Whats the best plug to be running to help the spark knocking in the AC ?
Whats the best octain to be running?
Should i be using additives???????????

I have an 05 400AC Manual seems to be spark knocking up hills or something ratteling in the top half.
It only has about 100 mi on and just changed the oil to the synthitic. And it seems to do it worse with 87 grade gas.

 

First thing I'd do, is talk to your dealer. Might need to have the timing looked at.

 

Timing can be checked but is not adjustable. Have dealer check valve clearance, sparkplug should be CR6E, I think Gap is .025 not positive though. Should run fine on 87 octane, make sure fuel is fresh and use fuel stabilizer year round.

 

There shouldn't be any knock if everything is working right. Get it looked at soonest.