So what's the criteria for boxing frame rails? Is it a good idea with the original suspension? I was under the impression that the original design of the chassis utilized the flexible frame as an integral component of the suspension. It makes sense to want to stiffen up the frame with an IFS install, but I'm not sure what ramifications there would be with a straight axle and leaf springs.

 

By George I think you've got it! Boxing a frame with the original straight axle would result in a rougher ride since you would lose some flexing of the frame. Boxing should be done only when adding an IFS or IRS because then the suspension does all the flexing...

 

Or when adding a high torque motor that will twist the frame when it revs up.

 

And that would be why I'm asking. I'm still trying to decide which, but my truck will either get a 351W or a 347. In either case, it will be cammed and built for maximum fuel economy and torque at low RPM. Think truck motor and not your average street rod. My truck will be expected to work for a living and will be subject to the same level of abuse as your average pickup truck with a 250-300 HP engine. I think it was Julie that recently posted that engine torque was tearing up her right spring and frame rail. I just want to avoid that. I guess that maybe the question that I'm really asking is roughly what is the threshhold at which engine torque can start turning the frame into a pretzel?

 

Well, it's not so much a matter of engine twisting the frame as much as it is a matter of the flexibility allowing the torque to beat the living day lights out of everything, and the interplay between all th efront end components - frame, motor mount system, cross members, springs, wheel size, etc. And remember, your engine doesn't know it's getting resistance (and subsequently translating the torquing moment to the frame via the motor mounts) because you've just floored it coming off a line, or if you are carrying a load and the engine torques. Moments are moments.

The major thing that caught my eye on the spring thing and torquing was this....I was in my driveway pointing downhill (I have a loop driveway and had pulled around th eloop to back into the garage - not dramatic drop but definately downhill) with about 500 pounds of concrete bags in the back. When I shifted the (automatic) transmission in reverse and hit the gas to back up the driveway, the entire front of my truck twisted from lower right to upper left, noticeably, about 20 degrees - like someone was holding the edge of a playing card and twisting it. The whole front of the truck.

Boxing is relatively inexpensive. It will help preserve the frame from high torque engines, but more, it will distribute the torquing momnets over a larger area.

A 351W is no wimpy little "pony motor" and should be respected when it comes to what it can do.

Honestly, if I were you I would box my frame from the front spring hanger all the way back to the firewall. Then integrate a good motor mount system into the boxing with a stiffening cross member.

Folks always say "well boxing will stiffen up the frame and make the ride rougher!"

Question is, have you ever driven one of these trucks with stock suspension? It's not going to hurt it - this ain't your moma's BMW. And honestly, the "solid" feeling you get as opposed to seeing your front end in "fluid" motion can elliminate a little nerve wracking, especially when you've had a motor mount fail (bad design and workmanship by PO). This is especially true when in long right turns (like freeway on ramps) and stepping on the gas to excellerate.

If the engine is out - just do it!