Why are Rotary's Fuel Hungry?
12-05-04, 11:04 PM
#1
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Why are Rotary's Fuel Hungry?
I never really got down too the True Reasons. Why Do they Require So much Fuel..I know if they run lean they Explode..However Im Sure with computers they can run 25mpg.. (with easy driving)...Anyone with the Real Reason rotary motors suck so much gas?
12-05-04, 11:48 PM
#3
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Originally Posted by Adam
I never really got down too the True Reasons. Why Do they Require So much Fuel..I know if they run lean they Explode..However Im Sure with computers they can run 25mpg.. (with easy driving)...Anyone with the Real Reason rotary motors suck so much gas?
16-17 city with mixed thottle input.
No complaints here. Some beg to differ on spent fuel.
12-05-04, 11:57 PM
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I think that the reason most 7's have lower gas mileage is because of how the drivers drive not the cars. Of course there not the best on gas but they are sports cars not a little civic made for gas mileage
12-06-04, 12:36 AM
#7
Originally Posted by Relisys190
I've a stockport engine with a Microtech Lt-8 running 168rwhp and STILL getting 19-20 MPG on the highway.
16-17 city with mixed thottle input.
No complaints here. Some beg to differ on spent fuel.
16-17 city with mixed thottle input.
No complaints here. Some beg to differ on spent fuel.
19mpg on the highway with a microtech and stock ports?
i got 22mpg on the way back from rotary resurrection (5 hour drive) on a large streetport and the stock ecu...
you should be able to get ATLEAST 25mpg tuning with a microtech on stock ports, cruising on the highway anyways.
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12-06-04, 12:40 AM
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Its just the way the motor is designed. But about the way you drive it doesnt really matter me and two other guys have try going easy on it and going hard on it and it really doesnt matter how you drive her. she uses the same amount of gas both ways.
A old rotary guru once told me "It costs money to have fun" and thats the way i look at it.
A old rotary guru once told me "It costs money to have fun" and thats the way i look at it.
12-06-04, 01:04 AM
#10
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Originally Posted by 300zx
she uses the same amount of gas both ways.
A old rotary guru once told me "It costs money to have fun" and thats the way i look at it.
A old rotary guru once told me "It costs money to have fun" and thats the way i look at it.
12-06-04, 01:41 AM
#11
Savanna Rx-7
emissions stuff also
The stock emissions of certain years makes a big difference also. Originally it was all about co, then nox started being a problem. The way a rotary is designed the trailing area right behind the apex seals causes the rotary to put out more nox emissions on a peripherly ported exhaust (all models except the RX-8) in order to overcome this they dumped excess fuel in and used a airpump to clean up the co side of the emissions.
thats why removing the emissins crap and rejetting a stock port carbed NA gets gains in both HP and fuel economy (but kills it emssions wise)
cheers
thats why removing the emissins crap and rejetting a stock port carbed NA gets gains in both HP and fuel economy (but kills it emssions wise)
cheers
12-06-04, 06:09 AM
#12
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well 5 gallons only gets me 63 miles... Iv done this many times.... 10 gets me 126 miles 15 gallons gets me 189 miles... and so on.. I remember Durring the Summer i got 260 miles a tank...Soon as it got Cold went down the Crapper... Not sure if its temp related but dayayayam
12-09-04, 11:07 PM
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I believe it has to do wit the fact that it takes a rotary one revolution (of the crank shaft) to perform the intake, compression, power, and exhaust strokes while it takes a piston engine 2 revolutions. Therefore, a rotary is pumping in twice the gas as a piston engine for the same rpm.
12-09-04, 11:53 PM
#14
Originally Posted by rs_1101
lack of thermal effeciency and strange combusion chamber shape.
The rotary just can't be as efficient as a piston motor.
The combustion chamber has a high surface area to volume ratio.
The fuel/air mixture can't burn right up to the side of the surface (try holding a burning piece of paper with pliers, it won't burn to the edge).
The high surface area to volume ration means a lot of heat energy is absorbed into the cooling system. Ever heard "rotaries run hot"?
Porting for power allows overlap.
Overlap=fuel/air mixture out of the exhaust.
Funky combustion chamber shape defeats complete combustion of fuel/air mixture and further allows more unburnt fuel/air mixture.
2 plugs mean 2 flame fronts, that can't be good...
Your 160hp na gets 28-31 mpg? Not, either you calculator is wrong or your speedo is wrong...
My 240hp Honda minivan (0-60 in 7.7 sec, 1/4 mile in 15.9) weighs 4300lbs and has a HUGE frontal area gets 22-23mpg on the freeway, 18mpg in town.
What mpg does a 2900lb, 238hp RX8 get? Answer=a little less than my minivan...
Jeff
Driving rotaries since 1987...
12-10-04, 04:18 AM
#16
Savanna Rx-7
While jeff you are correct in some respects....
[QUOTE=turbojeff]This is it in a nutshell. Anyone that says any different has no clue.
to make a blanket statment like that you are inviting critisism. I am not disputing any of what you have written jeff, but I just didn't go into 100% detail.....
first I assume you are reffering to swept volume possibly? this is the same problem that cause a large bore engine to get lower mileage.
the trailing spark plug is there to increase mileage, and decrease emissions, so it in itself has nothing to do with bad mileage, as it increases the mileage of the car.
the combustion chamber theory is relativly sound, and has been known about for a while.
I was simply pointing out the NON-Obvous things. this doesn't make me stupid, nor does it make you smarter. I know people who have "driven" a particular model of car for many years, and still don't know there backside from there frontside about the thing they drive, so the ive been driving bla, bla, bla doesn't mean jack. Now if you had said I have been a certified mechanic for x amount of years, I would accord your words a little more weight,
but I have been swinging my dick for better than 20 years, and that don't make me an expert on procreation though
Oh and I have been driving, racing, and wrenching on rotaries since 1985. But that doesn't mean I know jack, nor does your driving one since 1987 mean you know jack.
don't try and flame me, and I won't reply in a negative fasion.
to make a blanket statment like that you are inviting critisism. I am not disputing any of what you have written jeff, but I just didn't go into 100% detail.....
first I assume you are reffering to swept volume possibly? this is the same problem that cause a large bore engine to get lower mileage.
the trailing spark plug is there to increase mileage, and decrease emissions, so it in itself has nothing to do with bad mileage, as it increases the mileage of the car.
the combustion chamber theory is relativly sound, and has been known about for a while.
I was simply pointing out the NON-Obvous things. this doesn't make me stupid, nor does it make you smarter. I know people who have "driven" a particular model of car for many years, and still don't know there backside from there frontside about the thing they drive, so the ive been driving bla, bla, bla doesn't mean jack. Now if you had said I have been a certified mechanic for x amount of years, I would accord your words a little more weight,
but I have been swinging my dick for better than 20 years, and that don't make me an expert on procreation though
Oh and I have been driving, racing, and wrenching on rotaries since 1985. But that doesn't mean I know jack, nor does your driving one since 1987 mean you know jack.
don't try and flame me, and I won't reply in a negative fasion.
Last edited by kenn_chan; 12-10-04 at 04:22 AM. Reason: add some stuff
12-10-04, 04:27 AM
#17
Eats, Sleeps, Dreams Rotary
According to How Stuff Works, they say it's also because they have low compression ratios.
But I sort of doubt switching to high-comp rotors would be a viable solution... since that would make the combustion chamber shape problem even worse.
But I sort of doubt switching to high-comp rotors would be a viable solution... since that would make the combustion chamber shape problem even worse.
12-10-04, 11:49 AM
#18
Originally Posted by Valkyrie
According to How Stuff Works, they say it's also because they have low compression ratios.
But I sort of doubt switching to high-comp rotors would be a viable solution... since that would make the combustion chamber shape problem even worse.
But I sort of doubt switching to high-comp rotors would be a viable solution... since that would make the combustion chamber shape problem even worse.
Low compression ratios? The 89-91 cars have 9.5:1, that isn't low.
12-10-04, 11:58 AM
#19
[QUOTE=kenn_chan]
The "driving rotaries since 1987" comment means that I like them despite the bad mileage.
The trailing spark plug increases mileage on a rotary but does not make it better than one simple shaped combustion chamber like a cylinder.
Your right, adding the blaket statement did invite critisism.
I don't like people that claim "29-31 mpg" from an 89 NA. Sure they might have got that once when the tank wasn't filled all the way up or they drove DOWN a mountain pass.
The big reasons rotaries don't get good mileage are inherent in the physical design of the motor not the emissions. The particular emissions can make poor mileage worse but either way they will not perform like a piston engine. Plain and simple.
Originally Posted by turbojeff
This is it in a nutshell. Anyone that says any different has no clue.
to make a blanket statment like that you are inviting critisism. I am not disputing any of what you have written jeff, but I just didn't go into 100% detail.....
first I assume you are reffering to swept volume possibly? this is the same problem that cause a large bore engine to get lower mileage.
the trailing spark plug is there to increase mileage, and decrease emissions, so it in itself has nothing to do with bad mileage, as it increases the mileage of the car.
the combustion chamber theory is relativly sound, and has been known about for a while.
I was simply pointing out the NON-Obvous things. this doesn't make me stupid, nor does it make you smarter. I know people who have "driven" a particular model of car for many years, and still don't know there backside from there frontside about the thing they drive, so the ive been driving bla, bla, bla doesn't mean jack. Now if you had said I have been a certified mechanic for x amount of years, I would accord your words a little more weight,
but I have been swinging my dick for better than 20 years, and that don't make me an expert on procreation though
Oh and I have been driving, racing, and wrenching on rotaries since 1985. But that doesn't mean I know jack, nor does your driving one since 1987 mean you know jack.
don't try and flame me, and I won't reply in a negative fasion.
to make a blanket statment like that you are inviting critisism. I am not disputing any of what you have written jeff, but I just didn't go into 100% detail.....
first I assume you are reffering to swept volume possibly? this is the same problem that cause a large bore engine to get lower mileage.
the trailing spark plug is there to increase mileage, and decrease emissions, so it in itself has nothing to do with bad mileage, as it increases the mileage of the car.
the combustion chamber theory is relativly sound, and has been known about for a while.
I was simply pointing out the NON-Obvous things. this doesn't make me stupid, nor does it make you smarter. I know people who have "driven" a particular model of car for many years, and still don't know there backside from there frontside about the thing they drive, so the ive been driving bla, bla, bla doesn't mean jack. Now if you had said I have been a certified mechanic for x amount of years, I would accord your words a little more weight,
but I have been swinging my dick for better than 20 years, and that don't make me an expert on procreation though
Oh and I have been driving, racing, and wrenching on rotaries since 1985. But that doesn't mean I know jack, nor does your driving one since 1987 mean you know jack.
don't try and flame me, and I won't reply in a negative fasion.
The trailing spark plug increases mileage on a rotary but does not make it better than one simple shaped combustion chamber like a cylinder.
Your right, adding the blaket statement did invite critisism.
I don't like people that claim "29-31 mpg" from an 89 NA. Sure they might have got that once when the tank wasn't filled all the way up or they drove DOWN a mountain pass.
The big reasons rotaries don't get good mileage are inherent in the physical design of the motor not the emissions. The particular emissions can make poor mileage worse but either way they will not perform like a piston engine. Plain and simple.
12-10-04, 11:59 AM
#20
Eats, Sleeps, Dreams Rotary
Originally Posted by turbojeff
Low compression ratios? The 89-91 cars have 9.5:1, that isn't low.
I guess they weren't referring to the 7 specifically...
12-11-04, 03:44 PM
#21
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If you compare the fuel consumption of a 1300 cc rotary to a 1300 cc boinger (used in cars of the same weight, drive line ratios and vintage) it would appear at first glance that the rotary's fuel economy was terrible--- only about 1/2 the distance covered for the same volume of fuel.
But what rednecks who use this comparison to justify their disdain for all things rotary don't seem to grasp is that displacement isn'tthe deciding factor here. HORSEPOWER is. The most accurate way to compare apples to apples here is to compare Specific Fuel Consumption (SFC). This refers to wieght of fuel burned per hp produced per hour at a given power setting, typically full power, but sometimes cruise power settings.
Typical boingers in the early '80s had an SFC of around 0.45 pounds of fuel consumed per hp per hour. The Mazda 12A of the same era had an SFC of around 0.5 lbs/ hp/ hr. Not as good as a boinger thanks to the elongated combustion chamber and heat issue, but not terribly worse either. In fact, the difference is neglegible. For example, the 1980 RX7 got about the same fuel economy as the 1980 Datsun (Nissan) 280ZX. True, the ZX produced a little more HP, but this was offset by its extra weight. The ZX was quicker off the line with more torque down low, but the 7 returned about the same zero-to-sixty times.
Modern boingers consume only about 0.43 lbs/ hp/ hr, while the Renesis 13B has an SFC of around 0.45 lbs/ hp/ hr, again, not as good as modern boingers, but the difference is neglegible. This means that the rotary may get 25 mpg on the highway but a comparable hp boinger may get 27. Big F&%king deal.
The problem is that the rotary's SFC at low rpms (city driving) is even more pronounced than it is at cruise compared to that of a boinger, ie: in city driving a comparable hp boinger might get 20 mpg while the rotary only gets 15.
Combined city/hwy driving produces a difference in fuel economy of only about 3 mpg, but ONLY if you compare apples to apples, ie: two engines of the same hp in cars of the same weight and drive line gearing.
I think I'll stick with my rotary.
But what rednecks who use this comparison to justify their disdain for all things rotary don't seem to grasp is that displacement isn'tthe deciding factor here. HORSEPOWER is. The most accurate way to compare apples to apples here is to compare Specific Fuel Consumption (SFC). This refers to wieght of fuel burned per hp produced per hour at a given power setting, typically full power, but sometimes cruise power settings.
Typical boingers in the early '80s had an SFC of around 0.45 pounds of fuel consumed per hp per hour. The Mazda 12A of the same era had an SFC of around 0.5 lbs/ hp/ hr. Not as good as a boinger thanks to the elongated combustion chamber and heat issue, but not terribly worse either. In fact, the difference is neglegible. For example, the 1980 RX7 got about the same fuel economy as the 1980 Datsun (Nissan) 280ZX. True, the ZX produced a little more HP, but this was offset by its extra weight. The ZX was quicker off the line with more torque down low, but the 7 returned about the same zero-to-sixty times.
Modern boingers consume only about 0.43 lbs/ hp/ hr, while the Renesis 13B has an SFC of around 0.45 lbs/ hp/ hr, again, not as good as modern boingers, but the difference is neglegible. This means that the rotary may get 25 mpg on the highway but a comparable hp boinger may get 27. Big F&%king deal.
The problem is that the rotary's SFC at low rpms (city driving) is even more pronounced than it is at cruise compared to that of a boinger, ie: in city driving a comparable hp boinger might get 20 mpg while the rotary only gets 15.
Combined city/hwy driving produces a difference in fuel economy of only about 3 mpg, but ONLY if you compare apples to apples, ie: two engines of the same hp in cars of the same weight and drive line gearing.
I think I'll stick with my rotary.
Last edited by Aviator 902S; 12-11-04 at 03:50 PM.
12-12-04, 05:44 PM
#22
Eats, Sleeps, Dreams Rotary
I'm curious... would a lightly modified 13B get a higher or lower HP/Hour/Fuel rating?
Assuming it includes proper fuel tuning and mods that bump up mileage (ie, ignition upgrade), etc...
Assuming it includes proper fuel tuning and mods that bump up mileage (ie, ignition upgrade), etc...
12-13-04, 05:30 PM
#23
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It would certainly burn more fuel, but more fuel per HP? not unless the mod in question included larger exhaust ports, thereby increasing overlap and decreasing the # of degrees of power sequence before spewing everything out the exhaust.
