Difference between R7438-8 and R7437-8 NGK spark plugs

Had the opportunity to have access to both R7438-8 and R7437-8 NGK spark plugs for a side-by-side comparison. These are NGK’s racing series plugs and are pretty expensive for spark plugs. Typically used in the higher-end builds for track purposes.

The most notable difference between the two is the ground electrode design. The R7437-8 has what is a projected center electrode and standard ground electrode where it’s curved at about a 90 degree angle which is pretty typical looking. While the R7438-8 has a non-projected center electrode and slant ground electrode that points straight out over the center electrode at a slant.

Left is the R7437-8 and right is R7438-8

The electrode and insulator tip in the R7438-8 is also recessed more to account for the ground strap being closer. So the gap is the same on the two plugs. The other interesting piece is the R7438-8 thread body isn’t shorter than the R7437-8, the thread count is the same but the top above the threads on the R7438-8 is beveled. The ground electrode and center electrode don’t project as far into the cylinder on the R7438-8 as the R7437-8. I’m not an expert on combustion DI engines so won’t hazard a guess as to the effects of both electrode types.

When you use ngk.com to compare these two plugs the only differences are the following:

  • Center Electrode projection: projected vs non-projected
  • Center Electrode size: 0.6mm vs 1.0mm
  • Ground Electrode material: Nickle vs Platinum
  • Ground Electrode type: Standard vs Slant

The ground electrode material of the R7437-8 was surprising to me. I didn’t expect it to be Nickel and thought both were platinum. The 91006 NGK plug (RS7) is also platinum for the ground electrode for reference.