Bronco Forum - Full Size Ford Bronco Forum banner

6 Liter Tune Up Loss of MPG

1564 Views 35 Replies 13 Participants Last post by  mustangGT90210
Howdy y’all,

I just did the 6 liter tune up on my 96 351. I did not do the timing advance yet however. I gapped the new plugs to .54 and I am noticing a loss of about 2mpg from what I was getting before and not much of a noticeable increase in performance. Idle is fine, engine is running fine, just worse fuel eco. Any thoughts?
1 - 2 of 36 Posts
Also would skipping be the reason for my MPG dropping? I feel like I would notice it in how the engine runs, but it seems like it runs fine. Should I just go back to stock gap or maybe just decrease it to .05-.052?
If you're not going to bump the timing, return the gap to the factory spec. By running a larger gap, and not increasing anything else, all you're doing is mimic-ing a motor with worn out spark plugs.
  • Like
Reactions: 4
Like user Jermil said, the 6 liter may have only been designed for the 302. Is there any benefit for doing it in a 351?
I did the 6Liter on my '86 351 Bronco, and it's night and day better. Starts up better, and a little more power when cruising. The initial throttle tip-in response is far more reactive, she's a peppy truck at 13 degrees BTDC. I will absolutely do it to any other one of these old trucks I buy. It did help that when I bought mine, it already had the MSD coil and 6AL ignition box.
Never used an MSD but I've heard they arent that reliable. the factory system seems pretty good. I just keep a spare TFi module just in case, but I've never had to use it to date
Knock on wood, the MSD stuff in my Bronco has been flawless. I've had the truck for 3 years, put 8k on it, no concerns at all. The MSD coil and 6AL box were on it when I bought it, and they've been on it for years. Perks of buying it from someone you know, you get a little history.
1 - 2 of 36 Posts