The basic idea behind jet propulsion is to take a mass of air (weight of air) and “force” it in a direction causing and opposite and equal force in the form of thrust. The amount of thrust depends on the air mass flow rate and how fast the air mass’s velocity is accelerated within the engine. A turbojet engine takes a relatively small air mass and gives it a large acceleration to create thrust. This is inversely proportional to a propeller based propulsion system which takes a large air mass and gives it a small acceleration to produce thrust.
Fortunately for the turbojet engine increases in thrust can be achieved by using a exhaust nozzle to further accelerate the exhaust gasses. This is done by converging the turbine exhaust gasses into a focused “jet” hence the term “jet propulsion”. The “tighter” the nozzle, the faster the gasses have to travel through it to maintain the same mass flow. This of course requires more exhaust pressure from the turbojet engine, increasing it’s workload.
To design a jet pipe for the GR-7 I would need to know what diameter to make the jet pipe nozzle. On my GR-1 engine, the nozzle was sized to 70% of the turbine exducer’s diameter (at the wheel). This worked out great for the GR-1 but may not work for the GR-7. Different turbos have different flow rates at different pressures so it may not be as easy as a 30% reduction in the turbine exit profile. If the nozzle is too narrow it will not allow the engine to “breath” and cause surging, overheating and starting problems.
The turbine volute (scroll) inlet area seems to be an acceptable starting point for sizing a turbocharger jet pipe nozzle. On my VT-50, the turbine volute inlet (the 4-bolt inlet flange on the turbo) has about a 6.25 square inch area profile. This is equal to a round duct diameter of 2.82”. This value, in theory is a safe diameter to start with for the GR-7 jet pipe.
I wanted to build the new jet pipe out of 304 stainless steel. This would help the jet pipe resist corrosion and look pretty cool as well :0) Earlier I had purchased a v-band clamp and flange fitting to mate with the VT-50’s turbine exducer. The mild steel flange fitting that came with the stainless clamp would be a less than desirable choice to weld to a new stainless jet pipe so off to the internet to find a stainless steel flange fitting
After some searching, I located a local business called Burns Stainless that specializes in stainless steel and Inconel performance exhaust parts. Looking through their catalog I found a 5” v-band flange that is made of 304 stainless so I purchased the fitting to see if it would work on my VT-50. Once the part arrived I discovered it was not exactly what I was looking for. Seems that the new v-band flange was designed to fit a proprietarily designed flange system and would not fit the VT-50 turbo as is.
The close-up below shows a doubled straight flange that will not fit to the angled v-band flair that is on the Cummins VT-50 turbo. Notice the angle of the mild steel flange (lower photo) compared to the stainless flange.