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Hey, I have a 350 jasper rebuild that has 20,000 miles in it. Its in my 87' Chevy truck. All stock. I want to get as good of fuel economy as possible within reason. Any Mod ideas, building it up for mpg? Right now I'd guess it gets about 12. Thanks.
 

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Really could use more info about the truck and its regular usage. Big tires? 2 or 4 wheel drive, gearing, weight(do you carry a load regularly).

What transmission?

Saving fuel comes down to building low rpm torque and keeping the motor at the lowest rpm to do the job needed.

Start with doing a complete tune up( and a compression or leak down test), to make sure the motor is in tip-top shape.

EFI is the best path to MPG, if youre running a carb., a Q-Jet or a spread bore vacuum secondary carb(holley or Demon) work best for maximizing mpg with still having the ability to make power when needed.

Small tube headers(1 1/2" or 1 5/8") help build low end torque

Adjusting you driving technique is the least expensive and quickest way to improve MPG
 

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As was already asked in what truck model you have to work with, the potential for example of a two wheel drive half ton standard cab short box would be far ahead of what an extended cab long box 4x4 could ever dream of.

FmrStrtracer's last statement is one huge factor in the end result beyond being mechanically sound and how the truck is equipped gearing wise, etc. If one is looking for ultimate fuel mileage it comes down to driving like the proverbial old grandpa or another words envision attaching a raw egg to your throttle pedal and driving your truck without breaking it. I see it so often and with a lot of women too that are very aggressive drivers and can not emotionally handle it if someone passes them in town or on the open highway, that line of thinking never equates to good mileage !.

Some other items to look at on your truck, lets say its missing the front air dam under the bumper, install one as the less air that gets under a vehicle the less air drag. If the truck was lifted, lower it. Maintain as much air pressure in the tires as you can without causing uneven wear. Front wheels aligned, synthetic gear oil in the diff, a thinner but recommended viscosity of oil in the engine if a very heavy viscosity oil was used, use an engine fan that uses the least amount of energy but can still do the job of cooling the engine. Even removal of the tail gate although that may be impracticable as it reduces drag and weight. Even such a thing as a truck that was equipped with large mirrors like a highway tractor would use, take them off and use small ones for less wind drag.

I see before I posted, you have a 3/4 ton and assume its a light duty model as per the light duty transmission. I also assume that model would not come with the deep blade/high capacity cooling fan but I could be wrong.
 

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Merc, I would point you back to FmrStrtracer as he would know far more then I on carbs and most likely how to set them up tuning wise. All I know are comments from even many years ago on the misconception that a two barrel would produce better mileage then a four barrel when in reality a well set up four barrel used on ONLY the two barrels opening partially produced the best mileage because of the small bore size and high vacuum/velocity.

Whatever you may decide to put on, it will have to be rebuilt and/or tuned properly as otherwise its not worth doing.
 

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First figure out what mpg you are getting (no guessing). And how much is city and how much is highway driving.

Got a timing timing light? Timing is very important for mpg.

2 barrel carbs are not good for mpg. A Q-jet with a shock 4 barrel intake would be much better.

Headers help.

Check line pressure on the 700r.

3/4 ton trucks are heavy. 1000 lbs heavier than a 1/2 ton. I would not expect to get more than 10 mpg around town and 16 on the highway. If you are getting 12 mpg overall (city and highway), then it is doing well for a 3/4 ton truck.

Do you know what gear ratio (3.73, 4.11) and what size tire (for example 275-70 16)???
 

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Hey, I have a 350 jasper rebuild that has 20,000 miles in it. Its in my 87' Chevy truck. All stock. I want to get as good of fuel economy as possible within reason. Any Mod ideas, building it up for mpg? Right now I'd guess it gets about 12. Thanks.
Maybe start with an actual MPG number so you know if changes you're making made a difference?

How about giving some real details instead of having us try to ask you enough questions so we know?

Simple answer:
- build for low end, setup the drivetrain to match (the 700r4 has a low first which lets you get away with less gear in the rear, and an OD that helps even more)
- get as much cylinder pressure (comression, timing, valve timing...) as the fuel you want to run will tolerate
- if carb find something with the smallest primaries possible, with the bores open more at cruise you'll get better mpg
 

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Hey, I have a 350 jasper rebuild that has 20,000 miles in it. Its in my 87' Chevy truck. All stock. I want to get as good of fuel economy as possible within reason. Any Mod ideas, building it up for mpg? Right now I'd guess it gets about 12. Thanks.
Read through this, big truck (not huge) with a 350 and a two barrel, 12 seems decent for what it is.

A Jasper rebuild is probably pretty close configurationally to the OEM engine, that is low compression, mild cam.

The tranny should be a 700R4 with a lock up converter, is it? If so does the lock up work?

Where you drive has a lot to do with this stop and go eats more gas than freeway cruise speeds. Stop and go with a 700R4 should be about as good as it gets with the deep low of those transmissions makes it easy for the engine to move the machine without a lot of throttle. If there is a lot of freeway travel you need the lock up converter and 4th overdrive to extract the best mileage. Lockup converter itself should be a 10% improvement.

Increasing compression helps mileage the 87 350 was a pretty low compression engine, I expect that your Jasper is like all others I've seen where the deck has been zeroed and low height rebuilder pistons used along with a large chamber head. Problem with pushing the thermal efficiency up with compression is the cost of getting there probably won't be repaid in improved mileage over the remaining life span of the vehicle.

The old stand by of headers and dual exhaust offers some improvement to mileage if your driving habits (fast) and use cycle (i.e. more freeway that stop and go) tend to push the existing systems capabilites to where there is a back pressure against the pistons from using power to pump the exhaust overboard. But if you're a mild driver and the engine never gets into more than moderate RPMs and throttle settings there probably isn't enough improvement here to off set the system's cost.

I doubt a 4 barrel conversion would offer sufficient improvement to pay its way on either.

All-in-all 12 mpg for a 350 pushing this truck with a carb rather than Electronic Fuel Injection doesn't strike me as all that bad. If this was 9 mpg, which is pretty typical of a carbed 350 in a full size 3/4 ton truck, then I'd say go to work on it. But where it's at sure sounds pretty good to me and not cost effective from here to chase improvements assuming 1 or 2 mpg can be had. A data point for you, I run a 350 built for 400 horses on the engine dyno that is port EFI, high compression small chamber heads, D dish pistons, moly rings connected to a 4L60E with a 3.08 final, 1995 Z28 rims and tires in a slammed, 2 wheel drive 1989, S15, short bed for my daily driver. It gets 18 to 20 depending on how the ratio of city to freeway splits for any given tank of gas and if I've kept my foot out of the throttle. A couple "I'll show that import a thing or two" excersions will bring that average down to 13-15 really easy. It's not bad mileage for a 350 but it's in a small, light truck with a ton of expensive Rocket Science in it.

Bogie
 

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As mentioned, the tune is very important to mileage. What distributor is in use? If it's a HEI, does it have mechanical and vacuum advance?
2 barrels can get decent mileage, however a Quadrajet would be better.
If it were I, I would do the following. Change the rear diff fluid to full synthetic. Change the motor oil to full synthetic. Tune it up with the best pieces you can afford. I recommend spiral core spark plug wires and not the cheapy parts store solid core wires. Route the wires carefully.
If the truck has dual exhaust, have a cross over pipe added, if it doesn't have dual exhaust I would recommend doing so. Go over the spark timing.
With the vacuum advance disconnected and plugged, set the base timing to 12 degrees before top dead center. Reconnect vacuum advance, connect it to a full time vacuum source, or one that pulls vacuum at idle. Recheck the timing, it should be 18 to 24 degrees before top dead center.
Add a K and N air filter. Pack the front wheel bearings with synthetic grease.
Adding a set of small tube full length headers would also be a step in the right direction. Also as mentioned if the lockup converter function is not in use I would get a kit for it as the stock converter without lockup is not real efficient.
 

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If you were going to go EFI I would seriously consider a late 80's-mid90's TBI swap. It should be about as close to a bolt on to that setup as you'll find, you should be able to get the parts for cheap (should really be cheaper than a 4bbl carb swap unless you get really lucky with used parts), and will get about as good a mileage as you'll get out of it (you can get some killer MPG numbers out of them if you get them working right). The setup is all about low end, efficiency and driveablility and was used in heavy trucks and full sized cars with 305/350 from the factory.
 
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