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High Revving Engines vs. Lower Revving Engines

Discussion in 'Forced Induction' started by Bob_Ash, May 28, 2011.

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    In discussion of various engines, reviewers seem to get excited about higher-revving engines. Do higher revving engines deliver a unique advantage? Is there a performance advantage, or more of an audible-cosmetic advantage (sounds great)?

    Basically, engines with great horsepower and torque are very engaging to drive. Wouldn't the delivery of HP and torque be the criteria for judging an excellent engine, regardless of max RPM?

    Do higher revving engines deliver more power the faster they rev? This is generally true, but there are lower revving engines that can deliver similar HP numbers to some of the high revving engines that are discussed. Is the advantage of a higher revving engine that you get more power per unit of displacement? Thus the car can have a small, more efficient engine for normal driving, but more performance when pushed to high RPMs?

    As you can tell by the above, I am very curious about the dynamics of engine performance related to RPM. All the engine/car reviews that I have read that refer to higher redlines treat it as an end in itself without clearly (to me) explaining why they are better. So I thought I'd throw it out to this board to start the discussion and education.
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    The equation relating torque to HP is: (Torque x Engine speed) / 5,252 = Horsepower
    Look at http://www.howstuffworks.com/question622.htm

    Note that HP is directly dependent upon engine speed. Torque is a more fluid number that depends more directly upon displacement and other MEASURED engine characteristics.
    Even if torque starts decreasing after some point, the increase in RPM in the equation means it has to decrease quite a bit to offset the conversion of torque to hp.

    This is quite clear if you look at http://www.e90post.com/forums/showthread.php?t=34078, where torque and HP are graphed for an N54 engine.

    ForcedInduction guest

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    High revving automotive street engines are not the norm and are more often associated with race engines, thus the aural and psychological attraction by some folks. As an example the 8400 rpm redline on the M3 S65 engine is in race engine territory compared to normal redlines in the 6500-7000 rpm range. The S65 engine has moderate torque but nice HP as a result of being tuned for higher peak power than most traditional street engines.

    High revving engines can be more fun on the track then the street, but torque is always king when it comes to fun. I have never met anyone who didn't have a smile driving a high torque engine. As noted above torque and HP are related and when torque increases so does the HP at that specific point where the torque increases. The torque/HP increase however may not be at the HP peak. A typical example is the 335i N54 turbo which has excellent low end and a broad torque band with modest peak HP. These cars are fun to drive because of the broad torque band and you don't need to rev them high to enjoy the fun.

    cwbiii guest

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    torque at the wheels...

    Another way to look at it is how much torque is being applied to the pavement by the tires... This is a function of the torque out of the engine divided by the total gear ratio. A higher engine speed allows the use of a lower gear ratio for the rear end and this multiplies the torque to the wheels... given the same target top speed.
    That in turn boosts every gear by that same multiplier... which can be a big win in the end.
    The down side... it uses more fuel, and when things let go it can be pretty spectacular.
    Not to mention that balancing and blueprinting an engine is pretty costly.
    A flywheel blast shield is recommended if you are going to take the revs past a certain point.

    I had a 392 hemi out of a 57 Chrysler Imperial that was balanced and blueprinted. I never put it in a car though since I couldn't find something that would actually handle it on my budget. Eventually sold it to someone with a rail (dragster) for a very respectable sum. He said he would probably get a dozen runs out of it before he used it up. I believe he said he was getting 1200-1400 hp out of that engine with his blown nitro-methane setup.
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    As owners of E60 M5s have found, the unfortunate part of having a high revving engine design is the 99.9% of the time you are not at WOT between 6000 and 8000rpms. Even with the help of dual VANOS and modern fuel/ignition timing, the S82 is extremely inefficient at low rpm where you typically drive.

    The S82 is down over 40 ft-lbs on torque at 2k rpm compared to the S62 in the E39 M5 for example. It makes less torque (and therefore HP) than the S62 all the way up until about 5500rpms where its torque curve comes alive with a big bump. Hence in everyday driving, to achieve the same level of light to moderate acceleration, etc, it requires larger throttle openings, and it achieves horrific fuel efficiency as a result. It is as much as 30-40% worse in fuel mileage compared to the E39 M5, which wouldn't be a problem for most except the fuel tank is the standard E60 size, so you're constantly (it seems to owners) stopping to refuel.

    Anyway, if you want to really understand the difference between these two types of design (i.e. compare an E92 M3 to the new 1M), go ride a two-stroke and then a four-stroke motocross bike. The 2-stroke motox bike is ENORMOUS fun when you can "keep it on the pipe" in the sweet spot of its powerband otherwise its a PIA.

    Hence "that" is the problem with high strung engines...the 99.9% of the time you aren't winding them out in the sweet spot of their powerband. It gets old really fast. In daily driving, the 1M will be an incredible blast with boatloads of low to mid rpm power (torque) compared to the M3. If the M3 was to keep up with it, the M3 driver will have to keep that S65 on the boil above 6000rpms while the 1M driver might be a gear higher just enjoying the easy torque blasts when they're needed.:)

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