Spark Plugs - Too Many Choices

[MaddMike]

I have a '71 351C, with PerTronix Flamethrower distributor and coil, Edelbrock comp cam and Edelbrock 600 cfm carb but otherwise stock. I need to update and replace my plugs. My dilemma is there are too many choices out there - original copper, single or double platinum, iridium, C-cut, gold-platinum ...

With the electronic ignition sure, I assume I should upgrade the plugs also, I'd like to get as much bang for the buck as I can. I know I will get different opinions but anyone have strong feelings / suggestions?

 

[Stanglover]

Mike, I'm using Autolite 24's in my 351C4V M code. with Pertronix Ignitor II and Flamethrower II coil, matched with 8mm spiral wound plug wires. I gap the plugs at .045" and have had zero issues with burning plugs. Color is a good light brown.
I've also used Autolite 25's, but decided the hotter plug was not needed.
As you have a stockish motor, don't over think it. That's just my input from my experience.
EDIT: looking on Autolite's website under Ford Mustang 1971, they list the 24 for the M code and 25 for the R code OR 71 and 72 H code, OR 72 Cobra Jet Q code.

 

[Big Red Mach 1]

Don't remember if I got this on this forum or not. I run 24's based on compression ratio being higher than 10:1.

 

[c9zx]

Regular Autolite, Autolite Racing, NGK, or Brisk. Do an in-depth dive on
"How to read spark plugs". Chuck

 

[Ryunker]

I will never use Fram (autolite) plugs in anything, they really are sub-par, go with the Motorcraft stuff, copper. The "upgrades" are to get more life from them, not because they are better performance.

 

[Steve1973M1]

​

Hi MaddMike,
I used NGK UR41X#7401 which is a substitute for accel 276 (ARF42). The originals were accel 274 (ARF 32). These are hotter plug slightly but do not foul like others and have an Iridium Tip which is small and sharp. This means it takes less voltage to fire than round blunt platinum tipped plugs. I have an electronic ignition in which I designed with extended dwell as rpm increases. It also times out if no signal is sent from the distributor in 3 seconds without sending a spark during shutdown. Current on the secondary output is almost totally the same up to idle to 5500 rpm after that its starts to go down. It has a built in adjustable rev limiter. It uses a Duraspark distributor and an old oil fill Jacobs ignition coil. I have altered primary resistance for a 7.5 amp coil. ( the ignition also has adjustable peak current protection) With the older holley avenger 4 barrel fuel injection with new terminator computer my fuel ratio is 14.5 to 1 except wide open throttle which is 12.5 to 1. The engine is a 73 with open chamber heads and does not ping with 87 octane gas. The manifold is a older Edelbrock performer with a 1 inch spacer between it and the 4v avenger EFI . Its all abut just start up car and drive it. The timing is set for the best vacuum about 14-16 degrees advance but watch those hot starts. The Fuel injection will not prime if you start fast when hot (under 1/2 second). I am going to build hot start ****** circuit one day I have full circuit simulated. But all you really need is a timer that shuts ignition off for 1 revolution when hot it will be up to speed by then.
I still haven't made up mind on what to do with ignition design yet, it is all analog design and doesn't overheat like pertronixs ignition inside distributor or various ford versions ( have run full tests to thermal equilibrium output IGBT case temp 50C max @ 4500 -5500 rpm test small box. ) ( takes about 20 minutes at a steady test at 5000rpm to get it to thermal 50C and runs way cooler at regular rpm) some pictures below.
Steve
https://www.amazon.com/NGK-IRIDIUM-Spark-Plugs-7401/dp/B0043BNW0C

 

[Ron Tanzi]

My go to spark plug for 15+ years has been NGK in all the applications that I work on including my Mustang. Autolite and Champion are not what they used to be.

Ron

 

[Ron Tanzi]

​

Hi MaddMike,
I used NGK UR41X#7401 which is a substitute for accel 276 (ARF42). The originals were accel 274 (ARF 32). These are hotter plug slightly but do not foul like others and have an Iridium Tip which is small and sharp. This means it takes less voltage to fire than round blunt platinum tipped plugs. I have an electronic ignition in which I designed with extended dwell as rpm increases. It also times out if no signal is sent from the distributor in 3 seconds without sending a spark during shutdown. Current on the secondary output is almost totally the same up to idle to 5500 rpm after that its starts to go down. It has a built in adjustable rev limiter. It uses a Duraspark distributor and an old oil fill Jacobs ignition coil. I have altered primary resistance for a 7.5 amp coil. ( the ignition also has adjustable peak current protection) With the older holley avenger 4 barrel fuel injection with new terminator computer my fuel ratio is 14.5 to 1 except wide open throttle which is 12.5 to 1. The engine is a 73 with open chamber heads and does not ping with 87 octane gas. The manifold is a older Edelbrock performer with a 1 inch spacer between it and the 4v avenger EFI . Its all abut just start up car and drive it. The timing is set for the best vacuum about 14-16 degrees advance but watch those hot starts. The Fuel injection will not prime if you start fast when hot (under 1/2 second). I am going to build hot start ****** circuit one day I have full circuit simulated. But all you really need is a timer that shuts ignition off for 1 revolution when hot it will be up to speed by then.
I still haven't made up mind on what to do with ignition design yet, it is all analog design and doesn't overheat like pertronixs ignition inside distributor or various ford versions ( have run full tests to thermal equilibrium output IGBT case temp 50C max @ 4500 -5500 rpm test small box. ) ( takes about 20 minutes steady test runs way cooler at regular rpm) some pictures below.
Steve
https://www.amazon.com/NGK-IRIDIUM-Spark-Plugs-7401/dp/B0043BNW0C

Jesus, you seem very committed to the cause of getting **** as precise as possible. If that is actually possible. Cheers to you.

 

[Steve1973M1]

Hi Ron,
I have NGK iridium spark plugs in my 1998 mustang and are other car which is a ford focus.They are way better than the newer autolite and champion.
I had so many issues with the ignition stuff because the basic Duraspark had issues. I used the Napa box TP40 (Now ECH TP40 it has dwell extender built in). It is a good Duraspark ignition replacement. I just wanted to go out and start the car and not have to mess with carb and points . Instead I have had to mess with a 3.5" Holley display to tune the EFI which has it stuff too. The bad thing is it is not mass air it is manifold vacuum and I had a Ram air leak in one of the flappers which then caused the manifold sensor to mess up and the efi was off cal. The vacuum line to this sensor is now at the Throttle body base manifold connection instead of where the ram air connection is.
Attached is original point type Dwell extender which can be used with some care.
Thank You
Steve

 

[Ron Tanzi]

Hi Ron,
I have NGK iridium spark plugs in my 1998 mustang and are other car which is a ford focus.They are way better that the newer autolite and champion.
I had so many issues with ignition stuff because the basic Duraspark had issues. I used the Napa box TP40 (Now ECH TP40 it has dwell extender built in). It is a good Duraspark ignition replacement. I just wanted to go out and start the car and not have to mess with carb and points . Instead I have had to mess with a 3.5" Holley display to tune the EFI which has it stuff too. The bad thing is it is not mass air it is manifold vacuum and I had a Ram air leak in one of the flappers which then caused the manifold sensor to mess and the efi was off cal. The vacuum line to this sensor is now at the Throttle body base manifold connection instead of where the ram air connection is.
Attached is original point type Dwell extender which can be used with some care.
Thank You
Steve

The early style speed density systems Ford used worked OK but they were much improved when they went to mass air flow. I like Napa products. Napa also stocks many older electronic parts such as your Duraspark box (that dwell extender is pretty cool). I am running an original style Pertronix with an MSD 6A box on my Mustang. My 92 Escort GT that was rocking the Mazda 1.8 used a vane air flow sensor like a lot of the Toyotas of that era. It never had an issue in all of it's 280K+ miles. I have had Mazda products since. 2009 MS3 and 2017 6 with a 6 speed manual. Have a good night sir.

Ron

 

[Steve1973M1]

Hi Ron,
I have a 6A box but it was from my drag racing days. The spark is all of 500us above 3500rpm( single spark) but what a spark. I agree with you totally about the speed density systems. Our newer cars have Mass and it works superb and never had issues with them. Have 350K on 1998 mustang V6 (new long block at 280K) but all original injectors and all of the sensors, but the Ox sensors and cats are new. New plugs and wires of course when needed. 2006 Focus has plug on coils (this puts MSD to shame for the cost, tons of dwell).
Thank You
Good night.
Steve

 

[Stanglover]

Don't remember if I got this on this forum or not. I run 24's based on compression ratio being higher than 10:1.

View attachment 74034

That was the chart I was looking for, now idea where I put it!! thanks Chriss.
What doesn't make sense to me, is why the Boss 351 with 11:1 comp ratio, uses 25's, whereas H and Q code lower comp motors also get that plug, weird!

 

[Stanglover]

I will never use Fram (autolite) plugs in anything, they really are sub-par, go with the Motorcraft stuff, copper. The "upgrades" are to get more life from them, not because they are better performance.

Not to get into a pissing match, but Autolite 24/25 plugs are copper. Motorcraft equivalents are a good choice too. I've never had any issue with Autolite plugs, I will NOT use Champion.....not even in my lawn mower.

 

[Steve1973M1]

Hi all,
Just some information:
https://www.repairsmith.com/blog/spark-plug-types/https://www.enginelabs.com/engine-t...ark-plug-debate-separating-fact-from-opinion/

 

[Ron Tanzi]

My main issue with Autolite and Motorcraft plugs is that in daily driver use in the rust belt their plugs tend to corrode pretty badly. In all my years as a mechanic these are the only plug brand I ever had break off in a cylinder head (happened on a 97 5.0 and a 96 4.0) I am also not referring to those evil modern Ford 3V plugs that are known break, but they are indeed horrible. The NGK plugs hold up really well.

Ron

 

[tony-muscle]

Hi all,
Just some information:
https://www.repairsmith.com/blog/spark-plug-types/https://www.enginelabs.com/engine-t...ark-plug-debate-separating-fact-from-opinion/

So, is iridium the way to go?

 

[Stanglover]

Guys, I've been following this with interest and for sure I've learned something new today. The "Great spark plug debate" was quite interesting.
However MaddMike has a pretty much stock 351C from 1971. He does not have a 2000 and whatever car that has a totally differently designed engine that requires totally different ignition components. Like myself, he needs good basic plugs that will do the job without fouling or causing any other issue. He does not need a plug that will last a million miles either. He just need the best plug we can get that does the job while he drives about 2000 miles a year and only on sunny days.
So, unless the motor is far from stock. modified to the hilt, then surly the plugs that Ford specified ought to be a good start. Granted, our fuel is different now as it is unleaded, so that may require some change, but honestly, as I said before, I have run Autolite or Motorcraft equivalent with a good afr reading and plug color ever since I got this car.
I don't think we need to be overthinking this and I certainly don't think we need to get personal with our responses. All Mike wants is good advice, nothing more.
My rant over.

 

[Hemikiller]

I used to run nothing but Autolites, but the last batch of #24 plugs I received were made in China. Not wanting to risk a new engine build, I went with NGK Copper plugs which were made in Japan. I'm currently running the colder UR6 in my 351C which is 10.5:1 with an MSD ignition. I've switched to NGK when doing tune ups on my cars. The 429 in the wagon is running the WR5.

IMO, the only reason to run anything other than a high quality copper plug in a classic vehicle is for a very specific purpose. I pulled the OE AC-Delco iridium plugs out of my 07 2500HD 6.0 with 125K on them and they looked like they could go another 125k. If I wanted the ultimate plug that will never need to be changed, I'd go with iridium. Our cars will most likely never see a fraction of that mileage, so there's not much of a reason to invest the extra in a rare-metals plug.

 

[Ron Tanzi]

I used to run nothing but Autolites, but the last batch of #24 plugs I received were made in China. Not wanting to risk a new engine build, I went with NGK Copper plugs which were made in Japan. I'm currently running the colder UR6 in my 351C which is 10.5:1 with an MSD ignition. I've switched to NGK when doing tune ups on my cars. The 429 in the wagon is running the WR5.

IMO, the only reason to run anything other than a high quality copper plug in a classic vehicle is for a very specific purpose. I pulled the OE AC-Delco iridium plugs out of my 07 2500HD 6.0 with 125K on them and they looked like they could go another 125k. If I wanted the ultimate plug that will never need to be changed, I'd go with iridium. Our cars will most likely never see a fraction of that mileage, so there's not much of a reason to invest the extra in a rare-metals plug.

The plugs in our cars are much easier to change as opposed to many modern cars equipped with a long life spark plug set. And as you said we just don't do a lot of mileage on average. Fancy rare earth metal plugs are just superfluous to most folks needs. And a waste of money.

Ron

 

[mrgmhale]

Don't remember if I got this on this forum or not. I run 24's based on compression ratio being higher than 10:1.

View attachment 74034

I am not sure that chart originated from (either), but it looks useful to me. I have found Autolite spark plugs to be of very high quality. When ever I tried other plugs (Platinim, Iridium) I had some serious misfire issues. As soon as I went back to Autolite no more misfire.