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Old 08-24-2000, 06:25 PM
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Can you please explain to me why some of the MB engines have less horsepower than other engines, but have greater displacement.

Example the early 80s 380 SEL had 155 hp

the 90 300 SEL had 177 hp..

Also the 420 SEL had more hp than the 500 SEL.

Was wondering what made up for the difference

'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)
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Old 08-24-2000, 09:09 PM
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Moof !
Join Date: Jul 2000
Location: Seattle WA USA
Posts: 469
Often it is the engine compression ratio that will provide one of the biggest differences. Higher compression, given all other things being equal, yields greater bang in the combustion chamber. Of course, a multitude of other factors, eg, cam timing, lift, and duration, ignition strength and duration, fuel metering, air flow, supercharging, turbocharging, etc, etc, contribute to the total "package". A change of just 1 or 2 % efficiency in several of these factors will add up.

1979 240D
1990 300E
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Old 08-25-2000, 07:57 AM
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The main thing is, Volumetric Efficiency. An engine is nothing more than a pump. The more fuel air mixture you pump in, the more horsepower comes out. A Volumetric Efficiency of one, means that you pump in the same amount of fuel air that the engine displaces.

dakota mentioned many, if not all, of the variables that will ultimately determine the Volumetric Efficiency of a particular engine. Also dakota made a good point about compression ratio, the overall power as well as fuel efficiency is generally improved with an increased compression ratio as long as adequate octane fuel is available for the higher compression ratio.

Another factor that enters into this, is that the horsepower figure quoted is the horsepower peak. As you move the torque peak of a given engine to a higher RPM the horsepower increases dramatically. So, you could have a relatively detuned large displacement engine making good torque at low RPM, but make less horsepower than a smaller displacement engine with a lower torque value at a higher RPM.

As dakota said, there are beaucoup variables involved.

Have a great day,

Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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Old 08-25-2000, 08:47 AM
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thanks guys, never thought of that stuff. will a higher compression ratio on a car necessarily translate into lesser longevity.

For ex, some of the Corvettes ,I think , had a 11:1 and some had 9:1 (would the 9:1 last longer if ran the same way over time)?
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Old 08-25-2000, 09:14 AM
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Join Date: Jul 2000
Location: Toronto, CANADA
Posts: 1,193
Mercedes has come along way with their engines, and seem to make them better with each new engine. FOr example, a 99 E320 with the 3.2l V6 make more horsepower than my 4.2l V8 (and gets remarkably better gas mileage). As mentioned above, there are numerous factors including fuel injection, engine management, variable valve timing, etc.

Jason Priest
1986 420SEL
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Old 08-25-2000, 11:10 AM
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Join Date: Aug 1999
Location: ajax, ontario, canada
Posts: 773
horsepower ratings are mostly for marketing purposes and bragging rights (for racing purposes, it is a different matter, though)

what matters is usable power. By "usable", we mean at the rpms we use on a daily basis. This is determined more by the shape of the torque curve, and the torque peak itself. The higher the torque peak and the flatter the torque curve at the lower rpms, the more usable the engine power.

That's why those large trucks, buses and payloaders only have only so much horsepower, but have gobs of torque at what is practically idle speed to us car drivers - and most of that power is put to use.

the torque curve is flattened by the use of features like variable-length intake manifold systems and variable cam systems (also forced induction), which modern benz engines have. Probably the twin-spark feature also contributes to this.

horsepower is proportional to the torque and the rpm. Some cars like the Honda S2000 extract a lot of power from small engines but you have to rev the engine past 7000 or 8000rpm to do this. Not very usable in everyday driving. And you will likely spin your tires in the process.

so the bottomline is torque: speak softly and carry a big stick ...
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Old 08-25-2000, 12:25 PM
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Not to be a geek, but horsepower is not directly proportional to torque and RPM. As the torque moves up in the RPM band, the horsepower increases at a rate greater than a simple multiplication. I want to say that it increases exponentially, but I don't have the formula in front of me so I can't verify that.

In other words 100 ft/lbs at 4,000 RPM would not produce twice the horsepower of 100 ft/lbs at 2,000 RPM, it would be something greater than double.

There was also a reference to increased compression ratios and engine life. There were many cars of the sixties with 10.5:1 or greater compression ratios. Increasing the CR makes the engine more efficient, therefore, theoretically it should waste less energy in the form of heat. Thus, in theory, a lower CR would not lead to a longer lived engine, unless there was a structural defect that led to a failure.

Have a great day,
Larry the Geek
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