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Old 08-16-2007, 01:59 AM   #1
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Relating RPMs to FE

It's my knowledge that RPMs are almost always directly related to FE. If the engine is running at a higher RPM, it's going to burn more gas. But I have also seen posts contradicting this, notably concerning acceleration "sweet spots".

I bring this up because my car seems to run at a high RPM almost all of the time. At 65 mph in 5th gear it runs at 3100 RPMs, and around 2900 at 60 mph. In my Eagle, it would typically be around 2000 in 4th gear at 60 mph.. a difference of almost 1000. In fact, when driving Troy I rarely to never drive below 2k rpms, which in my old car used to be my maximum level for optimum FE.

I haven't had this car long enough to really establish a baseline, but out of general curiosity... is this normal? In every other car I've driven, 3k RPMs would be enough throttle for a hard acceleration, yet in this car it is a normal cruising speed. Normal acceleration in this car is anywhere from 3,500 to 4,000 RPM. I'm hoping that somehow my car is just an oddity, but it sure seems to suck down the gas for a 4 cylinder. Since this is the first standard shift car I've had, I assumed that it would run at an even lower cruising rpm since it has 5th gear... what gives?

Does Mitsubishi gear all of their cars this high?
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Old 08-16-2007, 07:36 AM   #2
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my dad who's a mechaninc tells me modern engines tend to be much higher revving then they used to be, mostly to get more power out of a smaller block.it's not so good for economy and engine life. most people are lazy on the shifter so they also might be set up to accomodate this. on the other hand you'r engine seems to have more power than the car needs, so if you drive with fe on your mind things shouldn't be to horrible.

it all depends on where the engines optimal power curve is and how the grear ratio is set up.

you may also have inherrited the engine response from the previous owner. my dad's usually quite rational so i was rather amused when he tolled me engines actually tend to adapt to the driving habbits of their owners...i think it's all a matter of subtle wear, dirt accumulation etc but apparently two identical cars with different engines can feel totally different just because of the way they're drivven. when i first bought my car from a very old lady who only seemed to have used it for church and shopping trips, it would slugg along even if i floored it... now i barely need to depress the throttle to get it going and i'm over the speedlimit before i know it.

so just drive as fe friendly as you can and within it's technical limitations the engine will get to know you and the other way around
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Old 08-16-2007, 07:59 AM   #3
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For the most part, you are correct, the less rpms at the same speed would equal metter FE. However, it possible due to cam/intake timing differences that a lower rpm would be out of the engine's powerband and make it less efficient that the FE would not be better.

My CRX for example, has not seemed to loose any mpg between averaging 65mph and 75mph on my commute. From this I'm asuming the engine is more efficient at the higher rpm to make up for the increase in drag.

Tiral and error, or if your car is an OBD2 so you could put a Scangauge on it would be the way to determine this.
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Old 08-16-2007, 11:50 AM   #4
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My car is a 1994 model... so no Scangauage for me.

Could it be due to the fact that it is is a twin cam engine? All the other cars I've driven are SOHC.
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Old 08-16-2007, 12:18 PM   #5
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Quote:
Originally Posted by atomicradish View Post
My car is a 1994 model... so no Scangauage for me.

Could it be due to the fact that it is is a twin cam engine? All the other cars I've driven are SOHC.
It depends on the car, but to get more HP, the car manufactures have been going to DOHC, 16V engines that rev higher. For example, my integra GSR is turning 4000rpm at about 80. For the power to weight ratio the car has, the ~30mpg it gets really isn't that bad.
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Old 08-16-2007, 09:26 PM   #6
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Quote:
Originally Posted by mrmad View Post
It depends on the car, but to get more HP, the car manufactures have been going to DOHC, 16V engines that rev higher. For example, my integra GSR is turning 4000rpm at about 80. For the power to weight ratio the car has, the ~30mpg it gets really isn't that bad.
Not bad at all.

If I can get 30 mpg out of my Galant then I will be more than happy. I'm not here to set a record or anything.
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