Brainstorming Session: Building a (relative) gas meiser

[79 silverado]

Start off by saying theres no way short of insane expenditures that we could ever expect to see a 70s C/K truck getting great mileage relative to an econobox. Thats said, how about looking at a repectable output? I'm considering 2 somewhat different platforms, and brainstorming, and would like to see other's opinions on the subject.

I picked this forum as I thought it'd be a nice general interest topic these days, but if the mods disagree (even though its not a solve my problem deal) feel free to shift it into tech....

In their stock forms:

Platform 1: 1977 GMC 4x4 1/2 ton 6' step, 250 I-6, TH350, part time 4x4 (would think its a np205 right?) non-ac truck, d44/12bolt rears

Platform 2: 1980 Chevy 1/2 ton 8' fleet, 350 v8, TH350, 2wd, a/c equipped, 12bolt posi

Again, very different starting points, but I can go with either one at this point, they're both in the garage already

Now, As a starting point lets keep the budget under $1000-1500 to work with for the sake of this experiment.

The final truck WILL have a tire with a profile of about 29" tall: either a 235/75/15 on the 2wd truck or most likely a 215or220/75/16 on the 4x4. Either rim would be aluminum in nature -my excuse to rationalize that one? have to also reduce rotating mass hehehe.

Beyond that, gearing and tranny are somewhat open-ended. I do not know the current gear ratios in either truck offhand, thats something that based on the final formula could be changed. Carbs? Again, open to speculation. Overall horse power is not the goal, just adequate get up and go so the engine isnt working to hard to move a nearly 5000# truck.

Before I start the brainstorm remember I'm talking true mpg after accounting for the fact larger than stock tires will show the odometer reading fewer miles than actually experienced (similar to how the speed reading is normally off)

Initial thoughts that came into my mind for the 1980 were a 4 to 2bbl adapter on the 350 to put a dualjet on there off a 305 (or maybe swap to 2bbl intake), coupled with around a 3.08 gearset, headers with less restricive exhaust and no cat, electric fan to cut the rotating weight on the front of the engine, and maybe even switching out the pulleys if its affordable for a starting package.

On the 77 I was thinking upgrade the ignition, custom mandrel header into a no-cat single 3" pipe exhaust, electric fan (again reduce rotating mass on engine), and somehow switch up the intake/carb as well. To be honest I had contemplated something exotic (although most likely problematic) where I know that some I-4 iron duke s10 guys use an adaptor to mount a carb onto their FI intake, maybe with a upsized injector to compensate for the extra cubes and a fabbed up adaptor a, early 80s s10 TBI unit could be adapted onto the I-6?

Again, the idea here is to have fun. Anyone who's tinkered with things to get better mileage I wanna hear em. I'd like to put together a reliable truck that will get in the mid to hopefully upper 20s while on the highway for a cross country tour at the end of the summer... I can see it now... everyone is gonna think im

That said... BRAINSTORM TIME

[79BIG10]

Here's what I would do- Take the 80 and regear it to a 3.08 or lower numerical number if it doesn't have it already. The 77 would cost more with the 4x4. Keep the engines in them. The 6 cylinder would be good on gas but really hate it when going up hills etc. Stick with a easy to tune carb like Edelbrock 600cfm. You can tune it easily with a meter and jet kit. Keep it as a 4bbl. but keep your foot out of it. Like you mentioned use better exhaust, electric fans etc. The one thing that would definately help would be to get a 700r4 or even a 5 speed stick. 700r4 would be the easiest to swap in since you already have a TH350 there. Yuu would save the most on gas this way if you plan on doing the cross country tour.

[C-10volvo]

it sounds like you are going for something like i already have.
my truck has a crate 290 hp gm 350 with the original 700r4 and 3.08 gearing. keep in mind i am running 235/75/15's on all 4's and at 65-70 mph i get around 22mpg.

i have a k and n filter aswell as a 3'' exhaust with highflow cat (gotta have it in oregon)

hope some of that info is useful...

[Slonaker]

One of the guys in the 67-72 section has a 4.3 liter V6 with fuel injection in a SWB. I think he has an overdrive transmission as well. I believe the 4.3 and the 305 make about the same amount of horsepower. That seems to me like the best option to optimize fuel economy and still have a little power.

Slonaker

[Jim85IROC]

Originally Posted by 79BIG10

Here's what I would do- Take the 80 and regear it to a 3.08 or lower numerical number if it doesn't have it already. The 77 would cost more with the 4x4. Keep the engines in them. The 6 cylinder would be good on gas but really hate it when going up hills etc. Stick with a easy to tune carb like Edelbrock 600cfm. You can tune it easily with a meter and jet kit. Keep it as a 4bbl. but keep your foot out of it. Like you mentioned use better exhaust, electric fans etc. The one thing that would definately help would be to get a 700r4 or even a 5 speed stick. 700r4 would be the easiest to swap in since you already have a TH350 there. Yuu would save the most on gas this way if you plan on doing the cross country tour.

Does an edelbrock carb have 4 same-sized butterflies like your typical holley, or does it have the big & littles like a Rochester? Reason I ask is because having the small main butterflies in a rochester will help you get good milage when you don't have your foot in it.

[Igotachevyor2]

rochester would be better gas mileage than the edelbrock primaries are smaller also the 305s are jetted different than the 350s stock rochester

[piecesparts]

Looking at some of the other entries into this foray and I am thinking that application of principal is the issue. First of all, evaluate what the motor needs to accelerate and achieve a running speed without using a large amount of fuel. If you gear to high (lower numerical numbers) or two low (high numerical numbers) then you hurt yourself in that realm. Second; if you run a to tall of tire or to short of a tire, then you throw the gear ratio off more. Third; what do you run for a tranny and what will work the best ---all around. Finally, what do you want for a fuel delivery system. Don't forget what you have on the vehicle for drag items for the motor to deal with (fans, alternator, A/C, tire pressure, and wind resistance).
Now to start with some of the things that I have tried or dealt with. I run a tire size around 27" tall (kind of small in your process) and I set up my gear ratio and tranny to that size. The trannys that I use are the 700R4 automatics, but if you look at the cars of today, the manual trannys (6 speeds) are much more efficient. This is due to the elimination of slippage in the torque convertor and the initiataion of overdrive gearing. The gear ratio in the rea axle is the MOST important. If you use something like a 3.08 geared rear end, with an overdrive tranny then you most probably are getting your engines RPMs to low to be useful or economical in an all around useage program. The reason I say this, my son's truck had that very same rear gearing and when we put the 700 R4 in it, we lost MPG. The installation of lower gears raised his mileage by 3 MPG. So a consideration of ratios is very important. If you keep the average GM V-8 motor RPMs at around 2100 to 2300 RPMs, then it is working at it's optimum power band. This will allow for the ability to pull the vehicle up to speed and maintain it without having to work hard to get it back up to that speed.

Next try looking at what is good for all around fuel economy in the fuel delivery. A TBI or TPI setup is the most economical, due to the sensors reading fuel in and exhaust out for balancing the useage. However, there have been articles written on individuals that can tune a carb to a very close tolerance and MPG realizations are good. The carb cannot adjust for changes in altitude, but most vehicles very rarely change locations that much. The cost of fuel injection compared to a carb is the decision pathway here.

The drag coefficient on the motor is another thing. The factory radiator cooling fan is major drag and elimination of that, by adding electric fans is a great change on the motor. However, you may have to add a little larger alternator, which will give some percentage of drag back. The A/C is a phenomenal amount of drag, --- Eliminate it totally. Tire pressure is a great drag problem also, keep it in the higher levels to lower drag. Wind resistance is a factor, many drivers working to achieve speeds around 200 MPH, in the past, have blocked the grille area off and fed air to the motor from a low point to get the resistance of the air caught in the grille area down and thus lower resistance. Lowering the vehicle eliminates air drag underneath. Air dams work well to keep a negative pressure under there, also. Another consideration is closing off areas of the truck bed, to eliminate drag issues. The guys that ran the GMC S-15 on the Bonneville Salt flats, put a tapered filler piece in their truck bed that went from the middle of the bed's floor to the top of the tailgate. Their wind tunnel tests showed them that this was the ultimate in wind/drag control. They ran that truck at 212 MPH on the flats.

[Captkaos]

My opinion get a 4.8L/4l60E out of a salvage yard. Previous owners practically give them away. You should be able to score one complete with a tranny for about $1000. The 4.8L runs around $300 used.

My stock '87 305 TBI/700R4 with over 100K miles (PO said it had 80K on it when I got it...) gets 22mpg on the hiway running 65mph. It has stock size tires and 2.73 gears.

Regardless of what you do with a 4x4, you won't be getting that kind of mileage.

[piecesparts]

I have a 383 stroker motor in my 84 with a 700R4 and 3.73 gears, that I get 15 MPG at an average of 70 MPH. My 91 5.7 liter, TBI, 700R4, and 3.42 gears gets 20 MPG at the same speeds. It can be done and with horsepower to boot. The weight is your worst enemy, as well as the wind resistance.

[84 400]

yes i defonatly agree with the above and say that your best bet is to steer clear of the 4x4 simply because of its added weight and addmovbing parts.

[79BIG10]

The Edelbrock uses the same butterfly sizes in both sets of the carb.

[piecesparts]

Edelbrock and Carter "AFB" style carbs are known as "Square Booe" carbs. That means all the butterfly plates and openings in that carb are the same size. (like a Holley square bore). The Edelbrock and Carter AFBs are actually mde for those companies by a company called Magnetti-Marelli under the "WEBER" line. This is a very economical and efficient line, since there are no diaphragms and vacuum lines to deal with on the secondaries. I have had a Carter carb on my truck with Edelbrock needles and seats installed in it. ( The Edelbrock price was cheaper for the same design parts) I have many miles on my carbs and they work real well.

Right now I have an Edelbrock Thunder AVS carb on my truck to get the 650 CFM size rating and a better air flow for my stroker motor.

[Pillow]

This may sound a little different, but why not try a 6.2 Diesel engine swap? Good mileage from what I understand and very cheap to aquire good used ones.

[magnethead]

Let me poke my head in after eading a couple reply's (not meaning to hijack a thread here, some of this may help others), i'm in a similar situation (10 MPG @ 50-60 MPH)

First of all, evaluate what the motor needs to accelerate and achieve a running speed without using a large amount of fuel. If you gear to high (lower numerical numbers) or two low (high numerical numbers) then you hurt yourself in that realm. Second; if you run a to tall of tire or to short of a tire, then you throw the gear ratio off more. Third; what do you run for a tranny and what will work the best ---all around. Finally, what do you want for a fuel delivery system.

I have 4.10's, stock tire size (~31"), TH400 right now, keep the carb.
Right now i'm running right at 3K RPM at 60 MPH- according to my math thats 300-400 RPM of slippage. What's the best automatic overdrive transmission (fairly stock, mind you) that can handle the occasional stresses i might give it (heavy pulls) with TH400 strength but gives me overdrive and a lockup convertor?

Don't forget what you have on the vehicle for drag items for the motor to deal with (fans, alternator, A/C, tire pressure, and wind resistance).

I still have the factory clutch fan, i think a 65 amp alternetor, the A/C is always off, need to check my tires, and i'm a brick in the wind. Since I'll have to replace the radiator and shroud before long, what is the closest known transplant in the way of electric fas from other vehicles?

The gear ratio in the rea axle is the MOST important. If you use something like a 3.08 geared rear end, with an overdrive tranny then you most probably are getting your engines RPMs to low to be useful or economical in an all around useage program. The reason I say this, my son's truck had that very same rear gearing and when we put the 700 R4 in it, we lost MPG. The installation of lower gears raised his mileage by 3 MPG. So a consideration of ratios is very important. If you keep the average GM V-8 motor RPMs at around 2100 to 2300 RPMs, then it is working at it's optimum power band. This will allow for the ability to pull the vehicle up to speed and maintain it without having to work hard to get it back up to that speed.

Again, I'm reading 3K. My power peaks in the mid to high 2000's. My math says overdrive will drop me to 2200 RPM with a lockup convertor. Am I correct to think that keeping it on te low side of the power curve is a good thing?

The drag coefficient on the motor is another thing. The factory radiator cooling fan is major drag and elimination of that, by adding electric fans is a great change on the motor. However, you may have to add a little larger alternator, which will give some percentage of drag back.

Back to the fans...Should I go up to a 85 amp alternator from the 65 or whatever i have now?

The A/C is a phenomenal amount of drag, --- Eliminate it totally.

Even when off?

Wind resistance is a factor, many drivers working to achieve speeds around 200 MPH, in the past, have blocked the grille area off and fed air to the motor from a low point to get the resistance of the air caught in the grille area down and thus lower resistance. Lowering the vehicle eliminates air drag underneath. Air dams work well to keep a negative pressure under there, also. Another consideration is closing off areas of the truck bed, to eliminate drag issues. The guys that ran the GMC S-15 on the Bonneville Salt flats, put a tapered filler piece in their truck bed that went from the middle of the bed's floor to the top of the tailgate. Their wind tunnel tests showed them that this was the ultimate in wind/drag control. They ran that truck at 212 MPH on the flats.

not much of that pertains to DD's. What CAN be done on a DD to help in that department?

[piecesparts]

The peak that you want for your RPMs and power curve should be around 2200 to 2400. The TH400 is a strong tranny, but there are numerous rebuilders that will tell you that it is a heavy tranny and uses more HP to turn over than most TH 350s and 700 R4s. The rear end gears you have with the 400 is a killer on gas, a 700 R4 or 200 R4 that has been built will stand up to a good motor, but you have to invest money into them, to get there. The update replacement for the TH400 is the 4L80E and it is really pricey to purchase. I am running 3.73 gears and my acceleration is quite efficient for my needs. I have a 383 stroker motor with a 2400 stall convertor (once again NON-lockup), with 3/73 gears and a posi rear end setup. I am getting around 15 MPG

The alternator that will handle electric fans should start at 100 amps and even go to 140 amps AND for a good reason. When you are at the stoplight --AT NIGHT, with the fans running, the headlights on, and the radio playing, you will want the charging power. I am presently running a 140 amp alternator, that carries 85 amps at an idle speed. Most alternators will only carry 35 amps at idle and that is why your fans slow down and your lights dim at the lights.

The situation on wind resistance is a realistic fact---WE DRIVE BRICKS. However, I discussed the truck at the Bonneville salt flats for a reason---The S-15 truck had a piece in the truck bed that ONLY filled the back corner of the bed. It ran from the floor at about the middle of the bed to the top of the tailgate. This was tested and it resulted in a smaller drag on the truck than using a full bed cover or even taking that tailgate off. I really hate those guys---ever eat one of their pieces of junk from their bed, because they are trying to save fuel by dropping the tailgate. The stuff that was used for the Salt Falts CAN be applied to the DD, because that is what we need to do to eliminate air resistance for us. I am using a full bed cover on both of my trucks and one is a soft cover, so I can roll it up when I need to haul big items. The thought of dropping the truck to the ground may be a big issue with some guys, but there is a reason for that too. The closer to the ground and the more air that is removed from under the truck, the lesser the resistance on the road. NASCAR is an excellent example of that. I had my bumpers and air dam off when I was doing body work, and I could feel the difference in the steering and resistance. The negative pressure under the truck helps on high speed driving control also.

[magnethead]

I wonder if something like that could be made using sheet metal and 2x4's (to make a deck lid type thing), hinge it in the middle so it can be folded up in half against the tailgate, and make it use the stake pockets so it's removable.

[piecesparts]

Why not use some aluminum square tubing and bolt it together in the corners, then put an aluminum sheet metal cover or just standard sheetmetal on the piece. We have a place in Wichita, Kansas; that sells scrap metal in aluminum or sheetmetal for a great price, I would think that there should be such a place relatively close to you. The square tubing is in every hardware store around here in their millstock area. This will keep the cover light weight and easy to move around.
Make the cover and just have it where it pins into the bed at the stake pocket area and has a couple of pins that go throught the truck bed floor. Design it so that it fits tight, so it does not rattle or llift up from the floor. Use the area under it for storage of items that you don't want people to see.

[84 400]

nice4 idea man^^^^^^^^.
it sounds like a good idea nad if your driving a 4 door dualkly it seems like it is worht the afternoons work to me.

[magnethead]

Good point.

Here's my CAD to a general sense of how it could be worked..