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Spark plug question


Richard Cranium

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So Wags and I are getting set to convert the dragster to fuel injection. The instructions call for resistor spark plugs and I've been running non resistor Autolite Racing AR51 plugs in the car and I'm happy with them. As best I can tell there is no resistor version of the AR51. Does anyone have any good resources to sort out an equivalent resistor plug?

 

Here are the specs on the AR51

 

Shorty Spark Plug: No

Resistor: No

Electrode Tip Material: Chromium nickel alloy

Electrode Core Material: Copper

Insulator Type: Projected

Spark Plug Thread Size: 14mm

Spark Plug Reach: 0.750 in.

Spark Plug Seat Style: Gasket

Wrench Diameter: 13/16 in.

Ground Strap Quantity: One

 

From what I've read online the last digit is the heat range, so I'm assuming 1, so it's a fairly cold plug.

 

For anyone who does not know the car it's an NA big block chevy, high compression running 112 octane. I don't have many more details than that as the motor is effectively the same as when I bought it turn key.

 

The reason I am happy with the AR51s are that they work well, and they are cheap. Something similar would be ideal.

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I saw that while I was googling around. I figured those would all be direct replacement similar (non-resistor) plugs. I'll scope through some of them and see if any of them are resistor plugs. Thanks.

 

Looks like my first link is a bust, they don't like traffic to their site I guess. Google "ngk chart" and there are a few images of charts that break down NGK's part numbering system based on specs.

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Is any part of the FI using radio or other transmission frequency to operate? Maybe Bluetooth or wifi to connect to the controller?

 

The primary purpose of resistor plugs, caps, et al is to reduce radio interference from the ignition system. Anybody who has ridden in a 50's Chevy missing it's radio shield can tell you about hearing the engine through the radio. You can get resistor caps too but I think those are just shielded rather than having an actual higher resistance.

 

The only other thing I can think of is that it wants a specific resistor plug to track misfires. If that is the csn you can just add a resistor inline to the plug wire and while it won't tell you if your candle is out, it will read normally.

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Is any part of the FI using radio or other transmission frequency to operate? Maybe Bluetooth or wifi to connect to the controller?

 

The primary purpose of resistor plugs, caps, et al is to reduce radio interference from the ignition system. Anybody who has ridden in a 50's Chevy missing it's radio shield can tell you about hearing the engine through the radio. You can get resistor caps too but I think those are just shielded rather than having an actual higher resistance.

 

The only other thing I can think of is that it wants a specific resistor plug to track misfires. If that is the csn you can just add a resistor inline to the plug wire and while it won't tell you if your candle is out, it will read normally.

 

No, nothing is using radio waves for signaling, everything is wired. I went back through the instructions and I don't see resistor plugs as a requirement. I have watched a lot of YouTube videos as well and I may have seen it somewhere in there. I assumed they were simply trying to eliminate as much interference as possible. It does not list any specific plug in the installation instructions. I'll have to do some more research and see if my brain is making this all up. It was pretty late last time I was looking through the instructions and videos. Thanks everyone.

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The resistor is just there as a precaution to help make sure there is no interference, voltage spikes and RFI are just not good with electronics.

 

Look at these. They are a heat range 2, might be a good starting point.

https://www.summitracing.com/parts/atl-4062/

 

Edit: a little more research and it looks like an AR51 is actually a heat range 3

 

So try this.

https://www.summitracing.com/parts/atl-63/

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The resistor is just there as a precaution to help make sure there is no interference, voltage spikes and RFI are just not good with electronics.

 

 

Also, are you running solid core or resistor plug wires (If not coil on plug)? Solid core wire can also cause interference. Resistance wire sets are also run for the above reasons.

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The resistor is just there as a precaution to help make sure there is no interference, voltage spikes and RFI are just not good with electronics.

 

Look at these. They are a heat range 2, might be a good starting point.

https://www.summitracing.com/parts/atl-4062/

 

Edit: a little more research and it looks like an AR51 is actually a heat range 3

 

So try this.

https://www.summitracing.com/parts/atl-63/

 

COOL! Thanks!

 

Also, are you running solid core or resistor plug wires (If not coil on plug)? Solid core wire can also cause interference. Resistance wire sets are also run for the above reasons.

 

Other good points. Not coil on plug. I believe it is an older MSD Pro-Billet Distributor. New plug wires seems reasonable for this project. Thanks!

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Based on Scott finding out the heat range is 3 for these. It looks like I'll probably go with the AR3924

 

Shorty Spark Plug: No

Resistor: Yes

Manufacturer Heat Range: 3

Electrode Tip Material: Chromium nickel alloy

Electrode Core Material: Copper

Insulator Type: Projected

Spark Plug Thread Size: 14mm

Spark Plug Reach: 0.750 in.

Spark Plug Seat Style: Gasket

Wrench Diameter: 5/8 in.

Ground Strap Quantity: One

 

Looks like the only difference is the wrench diameter size changes from 13/16 to 5/8. I don't think that change will be significant. Thanks again for the help folks.

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