Post #36 by motzingg » Thu Jan 02, 2014 12:13 pm
ah the m5 debate rages on! of course!
the one i pulled last weekend looks good, not a drop leaked and bright red fluid (doubt it was ever changed) with 121k on a truck used for heavy hauling... and i'm going to fully abuse the crap out of it, so i'll keep you posted. Even in the yard, the only time i could find them was in wrecked trucks, only one out of about 5 that i saw had the leaking plug. Most were clean and slightly greasy from leaking pushrod cover gaskets. one was in a thoroughly destroyed work truck. looked like it had been t-boned. I wish i had taken pictures.
Obviously, from an anecdotal standpoint, there are going to be failures of any mechanical component along a probability distribution of lifespan. You're a lot more likely to hear the 3% of 'early' failures before 150k miles (that is still a pretty decent run for a transmission) than the 97% that fall in the median, because the guys whose trans are still running don't have anything to talk about.
Maybe it is a weak trans, maybe they had bad QC or maybe the oil leakage problem is so severe on some, that oil gets out before the owner realizes and it gets run dry. Because ATF has much thinner film than gear oil, running dry or low for even a second could roach the journal bearings (also a weaker design point than the roller-heavy ZF), synchros, and roller bearings, whereas you can run a diff for 10,000 miles bone dry and not wreck it. I wouldn't even think of touching one that showed signs of leakage or running low on oil, maybe yours had already been cooked at 40k.
If you go on other ford truck forums people trash the reliability of the 300-6 because their dad had a fuel pump fail and dump gas into the crankcase in 1976 and kill the motor.... so its obviously a piece of crap! Ask me about Subaru 'EJ' motors sometime if you want to get controversial!! Or better yet, get 3 gearheads talking about 4.6 'mod' motors and you'll get 20 opinions (despite being ranked as a '10 best' engine for many years). Either way, ford wouldn't have put it behind a 200 ft-lb engine without a fair bit of testing. If their testing didn't simulate the aging and deterioration of the plug, or if they didn't realize it was being overtightened or had a mfg defect leading to a bad fit, some kind of molding flash, etc. that seems to me like a much more plausible situation.
but to the debate of fuel mileage, you'll be hard pressed to beat an M5
the thinner ATF will reduce frictional losses
the design is well suited for an efficient overdrive, with smaller lighter gears
its manual, duh, so there is no slip
it has narrow ratios so you can stay in the sweet spot of the power band longer
and its light weight, which, in the context of a flying brick weighing 3500 lbs might not matter much, but if you're really going for mileage, every little bit helps