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Old 09-23-2004, 08:03 PM
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351M on Propane... Fuel Mileage!!!

Hey Guys,

It ain't no hotrod but i have a 81 F-150. Its got a 351M on propane. It doesn't have much for power, but it doesn't burn oil either.

Basically, i just calculated my gas mileage after putting in $80 of propane. And I'm shocked, and looking for ways to improve.


From what i figure I am getting approx the equivalent of just under 6MPG if you think of it in terms of a motor running on gas.

Here my calculations:


I drove 267km (167Miles) on $80 of propane. Thats the reference.

Now, propane when i filled it was @ 52.9 cents/litre CDN

($1.56/gal US)

Now i calculated how much km or miles i was getting per $ unit of fuel.


$80CDN of propane works out to 151L or 40gal

So i got 267km (167miles) on 151L (40gal) of propane.


267/151 = 1.77km/L or 167mi/40gal = 4.175 MPG

Now I know a gallon of propane doesn't contain the same energy potential as a gallon of gas,

Well i found that propane contains 82,485 British thermal units [Btu], versus 115,400 Btu for gasoline.

So you need about 1.4x as much propane to make the same amount of energy as gas.

Even with that 4.175MPGp x 1.4 = 5.845MPGg




So you've probably skimmed over the numbers, well either way you look at it, its crappy mileage.


Now, im thinking of ways to improve this(other than an engine swap).


My air filer is good, i have dual exhaust. My plugs are good, as well as the wires.

My Propane carb deal is a 425cfm (name slips my mind).

The truck runs great, always fires right up, not a ton of power (3.73 single trac with 31s, no brakestands on dry pavement )

So any ideas what it could be. I mean im not expecting 20MPG or anything, just something half decent 15 would be awesome.

Just basically wondering if anyone has any suggestions on how to get some more mileage out of it (maybe tune the carb?????)

Thanks in advance,
Mike

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Old 09-23-2004, 08:07 PM
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Isn't propane sold by the pound?
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Old 09-23-2004, 08:08 PM
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Ya, if you're going to fill up your BBQ tank, but auto propane is sold by volume.
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Old 09-23-2004, 08:31 PM
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OK, don't really know anything about it...never had the interest.

I shall shut up now.
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Old 09-23-2004, 09:47 PM
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I don't understand governments and their taxes. We pay about $0.95 cents (Australian) a liter for gasoline, yet pay only about $0.40 cents (about 23 cents US) a liter for propane/butane gas (LPG), sometimes as low as $0.28 cents a liter. My 3.8 liter v6 Buick powered car consumes about 38 liters of LPG to cover about 250 km's (about 200 miles). Your f150 would probably get around 10 to 14 mpg on gasoline right? So in reality your truck should probably get about 7 to 10 mpg. Does the exhaust have a strong smell of gas when the engine's running as a very rich fuel mixture would cause this. Is your truck always loaded and is it mostly used in stop start traffic? The mixtures on LPG systems can easily go way out of range when the settings are altered so getting the thing tuned would be wise. Also, the fuel refiners blend the propane with inferior butane to reduce costs and to find a way of discarding it (butane). The gas in your bbq bottle is straight propane due to it being a lot more stable than butane. We've had problems over here when the fuel refiners have added more butane than the acceptable level, causing all kinds of efficiency problems.
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Old 09-23-2004, 09:51 PM
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I'm assuming that you don't have any type of overdrive. I'm also assuming that you're running a gas to propane conversion kit where the propane carb sits on top of the gas carb. If all of this is true you might be able to squeeze another 1 or 2 mpg but that's about the best that you'll get..To start with make sure that all the normal tune up stuff is in top shape. You can bump the timing up a little bit to take advantage of propane's very high octane rating. Make sure that you have at least a 180 degree thermostat and that hot coolant is circulating through the convertor. You'll want to make sure that all of the propane is getting vaporized. Find someone in your area that can tune the carb with a CO meter. Many propane dealers install these kits and should be able to tune it up. If you don't have any luck finding anybody you might see if a forklift dealership would handle it. Maybe one of their techs would do it one the side.
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Old 09-24-2004, 09:00 AM
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You will typically get less mileage on propane. It sounds like your setup is running a little too rich. What kind of carburetor are you using. I can tell you how to adjust the mixture. If you have an Impco setup there is a different cover for your vaporizor that will help with your mileage too. The impco stuff is all really old technology but it works.

Let me know if I can help.

John
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Old 09-24-2004, 02:34 PM
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I just reread your post and it looks like you have a impco 425. THat carb has two adjustments. The small slotted screw controls your idle and the 9/16 head bolt is the mixture. Set your timing at around 12*. You don't want more than 34 total on propane. Next screw the 9/16 bolt all the way in and back it out around 9 turns. Then you want to start it and use a vaccuum gage and adjust the slotted screw untill you get the best vaccuum. If the slotted screw dosent change anything your carb body is wore. Take the top off and if your lucky yours will have a teflon ring that the air valve cone rides in. Replace the ring and get a new air valve assembly and you should be in good shape. ( The air valve deal is the round ring that has a little cone on the bottom and the small diaphraghm on the top with the spring in the middle.

I am going to go dig around in my shop for the cover that you put on the vaporizor to help with fuel milage. I will post the pic when I find it.

Let me know if any of this makes sense.

Johnhttp://hotrodders.com/gallery/showphoto.php?photo=9853

Last edited by propaniac; 09-24-2004 at 02:57 PM.
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Old 09-24-2004, 04:17 PM
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HEY! Quite an odd occurance to find someone who knows LPG powered vehicles.

While I don't profess to be an expert by any means, I do know that my 305cid in my 1986 Chev 1/2 ton gets GREAT mileage on a strictly LPG only system. There is no gasoline carb on mine at all, and I seem to go forever on $20CDN ($0.44 / litre here)

The truck is relatively new to me, so I don't have any calculations to post yet. I can also tell you that it's barely strong enough to get out of it's own way, and the tank in the bed is about the size of my 60 gallon air compressor tank.....LOL

The IMPCO system on mine is all mechanical, there are no electrical components at all, just a vacuum line to activate the fuel flow when it senses the engine cranking, or running.

propaniac .... you may hear from me so I can pick your brain for more info on how this system can be optimized.
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Old 09-24-2004, 04:29 PM
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Hey no problem. Just let me know.

John
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Old 09-24-2004, 05:58 PM
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Not trying to Hijack the thread...some of this info is likely valuable to anyone running an LPG vehicle.

Here's a pic of my IMPCO 425 unit.

You can see both the small slotted screw in the foreground that is labeled "IDLE", and you can also see the 9/16" head bolt that is sticking directly out the back. Just like John was talking about.

My setup isn't too far off what John had suggested, but at 10* initial timing. I have also changed the T-stat to a 160* unit, and installed intake gaskets with the exhaust crossover blocked to try to keep it running cool.






I have heard that it is good to completely lock out the timing to stay at the base setting when using propane, since it needs no extra time to vaporize in the intake/combustion chamber. I've read to lock out the mech advance, and unplug the vac line from the distributor.

Any fact to this?

============================================

Interesting and related links....

http://www.propaneguy.com/

http://www.propane.tx.gov/commercial...mpcosystem.pdf
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Last edited by 19_Chevelle_69; 09-24-2004 at 06:05 PM.
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Old 09-24-2004, 08:21 PM
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Ok, thanks for all the info.

My truck is strictly propane, just used to get back and forth to work and a little 4 wheeling on the weekend.

I think its running very rich because at an idle you can really smell the propane, almost like it coming out of a tank raw.

I believe my truck has the impco 425 on it. I will check the timing and try and hunt down a vaccuum gauge.


Thanks for the help guys, I'll post back with my results.
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Old 09-25-2004, 09:17 AM
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Arrow

While propane is an excellent fuel it does have less BTU's than gas. The best way to build a strictly propane engine is to boost compression, Propane works best at about 12:1-14:1 compression. Most 351Ms have about 8:1 and are very fuel efficient initially, so the lesser BTUs make it even worse. A truly propane-only engine will need some internal mods to get better mileage and power. Some pop-ups for a 351C with enlarged pinholes would be a good start. Most propane conversions don't alter the gas engines mechanical design because they are often converted back to gas, which isn't an option on an engine that is set up for strictly propane. What you wind up with is a compromise, you get clean emissions, less oil contamination but less power and mileage. Since it is a truck the build options may not be feasible, but if someone was to build it for a rod or Muscle car the above parameters will make a lot of difference in the engine performance. A properly built propane engine can run with any gas powered engine. The mention of overdrive, while and external device, may not be worth it either, since the loss of power may make it not pull the extra gearing. One option we have used that is external and if set up correctly will work around the compression thing as well and that is Turbo charging/Supercharging, but then again there is the cost. A side note, Alchohol shares some of the same requirements, as in Alchohol/Propane dual fuel.
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Old 09-25-2004, 09:56 AM
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Great info...

I had to replace my heads (LPG burned the valves out). I ended up putting .015" steel shim gaskets under the heads to bump the compression up a little (not much with my low compression pistons, but it helped a bit I'm sure).

From what I understand, LPG has an octane rating in the 104 area, and loves higher compression... It's possible to closely match power with a gasoline engine, but as you say, internal engine mods have to be performed. Dome pistons and tight combustion chambers are needed to raise the CR up into the ideal LPG range, and a very long duration cam is optimal, which is not feasable on a dual fuel vehicle, or one that may be converted back in the future.
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Old 09-25-2004, 03:12 PM
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The unimog in my avatar is on straight propane and is turbo charged. It will now run 65+ on the highway. I used a $40.00 turbo off of a mitsubishi I got at a junk yard. I have it set at 5psi boost. The pics are in my photo gallery here.

John
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