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rpm vs. speed question
I've recently acquired a 05 MazdaSpeed Turbo, 6-speed, a retirement
gift for myself. Love it so far. It's been 40 years since as a single guy I drove a '64 Vette 4-speed. My driver for the last 13 years has been a 6-cyl '93 T-Bird auto transmission. Now the questions. At 60mph my Mazda is turning exactly 3000 rpm in 6th gear. My old T-Bird loafs along at 1750. Both have roughly the same hp. My 300 hp V-8 Corvette, with a 308 rear end turned about 2750 in 4th if I remember right. Anyone know a good source for the relationship between rpm, highway speed, horse power, number of gear speeds, etc. How can that old Ford engine cruise along at 1750 when it takes the MX-5 3000 rpm? Why do I need 6 gears in the Mazda when 4 worked fine in the Corvette? I probably knew all these answers 40 years ago, but I need a refresher. |
#2
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rpm vs. speed question
In article .com>,
"laocmo" > wrote: > Anyone know a good source for the relationship between rpm, highway > speed, horse power, number of gear speeds, etc. There's no precise relationship. It's simply the choice of gearing, presumably matched to the car's intended use, but also affected by marketing and fashion. The Miata doesn't start to make serious power until it reaches 4000 rpm; it also will run all day above 6000 without problems. It's how a small engine delivers good performance, which the factory assumes the buyer will value over quiet cruising. The Miata doesn't have much oomph down low, and more transmission gears makes it easier to stay within the narrower powerband. OTOH, modern Corvettes have 6 speeds, just because it's fashionable. They could probably get by with two. -- Lanny Chambers '94C, St. Louis http://www.hummingbirds.net/alignment.html |
#3
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rpm vs. speed question
In article .com>, "laocmo" > wrote:
>I've recently acquired a 05 MazdaSpeed Turbo, 6-speed, a retirement >gift for myself. Love it so far. It's been 40 years since as a single >guy I drove a '64 Vette 4-speed. My driver for the last 13 years has >been a 6-cyl '93 T-Bird auto transmission. Now the questions. At 60mph >my Mazda is turning exactly 3000 rpm in 6th gear. My old T-Bird loafs >along at 1750. Both have roughly the same hp. My 300 hp V-8 Corvette, >with a 308 rear end turned about 2750 in 4th if I remember right. >Anyone know a good source for the relationship between rpm, highway >speed, horse power, number of gear speeds, etc. How can that old Ford >engine cruise along at 1750 when it takes the MX-5 3000 rpm? Why do I >need 6 gears in the Mazda when 4 worked fine in the Corvette? I >probably knew all these answers 40 years ago, but I need a refresher. I agree with everything Lanny said. But the simplest relationship might be: smaller engine, higher rpm's. Engine torque is pretty much proportional to displacement (not considering force-induction engines which effectively multiplies displacement). So to make the same power which equals torque*rpm, they have to spin faster. If all else is the same with a car (weight, size, etc, which obviously it isn't here), it will take the same power to drive any given combination of speed and road grade. Thus the smaller engine must turn higher rpm. Now, in your case, it is force-induction, and makes a fair amount of torque. And its in a small car. So it should be able to be geared longer and turn lower rpm than 3000 @ 60mph. Lanny is correct that Mazda however chose to keep the rpms high in consideration of the sporting nature of the car. Also, it helps mask turbo lag. Basically, it makes the car more responsive, but at sacrifice of noise, fuel economy, and potentially some engine life. Frankly, especially given it is a 6-speed, I would have much preferred if Mazda made 6th much taller. Another reason why a manual might be geared shorter than an automatic is for perceived drivability. Imagine if it turned 2000rpm at 60. You press on the pedal ... nothing. You have to downshift. And automatic would do that for you, uh, automatically. Thus perceived better drivability. And, as Lanny also said, 6-speeds are in vogue. A Corvette, or any similar torque-monster, simply doesn't need them. Lexus is coming out with an 8-speed automatic. Why? Because MB has seven! Oy. Anyway, enjoy the car. If the noise gets to be a hassle on a highway cruise, do what a lot of us do: earplugs. |
#4
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rpm vs. speed question
The key de terming factor in the gearing of an engine is the torque
the engine produces. Push rod V 8 engines are designed to produce high torque, partly because the push rod design limits the design to 2 valve per cylinder. This design, while economical, limits the rpm that the engine can turn. They usually red line at relatively low rpm (5000 to 6000 rpm) Modern fours, using overhead cams can uses three or four lighter valves with better modern spring, can turn 7000 or 8000 rmp. But since the are smaller displacement they produce much lower torque. The formula for horsepower is: Horsepower = torque * revs/minute * minute/60 seconds * 2*pi * 1/550 Horsepower = torque * revs/minute * 1/5252 In a very simple way: Suppose your car requires 100 hp to move along at 90 mph. A v-8 rated at 300 ft-lb at 2000 rpm will be producing 114. An engineer will pick the gearing and rear end to produce the right rpm at the wheels. A smaller 4 cylinder engine, producing 140 ft-lbs or torque would have to turn nearly 4300 rpm to produce the power necessary to move the same car at the same 90 mph. As for the gears, the turbo engine puts out 28 % more power than the normal engine, but the full turbo pressure does not kick in until 4500 rpm. the red line is at 6500, so the maximum power band fairly narrow. If your gears are too wide, you can fall off the power band when you up shift, so having more gears allow you to keep the engine turning at high rpm while you accelerate. (its also more fun) On 20 Apr 2006 06:13:29 -0700, "laocmo" > wrote: >I've recently acquired a 05 MazdaSpeed Turbo, 6-speed, a retirement >gift for myself. Love it so far. It's been 40 years since as a single >guy I drove a '64 Vette 4-speed. My driver for the last 13 years has >been a 6-cyl '93 T-Bird auto transmission. Now the questions. At 60mph >my Mazda is turning exactly 3000 rpm in 6th gear. My old T-Bird loafs >along at 1750. Both have roughly the same hp. My 300 hp V-8 Corvette, >with a 308 rear end turned about 2750 in 4th if I remember right. >Anyone know a good source for the relationship between rpm, highway >speed, horse power, number of gear speeds, etc. How can that old Ford >engine cruise along at 1750 when it takes the MX-5 3000 rpm? Why do I >need 6 gears in the Mazda when 4 worked fine in the Corvette? I >probably knew all these answers 40 years ago, but I need a refresher. |
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