The previous owner of my 75 coupe pulled the original L-48 and replaced it with a GMPP crate engine(part #12486041) the original spec sheet reccomends R43LTS spark plugs. No auto parts store in my area has ever heard of this#. I contacted AC-Delco online to find a source and they advised that this is the wrong plug and the one I need is actually the R-44TX. Anybody got a clue whats up with this, 'cause these are two completely different plugs?

 

They prob don't make them anymore. Just guessing here, but it seems to me that the 44s would be a hotter plug than the orig 43s. If you're worried try running a R-43TX. It's not a big deal, if it were a monster powerplant on the verge of detonation you should be worried. But with a tame crate engine any plugs meant for a carb SBC will do fine.

 

I agree. Without knowing Delcos numbering, I'd venture a strong guess that the 44 is indeed only one step hotter than the 43. Shouldn't be a big deal on that engine to run either plug.

 

AC no longer makes the AC44 plug. When it was dropped most reference books indicate the AC43 as the replacement but the R43 is typically too cold of a plug for the street and tends to foul a lot.
The popular replacement is the AC45 which is a hotter plug and tends to do better than the AC43.

As for the "R" designation, that says it's a resistor type plug which was not stock on the older motors, they used resistor type ignition wires. As more and more electronics were added to vehicles they went to a resistor type plug to help keep electrical interference down. In your car it doesn't really matter much if you run a resistor or non-resistor type plug. Ask different people and you will get different opinions on the resistor or non-resistor type plugs. I've run both on my '65. It always had AC-R45's when I got the car and I continued to use them until I redid the engine bay and somehow developed a high-rpm miss that I couldn't get to go away until someone recommended using the non-resistor plugs - he swore it would help because of the added resistance of both the original resistor ignition wires and than a resistor type plug (the non-original R45's) would reduce the current so I tried them the non-resistor type and it seemed to help.

Alternatives to the AC45 (non-resistor type plugs) are also the NGK B4 and Denso W14-U

 

I am not trying to confuse the issue but is that the same plug recommendde for a ZZ4 crate motor? I have had problems finding the cross-reference info for the ZZ4 service items like plugs and filters.

 

Indeed, 43 indicates a colder plug than 44 or 45. The question I would have if I were you is what does the X mean? You'll want to verify the seal style and the reach. The plugs required on my stock 76 heads are TS (tapered Seal).

 

Tapered seat, extra long reach

 

My guess is that S is extended tip and X is extra long reach.

http://www.acdelco.com/parts/sparkpl...tification.jsp

 

New R44TX AC spark plugs on ebay now. take care

 

They are also available on Summit Racing!

http://www.summitracing.com/parts/ACF-R44TX/

Still can't figure out whatthe X in TX means.

Look at the Delco site below

http://www.acdelco.com/parts/spark-p...tification.jsp

 

I believe the heads on the ZZ4 use the same plug as the late 80's corvettes. Personally I found my best results using the Marine plugs that came in my zz4. They were MR43LTS.