AirGap intake mods for Propane fuel?

[tinnitus]

Back in my college days we converted a GMC 1500 with a TBI set up to run on Natural Gas as part of a SAE sponsored competition. We actually used a TPI intake with some modified gasoline injectors. We didn't know that wasn't supposed to work but it actually worked very well. We were fortunate to have a Siemens injector manufacturing facility local to us and they supplied what eventually became the "blue top" injector which was used on the OE CNG vehicles for a short while. I think the injectors eventually were marketed as a 160lb/hr gas injector. Not even close to as simple as a mixer but it had fantastic manners and driveability.

Excellent, yes the high storage pressure of CNG would allow for adequate flow thru a basic regular type injector. Great project.

No, it needs the pressure dropping first (single stage regulator), injectors won't even open against high pressure and a varying storage pressure gives a varying flow.
Storage pressure for CNG is anything from 2MPa to 4MPa depending on cylinder type and the country you're in.

I'm trying to remember the details (This was in 1992/93 time frame). I'm pretty sure we ran a single two stage regulator and not two independent regulators with the first stage bringing the storage pressure down to 1000-1500 psi and the second stage down to 100 psi or so. We had coolant circulating through the regulator to keep it from icing. Siemens was concerned about wearing the pintel in the injector without having a liquid fuel to act as a lubricant but the wear was negligible after a few thousand miles. Not a significant durability test but some data anyway.