I thought I would follow up on some of the earlier posts to this topic.
I had one of my old car hauler one ton trucks converted back in the mid 1970s when the first gas crisis was occurring for the increased fuel storage, 85 gallon tank. It had a 318 industrial poly motor in it and burnt the valves up real quickly. Major expense on installing harden seat and new valves that truck got 8.5 MPG on gas and 7.5 MPG on propane.
I later bought a M1500 I.H chassis Class A 26 ft. motorhome that was set up on propane only, it sucked on POWER and MILEAGE. I removed the system, converted it back to gasoline and never looked back and I never regretted it.
If you got 8.5 mpg on gasoline and 7.5 mpg on propane, you were doing very well. That meant that your propane system was giving you about 88% of your gasoline fuel economy, which is way better than the 73% predicted by their energy contents. As for the burned valves, are you positive that you wouldn't have had similar issues on gasoline in a heavily-loaded car hauler? The Poly 318 would not have been designed for unleaded fuel so it may not have been equipped with hardened valve seats.
As for poor performance in your motorhome, propane responds to the same performance mods that gasoline does. Are you sure that your fuel mixture and timing were properly set up?
You are spot on Cab. I worked for a shop doing propane/Cng conversions for the county short busses. Lots and lots of valve jobs were done on those rigs. I chalked it up to low lubricty of the fuel/ intake charge. But what do I know?
Is there a chance that those were GM buses in your shop? Prior to 2007, GM 6.0/5.3/4.8L engines had intake seats that were softer than the intake valve material. They fixed this from 2007 forward production. See Flash Lube Electronic Valve Saver Kit
In the 1992 CNG supplement to the 1992 Ram Van Field Service Manual, it states that internal changes to the CNG version of the Magnum 5.2 V8 engine were Chrome Plated Exhaust Valve and Hardened Valve seat insert.
I suspect that those CNG Ram Vans were expected to be in severe fleet service so exhaust valve and seat upgrades would be a good way to minimize warranty issues. These are good upgrades for any engine.
Burned valves often mean that the fuel mixture is still burning when the exhaust valve opens so it's important to get the fuel mixture and timing right. See Combustion 101
. For light-duty service that most cars and pickup trucks experience as basic transportation, the OEM induction-hardened valve seats that work fine on gasoline are generally sufficient for propane.