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?

The guy across from, me had a 1970 dodge truck slant six with a large tank in the bed he drove forever
It was done just like the lift truck we had at work

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 thought I would follow up on some of the earlier posts to this topic.


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?


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.


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.

My '73 Dart with it's 80s 318 runs on propane.
If one simply takes the time to properly tune the engine, its performance doesn't noticably go down that much.

How is timing changed on propane vs gas? I have my plow truck that starts great when its cold but wont turn over when the engine is hot on propane....could that be a timing issue?

Sounds more like an electrical issue if it doesn't 'turn'.
If it cranks but doesn't start, then it's a mixture problem, mostly because of heatsoak.

Propane ignition timing wants a little more low rpm advance than gasoline, a quicker curve and max. mechanical advance at around 2800rpm with 28-32° max. depending on engine-compression ratio.

That would be a electrical issue, imo battery cables are the problem since it was used in snow.

I have been around LP fueled vehicles since 1983, with my first exposure was a 1980 F350 with 351 gas motor on LP. It could also run on gasoline and used a old 300 mixer like they all did then like said above.
Then we had chevy caprise on LP and tanks in the trunk with rubber spring helpers in the rear springs. Then a chys 5th ave and chevy powered bobtails that had 427s in them that ran on LP gas. They ran straight off the 2600 gallon LP bulk tank it carried. tALK ABOUT NEVER RUNNING OUT OF GAS.
I did a forklift conversion years ago myself.

Yup years ago you did have to tune for one or the other but not anymore because it is done with a computer now.

I currently have 3 vehicles on LP gas and 2 are bi fuel systems and run the same on gas or LP gas. They are a little louder on LP gas but runs about the same.

I run ford V 10s because for 312 bucks I can get optional hardened valves and valve seats in these V10s.

Try to order that in new chevy or ram. Impossible to get from ram and chevy wants 12000 bucks for there CNG option which includes much more in a package for CNG. So if you want to run LP today ford is the way to go.


That 1980 f350 was used for 10 years straight and racked up 210000 miles during that time always on LP gas. No problems with valves and I sold the truck to a local guy and I saw it on the road for years later after that. LP fueled engine was a selling point, meant low carbon engine.

Currently I have a state of the art ICOM system and a XSI system on ford V10s, 2012 and 2014. They both run the same whether there on LP or gasoline.

The systems are both big bucks and not worth it when gas is 2.50 a gallon, but when it gets to 4 bucks it is a bargain.

But hey I am keeping the air cleaner and the trees greener!!... I think?

To the OP, if you got the parts then heck yes I would do it. It is not hard to do on a older motor and can be done cheap.
My only advise would be keep it bi fuel and run it once a week on gasoline for few minutes to keep the carburetor in shape. Good luck with it and keep us posted. The cloth braided hoses we used then had reusable fittings on it that were field installed. So if have those you can modify the hose to fit your setup better for free.
Now a days they do not sell those fittings anymore because too many misused them and hurt themselves. Plus the EPA made them change the way the build hoses and the ID is not always the same anymore. Plus ever notice how the new hoses want to coil and the old ones did not, Thank the EPA.

i just parted out a 1996 dodge 2500 that had factory propane---computer reflashed with dodge stickers--dual fuel----found the truck was ordered by texas parks dept probably as some type of government mandate-------the propane parts went to a friend that has a twin engine 1975 ford truck---he has twin big blocks with blowers on them----the rear engine sits backwards in the bed and drives the front axle--the front engine drives the rear axle---he can run either the front engine or rear or both---the hot rod guys around here all perk up when duane drives the truck to coffee....