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its all about the torque curve and gear ratios... Posted by Snowmobile [Email]  (#686)  [Profile/Gallery]
(more from Snowmobile) on Thu, 27 Feb 2014 08:55:54
In Reply to: Re: transmissions, dtechakacheaptech, Wed, 26 Feb 2014 16:13:13 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Torque curves come in all different shapes and sizes depending on the biases of the manufacturer in terms of engine design. Some people like high revving, others like low end torque. Traditionally, the Swedish brands (and probably most euro cars) liked to have flat torque curves (low end torque). Asian brands tended to produce peaky torque curves (high revving). There are exceptions but that is mostly what I have observed. Turbos tend to help flatten torque curves, but there are other ways to optimize this also (it's not just the turbo, but that is how SAAB/Volvo used the turbo - to get more torque sooner = flatter).
To get power out of the car, you need to keep the engine in the power band (near the peak of the torque curve). Ideally, you want to be cruising at the low end of the band but in a strong gear so when you hit the gas, the car goes fast right away (using most of the band) without shifting. The flatter the torque curve, the fewer gears you need to keep in the power band.
To get good fuel economy out of a car, you need to keep the RPMs low. Again, that means you want to be cruising at the low end of the power band (in a high gear).
Adding gears to the high end lowers RPMs (better economy, but the amount of benefit dependent on torque curve shape), but then doesn't let you sweep as far through the power band (so you don't accelerate as fast) without downshifting. Adding gears to the low end increases smoothness and off the line performance (but adding too many at the low end just wastes gears as you whiz through them fast, and at some point, shift time exceeds any benefit - think of starting at the top of a hill on a bike in granny gear).
So, one needs enough gears to be able to use the power band well to put power to pavement and still be at the low end of the RPM range at top cruising speed. In North America, 80mph is pretty much top speed. And anywhere, above those speeds (lower actually) fuel economy is significantly impacted by aerodynamic losses.
For a "modern" Saab, the torque curve is sufficiently flat that 4 gears is all that is needed to cruise at low rpms on the highway here. That is evident in the 9-5 where there is little difference between the 4 speed auto and 5 speed auto. Actually, vvack likes the 4 speed better because it holds in a lower gear longer. The 5 speed is perhaps a tad smoother and a tad more fuel efficient. Imho, it's already splitting hairs at 4 vs 5 in a Saab. If the 5th gear was taller (but I don't think it is), maybe it helps on the autobahn. I have a c900 with a 3 speed autobox. That car does great around town (relatively flat torque curve and hugely variable shift points), but it needs 1 more gear on the highway to reduce the cruising rpms (it's massive downfall).
For a car with a peaky torque curve (many Asian), you need more gears. Often waaay more gears. We rented several (non-turbo) Subarus with 4 speed transmissions and to get good off the line performance, they waste a gear from about 0-7mph (to sell cars on the short test drive). Cruising rpms were about the same as the 3 speed c900's, and fuel economy was equally atrocious. Those cars needed 5 gears minimum. don't buy an older subaru without a stick!
Another awful car we rented with a peaky torque curve was the Mazda5: It (successfully) tried to lower fuel consumption by going into 5th gear around town (eg at 25mph)... So the stupid tranny was constantly hunting in the city... I remember trying to go somewhere in a hurry and double downshifting on city streets. Ridiculous. You really felt the narrow power band! It was pretty fast off the line though with good fuel economy. Awful performance on the highway (due to the peaky curve and poor gear choices).
If the torque curve is not flat over most of the operable range, more shifting is required to make the power delivery "smooth". This is I think why many manufacturers are offering higher speed gear boxes or CVT these days. Your observations may be correct that more gears are required for a big SUV with a 2.0L motor, because, even turbocharged, they have a choice: do I want higher peak power, or a flatter band and they are probably going for a narrower band with higher peak and more gear usage to keep things feeling smooth anyway.
Regardless, the higher the mass of the vehicle, the more energy is required to make it move. That's horsepower. Big heavy vehicles need it. So the engine needs to supply it, and the tranny is just an interface to make the delivery smooth. If someone made a 400hp 4 cylinder engine with a wide flat power band, you wouldn't need 8 speeds... all those gears are to make up for band width or to add "refinement" (or because so+so competitor has it and is using it as a marketing tool and fools who just go by the numbers need to have it so we need to have it also).
On the flip side, adding a whole bunch of extra gears likely adds complexity and/or weight. They may be more prone to failure down the road. CVT's are not necessarily more complex, but come with their own drawbacks. All of this may be why fewer cars are coming with stick shifts.
, Sun, 23 Feb 2014 16:52:36, Fri, 7 Mar 2014 16:43:07, Thu, 27 Feb 2014 19:55:09
, Tue, 25 Feb 2014 09:47:25
, Tue, 25 Feb 2014 16:39:52, Thu, 27 Feb 2014 22:19:53, Tue, 25 Feb 2014 14:24:50, Tue, 4 Mar 2014 06:15:59, Wed, 26 Feb 2014 18:59:48, Thu, 27 Feb 2014 09:20:43, Mon, 24 Feb 2014 10:59:57
, Wed, 26 Feb 2014 13:22:03, Wed, 26 Feb 2014 15:56:30, Thu, 27 Feb 2014 08:55:54 <-- Viewing This Message, Thu, 27 Feb 2014 11:23:11, Fri, 28 Feb 2014 07:53:39, Fri, 28 Feb 2014 08:15:28, Thu, 27 Feb 2014 19:02:40, Thu, 27 Feb 2014 19:48:00, Mon, 24 Feb 2014 11:29:39, Mon, 24 Feb 2014 06:52:30
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