72 spark plugs?

[Gary S.]

In reading the archives, Duke said "All early SBs regardless of HP rating used the AC44 as OE without any exceptions that I can think of, but it was a bit on the cold side for street and highway driving and the slightly hotter AC45 provided better service. AC no longer makes non-resistor plugs so the best substitute if you want to use AC is the R45 as Dennis said." with regards to a question posed for a 63 FI. My car is a 72 LT1 and I want to replace the plugs. Can I assume that these same AC-R45's would be satisfactory for a stock, untouched LT1 that is street driven only, no racing, high speeds or other hard driving?

The car is a 36000 mile unrestored vehicle that is currently running AC-R44T's in it, installed by the PO. I am trying to find a rough idle situation and eliminating all of the other tuneup issues such as points, plugs, timing, etc, one at a time. Once the tuneup is complete, I will look at vacuum as a potential source. In addition, there is a very slight roughness while cruising at a steady rpm on city streets. Full throttle response and high speed bursts, from 40-75 mph seems quite acceptable and usually brings negative comments from my wife.

Any ideas?

Gary

[George Romano]

Re: 72 spark plugs?

Be sure the valve lash is adjusted properly before chasing down a rough running condition.
If you want to eliminate the possibility of the plugs being the trouble, replace them with Autolites. They will by far outlast any AC plug.

George

[Gary S.]

Re: 72 spark plugs?

George, thanks for the heads up on this. The rough running condition, is fairly new, having developed within the last two trips that I have taken the car on. I don't know if it was a gradual deterioration or something else. I will be checking the valve lash according to the "Duke/John H. method".

Which Autolites do you recommend? Since I have never used them, I only want to do this once. The #1 spark plug is a royal PITA, behind the PS pump and below the alternator. Last time I did this, I moved the alternator.
Gary

[Duke W.]

Re: 72 spark plugs?

Your owner's manual should list the original plug. My guess is the R44T was OE. Resistor plugs replaced non-resistor plugs in 1968 as I recall, and taper seats superceeded gasketed seats on small blocks in '71 if memory serves.

If you want to use an AC plug I would recommend the R45T. I doubt the plugs are causing your engine roughness situation. Do a thorough ignition system check including plug wire resistance, valve lash, and check for vacuum leaks.

Rough running at idle and part throttle with good WOT performance is a classic symptom of a vacuum leak on a carbureted engine.

Duke

[Gary S.]

Re: 72 spark plugs?

"Rough running at idle and part throttle with good WOT performance is a classic symptom of a vacuum leak on a carbureted engine."

Duke, thanks for jumping in here. The spark plug data from the GM service manual does show a R44t but I was just trying to cover my bases. I have been keeping notes on things like vacuum leaks and other interesting posts on this board, and that is why my original post says that I was, more or less, saving the vacuum trouble shooting till last. The gist of your post has been in the back of my mind for a while as a culprit in my idle quality.
Gary

[Mike McKown]

If wide open throttle performance meets expectatio

and it starts quickly, hot or cold, I'd forget about the tune-up first and go straight for a vacuum leak. You car has had plenty of time and heat to "cook" the hoses on the engine. I think your engine has a three way vacuum connector on the thermostat housing. If it does, check closley and make sure it is not cracked and sucking air.

About the plugs: I've used AC, Autolite and Champion all with good results as long as the heat range was correct and the timing and carburetor were functioning correctly. For most driving, the 45 heat range will work well.

When your car was new running on leaded fuel, you could expect 10-12,000 miles from a set of plugs before they would start to skip. With unleaded gas, I have seen similar engines go 3-4 times that long and still not skip.

Mike

[George Romano]

Re: 72 spark plugs?

I believe the correct number would be Autolite # 86. The parts counter guy can cross the numbers for you and get the right one for your car.

George

[Patrick H.]

Re: 72 spark plugs?

Gary,

The #1 is nothing on your car. Try #2,4 and 8 on the same car with AC. There's a reason my AC is not yet connected.

I use the AC 45's on all 3 cars without a problem. I've also used Champions with good success. I HAVE had problems with AC 44's and of course 43's. I've also used the AC RapidFire plugs, and currently have them in the 71 (Size #1 I believe). Heat range is more important than brand, IMO.

Patrick

[Patrick H.]

Re: 72 spark plugs?

Gary,

Block off the intake manifold vacuum port right at the engine, allowing none to go through the vacuum control systems. See if this eliminates the problem of idle roughness. If it does, then you can start troubleshooting.

Patrick

[Duke W.]

Re: 72 spark plugs?

"Heat range is more important than brand, IMO."

ABSOLUTELY CORRECT, Patrick!

The bottom line on spark plugs is GET THE HEAT RANGE RIGHT for your driving conditions, and you won't have any problems. All the rest is fluff, yet I don't think most "car guys" understand the importance of heat range.

Plug tip temperature needs to be between about 750 and 1650 F. Below 750 the plug will foul and above 1650 it can cause preignition. A "hot" plug will run a higher tip temperature for a given operating condition than a "cold" plug due to differences in insulator design. A hot plug has more thermal resistance to reduce heat transfer through the seat to the engine mass and cooling system, so it will run hotter for a given set of conditions.

The required heat range is based on your driving conditions, not how much power your engine makes. The OE selection of "4" was very conservative - a bit too cold and they are more prone to fouling in normal street and highway driving.

My experience is that AC heat range "5" is perfect for street driven small blocks regardless of tune. They are also fine for drag racing. For hot lapping a race track AC heat range "3" always gave very satisfactory performance for me.

My current brand preference is NGK and Denso, because their plain vanilla copper core plugs have a more corrosion resistant coating on the metal shell than AC and Autolite in particular. They also offer non-resistor plugs equivalent to the no-longer-produced AC 45.

Duke

[Larry M.]

Re: 72 spark plugs?

Hi Gary -

Long, long ago, I knew for certain what at this time I can only recollect with a degree of uncertainty: In the AC Spark Plug numbering system, all the letter combinations both before and following the heat range number held a specific meaning. In your examples, 44 and 45 are the heat ranges; the 'R' indicates a Resistor spark plug (Sorry! I don't remember what it resists!). 'R' or 'non-R' plugs of the same heat range (i.e. R45 or 45) were interchangeable. The 'T' suffix, I believe, indicates the new-for-that-era tapered seat design of spark plug body, which replaced the previously used crushable, round metal gasket that slid over the plug threads and seated on the flat bottom of the plug, to provide the seal between the plug and cylinder head. The 'T' and 'non-T' plugs are not interchangeable. If your car has R44T's in it now, the R45's would not fit, but R45T's would.

Please, anyone who can correct me on this, sound off! This bit of memory trivia was dredged from way down deep, and I wouldn't bet my life on its accuracy (although I would post it with cautionary warnings!).

Larry

[Jeffrey S.]

Re: 72 spark plugs?

Larry
You are correct that the "T" suffix is for tapered seat and that they are not interchangeable with AC45's. There is one slight correction I would make to your response and that is that the "45" is not the heat range. the "5" is but the first number (in this case 4) is the thread size. The number 4 is for 14 mm threads used by GM. Ford uses an "8" (i.e. R85T) for 18 mm threads. Just a small thing but since you asked for corrections I thought I'd chime in.
Jeff Salz

[Larry M.]

Re: 72 spark plugs?

Jeff -

Thanks for that bit of info. It certainly looks as if AC chose carefully all of the letter/number combinations that make up the merchandising number for their product. I appreciate your insight.

Larry