Decoding Spark Plugs

[Robert M.]

Hi all.

I read the posts and concluded that the AC R45 is the plug to use.
What I am curious about, is what to all the other letter designations mean?
I know R is for resistor, so what are the differences between an R45, R45S, and an R45TS ??

If anyone can point toward a site that could educate me on the subject, it would be much appreciated.

Bob

[Clem Z.]

Re: Decoding Spark Plugs

S= extended tip TS= tapered seat with extended tip

[Jack H.]

Re: Decoding Spark Plugs

Clem's on the money! In general, the suffix code following the plug's heat range designation (single or dual numeric code), describes the geometry specifics of the electrode. Some codes are rare (unique to a given application that may be non-automotive) and include:

B, D, E, F, FF, G, H, K, L, XL, N, P, R, S, T, TS, W, X and Y

[Duke W.]

Re: Decoding Spark Plugs

Manufacturer's catalogs have pages that explain their plug nomenclature, and most are now available online including cross references.

In the AC system "R" means the plug is the RFI resistor type; "4" means it's a 14 mm thread, and "5" is the heat range - suffixes as previously noted, which decode to other plug geometry features. Some manufacturers also use prefixes to the numerical heat range to denote geometric features. The higher the heat range number, the "hotter" the plug meaning that it runs hotter for a given load than lower number heat ranges. Most other manufacturers follow this convention, except Denso, where a higher number is colder.

Heat range "5" AC plugs usually are the best selection for normal road operation, regardless of HP rating. Engines with TI or HEI ignitions can tolerate a lower heat range since they have double the per spark ignition energy of the point type systems so they will tolerate considerable fouling before misfire occurs.

Colder heat ranges should be used for sustained high load operation like 100+ MPH cruising or race track hot lapping.

Duke

[Paul L.]

Pic of R45S

I just changed plugs in my 1967 327/300 yesterday. The bleeding caused by the heatshield brackets has now stopped.

The pic shows an R45S plug with extended tip. The NGK B-4 by contrast does not have the extended tip. Both are non-tapered.

[Dick W.]

Re: Pic of R45S

The NKG is a colder heat range plug, therefore the tip would not be extended quite as far. The R45S crosses to a BP5S or XR51X NKG

[Paul L.]

Re: Pic of R45S

Dick,
I am not sure what you mean. The lower the numeral in the NGK plugs, the hotter they are.

[Duke W.]

Re: Pic of R45S

Experience has taught many old hands that one heat range hotter than OE is best for normal driving. That would be the AC 45 for all SBs to '68, and this crosses to the NGK B4, which is also a non-resistor, non-extended electrode type. It and the Denso W14-U are the closest replacements to the discontinued AC 45, all being non-resistor type. I consider both the NGK and Denso to be equivalent high quality. FYI new Corvettes have had Denso plugs as OE going back to at least the beginning of the C5.

According to my 2000 NGK Catalog the AC 45S crosses to the R5670-5 (as do the 44S and 46S), which is actually cataloged as a "racing" spark plug, but the heat range is in the normal road driving ball park.

My 2000 catalog doesn't show a BP5S - just a BP5ES, which is 3/4" reach. Neither is the XR51X listed, but it's a resistor type due to the "R" in the prefix.

Also, I forgot to mention that NGK also, like Denso, uses and "inverted" heat range relative to AC. For both a higher number is a colder plug. Typical NGK street plugs for most cars are "5", so the B4 is on the warm side. No B5 is cataloged, but B2 and B6 are.

Extended tip plugs are generally thought to have a "broader" heat range. They may be more resistant to low load fouling than convertional, but Chevrolet never installed extended tip plugs, and the OE domed SHP pistons may hit the ground electodes if quench clearance is tight.

Duke

[Dick W.]

Re: Pic of R45S

Duke, I got those numbers from the online NKG crossover. I just checked it again and the 45's crossed over to the same number. Go figure.

[Duke W.]

Re: Pic of R45S

I just checked the NGK Web site and didn't find a cross reference function, but I did use their "part finder" to look up a 1965 Corvette.

The listed plug is B6S, which is a direct cross from the AC 44, and one heat range hotter on my paper catalog chart for this geometry plug is the B4. It also lists the XR51X, which is an "Iridium" plug - expensive - and also has a resistor. IMO common copper core, nickel electrode, "two-dollar spark plugs" are fine for vintage Corvettes and spending several times the price on some "trick" spark plug is a complete waste of money.

Finally the V-Power type B6Y is listed. V-Power is NGKs v-groove center electrode, which is a good technical feature. The lack of an "R" indicates it's a non-resistor type, but I would want to measure one to be sure.

In the case of the B6S the "S" doesn't mean extended tip like AC. It means "standard 2.5 mm electrode", so the maybe the B4 electrode design is a little different.

Can you post a link to the cross reference you used?

Duke

[Paul L.]

Re: Pic of R45S

Dick,
There was a recommendation here about three years ago to try the NGK B-4. It was at a time when I was fighting with my original Holley and AC and NGK plugs fouled equally. No discrimination. Sacrilege I know but I installed an Edelbrock #1406 that has been flawless since. And the R45S plugs for a "North American" reason. The 1967 ran well but I have always worried about that extended tip re piston contact. Paranoia perhaps as it never happened. So now I went back to NGK B-4. It's only been a day but the car idles perfectly and runs at 180F with smooth performance.... My hands are healing nicely.

[Duke W.]

Re: Pic of R45S

Extended tip plugs won't hit flattop pistons even if quench clearance is zero. It's only a potential issue with domed pistons and tight quench clearance.

The B4 is a functionally equivalent replacement to the out of production AC 45. That was my recommendation, and I stand by it. Also the Denso W14-U. As far as I can tell, NGK does not offer a direct heat range replacment to the extended tip, non-resistor AC 45S.

Denso does offer a direct replacement for the AC 45S - extended tip, non-resistor - W14P-U. NGK isn't the only game in town when it comes to correct heat range non-resistor replacement plugs just like Shell isn't the only game when it comes to CI-4 or CJ-4 oil.

Duke

[Paul L.]

Re: Pic of R45S

Duke,
I didn't wish to take your name in vain without your permission. But yes, it was your recommendation.

Same with the Pirelli P4000s.

My thanks.

[John D.]

Re: Decoding Spark Plugs

Bob, If you have decided that you want to use AC R45's in lieu of the straight AC 45's then you must have an early car. You didn't say what the application is. It would be helpful if everyone would give a little more info.

R45S: The S means extended plug. That may work on your car provided you don't have the hi-dome pistons.
The R45TS plugs are an entirely different spark plug.
My early 84 has the AC45TS plugs. Not even close in appearance and application for what you may want.
Only thing the same is the number 45. Other than that the plugs look entirely different. I have collected the 45TS since the real deals are discontinued. (for my 84.)
There is a long list of AC spark plugs that start off with 45. The suffix is the kicker. I have some that are stamped with FF. I think those are Corvairs. John

[Clem Z.]

just index the "S" series plugs when you install

them. mark the plug insulator in line with the ground electrode mounted side using a magic marker. install the plugs and as long as the mark is in the 8:00 oclock to 4:00 oclock position you will have no contact problem. you can just switch plugs to different cylinder if you find a problem.