Propane:  Page 2


How to Convert Your Engine to Propane

There are 4 major components to a basic propane fuel delivery system.  They are listed below in order of fuel travel.  Fuel starts out in the propane tanks and ends up in the engine.  Click on the pics for larger versions.

Propane Tanks This is where the LPG is stored under high pressure.  When I say high, I mean HIGH.  You want these things stored where there is NO CHANCE of them being punctured.  Even though they are tough cookies by design, who wants to deal with pressurized flaming bombs in their 4wd?  By using two forklift bottles, I can use one up, close it off at the tank, open the other one, and get someone to take the empty to get refilled if I have to.  It also serves as an early warning system ... Warning, you are now down to half fuel capacity.  Bottles are one way to go, or you can get a regular tank which will normally have a larger capacity.  There are legality issues to running propane in a vehicle though.  You should read here before you choose which tank to go with.
Lockout Valve:  Lockout valves come in many different forms.  Pressurized LPG goes straight from the tank to the lockout valve through a high pressure line.  Their function is very simple ... SAFETY.  The one seen in this picture is a vacuum actuated lockout valve.  It uses a diaphragm attached to a valve and a spring with atmospheric pressure on one side and vacuum on the other.  The source of vacuum can be from anywhere on your carb, really.  When vacuum is applied, the diaphragm is actuated opposing the spring, the valve is opened, and liquid propane is allowed through.  Without vacuum, no LPG gets through. 

There are also electric lockout valves.  They use an electric solenoid to open a valve which then lets LPG through.  The power source could either be switched with the ignition or manually switched.  I used an electric valve until it burnt out.  I had it manually switched as a theft deterrent.  By hiding the switch, no thief would be able to start my Jeep up unless they knew where to find the switch. 

If you know enough about the components of propane, you will know that this is not a required component.  You can run your high pressure LPG line straight to the converter and it will work great.  But if you have an older/used converter like I do, or even a new one with a slightly faulty valve in the converter, propane can leak out through your carb or converter.  Since propane is heavier than air, it will come to rest at the lowest point in the vicinity and collect there.  Then all it takes is a small spark or flame to ignite an invisible bomb!  So for $30 you can avoid that problem altogether.  Doesn't that seem worth it???

Converter:  The converter has a few different names that people call it.  Converter, vaporizer, and regulator are three that come to mind.  Most people don't realize that this device actually does ALL of these tasks.  As high pressure LPG enters through the top, it is heated by the coolant lines that you see entering through both sides of the top.  By being heated, it expands and is converted into a gas, or a vapor.  The gas then passes through the device into a chamber where it is regulated down to about 4psi.  (About what your average grill uses.)  The low pressure gas then sits, waiting for vacuum to suck open another diaphragm.  The more vacuum (throttle) that is applied, the wider this diaphragm opens, and the more gas is let through.  At this point, the gaseous propane is sucked into the carb/mixer.

You are looking at an IMPCO Model E converter.  You can see how all the bolts that hold it together are flat head screws.  Everything inside the converter is also flathead, making only one tool necessary for a complete rebuild.  Cost of rebuild kit:  ~$40!  The Model E is supposedly good up to ~325 hp, but people have reported using it all the way up to 400 hp.  This particular model features an electric choke, seen on the top of the converter.  The small tube coming out of the converter to the right of the two wires would be a small hose going to the base of the carb.  If your engine needs to warm up to idle nice and perfect, you could hook this up and flip a switch to apply power.  A small, unregulated source of propane gas would then flow to the engine, keeping the idle clean during a cold start.

The second photo is from the lower rear side of the mounted converter.  The large hose coming out of the bottom is the main fuel line going to the carb/mixer.  It is also the source of vacuum that actuates the large diaphragm inside the converter.

Lockout & Converter:  Here you see a pic of my lockout valve connected directly to the top of the converter / regulator / vaporizer.  I had the space to do it this way and the double-threaded pipe fitting to do it with, so it made it easy.  A HIGH PRESSURE line could just as easily connect them if you needed them to be in separate areas of the vehicle.


Another thing worth noting in the pic is the coolant lines running through the converter.  It doesn't really matter which one is in and out so long as you have them both.  They are important for two reasons.  One is that, as LPG expands into gas form, it absorbs heat from its immediate surroundings.  If the vehicle were run without the lines heating the converter up, it would eventually freeze into a solid chunk of dried ice.  The vehicle will run for a little while, just not very long.  The second reason is that, in the process of removing heat, it cools down your coolant.  It's like having an extra little radiator!  And if you're like me, running a big V8 in a smaller vehicle with not much space for a radiator, any added cooling is always welcome.

Carburetor:  Last, and certainly not least, comes the mixer/carb.  What you see here is an IMPCO model 425 carb on top of a Holley Spreadbore throttle body.  The model 425 is rated at about 450 CFM and is the highest CFM offered by IMPCO.  Although there are higher CFM carbs made by companies like Technocarb, the 425 is an industry standard and designed specifically for my application, so it's hard to beat.  If I desired a higher CFM I could use dual carbs.  If you're using a manifold that isn't made to fit the Spreadbore base plate, you will need an adapter.  I was using a stock small block Chevy cast iron q-jet manifold and did need an adapter.  I had to slightly modify my throttle cable bracket to work with the adapter.  Since the Model 425 is made to be a direct bolt-in replacement for Chevy 350's I was able to retain my stock throttle cable and kick-down cable.  Overall, installation of the carb is very straight forward and easy.  To find out how exactly a Model 425 works, click here.

Throttle linkage:  Here you see a few side pics of the carb's throttle linkage.  You can see how my stock throttle cable bolted right up.  My kick down cable was about 1cm too short, so I did what any good red-neck would do and bent a piece of coat hanger in a loop to get my extra length and to attach it to the linkage.  In case you're wondering, it works fine.  If you like manual trannies, this won't be a problem for you at all.

Fuel Flow Diagram

This is how your propane setup should look when it is done.  (Or a variation of it.)  There are actually computer-aided metering setups that can be had for reasonable prices to increase mileage, but part of the reason I was using propane was to get rid of computers, so that was worthless to me.  Obviously, since I did not use the computer, it's not shown.



Lockout Valve





Large low-pressure line




On to Final Impressions