I made a homemade LP setup for my 06 cummins out of a 20lb propane tank, 12v shutoff solenoid, adjustable regulator, and some 3/8 fuel line to the intake just in front of the turbo. The knocking sound is because the LP wants to detonate earlier than diesel fuel. Thus making an advance in timing affect. Like a "timing knock" from the added timing. I did many tests to make sure i wasnt dumping liquid LP into the engine and no matter what i did i couldn't get any liquid LP to come out of my setup. I made this system mainly for horse power but, I did set the regulator to 1lb and left it on wile on the interstate on a 25mi trip to get food one night and my truck showed that i was getting 45mpg! This was fine until i ran out of propane half way home... so i dont think this was very economic using a full 20lb tank in a 40-50mi trip!
As for the hp, i set the regulator at 16 lbs and after i'd get about 15psi of boost I would hit the toggle and it felt like a car ran into the back of us! Boost went to 50lbs and the exhaust would go pure black. I was able to put the truck on a dyno and my $25 LP setup gained me 110hp!!! It also lost me 250ft-lbs of torque witch also leads me to belive there was too much timing but, it did help on the high end.
The one thing i would like to address is to watch your EGT's !!!! Too much causes very high exhaust temps! I did lots of pulling with LP and my exhaust would be 1600+ WAY TO HOT! In the end, i won my share of pulls and got the 3 melted pistons to prove it.
I think if you could find a good way to control the way you use it, one could pull some better mpg's out of anything you use it with! Would be interesting to see how an 855 would react. Cant help but to think it would drink the stuff to be able to actually notice anything but, would love to hear someones findings on this. The 6.2 is a high compression engine at 20:1 and the knocking effect could be amplified because of it. Let me know your findings on anything propane related, love to here everyone's experiences with it!
