Propane for Older Mopars

Does anyone have any advice on running an older Mopar on propane. My truck has a 318 with a carburetor on it. I found some older propane fuel parts off a 350 Chevy. I have not looked at the parts yet, but everything is said to be there.

Ideally, I would like to be able to use propane or gasoline. Anyone have any advice?

Get with Hank Hill, he knows everything about propane and propane accessories.

this video may help.

Also there was a factory option to have LPG in dodge vans mid to late 90s

https://www.youtube.com/watch?v=cO-1M24qFiI

If you found a complete propane system from a GM vehicle, then you should be able to install everything on your truck. Be sure to get EVERYTHING, including the control panel and cables from inside the vehicle. The tank is most expensive part of a conversion and other propane system parts are still available and not expensive. Like shown in the video, an Impco Model 300 mixer is commonly used on dual fuel conversions.

With vehicles last equipped with carburetors in the late 1980s, carburetor conversion parts have gradually become obsolete. The challenge with a dual fuel system is getting the timing optimized for running 2 fuels. Dual Curve used to make timing controllers for dual fuel engines but now timing advance processors are only made by AEB (AEB Wolf-N for noncomputerized ignition systems). You can also switch to a computerized ignition system for better timing control. See Megajolt/E.

Before converting your truck, you should make sure that you have a readily available supply of low-cost propane motor fuel (HD5). Often, propane companies offer fixed-price supply contracts and the cheapest price can be locked-in during the summer. Ideally, the price of propane should be much less than 50% of the price of gasoline to have a quick payback on the conversion.

If your GM conversion is mono-fuel, it would be very similar to the system to the one I installed on my 78 New Yorker which I later transferred to my 77 Pontiac. See Model 425 Mono-fuel Carburetion.

Propane has higher octane but less btu of energy per gallon. That means, in theory, to get the same efficiency for propane vs gasoline your propane engine needs to have a higher static compression ratio. This is why the dual-fuel type propane mixers (like the Impco 300A mixer that I have on my Jeep) are said to be able to do both, but not ideally. The old 70's and 80's 8:1 compression engines are probably some of your worst candidates for propane. I haven't tried it yet, however my propane conversion came with a timing processor that is supposed to remove around 5 degrees of total timing but add roughly 5 degrees of timing around the 2000-2500 rpm mark. They say propane needs a little less total but with it all in sooner.

Wholesale propane prices by regions and states

https://www.eia.gov/dnav/pet/pet_pri_wfr_a_EPLLPA_PWR_dpgal_w.htm

Propane has been swinging in price more because a large fraction is being exported

Is it a good plan to convert to propane in a DD mainly for a mileage bennie? (mileage VS cost of the propane/the conversion parts & building a high comp eng to take advantage of it). I will go to the listed sites & read up on it.

I had a 93 Dodge 1/2 ton, club cab long box that was 318 with dual fuel (gas & Propane) . . . gotta say, it was a DOG on propane . . . I never looked at anything, cause it did run so well, but you would have to manually take it out of overdrive in order for it to maintain speed on hills, when it was running on propane. Did not get that great mileage on propane either . . . it probably needed that timing advance to get it running better on propane. Good luck with it . . .

Propane actually has 107% of the energy of gasoline on a mass-basis (ie, BTU/lb), which is how AFR is calculated. Since it is a less dense liquid than gasoline (68%), a gallon of propane has about 73% of the energy of a gallon of gasoline.

You will never get the same mpg fuel efficiency with propane with a carbureted propane system but it's not hard to get 75-80% of your car's gasoline economy on propane. However, the cost of operation should be much lower if you buy low cost propane. In Ontario, the cheapest places to buy propane is where the taxis fill up, which is around Pearson Airport.

Since HD-5 propane has an octane number of 104, you can safely run a higher compression ratio (eg 10.5:1) without any risk of knocking. Higher compression ratios increase the fuel efficiency of ANY internal combustion engine. See Fuels Forum: Compression Guide.

Mixer-based dual fuel engines will make less power than a mono-fuel engine because of the added flow restriction of the gasoline carburetor. However, both may be tuned to provide the optimum fuel mixture.

Someone out in oregon had converted his slant six truck to propane and used the MegaJolt/E ignition system. He found that his truck ran better on propane than gasoline and needed a lot less timing than commonly thought.
See Slant Six Forum: My timing map....

Propane or gas pick one not both.

My old neighbor had a duel fuel 460 F-250 and was nothing but issues burned the valves up towing his 5th wheel on propane then had it rebuilt specifically for propane. If you try to run both fuels its never making the best of either.

Not to mention he'd run on propane for months and months then try and switch to gas and all the issues with old fuel and dried out parts in the gasoline side of the system would show up.

In 1979 my girlfriend came over with her dads 74ish 318 Dodge Dart that ran on gas or propane. I asked what some dash switches were and she explained.

The trunk was full of two large tanks. She could switch from gas to propane on the fly. Car had a huge range if all fueled up.

Her dad was a career Navy guy, it was a government vehicle. Pretty wierd I thought.

It's a lot easier to set up a mono-fuel system than a dual-fuel system and you need to make sure that you get the best advance curve for each fuel.

Burned valves often indicate excessively rich propane fuel mixtures. Gaseous fuels do not have the evaporative cooling effect that rich mixtures have with gasoline. See Fuels Forum: Combustion Temperature.

Good point about old fuel. Gasoline will become stale if not used continuously while propane never becomes stale. Modern injection conversions always start on gasoline, which helps to keep gasoline in the tank fresh.

I've seen ads for old Mopar motorhomes that said they were converted to run on propane and always wondered what the point was since the cost for the fuel is pretty much the same. But I guess the point would be that they get better mileage and the fuel won't go stale sitting around like most motorhomes do most of the time. Is that right?

OP, don't waste your money or energy on the conversion
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

Go back up the page and read all of Frank Raso's posts. He seems to have the best handle on the issue.
You do not get a mpg increase going to propane. The only reason to go to propane is that the fuel costs so little that its miles per dollar comes out ahead.

Propane prices vary a lot. My parents had propane for heating house and water. They'd stock up when the price was low.

The problem as I see it with dual fuel systems is that an engine optimized for propane is different than an engine optimized for gasoline.

One thing that you all have missed is the decrease in air volumetric efficiency while on fully vaporized propane. Gasoline engines run best when the fuel enters the combustion chamber in the form of tiny droplets. The ratio between vaporized gas and liquid gas is about 1000:1. So to get the maximum amount of air into the engine you have to make the fuel take up less space. Propane is vaporized before or in the mixer and as the air/fuel mixture reaches the cylinder the fuel takes up a much larger percentage of the total, limiting air and thus limiting power.
BTW, the term "air volumetric efficiency" is something that I just made up. I don't know if there's a real term for this.

Here's my wonder....Why do we have to vaporize it before it enters the chamber? IMHO it might prove to be better if injected into the ports directly as a liquid. As it flashes to vapor (-156F) it'd greatly drop the temperature in the intake ports and increase the air volumetric efficiency. Even just a little LPG injected into the intake manifold near the start of the runners would supercool the air, increasing power.

I seem to remember that LPG stays liquid at 125psi at room temp.

As we now have Diesel engines in trucks running fuel rail pressures of 26,000psi, and the gasoline fuel injectors on cars have proven to be nearly bulletproof, the technology is there to give it a try. Watching Gale Banks' turbocharging video last night, he mentioned that using methanol eliminated the need for an intercooler.

How about using the liquid propane in an eductor setup to drag more air into the cylinder?


I think that we're ignoring the best uses for propane because it's easiest to just use what has been available for the last 50 years.

R.

Well, I goofed again. I did a search after I had posted rather than before. Google "liquid propane injection" and you get thousands of hits. Oh, well. Here's a great example:
http://www.eng-tips.com/viewthread.cfm?qid=129931

Liquid propane injection systems like you envision are on the market and available. However the cost of the injectors is so high that most guys looking into propane go back to the old standby Impco mixers after the sticker shock of the liquid injection systems. But yes, you're right in that the gaseous propane fuel is a restriction, in theory, however I have never seen any documentation to find out if this is an actual real-world factor. You can also get gaseous propane injectors that inject the propane gas right at the cylinder ports. Sort of a compromise between the old mixer and the liquid system, with the drawbacks of both. I have read successful stories of blower and turbo guys using propane, I would guess the boost would more than make up for the potential air volume lost by the gaseous propane. For a n/a engine, either a purpose built propane engine where peak hp isn't the goal but cost per mile is, a mixer could work well. Another thought I had is a n/a engine, high compression that runs race gasoline on the strip and propane on the street. Although you would basically need to do propane injection because the dual fuel mixers are all far too restrictive for a high hp single carb engine.

Best not to post if your not sure because you are wrong about plenty of stuff here dog

More later by me on this subject.

While gasoline leaves the carburetor as an atomized liquid, some or all will flash into vapour in the intake manifold, depending upon manifold vacuum and air temperature. As gasoline vapourizes, it cools the fuel mixture and the intake manifold. All OEM intake manifolds have a hot spot in the intake manifold to compensate for this cooling effect.

At full throttle, manifold vacuum is low and air flow is high so gasoline can enter the combustion chamber as an atomized liquid rather than vapour. Atomized gasoline displaces very little air. Because engines need as much air as possible in the combustion chamber (ie, volumetric efficiency), this helps the engine to make more power.

Propane expands 270 times going from liquid to vapour. I'm not sure how much gasoline expands when it vaporizes. In a gaseous fuel engine, the fuel displaces some of the air entering the combustion chamber so it will produce less power than a gasoline engine. If you're not running your engine at full throttle, you won't notice this effect.

On the street, a properly tuned propane engine will be very responsive because it doesn't require an accelerator pump and there will virtually no fuel mixture variation from cylinder to cylinder.

A simple performance improver for an Impco propane carburetor is the Impco VPV (Vacuum Power Valve). This device allows the engine to run slightly lean for better fuel economy and then enriches the fuel mixture when vacuum falls below 3"Hg.

As for liquid injection, I believe Icom NA has the only system available in North America. I think their focus now is on fleets and I'm not aware of their JTG system for retrofits. If they did, if would only be for EFI engines. Roush had the JTG system on their trucks about 10 years ago. See Roush Propane F150.

If you're not racing, propane is an excellent fuel for spark-ignition engines. Besides having excellent fuel distribution, its gaseous nature ensures that engines last longer because it doesn't wash oil off from cylinder walls.

While gaseous fuels produce less full throttle power because they reduce the engine's volumetric efficiency, the fact that they also reduce volumetric efficiency at part throttle means that the throttle must be opened slightly more, which reduces manifold vacuum. Propane gets a slight fuel efficiency boost by the slight reduction in pumping losses with lower manifold vacuum.

One issue with liquid injection is that the fuel is continuously circulated back to the fuel tank. The fuel then picks up heat from the pump, from the engine, and from radiation from hot pavement. If the fuel pump fails, then you need to empty the fuel tank (like an AC system) to fix it. Without a fuel pump, fuel will flow to a propane carburetor down to -40°F. Both vapour and liquid injection systems both need some additional tank pressure (ie, from ambient temperature) in order to run.

Propane carburetors also don't need a choke to start in cold weather and will idle smoothly immediately after starting. This is video of my Pontiac starting at 0°F:

[video:youtube]https://youtu.be/74CqA71DmRA[/video]

A drawback for some is an advantage for others.

A comment on valves, heads and seats. Gasoline as a "wet" fuel provides some lubrication and cooling to valves. Propane does not. Cylinder heads need to be set up differently to live on propane. Clearances and seals for example. A propane engine needs more oil on the valve stem - meaning less effective stem oil seals - to live and be happy.

A long life propane/gasoline engine can be done, but lots of R&D or trial and error is involved.

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....