Spark Plug Recommendation

[Don M.]

Guys, I have a car that is now just a Trailer queen, show car. Sooo it gets a LOT of idling and slow 1st gear driving. My plugs are shot, what would you recommend. It is a 327-350 HP, completely stock. Don

[Joe L.]

Re: Spark Plug Recommendation

Guys, I have a car that is now just a Trailer queen, show car. Sooo it gets a LOT of idling and slow 1st gear driving. My plugs are shot, what would you recommend. It is a 327-350 HP, completely stock. Don

Don------


Well, if originality is of concern to you, I'd suggest going with an AC 44 or 45. These are GM-discontinued but you should be able to find some on eBay.

If it were me, though, I'd use a Champion RJ8C.

[Bruce W.]

Re: Spark Plug Recommendation

Guys, I have a car that is now just a Trailer queen, show car. Sooo it gets a LOT of idling and slow 1st gear driving. My plugs are shot, what would you recommend. It is a 327-350 HP, completely stock. Don

Don, what plugs are you running in it now? how many time on the plugs you are using? probably need to go with an AC 45 or 46

[Frank D.]

Re: Spark Plug Recommendation

The 45s are a hotter plug and if you are idling and low speed driving a lot prob the better choice.
An alternative is the NGK-XR4 if originality is not a major concern...

[Gene M.]

Re: Spark Plug Recommendation

Guys, I have a car that is now just a Trailer queen, show car. Sooo it gets a LOT of idling and slow 1st gear driving. My plugs are shot, what would you recommend. It is a 327-350 HP, completely stock. Don

Don, Even a show car needs to be taken out and go for a drive. Things dry out and start leaking. Fuel from cylinders gets into oil and requires driving to burn off. AC44 plugs works just fine with suggested use. No plug is going to "make up for" non use.

[Duke W.]

Re: Spark Plug Recommendation

The OE AC 44s are too cold for normal street and highway driving. By the mid-sixties I figured out that the 45 ran hot enough to not rapidly foul. John Hinckley and I have been recommending AC heat range "5" or equivalent in another brand for normal driving, both small and big blocks, ever since the TDB has been around.

To the OP: Your use is the absolute worst thing you can do to a car. Rapid lug fouling and oil contamination can be expected unless you drive it sufficiently to get it fully warmed up for at least 30 minutes once in a while. You should drive it enough annually to consume at least one tank of fuel per year.

Duke

[Mark E.]

Re: Spark Plug Recommendation

Don------

Well, if originality is of concern to you, I'd suggest going with an AC 44 or 45. These are GM-discontinued but you should be able to find some on eBay.

If it were me, though, I'd use a Champion RJ8C.

Joe, In a prior thread you recommended iridium spark plugs. From the ACDelco site:

• Iridium tip for maximum resistance to spark erosion and corrosive combustion chamber environment
• 1-piece integral suppressor seal meters spark plug energy to help provide long electrode life and seals against compression leakage
• Spark gap remains virtually unchanged during the life of the plug for increased fuel efficiency
• ACDelco is an Original Equipment (OE) supplier and has the experience engineering and manufacturing components that achieve the highest-quality standards required by vehicle manufacturers
• Ideal plug for vehicles requiring platinum performance or older vehicles calling for conventional plugs
• Can give up to 100,000 miles of service in some applications

Not cheap at over $5 each. A good choice?

[Duke W.]

Re: Spark Plug Recommendation

Why do you need a "100,000 mile" spark plug on a vintage car that is only driven a few hundred to a few thousand miles per year.? A conventional "two-dollar" spark plug should last at least 30,000 mile with unleaded fuel and a conventional breaker point ignition in good working order.

Resistor plugs cost some spark energy, and AC no longer offers non-resistor plugs. That's why I recommend the non-resistor NGK B4 and Denso W14-U for those who are not concerned about having original brand plugs. Both are equivalent to the long out of production non-resistor AC 45.

If due to short trip, low speed driving you need an even hotter plug to avoid fouling install the non-resistor Denso W9-U, which is equivalent to the out of production non-resistor AC 46.

Duke

[Joe L.]

Re: Spark Plug Recommendation

Joe, In a prior thread you recommended iridium spark plugs. From the ACDelco site:

• Iridium tip for maximum resistance to spark erosion and corrosive combustion chamber environment
• 1-piece integral suppressor seal meters spark plug energy to help provide long electrode life and seals against compression leakage
• Spark gap remains virtually unchanged during the life of the plug for increased fuel efficiency
• ACDelco is an Original Equipment (OE) supplier and has the experience engineering and manufacturing components that achieve the highest-quality standards required by vehicle manufacturers
• Ideal plug for vehicles requiring platinum performance or older vehicles calling for conventional plugs
• Can give up to 100,000 miles of service in some applications

Not cheap at over $5 each. A good choice?

Mark------


I did not recommend the iridium plug in this particular case because of the very limited use of the engine and because I figured that originality might be more of a concern than performance and longevity.

For me, though, iridium is the plug of choice.

Price is no object here. He'll probably pay more than $5 a plug for NOS. old-technology originals if that's how he chooses to go.

[Gene M.]

Re: Spark Plug Recommendation

Original poster must be interested in flight judging his car and not necessarily performance. No matter what plug he installs (I assume for maximum correctness AC44 would yield best with the judges) they will carbon build up without real usage. Now the vintage of the AC plug will be difficult at best to determine on the show field since nothing can be removed. But a good judge with a small mirror can certainly determine an AC from the others.

Unless one does a lot of city driving the AC44 that GM engineers specified does the job just fine. The AC45 is not a good selection for highway or spirited driving that the Corvette was intended for. If fact some GM owners manuals suggest the AC45 for dominantly city driving. In MHO the GM engineers had their act together on the AC44 plug specification. Above all use GOOD fresh gasoline.

[Frank D.]

Re: Spark Plug Recommendation

Here are the 45 plugs out of my stock '61 283ci 270hp dual quad car running on 87 octane/ethanol-laced for 12,000 miles. They work perfectly.

[Duke W.]

Re: Spark Plug Recommendation

Unless one does a lot of city driving the AC44 that GM engineers specified does the job just fine. The AC45 is not a good selection for highway or spirited driving that the Corvette was intended for. If fact some GM owners manuals suggest the AC45 for dominantly city driving. In MHO the GM engineers had their act together on the AC44 plug specification. Above all use GOOD fresh gasoline.

I found out back in the sixties that the AC 44 is too cold for normal driving and constantly fouled. Installing 45s solved the fouling and did not overheat on freeway drives including cruising at up to 130 MPH (5000 revs with a 3.08 axle) for sustained periods, like the forty miles between the CA/NV border and the Vega strip. (There was no speed limit in Nevada back then so it was perfectly legal.)

When I hot-lapped Kent back in the sixties I installed AC 43s, which were just right for hot lapping a high speed race track.

John Hinckley found out the same thing and makes the same recommendation.

If the engine has TI ignition you can get away with heat range 4 in normal driving because the TI offers double the spark energy of the breaker point ignition and will fire a plug with more fouling deposits on the insulator that might misfire with the breaker point systems.

The heat range of the spark plug is not determined by maximum engine power. It's a function of typical engine load, and since Corvettes are relatively small, light cars with powerful engines, those engines rarely see high load for any sustained period. So whether you have a base 283 or a SHP big block, AC heat range 5 or equivalent in another brand is likely the best choice for typical vintage car driving, which likely consists of pretty easy driving with low average engine load.

Like most vintage engine spark advance maps, GM's spark plug heat range recommendation is overly conservative.

The 45s shown in post #11 with 12K miles look very good - little insulator deposit buildup or electrode wear. I'm sure they will be good for at least another 12K miles. AC heat range 5 is a good choice for the type of service.

Duke

[Don M.]

Re: Spark Plug Recommendation

Thank you guys! I think I will keep it simple and go with the AC45s. I hear what you are saying about driving it enough! Don

[Gene M.]

Re: Spark Plug Recommendation

I found out back in the sixties that the AC 44 is too cold for normal driving and constantly fouled. Installing 45s solved the fouling and did not overheat on freeway drives including cruising at up to 130 MPH (5000 revs with a 3.08 axle) for sustained periods, like the forty miles between the CA/NV border and the Vega strip. (There was no speed limit in Nevada back then so it was perfectly legal.)

When I hot-lapped Kent back in the sixties I installed AC 43s, which were just right for hot lapping a high speed race track.

John Hinckley found out the same thing and makes the same recommendation.

If the engine has TI ignition you can get away with heat range 4 in normal driving because the TI offers double the spark energy of the breaker point ignition and will fire a plug with more fouling deposits on the insulator that might misfire with the breaker point systems.

The heat range of the spark plug is not determined by maximum engine power. It's a function of typical engine load, and since Corvettes are relatively small, light cars with powerful engines, those engines rarely see high load for any sustained period. So whether you have a base 283 or a SHP big block, AC heat range 5 or equivalent in another brand is likely the best choice for typical vintage car driving, which likely consists of pretty easy driving with low average engine load.

Like most vintage engine spark advance maps, GM's spark plug heat range recommendation is overly conservative.

The 45s shown in post #11 with 12K miles look very good - little insulator deposit buildup or electrode wear. I'm sure they will be good for at least another 12K miles. AC heat range 5 is a good choice for the type of service.

Duke

In the 60's virtually all 327's went out the door with AC44 plugs. The fouling and overheating was not an issue. I never experienced it and still use the AC44's today and they run fine. Proper gap at .035 is essential along with good plug wires. As I said GM engineers know what they were doing. A poorly set up engine may suffer from fouling and overheating and be band aided using AC45's. Fix the problem. I could see how a dual quad 283 could have those issues. We are talking about an L79 327. They were not designed that way and didn't go out the door that way. Original poster was not claiming a TI option, only that his plugs were "shot". I don't know what that means. He didn't say carbon fouled, missing, or burnt nor engine over heating. Heat range 4 as designed and sold by GM is what the engineers specified. I can't see changing something in the original design just for sake of change or for fouling and overheating when neither is a stated issue.

[John D.]

Re: Spark Plug Recommendation

AC Delco R45S's in my 67 L79 for over 20 years.