Gas mileage INCREASED with aftermarket gears
04-19-2005, 08:38 PM
#41
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[QUOTE=CrazyPhuD,Apr 19 2005, 09:07 PM] Okay....so let's remember the simplest answer is often the correct one here....and this one is pretty simple.
Fact.....every time there is a combustion in a cylinder you use gas.
Fact.....more revolutions in a give time frame = more combustions by the engine.
Fact.....Higher gear ratios will result in a higher RPM at a given speed...
Well if these three things are true then how can anyone get better gas milage with shorter gears?
Well part is perception...one knock on the S2k is that you need to rev it have any real power, with shorter gears you feel like you have more power at lower RPMs this may mean that you don't feel the need to rev as high to get the power you want.
Fact.....every time there is a combustion in a cylinder you use gas.
Fact.....more revolutions in a give time frame = more combustions by the engine.
Fact.....Higher gear ratios will result in a higher RPM at a given speed...
Well if these three things are true then how can anyone get better gas milage with shorter gears?
Well part is perception...one knock on the S2k is that you need to rev it have any real power, with shorter gears you feel like you have more power at lower RPMs this may mean that you don't feel the need to rev as high to get the power you want.
04-20-2005, 12:12 AM
#44
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Hmm...
My shifting is SERIOUSLY like this most of the time- (Anything above 35 I am in six gear)
1st gear, rev up to 2.5/3 rpm, Shift to 2nd
2nd gear, rev up to 2.5/3rpm, Shift to 3rd
3rd gear, rev up to 2.5/3rpm, Shift to 4th
4th gear, rev up to 2.5/3rpm, Shift to 5th
5th gear, rev up to 2.5/3rpm, Shift to 6th
Do you really need to rev to 4k every casual shift? I find that very aggressive? (I know that most people do rev like this casually) Is my car different?! The acceleration seems fine revving it to 2.5krpms and shifting...
Infact most of the time when I am cruising and such my rpms are below 2k. (And NO I don't get that heavy vibration and car stuttering because of too high of a gear)
Agressively I rev to around 6k (before vtec), and when I feel crazy I go for 9k XD
My best fillup was around 27mpg and that includes tons of VTEC (I'd say at least 10, no 9k shift experience yet)
My last fill up was 22mpg with alot more 9k shifts (Probably 10, and about 90000 VTEC lol)
This fillup I'm going to drive as conservative as possible, NO MORE VTECing... Lets see if I can go over 30mpg! XD (Er, I already redlined at a light near the gas station leaving though... Twice XD NO MORE I SWEAR)
My shifting is SERIOUSLY like this most of the time- (Anything above 35 I am in six gear)
1st gear, rev up to 2.5/3 rpm, Shift to 2nd
2nd gear, rev up to 2.5/3rpm, Shift to 3rd
3rd gear, rev up to 2.5/3rpm, Shift to 4th
4th gear, rev up to 2.5/3rpm, Shift to 5th
5th gear, rev up to 2.5/3rpm, Shift to 6th
Do you really need to rev to 4k every casual shift? I find that very aggressive? (I know that most people do rev like this casually) Is my car different?! The acceleration seems fine revving it to 2.5krpms and shifting...
Infact most of the time when I am cruising and such my rpms are below 2k. (And NO I don't get that heavy vibration and car stuttering because of too high of a gear)
Agressively I rev to around 6k (before vtec), and when I feel crazy I go for 9k XD
My best fillup was around 27mpg and that includes tons of VTEC (I'd say at least 10, no 9k shift experience yet)
My last fill up was 22mpg with alot more 9k shifts (Probably 10, and about 90000 VTEC lol)
This fillup I'm going to drive as conservative as possible, NO MORE VTECing... Lets see if I can go over 30mpg! XD (Er, I already redlined at a light near the gas station leaving though... Twice XD NO MORE I SWEAR)
04-20-2005, 12:21 AM
#45
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Originally Posted by YellowS2kPwr,Apr 19 2005, 07:42 PM
I do something like:
1st gear, rev up to 4rpm, get to about 10 mph...Shift to 2nd
2nd gear, rev up to 3rpm, get to about 16 mph...Shift to 3rd
3rd gear, rev up to 4rpm, get to about 30 mph...Shift to 4th
4th gear, rev up to 3rpm, get to 40 mph. shift to 5th
And still get 16 mpg
1st gear, rev up to 4rpm, get to about 10 mph...Shift to 2nd
2nd gear, rev up to 3rpm, get to about 16 mph...Shift to 3rd
3rd gear, rev up to 4rpm, get to about 30 mph...Shift to 4th
4th gear, rev up to 3rpm, get to 40 mph. shift to 5th
And still get 16 mpg
04-20-2005, 01:01 AM
#46
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Hmm, well the bike analogy does yield one rather trivial piece of information. When is the power delivery (engine/muscles) most efficient. For a human on a bicycle it's pretty simple. High cadence (80-120 rpm) is your best efficiency, for a internal combustion engine.... well that's over my head. Just because a human is most efficient at high (relatively) rpms doesn't mean a car engine is. For example, you generate the fastest acceleration on a bike at lower rpms (40-60) and best efficiency at higher rpms. If IC engines worked like our muscular system we'd be racing at 3-4k rpm and doing groccery runs at 6-7k.
As far as the gears "taking a load" off the engine. How? Do they CREATE power? If not, then they aren't taking anything off the engine. When you go down a hill gravity accelerates the vehicle, thus taking a load of the engine. That's completely different than shorter gears. F=MA. On a downhill slope acceleration is partially (or completely) due to gravity.
I'd say where you are getting that gas mileage increase is due a difference in driving style (throttle/shift points). Based on constant torque (fairly accurate under low-throttle acceleration and instantaneous clutch engagement)
Acceleration=(Torque*GearRatio*(EngineSpeed/Radius_Tires))
So, raising the gear ratio from stock to aftermarket increases acceleration. Subsequently the forward Force (the engine) must also increase to preserve F=MA. This is why your aftermarket gears make a car feel more powerful. The higher the gear ratio, the more acceleration and forward force generated with a fixed power input. While this might seem contradictory, it isn't. WoT @5k rpm produces much greater acceleration in first gear than in fifth.
From the above it would seem we'd found the answer while defying a few major laws of physics. Since the vehicle accelerates faster it naturally accelerates for less time, requires less energy, which means it burns less fuel. Unfortunately, it isn't that simple. You aren't accelerating 0-40 in one gear. Your gear ratio is changing drastically with each shift. Infact, as time accelerating goes up the acceleration difference between 4.57 gears and stock gears quickly becomes less and less noticable. The shorter 4.57 gears simply mean you need to shift sooner than stock, which lowers the gear ratio. In an extreme example (infinite shifts, CVT trans.) 4.57 gears vs 4.22 gears would produce the exact same acceleration.
In effect, with the higher gears you will shift more frequently and each shift quite drastically lowers the gear ratio partially cancelling the effect of the 4.57 gears. Just how much it cancels depends on how long the acceleration (and number of shifts). For instance, assuming instantaneous shift times the 0-60 difference would be much greater than the 0-120. In the real world, where shifting certainly does take some time, choosing gearing (both trans and final drive) takes into effect the torque curve, common cruise speeds, and important bragging right speeds (60mph).
Now, while all of the above is certianly interesting (well, it is to me damnit... god i'm such a nerd =P) it really doesn't answer how changing final drive could possibly change gas mileage. Too many variables, and too many complex expressions for me! Fortunately, there is an easier way! If we go ahead and ignore certain contributions (namely anything non-conservative) we can use work-energy. So, for the moment, toss the driver, all types of friction, and engine efficiency out the window. This leaves two simple equations.
F=MA; M=F/A (1)
KE=1/2MV^2=1/2(F/A)V^2 (2)
In this simplified system the gearing won't make any difference at all. From (1) F and Acceleration are one-to-one inversely proportional which means in (2) when we take the ratio (F/A) it will be the same no matter what F or A is taken if (1) is to hold.
Therefore, any change in gas mileage is, as one would expect, due to the ignored complications. Furthermore, we can ignore air resistance and rolling friction. That leaves driving style, engine efficiency over the throttle and rpm range, and friction in the engine and drivetrain. How much each contribute is completely beyond me.
Now, let's see... back to that hideous lab report I've been putting off.
As far as the gears "taking a load" off the engine. How? Do they CREATE power? If not, then they aren't taking anything off the engine. When you go down a hill gravity accelerates the vehicle, thus taking a load of the engine. That's completely different than shorter gears. F=MA. On a downhill slope acceleration is partially (or completely) due to gravity.
I'd say where you are getting that gas mileage increase is due a difference in driving style (throttle/shift points). Based on constant torque (fairly accurate under low-throttle acceleration and instantaneous clutch engagement)
Acceleration=(Torque*GearRatio*(EngineSpeed/Radius_Tires))
So, raising the gear ratio from stock to aftermarket increases acceleration. Subsequently the forward Force (the engine) must also increase to preserve F=MA. This is why your aftermarket gears make a car feel more powerful. The higher the gear ratio, the more acceleration and forward force generated with a fixed power input. While this might seem contradictory, it isn't. WoT @5k rpm produces much greater acceleration in first gear than in fifth.
From the above it would seem we'd found the answer while defying a few major laws of physics. Since the vehicle accelerates faster it naturally accelerates for less time, requires less energy, which means it burns less fuel. Unfortunately, it isn't that simple. You aren't accelerating 0-40 in one gear. Your gear ratio is changing drastically with each shift. Infact, as time accelerating goes up the acceleration difference between 4.57 gears and stock gears quickly becomes less and less noticable. The shorter 4.57 gears simply mean you need to shift sooner than stock, which lowers the gear ratio. In an extreme example (infinite shifts, CVT trans.) 4.57 gears vs 4.22 gears would produce the exact same acceleration.
In effect, with the higher gears you will shift more frequently and each shift quite drastically lowers the gear ratio partially cancelling the effect of the 4.57 gears. Just how much it cancels depends on how long the acceleration (and number of shifts). For instance, assuming instantaneous shift times the 0-60 difference would be much greater than the 0-120. In the real world, where shifting certainly does take some time, choosing gearing (both trans and final drive) takes into effect the torque curve, common cruise speeds, and important bragging right speeds (60mph).
Now, while all of the above is certianly interesting (well, it is to me damnit... god i'm such a nerd =P) it really doesn't answer how changing final drive could possibly change gas mileage. Too many variables, and too many complex expressions for me! Fortunately, there is an easier way! If we go ahead and ignore certain contributions (namely anything non-conservative) we can use work-energy. So, for the moment, toss the driver, all types of friction, and engine efficiency out the window. This leaves two simple equations.
F=MA; M=F/A (1)
KE=1/2MV^2=1/2(F/A)V^2 (2)
In this simplified system the gearing won't make any difference at all. From (1) F and Acceleration are one-to-one inversely proportional which means in (2) when we take the ratio (F/A) it will be the same no matter what F or A is taken if (1) is to hold.
Therefore, any change in gas mileage is, as one would expect, due to the ignored complications. Furthermore, we can ignore air resistance and rolling friction. That leaves driving style, engine efficiency over the throttle and rpm range, and friction in the engine and drivetrain. How much each contribute is completely beyond me.
Now, let's see... back to that hideous lab report I've been putting off.
04-20-2005, 03:32 AM
#47
Registered User
For me I could not change gears with my daily driving.... I'm on the highway 100+ miles a day. Most of it at 80 mph when traffic permits. My rpms at 80 are 4k.. The extra shifting with the lower gears would drive me nuts I think around town. Shifting four times to get to 40mph? With the traffic in the city that would be crazy... the way it is now with the stock gears I sometimes start in 2nd if i'm rolling or on a decline.
On the other hand it must be a huge benefit in autox racing.... Does anyone make a quick gear swap kit for the s2k?
On the other hand it must be a huge benefit in autox racing.... Does anyone make a quick gear swap kit for the s2k?
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