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Old 11-24-2016, 06:36 PM   #1
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Default Converting back to gas from propane

I have 2005 Gmc 5500 Topkick with an 8.1 liter that is powered by propane and im wanting to put it back to gas simply because gas is easier to get than propane in the area I live. What all is needed to do this?
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Old 11-25-2016, 07:40 AM   #2
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I really can't help you much, but most of the time the gasoline stuff is still there. I have driven pick ups that were both propane and gasoline. All you did was flip a switch to go between the two fuels. I wish I knew how they were set up, but that was 16+ years ago........
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Old 11-30-2016, 08:36 PM   #3
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Welcome to the forum!

I am assuming by your description that you have an ex Schwann's truck? I have one as well, it is a 6500 with the same 8.1. I bough mine specifically because it was propane powered, as our family business was propane for most of my life so it was an easy choice for me. Plus, at the time I built the truck propane was about $2 per gallon and gasoline was about $3.50. Now, it is the other way around with propane at Flying J at about $3.29 and gasoline back to $2, so I feel your pain. If you are mostly driving local, you need to do as I have and find a local welding supply that will give you a good discount in exchange for regular business. My guy is about 2 miles from my shop and fills me for about $1.50 per gallon and the tanks are big enough that generally I don't have to fill anywhere else. The exception is a long trip, and those Flying J's get expensive.

I am also assuming you have the BiPhase system on your truck that Schwann's was using on that year. My understanding is that BiPhase used all the oem electronics and wiring with the exception of the Schwann's add on harness which plugged into all the oem connectors. You can generally tell the Schwann's harness from oem as it is a cloth covered as opposed to the oem tape and wire loom harnesses. It is my understanding from talking to folks who have done the conversion back to gasoline is that you will need the following from a gas powered truck:
fuel rails and fuel injectors
gasoline fuel tank with appropriate in-tank fuel pump
fuel lines to connect tank(s) to the injectors
All that will plug directly into the existing (oem) harnesses and use all the existing electronics, just eliminate the Schwann's harness. If you have a dual tank truck, the gasoline setup will use the same as the oem, ie the passenger tank does not feed the engine but rather just transfers fuel automatically to the drivers side tank that feeds the engine. If it a single tank setup the same dash gauge will still read from the sender in the gasoline tank, the ohm output is the same. Google Leo's Tractors in Wisconsin, that is where I bought our truck. He buys up a bunch of the retired Schwann's trucks and resells them, and converts a bunch of them to gasoline. He can hook you up with everything you need to convert back. Odd duck, but good guy and know's his stuff.

For information on your existing propane setup:
Bi-Phase | $mart Clean Propane - Home
You can download VERY complete manuals on your propane setup, and their tech line is great and they have any part you may need for propane on the truck.
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Old 11-30-2016, 08:44 PM   #4
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I am interested of course in what you did with the truck after you got it. I've seen a bunch of nice and not-so-nice conversions with the old Schwann's trucks. Mine is somewhere in the middle. Added a box and fifth wheel bed to tote our race car trailer, and then our fifth wheel camper.
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Old 12-20-2016, 12:41 PM   #5
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Converting to gas should not be too bad, since there are many 8.1 gas engines around. Are you wanting to stay with fuel injection or simple and carb?

To best of my knowledge, the older 7.4 intakes with TBI will bolt up and that is a pretty easy conversion, not sure of the computer interface vs using the 7.4 computer. The pre-Vortec 7.4 from early-mid 90's are the ones I am thinking.

As suggested by hot rod, you will also need fuel tank and pump changes. That guy that does the conversion is probably the best to talk with. A multi-port fuel injection would be better than TBI setup.
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Old 12-20-2016, 07:23 PM   #6
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The 8.1 (496) was an entirely new engine in 2001 to replace the 7.4 (454). Nothing interchanges, it is an entirely different engine. But it is already a multi-port fi, just need to rob the rails and injectors from a junkyard pickup/suburban/mdt.
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Old 12-20-2016, 10:04 PM   #7
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So I read up and learned there are some unique features of the 8.1. It is based on the old big block, but with a lot of differences. The block is a tall deck version, has the same std Chevy bellhousing pattern, but a unique flexplate. The cylinder heads have cathedral ports, kind of like the LS engines have. That is significantly different then the old 7.4 which had the small "peanut port" heads which were smaller than the regular oval port bog block heads. The 8.1 also does not use the std distributor, like most newer engines it has the distributorless ignition.

Most of the general comments I found said they make good power and torque, and especially like to burn gas while doing it.

So as stated by hot rod above, scratch my idea to use the 7.4 intake and TBI as an easy swap. It won't work. I was not aware of the tall deck and the unique cathedral intake ports. Now I know why nobody really uses the 8.1 for high perf builds. I'll stick with the old style big blocks that I am familiar with! Sorry for the confusion.
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Old 12-25-2016, 06:44 PM   #8
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I have done this conversion. In addition to the hardware that you need to "remove / replace" you will need to have the ecm reflashed by a gmc dealer or a shop able to do the same. The propane runs cooler, therefore the idle, engine operating temperature, transmission shifting is different. Look behind the seat in the door posts behind the plastic trim....byphase use to put the original fuel rails and injectors in the that position. This was in the "topkick" series from 1999-2003. I am not sure if they continued in the 2004-2010 series. Btw...schwains owns the by phase propane conversion company.
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Old 02-11-2018, 11:55 AM   #9
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[QUOTE=Pipeliner;46390]I have 2005 Gmc 5500 Topkick with an 8.1 liter that is powered by propane and im wanting to put it back to gas simply because gas is easier to get than propane in the area I live. What all is needed to do this?[/QUOprofane TE]
What was the verdict. Was it successful. I'm about to start the same project and was wondering if you have any advice?
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