Hey guys im lokkin at putting a propane conversion on my 1977 f250 with a 400m. My question is what kind of compression am i gonna need to run a strictly propane kit through it? Im not looking at dual fuel or anything like that either

 

The AKI (octane) of propane is 104 vs 87 the engine was designed for. Not knowing the specifics of the 400, bore, quench area, spark plug location and type, etc. its difficult to estimate. Does the 1977 Ford ignition system include a knock sensor? That would give you some extra detonation margin, using spark retard.

 

I have ran two diffrent 400's on only LPG I did not alter the factory compression which is around a very dismal 8-1 to 8.5 My engines ran very well both had aftermarket cam's and igntions. Now that being said if I was to raise those compresions up say 10-1 It would do nothing but benift from it. I believe you can go as high as 12-1 safely but I personally wouldnt go higher than 10-1 in case I want to throw a gas carb back on. Just my thoughts!

 

For a daily driver, I would keep the CR in the low 10:1 range.

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Hi guys,

new to the forum, appreciate everybody's feedback. I've got a 99 f-250 super duty set up for a dual fuel, gas/propane. I bought it last year and ran it on gas before finding a place to fill up on propane a few weeks ago. It was running well on propane (better than on gas) until the last few days. Now it will "bog" down anywhere between 35 and 60 mph and not want to shift into the next gear. If I flip back to gas, it runs fine. It just got colder here in WI, but it seemed to run fine for a few days in the colder weather before this started happening.

thanks, josh

 

Depending on how cold it is and sometimes how full the tank, starvation with gaseous fuels is not uncommon. When our CNG car gets low, the tank will chill due to pressure drop sometimes to the point of engine stoppage, then will restart a few minutes later its fine.

Your propane tank would have to get down below -40F or so to stop delivering vapor at all, but it may chill enough to not deliver sufficient amounts to run the engine at normal power.

 

Thanks for the reply. I thought at first it was temp. related, but don't think so any more. The temp has been up to 60 and the same continues to happen. I have good acceleration up to 35mph before it bogs down. I was reading another blog about k and n air filters (which i just cleaned) having too much oil on them. Apparently the excess oil can mess with the mass air flow sensor and inhibit proper air flow. I just picked up a can of sensor cleaner to try in the next day or so. Thanks again.

 

Its fuel flow thats messed up because the sensor is insulated and cant accurately read the amount of air flowing, giving the computer false readings. Usually if this is the problem it will throw a CEL code.

That is one "dirty" secret about oiled fabric air filters. The other is that they dont filter as well as stock paper, letting dirt in. This is shown by elevated levels of silicon (sand) in the oil of engines that have switched to that type of filter. Is it really worth it?

 

Hi, I cleaned the maf sensor to no avail. Actually, it helped for the first trip around the block, then it was back to bogging down. Do I need to clean it again? Another sensor I might be missing? Or is it something with the propane vaporizer? I have good acceleration up to the point it wants to shift into third, then it bogs down. Thanks for your help, josh

 

When the engine bogs down, is the converter cold? Do you see any frost?

 

The temp outside has been somewhere between 40 and 65 degrees. The truck ran okay during a few days of colder weather, then something happened to cause it to bog down. Since then the weather has warmed up into the 60's but its still bogging down. When you asked about frost, is this what you were getting at, or should I somehow be looking at the vaporizer as I drive? The truck revs up fine in neutral; its just under "load" trying to shift into third or overdrive.

thanks, Josh

 

I am trying to find out if your converter (aka vaporizer) is getting enough heat. The converter is a pressure regulator that has an integral heat exchanger. If the converter isn't getting enough water flow, its temperature will drop as it is converting high pressure liquid propane to low pressure vapor. If it drops low enough (when you see frost on the converter), the converter will start supplying the mixer downstream with atomized liquid propane. Since propane expands 270 times as it changes from liquid to gas, your fuel mixture will suddenly become over-rich and your engine will bog down as a result.

If this only happens when the engine is under load, you might find that the converter is become hot again by the time you can check it. If you have a spare water temperature gauge, you can hook it up on the converter's water outlet to monitor what is happening inside. Alternatively, if you have aftermarket Impco conversion, you could put the temperature sensor in the secondary test port (1/8" NPT) which is @ the 7:00 position on a Model E or @ the 12:00 position on the Model L to monitor gas temperature. If the problem isn't with the the water supply, then you would have to rebuild the converter, which is straightforward.

If you have factory (Teleflex GFI) system, you may be having problems with the Compuvalve. If you have an OBDII scanner, you could monitor your fuel trims to see what is happening when the engine bogs down. You would need to get your Ford dealer (or conversion shop that is knowledgeable with these systems) to repair the Compuvalve.

Because propane is harder to ignite than gasoline, you need to make sure that your ignition system is in top condition. I would make sure that the plugs are gapped to the low end of the spec or even 0.005" to 0.010" smaller.

 

I completely disagree about the spark plug gap. This engine does not have a magneto. A small gap DECREASES the total amount of energy in the spark. For hard-to-ignite mixtures, you want the largest gap possible for the fattest spark. Electronic ignition systems can easily provide the voltage to fire plugs at any rpm. There is no dwell issue like with points. Only if the wires or cap are bad would there be a problem.

Real world Ford example. I had a '74 Econoline with the 5.0 and Duraspark I (California only). It was very hard to start. The plugs were gapped .030. which was ok for the 49 state version with points. The smog certification sticker on the valve cover, once I wiped it clean and read it, called for .050. It started and ran like a champ after that.

 

I agree that the spark plug gap should be as large as possible for best combustion. However, propane has a higher ionization energy than gasoline, which makes it more difficult for the spark to jump the gap. Ionization energy increases with decreasing size of the fuel molecule.

To compensate for higher ionization energies, the spark plug gap is commonly reduced in propane and natural gas engines. High performance ignition systems like TFI and EDIS should have no problems producing the spark energy to ignite a propane fuel mixture with the gap specified for gasoline engines. However, a slightly smaller gap will produce a more reliable spark as the electrodes wear and will stress the ignition system less with its lower voltage requirement.

As for dwell, I agree that a variable dwell system like TFI will fully saturate the coil under normal driving RPMs. For a fixed dwell system like DuraSpark, the coil will not necessarily be saturated at higher RPMs.

 

Josh
I dont know what system you are running but there should be a filter inline usally at the lpg fuel lockoff change/clean it. Also make sure the valve on the tank is all the way open so it does not restrict you. I have seen excess flows on the tank valve get weak with age and also seen the rubber hose running from the tank bubble internally with age.
when you get it to bog stop as soon as it safe and inspect the system for frosting if you find any start looking there.
Good luck