I'm trying to find the correct plug to use on my 327. I do have the TRW pop up pistons and I'm running 64cc Edelbrock Performer RPM heads. I've narrowed my search down to these 2 plugs based on the fact that I have to use a plug with the following characteristics:

Gasket Seat, 14mm Thread, .750 in. Reach, Non-projected tip.

Option 1:
Autolite AR3935

Option 2:
NGK R5671A-7

I can't find any information that tells me the heat range of the Autolite plug. Can anyone tell me which is the hotter plug based on this chart?

 

I use the Champion RC12YC on my Dart Pro 1 heads.

Haven't pulled a plug yet to check its heat range , but I suspect it it is close.

This is a projected tip plug, much better all around choice, BTW.

I used projo tips on my old L79 and they fit fine.

Doug

 

The chart tells nothing. The NKG and Autolite plugs you listed are both the same heat range. They are equivalent to an AC 43 plug in heat range. A bit cold for the street, unless run your car car hard regularly.

 

14 MM 3/4 reach plug is the early BB plug. I dont know what Edelbrock uses, but thats a typical BB plug. Also Chrysler Hemis. In Champion it`s a suffix "N" Also a Harley plug. Many numbers have a "R" in front but all that means is resistor type.

 

I believe I have a set of AC 43 plugs left over from my 65 and 66, don't believe they were ever installed in a engine.

 

Been looking for a set. I'll PM you my address.....

 

Don't use the projected tip plug in those heads with the pop-up pistons. I ran the same setup as you are running with the NGK as you mentioned and the regular plugs just so cleared the pop-ups. And the combustion chamber around the plug comes very close to hitting the pop-ups, too.

 

OK, they are yours

 

The AC equivalent to that spec would be the "XL" series (3/4" reach, fully-threaded, non-extended tip); early big-blocks like the R45XLS, but that has an extended tip (the "S"). One for yours would be the 45XL or R45XL in a "5" heat range. "4" heat range would probably work too, but "3" is way too cold for the street.

 

Thanks guys. I should have mentioned that I have been running the Autolite AR-3935 and they clear the pistons just fine, but are EXTREMELY prone to fuel fouling on initial startup. I can't figure out all the heat range charts from all the different manufacturers because they all use different numbers and letters for their heat ranges. I've set the idle mixture using my vacuum gauge and it's where it needs to be. Could it be that I just need to adjust the float in the carb to avoid this problem?
Here's the specs on the engine:

TRW dome pistons
Performer RPM heads
Comp XR282HR-10 ROLLER Cam
Holley 650 Double Pumper

 

I doubt the floats are any problem, or it would be a problem when fully warmed up. Install a MSD box or equivalent and lower your heat range down to an AC 45 range. Your plugs are too cold for the mild cam you are using. Back in the day, we had to warm up the engine with a 45 and than switch to a 41 heat range plug to race. Why are you using a Ford plug in a Chevy motor? Yes I know plugs will not increase HP, But when the cross reference charts show the same plug for 3 different heat ranges I would think some thing is wrong.

 

David, Those will do nothing but foul on you. R45 is the way to go on stock heads.

Dave

 

It's the only one I had access to locally at the time of first startup.

 

Hey Dave,
I am under the impression that for my motor the AC 43 would be OK, because of the following:
1) I am Radio Delete, so I don't need an 'R' plug
2) Being my motor is a 365hp solid with Compression 11:1

A “cold” plug transfers heat rapidly from its firing end into the cooling system and is used to avoid core nose heat saturation where combustion-chamber or cylinder-head temperatures are relatively high. A “hot” plug has a slower heat transfer rate and is used to avoid fouling under relatively low chamber or head temperatures. What’s confusing is that a “hotter” (higher performance level) engine requires a colder plug because more power equals higher cylinder temperatures.

Critical factors affecting heat range include:

• Air/fuel mixture: Lean air/fuel ratios raise cylinder-head temperatures, requiring a colder plug. Rich air/fuel ratios require a hotter plug to prevent fouling. Mixtures that cause the plugs to read lean may contribute to pre-ignition or detonation. If not running an electronic engine management system, it pays to tune slightly on the rich side to avoid detonation.

• Spark advance: Ignition timing has one of the greatest effects on plug temperatures. It becomes more critical as compression ratios increase. More timing (I should be 10-12*) raises combustion temperatures, calling for colder plugs.

• Compression ratio: Increasing the mechanical compression ratio raises cylinder pressure, resulting in higher cylinder temperature. The higher the compression ratio, the colder the spark plug needs to be. According to Champion Spark Plugs, for normally aspirated, gasoline-fueled engines, a good rule of thumb is to go about one heat range colder for each full point in compression ratio increase from 9:1 through about 12.5:1, and two heat ranges colder for each point increase between 12.5:1 and 14.5:1. Beyond 14.5:1, 3-4 heat range reductions per point may be needed.

• Gasoline quality: With musclecar-era leaded gas, the lead is attracted to the hotter (core-nose) part of the plug, causing glazing. The spark runs down the core nose instead of jumping the gap. Going to a slightly colder plug helps prevent lead-glazing. However, with today’s cleaner-burning oxygenated unleaded gas, an equivalent engine needs to run plugs about 1-2 heat ranges hotter than originally specified (many plug manufacturers have revised their catalogs accordingly).

I thought because of this I'd grab em whilst they are avaliable from Tom......But, you already know I am just learning, so I am all ears...
David

 

Let me know how that works out for you.. I think you will find that you will foul them in short order. Dave

 

You will NOT be happy with 43's - they're WAY too cold for normal street use and will foul in town driving.

 

BTDT.

I ran 45 extended tip plugs on my last motor, and have tried 43 and 44 in the past.

43 is fine for a road race car, but will chug and miss around town.

Doug