Autolite,or a NGK FR5.. Just my thoughts.....
 
Been running only 91 pump premium for the past few months. I think they started switching to summer gas over April but I've found that winter gas is more detonation-prone. I already ordered the plugs in through NAPA, I'll see how they go but if no improvement I think I'll have to back the timing down a couple degrees.
 
MOPEkidD-3 said:
Been running only 91 pump premium for the past few months. I think they started switching to summer gas over April but I've found that winter gas is more detonation-prone. I already ordered the plugs in through NAPA, I'll see how they go but if no improvement I think I'll have to back the timing down a couple degrees.
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What are your cam specs? Dynamic compression ratio doesn't mean crap. Too many guys get caught up in that nonsense.
 
yellow rose said:
What are your cam specs? Dynamic compression ratio doesn't mean crap. Too many guys get caught up in that nonsense.
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I'd argue with you there since it's an easy way of saying "too much static compression and not enough cam duration/overlap". I have 10.5:1 static running a 213/220 @ .050" Voodoo cam at 5000' elevation with very dry air.
 
MOPEkidD-3 said:
I'd argue with you there since it's an easy way of saying "too much static compression and not enough cam duration/overlap". I have 10.5:1 static running a 213/220 @ .050" Voodoo cam at 5000' elevation with very dry air.
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Then why the issues? Your DCR is plenty low that it ought to run on beer and not have an issue. I can tell you my DCR is way above yours and my crap won't have an issue. I never once considered it when building my engine. But I damn sure planned ahead, and didn't buy a cam off the shelf.

DCR means ZERO.

I don't know your entire package, but I can tell you if you are fighting it like it sounds, you missed something. Not trying to be a dick, but you banked on DCR and got bit.
 
yellow rose said:
Then why the issues? Your DCR is plenty low that it ought to run on beer and not have an issue. I can tell you my DCR is way above yours and my crap won't have an issue. I never once considered it when building my engine. But I damn sure planned ahead, and didn't buy a cam off the shelf.

DCR means ZERO.

I don't know your entire package, but I can tell you if you are fighting it like it sounds, you missed something. Not trying to be a dick, but you banked on DCR and got bit.
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Based on what I learned on this forum 8.75 DCR is near the limit for pump gas, my old post wasn't including air density so theoretically yes I'm in the 8:1 range but you're right none of it matters now LOL. I built up a 360 LA short block with flat-tops and factory Magnum heads for quench and reused the small Voodoo cam I had in my 318 because I wanted it to be torquey for tall gears. The big mistake looking back was not taking the time to check the assembled quench distance on all the cylinders, having reused the factory rods I'm sure it was off enough to make some pistons too far down for it to work right.

I might just bite the bullet here soon and get a Crane H-272 I've had my eye on that for a while. Luckily the pinging isn't terrible and I watch out for it all the time, I still got some other mods to help as well I haven't done (cold-air hood scoop, 3.55 gears and shorter tires, 2800-stall converter).
 
MOPEkidD-3 said:
Based on what I learned on this forum 8.75 DCR is near the limit for pump gas, my old post wasn't including air density so theoretically yes I'm in the 8:1 range but you're right none of it matters now LOL. I built up a 360 LA short block with flat-tops and factory Magnum heads for quench and reused the small Voodoo cam I had in my 318 because I wanted it to be torquey for tall gears. The big mistake looking back was not taking the time to check the assembled quench distance on all the cylinders, having reused the factory rods I'm sure it was off enough to make some pistons too far down for it to work right.

I might just bite the bullet here soon and get a Crane H-272 I've had my eye on that for a while. Luckily the pinging isn't terrible and I watch out for it all the time, I still got some other mods to help as well I haven't done (cold-air hood scoop, 3.55 gears and shorter tires, 2800-stall converter).
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Again, just so I am clear, you need to call a cam company and NOT get an off the shelf grind. You already tried that and how's it going. Get every single fact together and get on the phone and talk to someone who grinds cams for a living, not just a phone desk jockey. I use Jim at Racer Brown because Jim is a Chrysler guy, BUT that is not all he does. He doesn't have a cam on the shelf. And Jim grinds every single cam himself. When you call Racer Brown, you get the guy who grinds the cam.

I also use Cam Motion. They do not have cams on the shelf.

I can not emphasize how important cam selection is. If you are going to the trouble of a cam swap, get a custom cam for YOUR application. That would help end your issues and make getting the tune up in shape.
 
OK suppose I'll give it a shot it's the same price anyway right? I just wanted to learn for myself what the different effects would be trying to run cam A vs. cam B, as long as the guy on the phone explains how and why it works so I learn something that's better lol.
 
I think your problem is much more related to elevation than anything else. You didnt mention you run at 5K feet until end of posts. The reason it pings on sunny days is not due to heat but due to thin air with no moisture content and much less ability to suspend atomized fuel. The NGK plug is the correct plug, I have run them for 20 years without issues. If you had injection, you would not have a problem. Increase your jetting a notch and give serious consideration to getting FI if you plan on living at elevation, or at least if you want your engine to live at elevation....
I would also call Jim as Yellow Rose suggested, the same guy that helps us design all our cams we sell which are not available anywhere else but from us.
 
magnummopar said:
I think your problem is much more related to elevation than anything else. You didnt mention you run at 5K feet until end of posts. The reason it pings on sunny days is not due to heat but due to thin air with no moisture content and much less ability to suspend atomized fuel. The NGK plug is the correct plug, I have run them for 20 years without issues. If you had injection, you would not have a problem. Increase your jetting a notch and give serious consideration to getting FI if you plan on living at elevation, or at least if you want your engine to live at elevation....
I would also call Jim as Yellow Rose suggested, the same guy that helps us design all our cams we sell which are not available anywhere else but from us.
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Interesting, so water injection would probably have a big effect then right? I run a wideband O2 and it pings even if the AFR is 12:1 or richer if it's sunny outside.

BTW I got a set of the regular-tipped FR5 NGK plugs this morning, just left for a work trip though I should get them in next week.
 
When does it ping, off idle, tip in throttle at cruise or only if you stomp it?
What is base timing? centrifical? vac timing? total at what rpm?
If its pinging with those plugs, doubt FR5 will stop it but I have been wrong on occasion.
 
It only pings when I stomp on it, worse in higher gears too it's definitely load-related. I haven't checked since last year but I think the timing is like 10 initial, 32 total all in by 2500 RPM; next chance I get I'm gonna back down the total timing 2 degrees or so and I need to put a stiffer spring in the dizzy to slow the centrifugal advance some more (it was coming in at like 2200 RPM before). But I don't think that will make much of a difference because it will still ping if the conditions are right and I hammer it at 3000 RPM after all the mech advance is in. I know my cylinder pressure is really high with the 10.5:1 static comp and small 213/220 @ .050" cam, underhood temps are also high with the uncoated shorty headers and I don't have any kind of cold-air induction just an open-element air cleaner under the stock hood.
 
When it's fully warmed up it pulls almost 17" of vacuum at an 900 rpm idle in neutral/park... pretty impressive considering the low air density. It can easily idle at 600 rpm but with the fast-ramp Voodoo cam and 1.6 Magnum rockers I like the peace of mind with regards to cam oiling of the higher idle speed. It would also be harder to keep running when cold esp. when in gear (running stock-stall 904 auto). FWIW out of necessity I have been driving my Duster as my only car for the past 2 months since my '93 Cherokee driver is in the process of getting a new clutch and it's great, I average 17 MPG with 40/60 city/hwy miles. It literally runs perfect except for the pinging issue and it's really a treat to drive.

I also just got a brand new 750 cfm Street Demon carb to replace the old 625 cfm Carter AFB on it now; nothing wrong with that carb it just doesn't behave well the way I have it tuned for lean cruise and I know my warmed-over 360 could use more carb as it still pulls almost 2" vacuum at WOT.

One more thing the gearing is super tall, 2.94 gears with 28" tall tires puts a lot of load on the engine it really needs 3.55's. It's perfect on the highway though, 3000 RPM at 80 mph and theoretically tops out around 150 but I haven't had it that fast yet lol.

EDIT: when I go to install the new plugs I'm going to do a cranking compression test I have not yet done one on this engine and it's been running 4 years now with probably 20k miles, at least.
 
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