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  1. #1
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    Engine questions. Any armchair engineers/mech?

    Hey guys, I drive a 2000 Chevy Blazer with 125xxx miles on it, I get "decent" mileage for what my truck is, 4.3L V6, 4x4, 4000+lbs body on frame ect. I average around 22-24 mpg combined city/hwy. This is irreverent to my question.

    I was wondering hypothetically, what gets better gas mileage in the same vehicle? A much higher revving 4 cylinder or a torque-y much lower revving V6? Same gearing, the V6 might cruise at 1700-1800 rpm or the 4 cylinder might cruise at 2500 rpm.

    Granted the V6 is pushing 2 bigger, heavier cylinders all the time but its not working as hard as the 4 cylinder

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    There are about a zillion factors that come into play, but you can get some fantastic gas mileage out of a v8 on the highway.
    I have an 86 mustang GT that on long trips would get 29-31mpg on the highway.
    I also have a 91 galant (turbo awd) that has never gotten better than 22mpg ever.
    Both cars are about the same weight and both pretty much lack any aerodynamic characteristics
    The mustang cruises at like 1600rpms on the highway, the galant often crosses the 4k rpm line. Both are 5 speeds.
    Even my 6 speed miata with a dinky 1.8L in a car that weighs like 2500lbs only gets 25mpg on the highway, but again 4k rpm's is normal.

    But around town the miata still gets 26mpg but the mustang gets like 15

    I also have a 91 suzuki bandit 400 that gets 17mpg on the highway (100mph = 10,000 rpms) but would get like 35 mpg around town. haha I learned to stay off the highway, it wasn't geared for it at all and even if it was, it didn't have enough power to maintain highway speed.

  3. #3
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    Chevrolet 4 cylinders are not known for outstanding mileage in anything other than compact vehicles .

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    my 05' tacoma 4cyl gets 25-28 mpg a equal V6 tacoma gets 18-21 mpg explain that......
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    Biggest factor would be how you drive it. You could get better mileage with a less powerful engine. I get around 15 mpg highway in my 2400 pound low power car, but it has no overdrive.

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    I6 turbo diesel ftw.

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    I'm an engine builder. I design, build and test high performance engines for a living that are used in street rods, drag, rally and road race cars.

    It's all about where the engines' most efficient RPM is. A smaller engine will typically be at peak efficiency at a higher RPM than a larger motor. This is where driving makes the biggest difference. An example is someone that lugs the motor by being in too high of a gear, or not downshifting. Yes, the engine is turning fewer RPM but it must work MUCH harder to move the mass of the vehicle and burns much more fuel. This similar scenario also rears it's ugly head in the form of underpowered vehicles. Small engine, heavy vehicle means the engine has to work nearer to it's max potential ALL the time which kills mileage. It's kinda like you pedaling your bike and always having to use the big ring.

    The auto makers and EPA need to get together on the same page. My V6 Tacoma struggles to get over 21mpg on the highway yet in other countries the same truck can be had with a turbo diesel that gets over 30mpg.

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    Turbo Diesel Ftw!!!!!!

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    Quote Originally Posted by ae111black
    my 05' tacoma 4cyl gets 25-28 mpg a equal V6 tacoma gets 18-21 mpg explain that......

    Apparently you cannot tell the difference between CHEVROLET and TOYOTA .

  10. #10
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    THIS is where turbos come into play. The 4cyl would do "better" if you didn't have to drive very fast or move a lot of heavy cargo, and even if you did you'd have to do those things quite a bit to make the milege worse than with the 6cyl (referenced by the earlier note, but you'd have to be driving the "weaker" 4cyl "hard" to make it worse), but if you have a turbo-4 you would have much better efficiency than the 6cyl when you didn't "need" the extra power, and you'd have just as much power as the 6cyl when you did need it.

    On the other hand, an SUV/truck like that has the aerodynamics of a brick (and it's probably not light) and the efficiency isn't going to be great no matter what engine you put in it. This is why you might not see a big change no matter what you do.

    Those new corvettes are pretty attractive, big V8, around 30mpg on the highway (reported by most owners). Even though the engine's RPM range plays a big role as explained before, the corvette is one of THE slickest/small frontal area-cars out there on the road, so very little drag and relatively good efficiency, especially for that kind of car. The newest corvettes seem to be the american car you go to when you're done screwing around with the "pretend" american sports-cars, like the overweight camero and charger.
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    I agree completely turbo diesels are where the best fuel economy/power ratio is at. My next car purchase (when I graduate in 2ish years and hold a job) will be a TD, I am hoping for a subaru forrester/outback or toyota truck

    And I guess i neglected to think about what RPM an engine is most efficient. I assumed it was always a lower RPM. So thanks Tony B for that insight

    and also I shouldnt have even posted my current vehicle, i was going for the more hypothetical approach, same car 2 different engines same gearing what gets better mileage?

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    Quote Originally Posted by Jayem
    THIS is where turbos come into play. The 4cyl would do "better" if you didn't have to drive very fast or move a lot of heavy cargo, and even if you did you'd have to do those things quite a bit to make the milege worse than with the 6cyl (referenced by the earlier note, but you'd have to be driving the "weaker" 4cyl "hard" to make it worse), but if you have a turbo-4 you would have much better efficiency than the 6cyl when you didn't "need" the extra power, and you'd have just as much power as the 6cyl when you did need it.

    On the other hand, an SUV/truck like that has the aerodynamics of a brick (and it's probably not light) and the efficiency isn't going to be great no matter what engine you put in it. This is why you might not see a big change no matter what you do.

    Those new corvettes are pretty attractive, big V8, around 30mpg on the highway (reported by most owners). Even though the engine's RPM range plays a big role as explained before, the corvette is one of THE slickest/small frontal area-cars out there on the road, so very little drag and relatively good efficiency, especially for that kind of car. The newest corvettes seem to be the american car you go to when you're done screwing around with the "pretend" american sports-cars, like the overweight camero and charger.
    I dont know so much if the charger/camaro are supposed to be sports cars per say, but more muscle cars reminiscent of "back in the day." They do emulate those quite well, big heavy cars with brute power, not the quickest things in the world but Im sure they bring a smile to your face at a stoplight

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    What you are looking for is an Efficiency Map for your engine. An example of one is here: http://rubydist.com/images/EngEffCurve.pdf.

    Different engines have different points where they are most efficient. One of the problems with smaller engines not providing significantly better mileage in some vehicles is the transmission, not a function of the smaller engine by itself. The transmission can make it difficult to operate the engine at its best point if there isn't a gear and rpm combination that lines up with the speed you wish to travel.

    GM, Pontiac specifically, was famous for offering smaller (cheaper) engined versions of the same car without any performance benefit. Some thought this was to better entice buyers to upgrade to the larger engined option to increase profits. Others thought the price difference between the 4 and 6 cylinder engines wasn't enough of an offset, so an even cheaper (few gears) transmission was also paired with the 4-cylinder option to bring overall vehicle price down. This compromise ended up killing the mpg difference in the end.

    Theoretically, which ever gasoline engine is operating at wide open throttle (WOT) is going to be more thermodynamically efficient because it will be experiencing less pumping losses through the throttle. (WOT is not the same as max rpm though.) This is another reason why the Audi R10 was able to go so long between refueling, because under caution, the diesel engine was experiencing fewer throttling losses than its gasoline engined competition.

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    Its all about finding what engine will run the most efficient for the vehicle. In some cases a v8 will be more efficient than a 4cyl or 6cyl, take the 98-02 Camaro's the V8 would get 3-4mpg better than the v6 do to the the v8 being able to do the same MPH, but at much lower RPM. Now with that being said dont think that the lower RPM the better thats not the case. You want to find where the torque curve comes on, whether it be 1000 rpm or 5000rpm, b/c lugging a motor will kill MPG quick.

    That being said if I where going to swap in a engine in the blazer, nothing really beats a 4bt Cummins w/ the ppump inction setup for power and great MPG
    s.

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    Quote Originally Posted by spazzy
    Hey guys, I drive a 2000 Chevy Blazer with 125xxx miles on it, I get "decent" mileage for what my truck is, 4.3L V6, 4x4, 4000+lbs body on frame ect. I average around 22-24 mpg combined city/hwy. This is irreverent to my question.

    I was wondering hypothetically, what gets better gas mileage in the same vehicle? A much higher revving 4 cylinder or a torque-y much lower revving V6? Same gearing, the V6 might cruise at 1700-1800 rpm or the 4 cylinder might cruise at 2500 rpm.

    Granted the V6 is pushing 2 bigger, heavier cylinders all the time but its not working as hard as the 4 cylinder
    As the others have said there are a lot of variables.
    All other things being equal though a smaller engine will usually give better gas mileage as long as it isn't so small that you are flooring it all the time and the fuel injection is enriching the mixture.

    The reason for this is a smaller engine will need more manifold pressure to give the same power output which will mean lower pumping losses.

    Ideally you wouldn't want a smaller engine that you rev high, you'd want a smaller engine at the same revs.

  16. #16
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    Quote Originally Posted by willevans
    What you are looking for is an Efficiency Map for your engine. An example of one is here: http://rubydist.com/images/EngEffCurve.pdf.

    Different engines have different points where they are most efficient. One of the problems with smaller engines not providing significantly better mileage in some vehicles is the transmission, not a function of the smaller engine by itself. The transmission can make it difficult to operate the engine at its best point if there isn't a gear and rpm combination that lines up with the speed you wish to travel.

    GM, Pontiac specifically, was famous for offering smaller (cheaper) engined versions of the same car without any performance benefit. Some thought this was to better entice buyers to upgrade to the larger engined option to increase profits. Others thought the price difference between the 4 and 6 cylinder engines wasn't enough of an offset, so an even cheaper (few gears) transmission was also paired with the 4-cylinder option to bring overall vehicle price down. This compromise ended up killing the mpg difference in the end.

    Theoretically, which ever gasoline engine is operating at wide open throttle (WOT) is going to be more thermodynamically efficient because it will be experiencing less pumping losses through the throttle. (WOT is not the same as max rpm though.) This is another reason why the Audi R10 was able to go so long between refueling, because under caution, the diesel engine was experiencing fewer throttling losses than its gasoline engined competition.
    Yup simple....

    The key to the map is this, most engines have peak efficiency near the torque peak...So to get the most efficient performance from the engine operate at an RPM slighty above torque peak....AND AT WIDE OPEN THROTTLE....That means a tiny engine if your going to cruise around at 60 mph....That is why disabling cylinders works so well.

    Next efficient lose the WOT, now you can have a big engine and tool around at 60 mph...

    My old Suburban has a torque peak about 2200 RPM the auto tranny works pretty well to hold the engine speed close to this mark....it gets about 22 to 25 MPH....

    We buy stationary gas engines to drive compressors we try to hold the load at about 50% to 85% of peak power, the torque curves are very flat, and the engines operate between say 900 to 1200 rpm, a very small range....for efficient operation.

  17. #17
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    Quote Originally Posted by spazzy
    I dont know so much if the charger/camaro are supposed to be sports cars per say, but more muscle cars reminiscent of "back in the day." They do emulate those quite well, big heavy cars with brute power, not the quickest things in the world but Im sure they bring a smile to your face at a stoplight
    Well, a better power-to-weight ratio puts an even bigger smile on my face.
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  18. #18
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    Quote Originally Posted by willevans



    Theoretically, which ever gasoline engine is operating at wide open throttle (WOT) is going to be more thermodynamically efficient because it will be experiencing less pumping losses through the throttle. (WOT is not the same as max rpm though.) This is another reason why the Audi R10 was able to go so long between refueling, because under caution, the diesel engine was experiencing fewer throttling losses than its gasoline engined competition.
    That's exactly why I floor it up to the speed limit everywhere.

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