The Stovebolt Site Forums The Shop Area Engine Shop Propane conversion

I am not sure if this is the correct place to ask this, but here goes. Due to the high price of gas, I intend to convert one of my pickups to propane. To get the most out of propane, I believe you must raise the CR. My question is, how much ratio should I be shooting for? The engine will be a 10.25:1 396. I don't intend on spending a lot of money on doing this, surfacing the head will be it, no new pistons. I am thinking 12:1 would not be too high, but I may be missing something. I also doubt if I can get that high by simply surfacing the head, anyone have any ideas what I can gain?

I don't think you need to do anything to the engine to run it on propane or natural gas. I converted a car in 1990 to run on natural gas. It was very successful. There was a slight loss of power because the natural gas doesn't produce the same BTU's as gasoline so I was told. The following is something I found on the internet which says basically the same thing. I realise you have said you want to get the 'most' out of the propane but I think it's not worth tearing down your engine to modify it.
Cheers George

OK, so what's involved in making an engine run on propane?

Basically, instead of delivering vaporized gasoline as the engine sucks in the air/fuel mixture during the "intake" cycle, the engine delivers propane/air mixture. The engine itself is not modified in any way. It still sucks in the air/fuel mixture, compresses it, ignites it and converts the resulting explosion (internal combustion) into rotary energy during the power stroke. Thus delivering work energy to the item which requires power (to rotate a generator armature for example).

The waste products are still expelled in the exhaust stroke, and the process continues, over and over. It's like using little controlled explosions to rotate the output shaft. Suck it in, compress it until it's explosive, blow it up, and use the explosive energy to rotate the output shaft. That's the way an engine works!

OK, enough said... It's really that basic. A propane conversion just swaps one kind of fuel for another, and the other principals are the same.

Propane is 110 octane. You can use any compression ratio up to 12.5:1 if you choose, but realistically, about 11:1 works best. Higher ratios just work the starter to death, and don't really gain much more performance. A couple of modificaztions are necessary to make things work right. One is installing hardened valve seats, at least on the exhaust valves. If the engine was not set up for unleaded fuel originally, it probably won't have hard seats to begin with. While the engine is down for the compression ratio change would be a good time to go with stainless steel valves and hardened valve seats. No, you can't get that much compression increase by milling the head. A good set of high-compression pistons will be necessary.

You will also need a heat exchanger to vaporize the Propane and feed it to the carburetor. Propane vapor displaces a certain amount of the air the engine would breathe otherwise, so if no other modifications wre done, such as the compression increase, just converting an engine to propane results in a 10% power loss due to impaired breathing. Raising the compression regains this loss, and then some. The downside to raising the compression is that even premium gasoline will detonate if you have to use it in an emergency, like running out of propane a long way from a refueling facility.
Jerry

Hot-rod is correct. I used to maintain a fleet of propane fueled trucks. It would pound the seats out in 2k miles. If your enginewas built before 1971 you will need to change seats. Its to bad I threw away several changeover sets.

Thanks for the replys. The heads will cost $325 to work on propane, including the $48 charge for surfacing. I know this will cost a little but I think I can pay for it in 5-7000 miles. Fortunately I have a tank, regulator, 4 barrel carb I can use. Only other thing will be a few fuel lines, but of course there are a few small things a guy never thinks of, as well as labor if you want to figure paying yourself. I think if 12.5:1 will work, I will just tell the machinist to remove as much as he can and hopefully I can get somewhere near 11:1. One other thing, I don't believe the CR is actually as high on most engines as advertised. I will measure the chambers and calculate actual compression just for curiosity.

I used to install the propane kits on new Schwans trucks, we were directed to up the ign. timing to around 22 degrees at idle......... easier than upping compression!
lots-O-trucks ran fine like this

While we are on the subject of propane conversions, anyone know of a source for the conversion kits? I'm thinking of converting my old C60 to propane. Thanks,Mike

Our local LPgas co. sells them, maybe yers does too?

I'm thinking of reviving my '67 K/10 Suburban to run on propane. It currently has a 454 that probably needs rebuilding. I'm also thinking of building up a 292 L6 to swap in, instead and maybe turbocharging the 292. I want dual fuel capability (gasoline 91-89 oct/ propane) and if possible, switch-on-the -fly capability. Is this all possible?

That's interesting. The power reduction is only 10%? I thought it was more, like almost a 50% reduction? Something I read once.

If it were only a 10% power reduction, I could see converting my 11.25:1 427 Corvette to propane someday...if race gas becomes prohibitively expensive.

And if you kept a fuel tank onboard just in case there were no propane stations and you were stuck, all you'd have to do is put some race gas in with pump gas, in your gas tank and switch over.

Or is there more to it than that? For example, if you run methanol, you have to run much bigger fuel lines because of the reduction in BTUs. I am almost certain propane would have to have a much different transport into the engine, so switching between gas and propane would not be possible on the fly. Not without other changes on the side of the road. Right?

Hot Rod Dad!

There are conversions available that allow switching 'on the fly' from gasoline to propane and back.

I ran a Crown Victoria on CNG/gasoline (compressed natural gas) and the switch back and forth was almost seamless. Had about 80,000 miles on that car before moving to an area where the CNG wasn't available. I commuted to work nearly everyday and the cost of natural gas was way less than gasoline. At the time, the trip on gasoline cost me just over $18. On the CNG it was under $8! They had a device available called a VRA (vehicle refuelling appliance) that could be hooked up to your house line and then pump up the tanks in the car. Took a long time to fill them up to 3000lb pressure. The cost of running the appliance was akin to operating a clothes dryer for a few hours at a time.
Although the economies of operating on CNG were great there was the drawback of weight. The tanks were big and heavy. They were certified at 6000lbs pressure but you could only fill to 3000lbs. I had to put cargo springs on the car and of course the brakes took a beating because of the extra weight.

Cheers, George

Hmmm. Extra weight in a Corvette might not be a good idea. I might be better off converting it to the new all-aluminum 427 small block LS-6 motor that GM is making for the new Corvette. Still uses gasoline, but makes a gallon go about 3 times farther, without much of a decrease (if any) in power.

On a truck, I could see the merits of such a system. Might be something to think about in my Stovebolt someday.
Did you feel a noticeable drop in power when running CNG? Or was that too, almost seemless?

Yes! There was a drop in power that was somewhat noticeable. For instance, I always drove in 3rd rather than O'drive around town. Passing on the highway was a bit different than on gasoline. Switiching from gasoline to CNG advanced the timing quite a bit too.
The system was OK for a full size car like a Caprice, or Crown Vic, or any trucks. Today's tanks are made from composite material rather than thick steel I believe.
Propane is a different story in that it is not such a heavy system. Not as safe though.

I am not sure there is a power loss if this conversion is done correctly. Years ago when tractors were available with gasoline or propane the propane burners produced slightly MORE power. I am not positive that in every single case this occured, but as a rule propane had more power than gasoline. The thing is it must be done properly. On the tractors, the manufacturers always upped the compression, and I am sure the manufacturers got the timing figured out for maximum efficiency and power as well.

I will guarantee you that if you use a dual fuel carburetor there will be a power loss. The only way to do it right is to use a carburetor designed for LP.

I am convinced that if you convert an engine with low compression ratio to run on today's gasoline, along with running a dual fuel carburetor you are not going to be happy with the results either from a power or efficiency standpoint. In the past when I have converted an engine, I have not been happy with the results but it was always with a low compression engine.

Propane conversions on tractors and farm trucks were very common in the Texas Panhandle in the late 50s. The manufacturers even got in on the act.

Of course it was butane then. LPG changed to propane later.

Clean burning, most modern engines are fine with the valves, and the ideas about higher compression or advanced timing are correct, too.

The actual propane carb is dead simple, just a rubber diaphram and throttle valve. On dual fuel systems, the diaphram carb was placed on top of the gasoline carb, and the gas butterflies regulated flow.

These systems would switch between gasoline and LPG without a burble. Some would even switch to gasoline to shut down and restart in the morning, as the LPG vaporizer worked best after the coolant water warmed up.

I have no idea how such a system would work on any kind of (gasoline) fuel injection or throttle body.

IF you go dual-fuel: Make sure and switch back and forth often. When the gasoline system is rarely used, the gas evaporates, leaves residue, and the gaskets shrink. None of the above is a good thing.