stupid question about my rpm
#1
08-20-2006, 09:30 PM
stupid question about my rpm
alright i have a question. its probably stupid but whatever. so i have a 2005 c230 kompressor and i noticed that when its at exactly 3000rpm its only at 73mph whereas my 05 accord at 3000rpm will be at 80mph and so does my corolla. is it cuz the benz is supercharged? thanks in advance guys.
#2
08-20-2006, 10:06 PM
RE: stupid question about my rpm
The gearing of your transmission mainly determines how fast your wheels rotate for a given engine RPM. In general, larger, more powerful engines will be able to support a higher gear ratio (more turning of the axels and wheels for each turning of the crankshaft), and thus can drive your car at speed with a lower RPM. I once owned an 87' Acura Integra, with a (then) teeny 1.6L engine; it would buzz along at almost 4,000 RPM when I was at 70 mph. My '03 Range Rover would be just above 2000 RPM for the same speed (big honkin' V8).
Keep in mind that a performance oriented car may have been set up to be a little inefficient in the final gear anyway, as a sports car would need to have ready horsepower available on tap should you ask for it. A car maker might feel a lot more comfortable pushing for maximum efficiency in an econobox, but the owners of such a car would expect the thing to be doggy going up a slight incline or into a head wind. The same engine in a sportier car would likely be geared less efficiently (cruise at a higher RPM), but would have more reserve on tap when asked.
The supercharger on your car is a belt driven device that forces (rams) air into the engine, producing a good bit more power than could otherwise be achieved from your engine size. (A turbo would be an exhaust gas driven device that does a similar thing: exhaust spins a fan, and on the other end of the fan's axel is a second fan that forces air into the engine). The degree to which your supercharger impacts your engine performance is in large part tied to the engine RPM, since it's revolutions are directly linked to the engine's. Superchargers usually have little impact at low RPM, and ramp up in effect as the engine speed does; a supercharged 4 cylinder engine feels very much like a 4-banger at low RPMS, and maybe like a small V6 at high RPMs. Given that, it's possible that the trasmission gearing was set up to keep the supercharger in more of it's "sweet spot" as well.
So I guess the answer to your question is maybe "yes". I strongly suspect that the car designers set the transimission ratios up so that it would meet their expectations for both fuel efficiency and overall vehicle responsiveness, and the supercharger's presence definitly impacts that.
Final note: don't get to thinking that engine RPM is the only factor when it comes to fuel efficiency. The amount of fuel needed to hold an engine at a given RPM varies; for an increased load (say a headwind, or when you're climbing a hill), you press down on the gas pedal and spend more fuel just to keep the RPMs and vehicle speed constant. With a tailwind or on downhill slopes, you can back off the gas an still maintain speed, and the same exact engine RPM. The vehicle aerodynamics, rolling resistance and base engine size all have the a big impact on the overall efficiency, and it's possible to have a car that cruises at a higher RPM be more fuel efficient than one that cruises at a lower one (just think of any car mentioned vs my Land Rover and it becomes obvious that low crusing RPM does not equate to fuel efficiency).
Keep in mind that a performance oriented car may have been set up to be a little inefficient in the final gear anyway, as a sports car would need to have ready horsepower available on tap should you ask for it. A car maker might feel a lot more comfortable pushing for maximum efficiency in an econobox, but the owners of such a car would expect the thing to be doggy going up a slight incline or into a head wind. The same engine in a sportier car would likely be geared less efficiently (cruise at a higher RPM), but would have more reserve on tap when asked.
The supercharger on your car is a belt driven device that forces (rams) air into the engine, producing a good bit more power than could otherwise be achieved from your engine size. (A turbo would be an exhaust gas driven device that does a similar thing: exhaust spins a fan, and on the other end of the fan's axel is a second fan that forces air into the engine). The degree to which your supercharger impacts your engine performance is in large part tied to the engine RPM, since it's revolutions are directly linked to the engine's. Superchargers usually have little impact at low RPM, and ramp up in effect as the engine speed does; a supercharged 4 cylinder engine feels very much like a 4-banger at low RPMS, and maybe like a small V6 at high RPMs. Given that, it's possible that the trasmission gearing was set up to keep the supercharger in more of it's "sweet spot" as well.
So I guess the answer to your question is maybe "yes". I strongly suspect that the car designers set the transimission ratios up so that it would meet their expectations for both fuel efficiency and overall vehicle responsiveness, and the supercharger's presence definitly impacts that.
Final note: don't get to thinking that engine RPM is the only factor when it comes to fuel efficiency. The amount of fuel needed to hold an engine at a given RPM varies; for an increased load (say a headwind, or when you're climbing a hill), you press down on the gas pedal and spend more fuel just to keep the RPMs and vehicle speed constant. With a tailwind or on downhill slopes, you can back off the gas an still maintain speed, and the same exact engine RPM. The vehicle aerodynamics, rolling resistance and base engine size all have the a big impact on the overall efficiency, and it's possible to have a car that cruises at a higher RPM be more fuel efficient than one that cruises at a lower one (just think of any car mentioned vs my Land Rover and it becomes obvious that low crusing RPM does not equate to fuel efficiency).
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