MPG 302 vs 351
#16
01-01-2016, 08:55 AM
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We had two EB Broncos, a '93 5.0 and a '94 5.8. Both 3.55s, same size tires, etc. The 5.8 consistently got 2 MPG less than the 5.0 (13-15 w/5.8, 15-17 w/5.0). I had no complaints with either motor, and while there is no doubt the 5.8 has more torque (and HP), it didn't feel much different from the 5.0. Towed relatively heavy, for a Bronco, with both and again, no real discernible performance difference without driving them back to back.
#17
01-01-2016, 09:16 AM
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We had two EB Broncos, a '93 5.0 and a '94 5.8. Both 3.55s, same size tires, etc. The 5.8 consistently got 2 MPG less than the 5.0 (13-15 w/5.8, 15-17 w/5.0). I had no complaints with either motor, and while there is no doubt the 5.8 has more torque (and HP), it didn't feel much different from the 5.0. Towed relatively heavy, for a Bronco, with both and again, no real discernible performance difference without driving them back to back.
#18
01-01-2016, 09:21 AM
Logistics Pro
Engine inefficiencies are obviously the key to why one engine will get different mileage than another in the same truck. Same truck, same speed, same load, same amount of power/work needed, so same amount of fuel USED. So the difference has to be in how much fuel is WASTED.
Raising compression ratio and increasing timing advance are two things that will improve engine efficiency, until the engine starts to knock, when it drops off an edge. Obviously the more you do of the one, the less you can do of the other, so finding the best balance of both is an art.
Leaning out the mixture also improves efficiency, until it gets too lean. Not such an issue with EFI because the computer does a pretty good job of optimizing it.
Internal friction is another source of inefficiency. All else equal, bigger displacement engines have more ring area traveling farther, so they have more friction. So bigger engines lose there.
Combustion efficiency is another source of loss. Engines are generally the most efficient at turning fuel into work at the torque peak (that's why it's the torque peak is there). So running near the torque peak helps your efficiency.
Pumping losses are another source, that can be kind of non-intuitive. Gas engines have throttles. It takes energy to drag air past a closed throttle plate. So an ENGINE is most efficient at wide-open throttle. Emphasis on the "engine" because WOT makes more power, more power uses more fuel, so the VEHICLE may (probably will) get fewer miles per gallon, but a higher percentage of the BTUs in the fuel are going into doing work on the vehicle, so the engine has a lower brake specific fuel consumption.
And this points to another reason small engines use less gas in a given vehicle. If it takes 25 horsepower to push a truck at a given speed, a 460 will do that with the throttle barely cracked open while a 300 needs the throttle to be opened a lot farther for the same power output. More open throttle, less pumping losses, less fuel used.
Raising compression ratio and increasing timing advance are two things that will improve engine efficiency, until the engine starts to knock, when it drops off an edge. Obviously the more you do of the one, the less you can do of the other, so finding the best balance of both is an art.
Leaning out the mixture also improves efficiency, until it gets too lean. Not such an issue with EFI because the computer does a pretty good job of optimizing it.
Internal friction is another source of inefficiency. All else equal, bigger displacement engines have more ring area traveling farther, so they have more friction. So bigger engines lose there.
Combustion efficiency is another source of loss. Engines are generally the most efficient at turning fuel into work at the torque peak (that's why it's the torque peak is there). So running near the torque peak helps your efficiency.
Pumping losses are another source, that can be kind of non-intuitive. Gas engines have throttles. It takes energy to drag air past a closed throttle plate. So an ENGINE is most efficient at wide-open throttle. Emphasis on the "engine" because WOT makes more power, more power uses more fuel, so the VEHICLE may (probably will) get fewer miles per gallon, but a higher percentage of the BTUs in the fuel are going into doing work on the vehicle, so the engine has a lower brake specific fuel consumption.
And this points to another reason small engines use less gas in a given vehicle. If it takes 25 horsepower to push a truck at a given speed, a 460 will do that with the throttle barely cracked open while a 300 needs the throttle to be opened a lot farther for the same power output. More open throttle, less pumping losses, less fuel used.
#19
01-01-2016, 09:27 AM
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Engine inefficiencies are obviously the key to why one engine will get different mileage than another in the same truck. Same truck, same speed, same load, same amount of power/work needed, so same amount of fuel USED. So the difference has to be in how much fuel is WASTED.
Raising compression ratio and increasing timing advance are two things that will improve engine efficiency, until the engine starts to knock, when it drops off an edge. Obviously the more you do of the one, the less you can do of the other, so finding the best balance of both is an art.
Leaning out the mixture also improves efficiency, until it gets too lean. Not such an issue with EFI because the computer does a pretty good job of optimizing it.
Internal friction is another source of inefficiency. All else equal, bigger displacement engines have more ring area traveling farther, so they have more friction. So bigger engines lose there.
Combustion efficiency is another source of loss. Engines are generally the most efficient at turning fuel into work at the torque peak (that's why it's the torque peak is there). So running near the torque peak helps your efficiency.
Pumping losses are another source, that can be kind of non-intuitive. Gas engines have throttles. It takes energy to drag air past a closed throttle plate. So an ENGINE is most efficient at wide-open throttle. Emphasis on the "engine" because WOT makes more power, more power uses more fuel, so the VEHICLE may (probably will) get fewer miles per gallon, but a higher percentage of the BTUs in the fuel are going into doing work on the vehicle, so the engine has a lower brake specific fuel consumption.
And this points to another reason small engines use less gas in a given vehicle. If it takes 25 horsepower to push a truck at a given speed, a 460 will do that with the throttle barely cracked open while a 300 needs the throttle to be opened a lot farther for the same power output. More open throttle, less pumping losses, less fuel used.
Raising compression ratio and increasing timing advance are two things that will improve engine efficiency, until the engine starts to knock, when it drops off an edge. Obviously the more you do of the one, the less you can do of the other, so finding the best balance of both is an art.
Leaning out the mixture also improves efficiency, until it gets too lean. Not such an issue with EFI because the computer does a pretty good job of optimizing it.
Internal friction is another source of inefficiency. All else equal, bigger displacement engines have more ring area traveling farther, so they have more friction. So bigger engines lose there.
Combustion efficiency is another source of loss. Engines are generally the most efficient at turning fuel into work at the torque peak (that's why it's the torque peak is there). So running near the torque peak helps your efficiency.
Pumping losses are another source, that can be kind of non-intuitive. Gas engines have throttles. It takes energy to drag air past a closed throttle plate. So an ENGINE is most efficient at wide-open throttle. Emphasis on the "engine" because WOT makes more power, more power uses more fuel, so the VEHICLE may (probably will) get fewer miles per gallon, but a higher percentage of the BTUs in the fuel are going into doing work on the vehicle, so the engine has a lower brake specific fuel consumption.
And this points to another reason small engines use less gas in a given vehicle. If it takes 25 horsepower to push a truck at a given speed, a 460 will do that with the throttle barely cracked open while a 300 needs the throttle to be opened a lot farther for the same power output. More open throttle, less pumping losses, less fuel used.
#20
01-01-2016, 10:55 AM
If you increase your torque to a point that your truck is running as close to idle in 5th at about 60 mph as possible then it'll help with the gas mileage. All I did with mine for now is put a Comp Cams 35-255-5 cam in it, put an Edelbrock Performer truck intake on it, bored it .030 over, and shaved .010 off the head deck on the stock E-7 heads. One thing you can do to help it is get a set of heads off of a 1999-2000 Ford Explorer 302, I was going to do that until I decided that I was going to put a 408 stroker kit in mine and knew that I'd need bigger heads, but for a basically stock 351 they're excellent and the only thing that you'll have to do to make them fit is get the head bolts holes drilled out because a 351 uses bigger diameter head bolts, but I'd suggest shaving .010 off the head deck because 351's have always had low compression and the only other ways to increase compression is to bore the block, put in flat top or domed pistons, or both. The intake and cam that I put in mine increased the torque by about 70 ft lbs, I also put a set of long tube headers on it that increased the torque by about another 25 ft lbs, and I don't know what the shaving the heads or boring the block did for it, but it has so much power going to the tires now that it's hard to take off without wheel hopping the rear end, so it all done some good ****, more than enough addition to power to make up for the 2 miles to a gallon loss by going back to the 351. I can't wait until next year when I put my 408 kit in it and my new big valve heads on it, then it's really going to be a monster. Oh, since yours is a '97 351 then you can put a hydraulic roller cam in yours and get more juice out of a basically stock motor than I did with the cam that I put in, probably not a major difference, but it will probably help yours on gas mileage over mine and add probably another 5-10 hp because mines just a hydraulic mechanical cam which means mine has more friction on the bearings than what yours would, but if you get the hydraulic roller version of mine I guarantee that you'll love the torque gain.
In the meantime I am in the process of a free flowing 2.5 inch mandrel bent exhaust, efan, and a "6 liter tuneup" for the improved spark and efficiency improving timing bump.
#21
01-01-2016, 11:04 AM
You guys are totally overthinking the issues here and making some bad assumptions. With everything else being equal.. same truck with the same trans, gearing, and tires, it doesn't matter what engine is under the hood it will turn the same rpms at any given speed and it will rev to the same point before shifts because that is how the trans is calibrated. So in stop and go traffic the larger the displacement the more fuel it'll burn simple as that.
At highway cruising speeds again engine rpms will be the same for either engine, it can't be different because everything else is the same, and given that both of these engines wear the same heads and use the same injectors fueling is going to be pretty close to the same.. not identical but at these relatively low rpms it's pretty close. So it is possible that the larger displacement motor could actually get better overall hwy milage simply because it makes more torque at that rpm and the trans will downshift less on grades and such. But again the numbers are going to be pretty close between the two so the 5.0 will likely do better on totally flat ground while the 5.8 may do slightly better in a little more challenging terrain.. rolling hills and such.
I had both motors in my '90 with similar mods(cam, head work, longtubes) and the results I saw were right in line with this, the 5.8 was about 3sec faster 0-60 and a lot more fun to drive and both motors got nearly identical fuel milage with the 5.0 doing slightly better overall.
At highway cruising speeds again engine rpms will be the same for either engine, it can't be different because everything else is the same, and given that both of these engines wear the same heads and use the same injectors fueling is going to be pretty close to the same.. not identical but at these relatively low rpms it's pretty close. So it is possible that the larger displacement motor could actually get better overall hwy milage simply because it makes more torque at that rpm and the trans will downshift less on grades and such. But again the numbers are going to be pretty close between the two so the 5.0 will likely do better on totally flat ground while the 5.8 may do slightly better in a little more challenging terrain.. rolling hills and such.
I had both motors in my '90 with similar mods(cam, head work, longtubes) and the results I saw were right in line with this, the 5.8 was about 3sec faster 0-60 and a lot more fun to drive and both motors got nearly identical fuel milage with the 5.0 doing slightly better overall.
#22
01-01-2016, 11:29 AM
I am basing this on my manual ZF5 which can be left in overdrive (5th) if I choose.
True, stop and go will eat fuel regardless, but the goal is to improve the bulk of normal uninterupted highway driviing mpg like most long trips or highway commutes would be. If someone commutes through a big city, they might as well get a prius.
True, stop and go will eat fuel regardless, but the goal is to improve the bulk of normal uninterupted highway driviing mpg like most long trips or highway commutes would be. If someone commutes through a big city, they might as well get a prius.
#23
01-01-2016, 12:10 PM
Well there is lots to gain with upgrades on both of these motors, none of the stock cams optimize airflow through the stock heads so the cylinders don't move as much air as they could and the motor therefore doesn't make as much torque as it could(poor pumping efficiency). The more torque you can generate from any given motor the better the fuel milage will be so cam changes, exhaust upgrades, and cylinder head improvements will all reap rewards.
The E6SE heads found on the '86 5.0 had a deep kidney bean shaped chamber that produced a lot of swirl and thus better A/F mixture so they are can generate more torque and are less detonation prone. Only problem is the chambers are too deep and shroud the valves badly and the ports are also much too small so these heads have terrible overall airflow. I had a set that were heavily ported and they were great torque producers but a completely untouched E7TE casting easily out flowed it so I wouldn't suggest using them, but you will notice all the higher quality aftermarket heads have shallow kidney bean shaped chambers of some sort.
Short of changing heads which isn't exactly a minor tweak, 1.7 rockers and headers are your best upgrades for the stock motor, longtubes are best for low rpm torque production but they can be difficult or impossible to fit with the solid front axle and leaf spring suspension so shorties are the next best option.
The E6SE heads found on the '86 5.0 had a deep kidney bean shaped chamber that produced a lot of swirl and thus better A/F mixture so they are can generate more torque and are less detonation prone. Only problem is the chambers are too deep and shroud the valves badly and the ports are also much too small so these heads have terrible overall airflow. I had a set that were heavily ported and they were great torque producers but a completely untouched E7TE casting easily out flowed it so I wouldn't suggest using them, but you will notice all the higher quality aftermarket heads have shallow kidney bean shaped chambers of some sort.
Short of changing heads which isn't exactly a minor tweak, 1.7 rockers and headers are your best upgrades for the stock motor, longtubes are best for low rpm torque production but they can be difficult or impossible to fit with the solid front axle and leaf spring suspension so shorties are the next best option.
#24
01-01-2016, 12:35 PM
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So that comp cams is a "torque cam" then? I suppose if it put my torque peak right at highway (2krpm?) cruising speed I could see some gains.
In the meantime I am in the process of a free flowing 2.5 inch mandrel bent exhaust, efan, and a "6 liter tuneup" for the improved spark and efficiency improving timing bump.
In the meantime I am in the process of a free flowing 2.5 inch mandrel bent exhaust, efan, and a "6 liter tuneup" for the improved spark and efficiency improving timing bump.
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