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Old 12-14-2006, 12:41 AM   #1
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Gas buses typically get between 4-7 mpg talking to guys at the bus barns. I'm sure my northern figures will differ, but you get the idea. With propane having less energy per gallon, I think your mileage is right inline. To get better mileage I would either convert back to gas or start considering building a propane motor with good heads, high compression, a hot spark, and advanced timing. Oh yeah...and hardened valveseats.

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Old 12-18-2006, 05:34 PM   #2
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whats the highest compression you can run with propane, I know its got the equiv of like 110 octane so arounds 11 to 1? 12 to 1? thanks....

Heres another question, what propane setup is getting the best mileage? talking chevy engines here, does a small block with high compression rock, or are the big block 427/454's getting the better mileage with proapne.... When I rebuild this motor which I beleive is a 366 I'd rather only do it once and if I got to toss the 366 block my feelings wont be hurt. I want to put in a the best mileage propane setup I can get in the GM family.... thanks....

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Old 12-18-2006, 08:16 PM   #3
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I don't know that the difference in mileage will come from one being a big block versus one being a small block so much as with the fact that 454 ci will use a lot more fuel than 350 in theory. However, a 350 will struggle darn hard to move a bus. Big blocks also typically have some fat low end torque.

12:1 is reasonable for 105-110 octane, but for this set up, I would stick to 11:1 or less. That motor is going to be straining a little and that might help your rods live a little longer.
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Old 12-18-2006, 09:17 PM   #4
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ya I figured you have to find a balance for the perfect torque to move the thing effeciently Anybody know off the top of their heads what the specs are on the old 366's.
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Old 05-20-2009, 05:13 PM   #5
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Re: 3.44 MPG on Propane -- Yikes!

If that's not a factory conversion (I dont know of any school districts that would normally run propane?) thats probably about what i'd expect. Propane is higher octane but lower btu, so unless you have higher compression you get worse mileage than normal because it's like a gas engine with inappropriately low compression wasting power.

In theory a smaller engine is better mileage than a larger one but if that engine is overloaded it can sometimes be worse. I've heard of cases of people putting in larger engines into highly loaded vehicles and improving both mileage and power. I know my dodge caravan 4 cyl only gets about 17mpg even empty, the damn V6's have twice the power and get better mileage. Hell some of the V8 full size vans get about that mileage!

One option might be a turbocharger. That would raise the power, compression, and probably the economy by giving an alternative to 100% throttle all the time. It's also self adjusting to load, ie - on a hill the boost would kick in, when going downhill the reduced load tends to drop the boost. It needn't be excessively expensive, i've seen homebrew systems for under $1000 made, and if you did a single rear mounted turbosystem (wouldn't have to be at the rear of the bus, just to the rear of where the pipes likely merge into a single exhaust) it would probably be easier to plumb than twin turbos under the hood although it would be harder to find a scrapyard turbo with the right Aspect Ratio to work in a rear mounted setup well, hmm. See STS turbosystems for how rear mounted turbosystems look and work. The 366 would likely be able to take a fair bit of boost especially on propane since theyre tough engines.

Dont know what the factory hp of a 366 is but probably pretty abysmal. Rule of thumb for truck type gas engines was cubic inches = lb/ft of torque, and hp = half that and you wont be too far off. Probably 175hp. Be surprised if it's less than 160hp or more than 185hp. Torque peaks typically around 2400-2800rpm and hp peaks typically by 2800-3200rpm for a truck of the 70's to 80's range.

The only thing that might majorly improve mileage is either going to be powertrain modifications or aeromods and there's limits to how far the latter can go.
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Old 05-20-2009, 09:04 PM   #6
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Re: 3.44 MPG on Propane -- Yikes!

GM spends lots of development time and money optimizing their drivelines and power packages, if it is a factory lp or dual fuel package it is probably optimized for the best power and longevity. I think that the only thing i would change from stock would be to try advancing nthe timing a couplle of degrees and see how it affects power and mileage.
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Old 10-15-2009, 11:03 AM   #7
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Re: 3.44 MPG on Propane -- Yikes!

i agree (check your timing) check your spark plug gap . you may want to try a MSD ignition.. MSD will alow you to run a wide gap on your plugs. check plug wires. check air filter. my father once had a diesel and found a mouse nest in the air fliter... lol check for LP leaks . can you smell propane?. you may want to try to lean your mix.. you may also check the compression in all cylinders..
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Old 03-11-2010, 10:45 AM   #8
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Re: 3.44 MPG on Propane -- Yikes!

For what its worth, my short bus only gets about 7mpg (overloaded), so half that for something three times the size seems reasonable.

Tell you what, though, it sure has it's upsides. My engine still sounds like a sewing machine after 32 years of running on propane- clean as a whistle.

I'm going regular diesel with my next bus, though. In industrialized areas, finding LPG is not too hard, but trying to explore the midwest and places where it can be many miles between services, it can be kindof hairy sometimes finding the stuff before you run out, ESPECIALLY if you travel at night when it is cooler. I spent years doing this and never was totally stranded, of course, but I have had to run on the shoulder off a redneck adapter hose trying to get every last bit out of my cooking tanks hoping to get to a station.

The only place where running LPG was actually [pretty] easy was Denver- they have/had gas stations with LPG pumps that you drive up to, put your card in, and just start pumping (sortof) like all the gas vehicles around you. That is much easier than dealing with some of your average attendants who aren't used to motor fuel LPG and try to make everything a hassle.

Anyway, just thought I'd chime in a bit, carry on.
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Old 08-04-2011, 09:51 AM   #9
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Re: 3.44 MPG on Propane -- Yikes!

could try these guys out. they have the know how, just not nearly as active forum as this is.

also couldnt hurt to check around at heavy truck repair and AG repair places to see if they can tune it for ya. I know in my town I cant get my mini van looked at cause "its not a 3 ton or larger" bs
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