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General Site Info => General Discussion => Topic started by: 78 Chevyrado on January 08, 2013, 10:36:52 pm
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I Think this summer (or at least when it warms up) I'll be replacing the stock 350 and th400 in my truck. I would like to go the most fual efficent route... I know, thats a stretch...
I'm now a general contractor and as such, I sometimes drive 10 miles one way or 110 miles one way to the jobsite. Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...
I'd like to keep it with a SB chevy. I'd go with the new LS engines, but if something doesnt work out, The truck absolutely can't sit around waiting for parts and such. If the trucks sitting, I'm losing about $300 per day.. I'm thinking so far to keep the 350 but put in a 700R4 Built heavily or a 4L80E. It will have OD. I need to be able to go at least 75MPH without the twice the fuel consumption penalty. I know it will use more fuel, but not double I hope...
78, C20 with 1 ton suspension, 33.6" tires, 3.73 rear, and it weighs about 7000lbs with all my junk in it which I have to carry all the time.
I'm thinking a 350 with an OD trans and something like that FAST EFI Setup.
I know the LS engine like a 5.3L would do best, but if I do that, it has to be up and running in a week. I could do that with a SB and EFI. I'd want a roller cam block or a retro cam.
What do you guys think?
350?
400?
5.3L
6.0L
4L60 or 4L80E?
I can do the work, but after I get my parts together, I want it finished and running and ready for work in 7 days at the most. I'll lose a lot of money otherwise. It could go a couple of days more, but if it goes much past this, I'll have to rent a truck...
I can go a couple of days without the truck, as I can drive the car if the job doesnt require too many tools, but every time I dont have the truck, the tool i need is 100 miles away in the truck... ya know...
So with these needs, SB or LS? 700r4 or 4L80?
Thanks Fellas!
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What mpg's do you get today? What kind of shape is the 350 in that's in your truck now? How about just dropping a 700R4 in and see what that does for you? A th400 is about the toughest automatic ever made, but one of the worst for fuel economy. That alone would make a big difference.
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A 4bt Cummins with 47RH would probably get 20 mpg in your truck. I'm a Chevy guy to the core, but I'm also a diesel guy.
It would be a easier swap than an EFI swap would be.
Here's a link to some fuel mileage numbers, the bottom truck is a two wheel drive Durango, all the other are 4x4 jeeps with 33" tires.
http://www.4btswaps.com/forum/showthread.php?10409-4BT-MPG-Challenge
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Very curious on the outcomes of this topic!
I'm in a comparing situation as you're in. I want to use my truck for work (starting general contractor) after it's ready for the road and the Dutch climate.
I've almost the same setup as you have: '77 C20 Camper special, so 3/4 ton and 1 ton suspension, sbc350, th400 and probably 3.73 or 4.10 gears. I only drove the truck 40 miles home so I don't know the MPG's, but probably not so good ;)
My formal truck with the same setup, except for a LPG installation and 4.10 for sure did on a good highway day: 7 MPG but more often under 5.5! And especially in Holland fuel isn't cheap.
I was thinking of swapping my th400 with a '87+ th700r and just see what happens. Friend of mine had pretty good results with this swap and is also very pleased with the low RPM's.
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I to have a 78 C28 350 sm454 no clue the gears in the rear.
I plan on replacing the sm454 with a sm456 Going to freshen up my 350 rebuild the carb, new exhaust manifolds, replace the hoses and belts, full tune up, make sure it's in the time, tighten the valves. new oil pump new water pump. etc then rebuild the rearend with better gas friendly (if I don't have it already) gears. then just the general stuff. make sure the tires are good and filled right make sure it's aliment is good Etc.
That will get me about 15-17mpg if I do it right, but in the long run when I fully restore her I am going diesel when I find one that I like.
Mariah =^_^=
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I agree with jaredts on swapping in a 700R4 for cost reasons alone. Get a later transmission because the earlier ones were prone to cracking around the bellhousing, IIRC. I don't know how to ID the difference, however. I'm sure others will know. The disadvantages of a 4L80E are that it is usually more costly and you will need a stand-alone computer to control it. If you decide on an electronic transmission, consider a 6L80E or 6L85E if cost isn't critical to you.
Rusty
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Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...
No engine is going to get good mileage at 75 mph in our non aerodynamic trucks. That said, OD is a good start but there is a lot more to gain through aerodynamic mods.
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Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...
No engine is going to get good mileage at 75 mph in our non aerodynamic trucks. That said, OD is a good start but there is a lot more to gain through aerodynamic mods.
This is true, so I'm looking for the best possible while driving a brick $%^& house down the road...lol
Right now, I have the OE 350 and TH400 and get about 12-14MPG... as long as I stay under 65MPH. The Truck has 143K on it so does the 350 and TH400. They still run good only smoke at startup... bad valve seals.... all else is good. don't know how long it will last though, as some days I'm so ready to get home, I'm running 85MPH with traffic just to get to my shower n bed faster.... It's gonna blow one day, I knoiw it and expect it, I just hope to be as prepared as posible when it does go.....lol
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If you wanted the BEST mpg while still having the power to pull a house you'll need turbos. No other way, small displacement gets you mileage while turbos give you the power when you need it. I would look into the new Camaro v6 and turbo it. 300hp and 24mpg factory, throw in the turbo and upgrades associated, 500hp and 26-28mpg.
Pretty sure it comes with a nice 6 speed auto so you'll always have the right gear too boot.
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Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...
No engine is going to get good mileage at 75 mph in our non aerodynamic trucks. That said, OD is a good start but there is a lot more to gain through aerodynamic mods.
This is true, so I'm looking for the best possible while driving a brick $%^& house down the road...lol
Yeah, I understand what you mean. Keep in mind that at 75 mph, a tremendous amount of energy is being used to overcome wind resistance. Our trucks are not aerodynamic so a newer 5.3L in a newer Chevy truck is not going to get same mileage at 75 mph in our trucks. Also, gas engines tend not to be very efficient under heavy throttle. A lot of the fuel is being used to prevent pinging.
The best possible mileage in your truck at that speed is going to be either the GM 6.2L diesel or Cummins 6BT. I can comment on both of these since I own each. My 1982 Suburban (2 wheel drive, 3.08 axles, 700r4) with the 6.2L gets close to 30 mpg at 55 mph and at 75 mph, it'll get around 20 mpg. The Cummins 6BT in my 1990 Suburban (4 wheel drive, lifted) gets 26 mpg max and at 75 mph, 18 mpg is what she'll do. However, I have a feeling that the Cummins might even do a little better than the 6.2L if it were in the 2 wheel drive Suburban.
If you want to stick with gas, I would probably put in a 5.3L or 6.0L engine. However, at 75 mph, I doubt you will get very much more than 10-11 mpg. I have a friend with a newer Chevy 3/4 ton that has the 6.0L and he complains of only getting 12 mpg on the freeway but he drives fast.
Also many times, a bigger engine will get better fuel economy when worked hard. Several weeks ago, my friend and I were following each and he's got a 2012 Chevy Impala and I've got a 1997 Lincoln Mark VIII (295 hp engine). With drove 80 mph the whole way. When it came time to fill up, he got 23.5 in the Impala and I got 24.1 in the Lincoln. That said, his Impala will get over 30 mpg driven easy but the most I've ever been able to squeak out of that Lincoln is 26 mpg.
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Is 7000lbs. the max you will ever need to move?
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As for aerodynamics and fuel efficiency our trucks have about a .41 drag coefficient vs say a Corvette with .30 vs say a mid 80's Caprice at again, .40... Needless to say, our biggest issue is frontal area. And there is a formula that will calculate the drag limited top speed of a car and it is based on the horsepower vs Cd.. That being said most trucks are going to max out at about 22-25mpg if driven at around 65-70 mgp. My 1987 got 22mpg on all the trips we took, it was an old underpowered 305, but it got 22mpg all the time on the highway, In the city, it typically got about 15-17 with 55mph speeds. That being said, If you are after efficiency with gas you can go with a big motor making alot of torque that can be lugged around all day turning no more that 2000rpms or a small motor that can make good torque and be overdriven under low speeds.
If you are wanting to move away from pre GEN III motors, I would recommend a 4.8L or a 5.3L with an overdrive if mpgs are your goal. You should be able to achieve 22mpg, and if you are after cost, find a low mileage pullout 4.8L/4L60E combo as they are skipped by all. A friend got one to put in his 70C10 for less than a grand and it had 15K miles on it.
If you are wanting to stay with a GEN I, I would build it for efficiency and not power. I would probably start with an overdrive unless you are geared high already, but if your numbers are 12-14 in the city, I don't think it will improve much with either option.
If your choices are a GEN I with an OD and an aftermarket system OR a LS motor and OD, HANDS down I would go the LS route as the EFI systems on them were designed to be driven under varying conditions and have a 100K warranty. aftermarket injection systems were built to achieve Power in Hobby categories. If the GM ECM dies you can get one for nothing. Not going to be the case with an aftermarket setup...
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The best fuel economy at 75 mph I know of is a guy on a different board who has a early 80's Chevy 6.2L diesel that has been lowered. It's got a 700R4 and 2.73 gears. He's claiming 27 mpg with the setup and I believe it. I've thought about lowering my Suburban to gain fuel economy at fast speeds but I'm usually not in a hurry.
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No doubt if you are after mileage and your options are not limited to gas, the 6.2L is going to be hands down the best option as that is why it was originally created. 25mpg is the norm. Even 4x4's get up to 20mpg with a 6.2L.
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No engine swap it discounted here...
I want the 75MPH mark, because at 65MPH I get 15 or so MPG... at 75mpg I get 8 or 9MPG... I'd like to go 75MPH just to save some time, and time is money. I realize these aren't effiicent trucks, but I guess I just want to get the best mileage possible... BEcause gas saved is money in my pocket... Even if I got way better mileage at 65MPH. I get about 14-15MPG now driving 60-65MPH everywhere. I just figure (HOPE) an LS 5.3l or 6.0 with OD would do better than my current worn out 350 and TH400. Grandad already teared up on me about using the truck he retired from working with to start my career, so this C20 is a work truck from here on out....
I heard 6.0l suck gass bad... I picked a 5.3L with OD because I havent heard anything about them being badly inefficient.. I don't haul a triler often enough to consider that on my new engine...
For instance, today, I drove 65m to marietta, then 37m to snellville, then 42m to flowery branch, all to some odd small jobs.,.. So with the possible 144m+ per day, I want to save $$ where I can. But I have to have my truck and all my tools pretty much wherever I'm sent... never know what your gonna need.... So thats my situation. I had to add 15psi to the back tires to make up for the load imposed on the truck by my tools, etc... Thats how much I haul daily....
I'm in my truck 3 or 4 hours daily... couldnt be happier, but i need to save gas all I can... Right now I find myself driving 55mph often... well thats too slow to make a living daily... If I take a day off I lose around $300-400 So whatever the cost in $$$, but I'm limited by time... So I wanna go the 350 with efi route because it bolts in with a minimum of problems.... the ls could mess me up time wise... Though the ls is the ideal.
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So, I have an 87 C20, 350 TBI, TH400, 4.10 gears. And on my best day I have only gotten 14 mpg at 60 mph. But I normally get 12mpg. So I don't think efi is going to be a big help. Now for the Ls engines both my brothers drive newer Chevy's. Both are 1/2 ton 2wd, one is a 4.8 with a 5 speed (nv3500) and he avg. 20 mpg. The other brother's has a 5.3 with a 4 speed (4l60e) and he gets 16-18 mpg.
My plan is to swap in a 5 speed or overdrive auto to hopefully get me up to 16 on a good day.
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Just remember, any engine you put in this truck will lose a significant amount of fuel economy at 75 mph vs 60 mph. It just takes so much more power at 75 mph to overcome wind resistance. Even with the most fuel efficient engine (the 6.2 diesel), I'm barely able to squeak 20 mpg at that speed. That extra 15 mph uses a third more fuel.
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1) Is 7000 really the weight you are at?
2) i don't think there is anything you can do to improve mileage significantly (see my mpg thread) with the exception of going diesel, going down on cubes or LS.
If you are grossing $300-400 per day, then it may be cheapest to do nothing.
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1) Is 7000 really the weight you are at?
2) i don't think there is anything you can do to improve mileage significantly (see my mpg thread) with the exception of going diesel, going down on cubes or LS.
If you are grossing $300-400 per day, then it may be cheapest to do nothing.
1 - yes right around there
2 - that sucks...lol but i know I'm moving a lot of weight and low aero-anything....
3 - the only problem I have with going diesel, or even LS, is there may be signifigant downtime, as I'd be treading new ground.
I have a special interest in driving this particular truck though. And I want to keep it working. I know whatever I do I'll pay a penalty for that. And whatever MPG the newer trucks get, the older trucks are lighter, so I'd get a few more MPG.
I checked on it one day, my 78 C20 Weighs 5200lbs empty, and the corresponding 2012 model 3/4 ton HD truck weighs 6800lbs empty. So I have a 1600lb weight advantage to start with. The newer trucks are better, but are heavier. So basically my loaded truck weighs what a newer 2500HD does empty. I'd like to go 75MPH just to get mnore work done faster. Faster is better, but no matter what I'll have to find the balance.
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There is not much mpg penalty for weight on the freeway at a steady speed. The main reason the newer trucks get better fuel economy is better aerodynamics and a slightly more efficient drive train (the big one being a lock up torque converter).
Have you thought about installing a 700R4 transmission? You might also have a company grind you a cam that optimizes fuel efficiency at 75 mph. I know this is done with boat engines.
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In that case, the easy/cheap things would be to change the gear to 3.08 or higher. Switch to smaller tires---235/75 or smaller.
2) i think aerodynamics can help greatly; i have not had time to experiment further and probably will not.
3) If you want to get more drastic, i would suggest in order of level of seriousness:
a) get a 4.3 v-6 from a junkyard, get it running and tuned on a stand, or a tire or cinderblocks etc. This, i feel, is the most plug-and-play out of all the engine swap options that will bring large mpg gains. i'm not sure about the mounts/exhaust, but everything else is plug and play except for the fan shroud. This route, i feel, results in the least amount of down time.
b) If you want to go LS, then i would also get it running on a stand first. Then work on each department separately; For example everyone says you need to install an electric pump, but it's not that simple. You need to mount it correctly, install a relay properly, wire it properly etc. Then move on to the next department. Things like this can be done while still driving the truck daily.
c) Same with diesel. Research what is need for each step. For example Hydroboost brakes---what parts are needed and what exactly needs to be done? What type of torque converter and how to get one. All things like this can be done and parts and tools purchased while still driving the truck.
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I'd have to agree with the OD option. Keep the 350 and go with a 700r4. Its going to be a big help forsure plus less down time doing the tranny swap, save the LS swap for down the road when you have more time. Going with the 700r4 will be easier, cost less and you dont need much to make the swap happen. Whats the gear ratio options for your rear end. Changing the ratio might help out as well.
My buddy has a 3/4 ton 2wd with a good running 305 and a 700r4. He carries all his tools too(mobile welder fabricator jack of all trades guy). He uses a contractor cap on his and gets about 15 to 17 mpg. The cap dont look real eye pleasing but it helps.
I think Ls swaps are cool too, more efficient, better drivability and make more power no doubt. But are you going to gain enough mpg to make it worth while doing that swap.
Refridgerators are only so areodynamic that how I look at it.
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What is a contractor cap? I agree, best thing is to do is in steps. Especially if you get more money down the road. Get the 700r4/modern equivalent built up first, then install. Next would be gears, then motor. Get a plan down and stick to it. You want this done right and last because it is your livelihood. You want strong reliable parts used. Break this up in chunks and it will cause less downtime.
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I have looked into 700r4s and it would workwell, even has a mechanical lockup, so no electronics needed. Lowestgears i can find for my 14bolt is 3.23 or something close to that. Not enough to warrant a swap from a 3.73. Im thinking build the torquey engine i want, back it with a hevilybuilt 700r4 and add FAST EFI to itfor driveability.
The 75mph mark is just a goal. Anything will be better than the worn 350 and th400 i hav3 now, even if i replaced everything to stock. Id love a gear vendors unit but not for $3500. Theyre insane....
I wondered if adding the airdam off an 87 wouldhelp? I know i lost mileage on my 87 when i took it off for looks...
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I wondered if adding the airdam off an 87 wouldhelp? I know i lost mileage on my 87 when i took it off for looks...
Yes it would especially at 75 mph. Also, lowering helps even more.
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One thing to think about is that driving a 100 miles at 75 mph vs 65 mph you will only get there about 15 to 20 minutes faster by driving 75. Is that time saved worth spending about twice as much on fuel? Probably not
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Yeah I don't understand the need of 75 MPH. At that speed, that's death in these trucks. I would hate to drive a 7k+lb 25 y/o truck at that speed, for what? A few more minutes and more money spent on gas, endangering others around you. The new trucks can survive a crash then older trucks. Not to mention they stop faster.
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This is true about the time savings... hadntthought about it that way. I have no problem getting up a halfbhour earlier, i just like to go with the flow, which aroundhere can be 55mphto 85mph depending on the area. And for safety, the faster i go the farther i am from the car in front of me. Im not worried there, as my brakes are good enough, if i ever panic stop, i feel extremely nauseated for the next 4 hours... i almost puked one day, it stopped so fast. Itll stopabout the same distance as my 2008 impala. Love the big brakes and the bfgs.
Like everyone else, i just wanna see how to get the most out of what i got.
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i wanted to add another suggestion that would be fast and easy and raise mpg---pretty much a bolt in affair except you would need to add a fuel pump for a carb which is easier than adding one for fuel injection.
i think the 91-96 impalas had a 262inch v-8. i think it was code L99 or something. Run this with carb instead of fuel injection and this could be bolted in in less than 8 hours. i think the engine will have enough pep to move everything.
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My thoughts, based on a lot of research into the matter and trying to maximize the dollar while keeping an old truck...
Just as a point of reference: My 2005 Chevrolet 2500 HD, 6.0L, 4x4, 4.10's, 5-speed NV4500 gets about 14 mpg on the highway. I can get 15 mpg if I watch my speed or as little as 13 mpg if I don't care.
Prior to being transferred, my daily commute was 36 miles roundtrip. I tried to maximize my fuel economy in the 2005 and could only get 14-15 mpg. The reason I initially swapped the '73 GMC K2500 to to run on propane was the price of fuel, $2.00/gal vs. $3.85+ (at the time). The '73 GMC with a 4" lift, no overdrive, horrible aerodynamics, iron exhaust manifolds, worn valves and 4.10's cost less to drive to work than the 2005. The price to swap to propane was significantly less than an aftermarket EFI system, with rebuilding used items it was closer to what it would cost to swap a factory EFI (but a lot simpler to do). After the swap, I could start the truck up at 10 degree outside temperature and pretty much drive off.
I liked the propane enough that I installed a propane system on my '74 Chevy K10. I haven't measured the fuel mileage on it yet, but I can start it and drive it like it was fuel injected.
I'm currently planning out a rebuild for the '73 GMC. At this time I plan for a 700R4, keeping the 4.10's and rebuilding the 350 (355 or 383). The key part of my rebuild, besides the overdrive, is to up the compression ratio from 9.1:1 to 9.5:1 or more, using a cam that maximizes low end torque along with better flowing heads and exhaust. With that, I should be able to take advantage of overdrive at highway speeds be able to use the 110 octane available from propane, yet still run on 92 octane gasoline if I need to. With the low price of propane (compared to gasoline) combined with better efficiency, operating cost should be closer to the '01 Tahoe with a 5.3L and overdrive (averaging 20 mpg city/highway) than the '05 2500. From research, if I were to go with a dedicated propane system and lose the gasoline carburetor I could get even better mileage and performance.
Overall, my idea is maximizing the torque at realistic engine speeds (i.e., <5,000 rpm), have overdrive to keep rpm's down at highway speed, have a C/R that can better squeeze the propane but still be able to run pump gas and have the lower gears. Really, I'm trying to have my cake and eat it too...
If you were to consider propane there is an added bonus: there are tax credits that are offered by the federal government as well as many states to swap to an alternative fuel (e.g., propane). So a good portion of your initial investment can be returned by tax deductions while saving money on fuel.
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Actually, scratch all the stuff i listed for now.
i think it would be best to do the cheap/easy stuff first in order:
1) Since you do alot of driving, i think using this guide and trying to find stations that only sell 100% gas is a good start:
http://pure-gas.org/
2) Focus on saving on gas first, then improving mpg; So i would first look into credit cards that offer some sort of rebate on gas. Exxon had one that rebated .15 per gallon off sign price. This just ended this past october, but now they are offering a card that rebates .06 cents per gallon. There are credit cards that offer 5% rebate at any gas station. At current prices this is about .15 off per gallon.
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Jason S will your set up run on natural gas as well? With me living in central pa where they are doing a lot of gas drilling right now they are starting to put up natral gas stations around here.
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Jason S will your set up run on natural gas as well? With me living in central pa where they are doing a lot of gas drilling right now they are starting to put up natral gas stations around here.
Some of the parts will work with a Compressed Natural Gas (CNG) setup, but overall no. CNG is usually at 2,000 to 3,000 psi, whereas Propane (actually LPG) is under 200 psi. Tanks, lines and regulator would be different.
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ok thank you I am just starting to think about doing this with my chevy and dodge.
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Jason S will your set up run on natural gas as well? With me living in central pa where they are doing a lot of gas drilling right now they are starting to put up natral gas stations around here.
Some of the parts will work with a Compressed Natural Gas (CNG) setup, but overall no. CNG is usually at 2,000 to 3,000 psi, whereas Propane (actually LPG) is under 200 psi. Tanks, lines and regulator would be different.
i had no idea you needed such a pressure for natural gas... the tank to store natural gas must be extremely heavy do to the thickness of the metal to hold that pressure... kinda like an acetylene tank... sounds a little dangerous for it to be mounted in a vehicle
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I didn't want my posting on LPG to defer anymore from the overall question 78Chevyrado wanted to have answered so I started a thread on Alternative Fuels here http://forum.73-87chevytrucks.com/smforum/index.php/topic,25394.0.html (http://forum.73-87chevytrucks.com/smforum/index.php/topic,25394.0.html) in case there are any questions about LPG and/or CNG that I or others may be able to answer.
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As far as propane tanks go.... Their not common enough around here to be cost effective... Around here its gasoline of diesel, even E-85 is hard to come by.... Propane would cost me about what it would to get a tank for the grille, which is too much.
I'm just wanting which will be the best mpg setup. I know 6.2lD is it, but i drive 60-65mph regularly, though i was in a hurry the other day and ran 80mph to get there quick.... i dont like going that fast on NON-OD. But it was that or lose $450 Thats the kinda thing I have to deal with.... Yes I got $450, but whens the 350/TH400 gona leave me stranded... I know it will, its a mathematical reality, as I got the truck with 84K on it, it now had 144K on it, but what do I do when it happens...lol
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For when it does quit... I got a lil taste the other day... The OE starter quit on me. and in the worst part of town possible... I didnt like my boss standing over me while i fixed it with a claw hammer and a 45. but it was reassuring... im glad somebody besides me had a gun... cuz all i coulda done at that point with mine was shoot out the fender and floorboard if something hadda gone wrong...lol
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I know MPG will suck... which setup will get me the best? I drive 60-65MPH usually but sometimes I wanna get home quick, so I go with the flow and go 80MPH. What will get me there the best? I basically like to go with the flow weather its flow at 55mph, or flow at 80mph. The cops around here run 80-85 and dont stop you unless you do something stupid like pass them, or such...
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Is there any reason why you can't look for another truck with a 6.2?
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My being sentimental makes me wanna drive it. It was the last truck grandad used when he quit building, and id love it to keep on doing what it used to do. I just think its cool is all.
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One thing that really stands out to me that has not been mentioned is your tires. You have wide, aggressively treaded BFG's, and decently wide wheels. Seriously think about going to a skinnier tire with highway treads. If you could reduce the overall weight at the same time that will help even more.
I am thinking something along the lines of 235/85-16 with factory aluminum wheels, PYO's I believe most people refer to them as, would look good on your truck. They will be easier for your truck to turn, have less rolling resistance, and probably be lighter. It could also facilitate a small drop in height that could have a possible effect, albeit not much.
When I had my 2005 Sierra, I went from factory 265/70-17 Bridgstones to 285/70-17 BFG A/T's. Bigger, wider, heavier, and I lost 1-2 MPG's.
4x4 Garage did a test between the Michelin LTX M/S2 and the Dick Cepek FC-II's. Same size tires, 285/70/17, on a F-150HD. On the 182 mile loop they drove, the Michelin's delivered 17.16 mpg, and the Cepeks delivered 14.93 mpg's. That's a 2.23 difference! This was in a controlled test, and the article did state that they kept records for everyday mileage also, and the difference was more like 1.75 mpg.
One other thing to keep in mind, at 15,000 miles a year, the difference between the 15 and 17 mpg is only $353 a year at $3 a gallon. (the mags math, not mine.) If you are doing anything purely for mileage, make sure you calculate the rate of return on your investment. If your engine goes south, then it really doesn't matter.
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What type of fan you got?
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To get back to your original question about the engine itself, have you checked out the 383HT? I believe you said you are running the original 350 for your truck, so I am going to guess it has a bad compression ratio (as in lower than advertised, possibly sub 8:1), not the most efficient heads available, and old cam timing tech. I can't remember specs off the top of my head, but I believe it was 9:1 compression (will be more efficient than what you are running now), L98 style heads (good for slow speed power and mpg), and a cam that makes good low end power so you can run it at lower rpm's without lugging it. It would be a straight forward weekend swap. Pair that with an OD transmission, electric fans, different wheels/tires, and every other mpg trick and over time I bet you could pull down some decent numbers.