I have a 74' F-350 2wd with a 460 that currently runs on straight propane and has just over 100k miles. I know that running on propane is cleaner than gas so the overall shape of the engine should be in better shape for quite some time but there are a few things I am curious about as to what I need to expect with using propane.
What should I expect for HP and MPG compared to gas?
Can parts be upgraded and the same performance boosts be expected with propane? (I.E.- Carb, Manifold, Cold air intake, ect.)
Are there any parts that I should expect to wear or fail sooner while on propane?
Thanks!

 

You will need to do occasional maintainence on the convertor/vaporiser, change the fuel filter etc. As for performance gains free flow exhaust system will help, If your distributor and timing is not set up for straight LP doing so will help alot. Since Lp is already a vapor intake manifolds dont help as much unless you have a really restrictive one. If its set up right you will get pretty close to gasoline mpg and most time have a little more power.

 

I used to do a lot of propane conversions and built a few propane specific engines and I'm going to agree with part of Jim's post and disagree with part. A free flowing exhaust system is certainly a benefit as it is on any engine regardless of fuel. As far as intake manifolds, the fact that its gaseous will allow you to get away with "too much" intake manifold-such as a tunnel ram- and not have the fuel separation/wetting out issues you'd otherwise have, but does not diminish the advantage of having an intake properly matched to the cam and exhaust. The same goes for cams, you can get away with more and not have such a sloppy bottom end a gas motor would. I'm not advocating over camming or over manifolding, as its still a waste of efficiency. Just pointing out that a propane engine won't suffer the negative results to such a great degree as a gasser. Propane has a faster flame propagation rate than gasoline so it doesn't need as much timing advance. About 32 degrees total timing is all you need. Much more than that will only generate heat and cost power because peak cylinder pressure will be occurring too soon. It should peak about 20 degrees after TDC. The interesting thing with LPG is that is has a high octane rating which makes it quite resistant to detonation/pinging. A gas engine will tell you when the spark is too far advanced by pinging. Not necessarily so with a propane motor, you will just lose power. Also on the subject of octane, if you're ever building a propane specific engine, they love compression. I suggest at least 10:1 -- even 11 is not excessive for a street motor. Propane is a low density fuel - around 4.4 lbs./gal. as opposed to gas which is right around 6. As a result it produces less BTU/Gal. than gas and therefor you have to burn more to produce a given amount of power. Usually around 20%. Because of this lower charge density you have fuel displacing a greater percentage of the cylinder volume and therefore less air/oxygen. LPG will never produce the same power as gas, its physically impossible-- sorry. Propanes heyday was when gas was over $1.00 and LPG was around 40 cents. This was in the early 80's, and I did a lot of conversions back then. As far as engine life, propane is very clean and produces very little oil contamination and/or carbon. Exhaust emissions are greatly reduced also. About the only mechanical downer is valve life. LPG engines have high exhaust gas temps and are prone to stretching valves and sinking seats. This can be solved with hardened seat inserts and inconel valves, but requires engine teardown and a considerable infusion of cash. The more you attempt to increase the performance of the engine, the sooner you're going to have to deal with the valves.

 

P Bronner well said, I just didnt go into as much detail as you. I have also found that engines designed to run on unleaded gas dont have as much valve issues

 

Great stuff Bronner, thanks!
I'm pretty sure that this truck was swapped over in the heyday of the early 80's and the people I have talked to said it was for the cost like you mentioned.
An upgraded exhaust system is definitely on my to do list, I just didn't add it to the first post. Any suggestions on a good brand to go with? I'd like to replace the headers and run true dual exhaust on it.
I don't plan on trying to really make this engine high powered, just running good with a few extras for a little boost. My dad did say that when he would use the truck, especially when loaded heavy, the engine would put off very high temperatures which I would guess is related to what you were saying about high exhaust temps. How often would those seals need to be replaced and is there anything else that would cause higher temps while running the engine and is there anything that could fail or malfunction because of that?
Again, thanks for the info, I've never dealt with a propane engine before and anything I can find out is a big help!