408+ and a rear mount......... Zero back pressure Issues...

 

Are there any durability issues with the 408 vs 370? If the only issue is buying a forged crank vs using a stock cast unit, I'd seriously consider the longer stroke. You're talking some pretty stout boost #'s. A forged crank certainly wouldn't be overkill. As long as you're upgrading the crank, might as well put a little longer arm in it. More cubies never hurt anyone. Yes, there's more stress at a given rpm, but you don't have to run the bigger motor up so tight. I've heard of lots of 408's making some serious #'s n/a. Why wouldn't it work f/i? HP is HP......right??

 

I've gone through a super charger and am on my second front mount set up and I am not going to be changing that out at this point. It gets expensive. Im going to keep the Trick hot parts, with 3.5 dp and go from there. I do worry about the 38mm waste gate being able to keep up though if I do go to the 408.

 

In my case, I'm running 18 degrees of timing at 7 psi to avoid detonation, and we have 93 octane here. It has been awhile since I was running 9-10 psi and I don't remember how far I backed out of the timing to get rid of KR, but it was rediculous.
My point is that as a 408, my engine needs more octane than as a 6.0, with close to the same CR. I, too, was running +- 11 psi as a 6.0.
P.S. You might be better off with the AFR 225s, I am running the 205s. My detonation comes in the midrange, the 225s might be a little less efficient in that rpm, and therefore less prone to detonation. Just a thought.

 

I already have the 225cc heads. They are currently bolted to the motor.

What kind of power numbers are you making at 7# with your 408 and what compression are you at?

 

I know, that's why I said that. 9.3:1, and my excuses for not going to the track or dyno are: 1. Every time I think I have a good enough fuel system another pump takes a crap so that 2. I can't sort the tune out, 3. The exhaust is stock behind the cats, not sure if is a restriction, and 4. (Should be #1) I am lazy.
My seat of the pants dyno says that it has way more low and mid range power than before, and probably not much more top-end. That's why I am leaning toward the exhaust being a restriction.

 

Get the cubes...nuff said!

 

I had significant backpressure on my 408 with the stock manifolds.

 

408 all the way!!! I am putting out over 700 hp and averaging 650 ft/lbs of torque at just 5.5 lbs of boost with the TVS 1900. This engine will last for years running such low boost, no issues with detonation or bad fuel. I was only getting 12 mpg with my 6.0 L anyways. I am getting about the same now, of course I can't keep my foot out of it. Torque is what trucks need, go with the torque monster, the 408.

 

He's running a T6 flanged turbo too.