We've all heard that running straight pipes will burn valves but does anybody have actual proof? And if there's someone out there who can explain back pressure and scavenge to me that'd great too.

 

Welcome to FTE.

Open exhaust burning valves....Not really. If you run shorty stacks right out of the head, you may suck some air back in at certain engine speeds that could cause valve problems, and cold air in there after shut down might cause an issue, but I doubt it. The major issue would be debris down the pipe. Straight pipes that reach the rear axle or the sides of the vehicle, or even open headers, I doubt you would have any problems at all.

Back pressure: An FE car motor would be a good example of manifolds that would have enough pressure built up that exhaust gas could re-enter the engine while the exhaust valve is still open. This is a power killer as it would dilute the charge in any cylinder, and would cause the engine to have work harder to pump out exhaust.

Scavenging: Making the exhaust gas travel in such a way as to create reduced pressure behind it, which will bring more gas along and help draw air/fuel into the cylinder.

Search and find more like this:

http://en.wikipedia.org/wiki/Scavenging_(automotive)

 

welcome to FTE. can it happen? yes, if left open exhaust long enough.
example: years ago on the farm we has an old surplus military irrigation pump that had a chrysler 331 CID industrial hemi engine in it.
it had a cracked manifold rite on top of #3 cylinder.
after 30 years of almost daily use, it finally burned the #3 exhaust valve.
i really doubt it was from the cold air coming in after shutdown, i think it was from the lean condition from running it out of gas for 30 years to shut it off.

 

I'm thinking about doing true dual straight pipes on my truck and I plan on having side pipes that end before my rear tires. So it's be about 6 ish feet of piping should it be ok for a daily driver? (I'm not too concerned about performance. It's slow anyways. I just want it to be loud)

 

Any real danger of burnt exhaust valves comes from running open exhaust right at the head, like without a manifold or header.

Open pipes, never. (ok, I should never say "never)

If you want loud, you're gonna get loud with open pipes

 

I think the closest you get to that is when people change the exhaust system and neglect to fatten up the fuel mixture. This will probably not be a problem if you plan on using the factory "log" manifolds.