True duals or H pipe?
 

My 8.1 truck has true duals, no cats using 2.5 inch pipe into flowmaster mufflers. I use it for towing a small lawn trailer. Would this truck benefit from an H pipe?

I don't think it would make one bit of difference.

I've read that 8.1's do benift from a H pipe.

The biggest problem with late model trucks and an H or X pipe is that it is nearly impossible to get the correct location for proper tuning. The engine produces exhaust pulses, and they're spaced according to the firing order. The problem is the exhaust has to run down the passenger side of the truck. So, the driver's side tube is longer than the passenger side, because it has to cross over to the passenger side.

Here's the example for my truck, for this exact problem. My 8.1 calls for a total primary length on the header of around 42", with a collector length of 22" (I can run an 11" collector but it will raise my RPM band). And the H pipe needs to be at the end of the collectors. Or, alternatively, you can put an X pipe at 6' behind the collectors. I can tell you that there is no way to put an H pipe at that location in my truck, it will hit the Allison.

I have not yet figured out how to run an H pipe or X pipe on my truck, I'm working on it. I'm not sure I can get one in the correct location at all.

Now, that is the correct tuned lengths for MY modified 8.1, so unless you have the same camshaft, and the same intake I have, and tuned headers, those dimensions are not correct for your truck.

However, what is important is the tuning lengths. For example, if I were to put an H pipe one foot back from the collectors, that is the worst possible location for MY truck, and would hurt torque and HP, not help it.

You CAN help power and efficiency with an H pipe or an X pipe, but it has to be in the right location. Not having one at all is better than having one in the wrong location.