Spark Plug Recommendation
 

I have 496 strokers with AFR aluminum heads should I use a stranded tip or a projected tip plug?

Brian,

For N/A applications start with;

NGK R5671 A-9
NGK R5671 A-8 Hotter
Autolite AR 3933
Autolite AR 3934 Hotter

Blower AFR Plug
Autolite AR 3935

Bob

Why hotter with a blower?

Definitely +1 on the Autolite AR 3933, the shorter electrode arm reduces the chance of detonation. I talked to an old time off shore racer and they used to saw the tips of their plugs to prevent detonation.

-8 is hotter than -9....that's all that means. A -8 is not a 'hot' plug.

NGK rates heat range from -1 (hottest) to -9 (coldest). Therefore, -8 is still a pretty cold plug. As another reference, many street engines use -5 or -6's.

BTW: - is read as ' dash'....not minus'

The Autolite AR 3933 is listed as a non-resistor type plug. Will this cause interference with the stereo in my boat? If so, is there a resistor plug that can be used in its place?

Bud - totally going by the top of my head here. Autolite 3923 ? Check to see if I'm right.

Edit in: Damn, I was close. Eveything is right, other than being a projected tip plug. Damn...close.

Bud,

Brain is utilizing the 305 CNC chamber head which carries a different baseline spec. Your base plug is the 3935.

Bob

Bob, so it is the same plug as listed for the blower plug? AR 3935? I assume that is also a racing, non-resistor plug. I know most street cars run a resistor plug to prevent radio interference. Is that an issue on boats, or is the fact that the engine is 8 feet away behind the location of most stereo equipment enough distance to prevent interference?

I also see a Champion plug listed on the AFR site.

Thanks!

I have been running non resistor racing plugs, with CD ignitions for quite a while now. No issues.

If I was running a blower with Autolite plugs, I'd run the 3933 or 3934. The lower the number in Autolite is colder. NGK, the higher the number, the colder. As in, a 3933 would be a colder plug than a 3934. A -9 would be a colder plug than a -8 in NGK. You can get a -10 heat range in a NGK racing plug.

Mercury used a MR41T plug in their supercharged 525SC, 600sc, 800sc. The Standard non supercharged plug was the MR43T. A comparable heat range plug to a MR41T, would be a -8 NGK or AR3934 autolite. A comparable plug to the MR43T, would be a AR3935, or NGK -6. Thats for heat range purposes only, as the MR43T and MR41T were .460 reach, tapered seat plug for the GM iron heads. The AR3933, AR3934, AR3935, NGK 5671 series, are .750 reach gasketed style non resistor, non projected tip. The AR133, AR134, AR135, NGK 5673 series are .460 reach, tapered seat, non resistor, non projected tip. I would not run a projected tip in a forced induction engine, or actually any marine engine in my opinion. :helmet:

i wish a mutuial friend would have followed the last part of your post.

Anyone had any issues with GPS or Depth Gauge interference with non-resistor plugs? Found this on NGK's site, so that's why I'm asking.

"NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.

They are also recommended on any vehicle that has other on-board electronic systems such as engine-management computers, two-way radios, GPS systems, depth finders or whenever recommended by the manufacturer."

Is there a particular reason that the non-resistor racing series plug is recommended with the AFR heads, versus a plug with the same reach and heat range that might be more readily available?

Tom,

Go with a Champion
RC 12 YC
RC 14 YC_(Hotter)

or cross reference to you brand of choice.

Bob

I have never had any issues with non resistor racing plugs. No interference with my stereo system, my GPS chartplotter, MSD or Crane Ignition boxes, etc.

Is there any reason you don't want to run the spark plug AFR recommends? From what I see on their website, they recommend an Autolite 3935. That is a non resistor, non projected tip, copper racing plug.

Hey MT, I was just mostly going by Bob's recommendations. I figured the plug recommended by AFR on their site is for automotive applications, which may differ from marine. I was concerned about radio interference with the non-RF plug because we run with the stereo on frequently, use the VHF radio, and I have a depth gauge with the transducer right under the motor. I didn't want to install a set of $4 plugs in each hole only to find that they cause me problems and will have to be switched out. The RC12YC's that Bob recommended with his most recent post are actually a resistor plug that is available almost anywhere for around $2.00, but they ARE a projected tip plug. I noticed that you mentioned that you don't like to run a projected tip plug in a marine engine - is that due to concerns about detonation, or is there some other reason?

I too run with the stereo, chart plotter, etc going. I buy my NGK racing plugs from o'reilly's or rockauto for under 2 bucks each. I don't care for projected tip plugs because of the constant load high cylinder temps we see in boats. I don't like any detonation or hot spots , they ruin your good time lol

Something to think about. Guys will install super low 50 ohm per foot plug wires . A 5k ohm resister in the spark plug kinds defeats the idea of comparing a 50 ohm per foot msd plug wire compared to a standard automotive off the shelf wire. Will any of that really matter as far as power goes, not really. Just a thought.

I don't think you will have any issues with radio noise unless you are running solid core wires