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What's redline got to do with it? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 19 Mar 2007 10:31:53
In Reply to: The 9000s revv quite high compare to other cars, Pat98, Sun, 18 Mar 2007 18:49:16 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I've seen two definitions of 'redline.' One is when the horsepower curve starts to drop. The other is a do-not-exceed speed, usually based on the fact that the valve train can't keep up. You'll typically see a higher redline on an overhead cam engine versus a pushrod engine. You'll typically see a higher redline on a 4-cylinder engine versus a V-8, but if you design it right, you can get a 16K rpm redline on a V-12 Formula 1 engine.
The cruising speed in top gear is an engineering and marketing decision. Ideally, for fuel mileage, you want a high gear ratio, so the engine turns slowly at speed. Chances are the engine is quieter then, too. People like that. The bad news is that at lower RPM, a smaller displacement engine has less power and is less responsive. So if you want to speed up to overtake a car, it means a downshift. Yes, Saab has a turbo. But at cruising speeds, the throttle is pretty much closed. Not a lot of flow means the turbo isn't spinning very fast, so if the engine is at low RPM and low flow, it will take time for the turbo to spin up. So if you want a more responsive engine, have the engine turn faster at cruise.
Of course, you can add a cruising gear. Say a 5 speed automatic. You still have the response issue, but you can cruise at lower RPM. All that takes is adding a gear. At the time of the 9000, 5 speed automatics were few and far between, if any.
Yes, there are other 4 speed automatics that turn fewer rpm at cruise. And all of that is a trade-off of gear ratios. I've driven some cars with really lousy gear ratios, tuned for low rpm, high mileage, and crappy engine response. They hunt between gears, downshifting every time you ask for a little power.
I've not seen a lot of small-displacement engines that turn low rpm at cruise AND have any decent throttle response. Could Saab have done better? I'm sure each of us has a few suggestions.
As to a different final drive, the gear ratios were unchanged from '88 to '98; (I don't know the specs on the '86 & 87, but I doubt it was different). Even the V6 autobox had the same ratios as the four-cylinder.
posted by 192.249....
, Mon, 19 Mar 2007 10:31:53 <-- Viewing This Message
, Mon, 19 Mar 2007 08:19:56, Sun, 18 Mar 2007 22:23:32
, Sun, 18 Mar 2007 19:49:52
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