Yes it does add boost, my setup added about 2PSI. I could hit 13-14 psi without LPG, and would peg the 15 PSI boost gauge with LPG:
Here are the actual dyno numbers:
First pull, no LPG
HP 183.15 Torque 325.45
2nd and 3rd pull with LPG
HP 205.35 Torque 362.64
HP 205.51 Torque 361.55
So 22.96 increase in HP
and 37.19 increase in Torque.
I did blow a head gasket but I knew that it was going for about a year as I was loosing coolant but still didn't take it easy on the truck. No idea how many miles the motor had on it, I was running LPG for 4 years on this motor.
I have had this LPG setup on 2 different trucks with 3 different motor combos.
First
1987 F250 6.9 NA
Second
Big Ed with 7.3 Banks Turbo Diesel
3rd
Big Ed with the Cummins
I have only dynoed the 7.3 Banks Turbo so far.
This is a single stage setup that is either ON or OFF, no boost controller to add more propane at higher Boost or RPM's. It kicks on at about 8 PSI and 3/4 throttle. I have it set pretty conservatively. I would like to add a second stage to it someday that kicks in more propane at about 25 PSI but I really don't
need it. The Cummis does like the stuff!