NGK (7397) TR5IX Iridium IX Spark Plug, Pack of 1

These replace the former NGK part TR55 iridium spark plugs. Not sure of what the difference is, but I suspect it is slight. Installed 8 of these gapped at 0.040" on a 2004 Pontiac GTO, along with 8.5mm MSD wires (generic LS1 kit) and K&N Cold Air Intake kit (57-3044). The car was formerly throwing engine codes of right and left banks running lean/rich. New plugs and wires cleared that code. I've read on forums that the TR5IX plugs show a 5HP gain over stock plugs if your car is already modded, but may be only as good as stock plugs if the car is stock otherwise. Car is perhaps a slight bit more responsive, but I think that is due to clearing the engine code and letting the engine relearn itself. Removing stock wires on an LS1 is a ridiculous job (you just have to rip them out), so that added time to the whole job. Plugs and wires took 3 hours (yes, I know, that is retarded long), but the new MSD wires have a much longer boot on them which will facilitate easy extraction - the...Read more

I have a 2005 GTO with headers, exhaust, and CAI, I replaced my old acdelco plugs, and I noticed a noticeable increase in smoothness and acceleration, I don't know if I've had any fuel efficiency increase yet, but I've only had them a few days.

i purchased these thinking it would work on 2004 gto as claimed on NGK website.. however, i suffered from rough idle and hiccups. Installed back the OEM ac delco, and everything became smooth again. Mods are: CAI, Shorties, Catted mids, Magnaflow exhaust system, and tune.