I just would like to know which one of these two is better and why?
I mean which one would return better MPG, would be cheaper to maintain, better torque at low revs and would run to higher miles?
Please also provide any other information you think we would find useful!
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In principal 16v allows a larger valve area for a given cyl bore, and if designing an engine from scratch I reckon 16v is the way to go (as is cam chain though that is another story) however there are so many variables from engine to engine that you cannot simply say 16v is *better* than 8v.
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so many variables from engine to engine that you cannot simply say 16v is *better* than 8v.
Yes. When I was young twin OHC engines were at least sporting, and when small tended to be racing screamers. When 16v heads came in those engines were even more like that.
Now a I have a twin-OHC 16v car which is torquey and fairly economical and doesn't mind wafting about at low speeds in high gears. I've never tried to make it be a racing screamer but I don't think it would fulfil the role all that convincingly.
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Generally on a normally aspirated engine a 2v will be more torquey at low revs and a 4v will breath better at high revs, again rather a generalisation though.
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Cheddar, could what you said be expressed as...?
"Depends what the engine is intended to do. To get a higher bhp per litre four valves per cylinder would be useful beacause they would confer better breathing at higher revs. By implication, such an engine would need to be revved more to get the best from it."
By this reasoning the higher-output (4-cyl) diesels would be 16V. Correct?
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So we're saying that 16 valve diesel engines are more suitable for heavy footed people or those who like to see the revs go up to 4000+ before they change the gear? :)
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IIRC, 16 valves give a better, central placement for the injector. For petrol engines, breathing can be better at higher revs (6000+ rather than 4000!) at the expense of mediocre low-end torque, and the valve weight reduced etc. All irrelevant with low-revving turbo-charged diesel.
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IME, 16v engines are more free-revving, and don't run out of breath as easily when you're giving it some beans. This applies for diesel as well as petrol. My 16v 1.4 petrol is very sluggish at low revs though, and usually requires revving up to 4000 revs before changing gear to make decent progress.
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>>Cheddar, could what you said be expressed as...?
"Depends what the engine is intended to do. To get a higher bhp per litre four valves per cylinder would be useful beacause they would confer better breathing at higher revs. By implication, such an engine would need to be revved more to get the best from it.">>
Yes, until you introduce a turbocharger which enables a 16v engine to offer loads of torque at low revs.
>>By this reasoning the higher-output (4-cyl) diesels would be 16V. Correct? >>
Not necessarily, because they are turbocharged.
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Some figures for 12v and 24v Mercedes diesel engines. Both are otherwise comparable, fairly low tech, indirect injection, with purely mechanical injector pumps.
cc Valves Fuel Layout Power(PS) Torque(Nm)
3.0 12V D S6 113 191
3.0 24V D S6 136 210
(Doubtlessly this site will mangle the layout of the table!)
Number_Cruncher
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Based on driving experience I'd say that 16v diesels do seem to rev a bit better than 8v ones, but there again, as most people drive diesels in such a way as to make maximum use of the torque, the practical difference isn't great.
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My 8v TDIs used to get to 5000 if required, but as paulb says, it isn't really necessary. Limitation of maximum revs is by governor, so the ultimate rev range is presumably very similar. If you mean the rate of change, maybe the 16v gets there quicker. But isn't that a feature of the greater power available to later engines, due to various developments like PD and CR Mk 11?
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4 valve heads are becoming the norm on modern farm tractor engines. These are not revy little car diesle engines and generally get a very hard life, when a family of engines is used in an agricultural application it will have a lower rated power than a similar engine in say a LGV, this is because they are expexted to run under full load for prolonged periods. But they don't rev. High idle would be 2500rpm max and they would normally run at 2200 with the throttle wide open.
I'm not sure what advantage having 4 small valves is at 2200revs compared to say 6000rpm in a car or 14000rpm in a bike engine.
We very rarely ever touch valve clearences, but specific fuel consumption is a issue. The amount of torque our modern engines serve up would make your eyes water!
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What implications 4 vales per cylinder have on fuel compared to 2 vales per cylinder?
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16 small valves as opposed to 8 larger valves helps to keep the reciprocateing weight down. Then there is also variable valve timing to help out across the rev range.
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VVT on petrol engines, to fill out the hole in the power band when the intake velocity drops:) Don't think VVT or VTec etc is required on turbo-diesel engines, but no doubt there is some exception.
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4 valves/cylinder is also a good marketing tool :-)
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