Higher torque in a normally aspirated engine can be achieved by narrowing the inlet tracts thus ensuring that air flow speeds are maintained at low revs. Hence why an 8v engine often feels more gutsy at low revs than a 16v of the same capacity, this also why high perforamance carburettors used to have twin chokes, one of which remained closed at low revs to maintain airflow. A similar though more sophisticated approach has been taken on many fuel injection systems.
Taking into account Number Crunchers points it could be that the 150lb/ft mark is the optimum when using technology to offset the conflicting requirements of high speed airflow at low revs v high volume airflow at high revs.
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Variable valve timing (as mentioned in the original post) actually plays a huge part in extending the torque plateau, especially when implemented on both the inlet and exhaust camshafts of a twin cam engine.
With V-Tec, Honda (and probably others) have also tried the trick of stopping one inlet vale from working at low engine revs, thereby forcing the other one to flow faster. The disadvantage of this, especially on a motorbike like the current VFR800 rather than the Civic where I first remember V-Tec from, is the sudden change in power deliver when the second set of inlet valves chime in.
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Indeed, it is the extension, towards higher revs, of the volumetric efficiency plateau, hence torque plateau which explains the increased power output of the more technologically advanced engines quoted in the original post.
number_cruncher
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In english, does that mean more valves = less torque?
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No
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Without VVT, more valves -or making use of them- will tend to mean the torque peak is shifted upwards in the rev range. This means the maximum power output figure is increased, but torque may be reduced at low revs.
VVT allows the torque peak to be spread over a wider band of revs.
But as in my original point, the actual maximum torque value hardly seems to be getting any higher as time goes by, for the reasons NC has stated.
With given octane ratings(?) and lambda-loop, and VVT now established, is there any further specific-torque to be achieved?
Have I noticed a teensy-increase on these new direct-injection engines??
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In english, does that mean more valves = less torque?
Inherently larger inlet valve area relative to cylinder capacity = less torque however twin choke carbs, variable length inlet manifolds, variable valve timing and lift etc etc can cancel this out and then some.
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Inherently larger inlet valve area relative to cylinder capacity = less torque ...
Hi Cheddar,
I don't think I can agree with your logic here in any general sense. Larger valves will, when averaged over the whole speed range, allow more air to flow, allowing *more* torque to be produced.
Depending on how the valce timing has been optimised, this may, however, lead to poorer low speed running for the reasons you stated above.
number_cruncher
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Hi NB,
larger valves will, when averaged over the whole speed range, allow more air to flow, allowing *more* torque to be produced.
Infact larger valves will, when averaged over the whole speed range, allow more air to flow, allowing more POWER to be produced, this is not the same as torque.
Regards.
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Hi Cheddar,
At the risk of kicking of August's Torque / Power thread again - was it DD who mentioned Groundhog day earlier? :-)
It isn't an either or thing between torque and power - you need both torque and speed for an engine to deliver power.
Taking the valves to an extreme, if you have tiny valves, you can't produce much torque, they would effectively throttle the engine. As you increase their size from being tiny, torque, and hence power both can increase.
Typical modern engines make good use of the available area above the piston, and potential increases in valve area are marginal at best.
number_cruncher
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>>Taking the valves to an extreme, if you have tiny valves, you can't produce much torque, they would effectively throttle the engine. As you increase their size from being tiny, torque, and hence power both can increase.
I know you dont need me to agree. but well said
--
Steve
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Hi NB,
Taking the valves to an extreme, if you have tiny valves, you can't produce much torque, they would effectively throttle the engine. As you increase their size from being tiny, torque, and hence power both can increase.
>>
Let's put it another way, the optimum inlet valve size (or valve area v cylinder capacity) for max torque is different than for max power. Hence the challenge faced by engine designers who try to make engines flexible at low revs and powerful at higher revs.
Regards.
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Let's put it another way, the optimum inlet valve size (or valve area v cylinder capacity) for max torque is different >> than for max power.
No, really, No! If you look at a racing engine, which still do / must produce torque, the inlet manifold is designed purely to avoid being a restriction; they are quite short, and aerodynamically excellent.
The thing is, racing engines can produce their maximum torque (which *isn't* poor) at high revs, and then their maximum power at even higher revs - they don't need to tick over smoothly, so this part of the rev range is not designed for.
Racing engines produce both high torque figures and high power figues. They can't make power without torque!
Hence the challenge faced by engine designers who try to make engines flexible at low revs and powerful at higher revs.
Yes, you can imagine the VTEC system, for example, as a means of making a screaming engine, optimised to produce both torque and power at higher engine speeds, tractable at low revs. For obvious reasons, it isn't marketed this way round.
number_cruncher
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Hi NB,
Perhaps we are talking almost at cross purposes here. You are right a racing engine can produce high levels of torque albeit at high revs. However in a road enging the challenge is 1/ to provide drivability at low/medium revs and 2/ a strong band power up to and beyond 6000rpm or so. 1/ can be achieved with an 8v engine with relatively small inlet valve therefore ensuring reasonable airflow at slow engine speeds however this engine would feel breathless at higher revs. 2/ can be achieved with larger inlet valves allowing the engine to breathe more freely at higher revs however the slow airflow at low engine speeds would make it seem gutless at low revs.
It is the need to acheive 1/ & 2/ together, i.e an engine that is flexible at low revs, has strong midrange and is powerful at high revs that has lead to the development of devices such as twin choke carbs, variable length inlet manifolds, VVT etc.
Regards.
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Perhaps we are talking almost at cross purposes here.
Yes, there's always a good chance of that happening! :-)
Cheers,
number_cruncher
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It is true to say that for much more complex reasons exhaust back pressure has a lot to do with torque hence the reason for Yamaha's development of the EXUP valve, this is a valve that sits in the exhaust system and is controlled electronically varying the effective diameter of the exhaust. Yamaha have been fitting this system to motorcycles for 15 or more years, more recently other manufacturers have introduced there own version of the idea.
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Upon reading your post i had the following thoughts, please correct me if i am wrong:
If you have big valve area, will not flow at low speeds be worse than a smaller valve and hence you achieve lower volumetric effciency? Further still if you have achieved atmospheric plus a few per cent, increasing valve area would not increase torque as the celing would have been reached. Hence, as SJB points out manufacturers have sought to decrease valve area at low speed to increase torque.
Also exhaust tuning can significantly effect torque- you can't introduce new charge if some of the burnt charge remains.
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