Just changed the plugs in my 06 LTZ about a month ago, needed it bad...orig plugs at 110k mi :/. I used the most expensive Bosch plugs at the store (Platinum coated, 4 prong, no gap adjust) And shortly after, I found a bunch of people on here that swear against the use of Bosch. Im wondering whats the big deal? After running my car for a few days with these plugs in, I had gained 4+ mpg and the throttle seems peppier, so I can only imagine theyre doing what theyre supposed to be doing.

 

Every engine is different - even the same CID - so some will have good luck with a particular brand and bad luck with others. The 4.3 crowd has the same mind set that nothing will work in those engines except AC Delco. I put them in my engine and it ran like crap. Swapped them out for E3 plugs and the engine ran like a finely crafted swiss watch. I put AC Delco in my Monte when I tuned it up because they were on sale, but next tune up it will get E3. If the Bosch are performing well in your engine then stick with them.

 

I have always been told that the best plugfor the LS4 is the stock plug AC Delco Iridium plugs. Thats what I used in the Deuce when I changed them out. I've been happy with them but then I never tried the Bosch cuse of the bad PR I have heard about them

 

Personally I'd never buy a Chinese AC Delco don't believe anything AC Delco is made in the States anymore. But I'm probably wrong.
My car came with NSK Iridium plugs. That's what I have.

 

The 3.8s seem to not like Bosch

 

I have had first hand experience with Bosch plugs in 3100/3400/3800 engines, they suck. Either a bad plugs in six new ones or about 6 months later the car begins to run for crap (such as misfiring). I will never run Bosch plugs in my GMs again (some of these experiences were in my cars, others were helping friends/family with there cars).
I tend to favor AC delco plugs, but I have recently shared equal satisfaction with NGK (which turns out AC has farmed out plugs to NGK as I have a set of old plugs in my garage that are dual branded). Bosch makes other good stuff (such as there plug wires work great).

 

Wow....A lot of opinions here, wish someone with a 3.9 would chime in if they've had bad experiences. But the 3.9 and 3.8 are very similar so maybe problems could arise? If I start having issues Ill probably just change em out until I get it right (probably new wires as well). But a month has gone by since I've installed them and no issues (fingers crossed).

As a funny side note, when I took the original AC delco plugs out, three of them crumbled in my hands as I pulled them out...I was freaking out that I had just dropped a bunch of ceramic dust down into my pistons...Luckily I had a cheap little borescope to put my mind at ease

 

The 3.9 and 3.8 are not related at all. The 3.8 is 90 degree engine and the 3.9 is a 60 degree. If I recall, the 3.9 is more closely related to the 3100/3400/3500 family.
All my experiences with bosch plugs have been bad (hence why I won't touch them ever again). Since you already have'em it's hard to justify replacing them if. Are not having trouble. But if you develope ignition/spark related issues, bosch plug replacement is typically my first piece of advice. Hopefully your bosch plugs run without pre-mature issues.
As I mentioned, my experience comes first hand (such as I helped replace a set of 6 month old bosch plugs with few miles on them for bad misfiring).

 

Tried many plugs in the Monte, some pretty spendy ones to. We are having great luck with Autolite plugs.

 

The problem with the Bosch plugs is that they're gimmicky, unadjustable and WAY overpriced.

Think about Electricity 101. What's one of the first things that you learn about electricity? It takes the path of least resistence every time. Since this is the case, why does a spark plug need more then one prong? That's one of my main beefs against E3 plugs as well. This is why Bosch platinum 4 plugs are gimmicky.

Next problem is that they're unadjustable. Like already mentioned, every engine is different so it's nice to be able to set your plugs at the best gap for your personal engine. Yes, this does require some work taking the plugs in and out through trial and error, but you can find the best gap by doing this.

Then, there's the price issue. I bought Bosch platinum 4 plugs ten years ago for a different car for 6$ per plug. Ouch! If you look around you can get six good plugs for under 15$ (for all six plugs) total. So now you've saved yourself over 20$ by not buying Bosch platinum 4 plugs.

Another thing to consider that I didn't mention is the spark plugs heat ranges. It's been a LONG time since I looked at Bosch platinum 4 plugs so I don't know if they've changed, but if you can't select the proper heat range for the plugs, then that's a problem. The last thing that you want is a spark plug to break/melt in the combustion chamber.

Those are some of the main reasons why I would NEVER buy a Bosch platinum 4 plug. Any plug will perform better then an old nasty plug, but will it be worth it in the end?