Spark plus and fuel economy

[GTP-1]

I need some experianced opinions. I can't beleive what I am seeing on my 99 GTP. I changed plugs the other week, took out the factory NGK's. I put in a set of Bosh +4's that are one range colder. I have the 3.4" pulley and CAI as the only mods. I noticed a huge drop in fuel economy right around the same time. The car has about 45,000 miles on it. I picked up some lost power with the plugs, especially up top. But what gives with the mileage?? Ideas?? I just got back last night from a 160 mile all interstate trip with criuse set whole time at 74mph. I clicked off a very pathetic 20.5 mpg. Can plugs really cause this much of a loss?? Car runs great and has lots of power and idles very smooth.

 

[T-Keith]

It could very well be the plugs. I haven't heard many good things about bosch platniums and GM DIS cars. The +4s tend to have a huge gap also. It could always be something else, but try putting the old plugs back in for a couple tanks.

 

[GTP-1]

Welp. I bought a cheap set of standard $.99 plugs and dropped in it just to see what would happen. My mileage jumped right back up to where it is from the factory. Seems after some checking and asking around, you can't use platinum plugs in any blown vehicle for some reason. Maybe this is why the S/C engines get the NGK's and the natural 3800's get the AC Delco's. Thanks T-Keith.

 

[SubZero350]

i think the reasoning for the platinum "blow out" has to do with cylinder pressure. my 87 gta has 10.7:1 approx compression and I blew the platinum tips right off all 8 of my new AC Delco Platinum plugs. Turbo guys can't use them either. perhaps it has to do with cylinder temp also.