My mum bought a StreetKa recently, and I got a go in it for the first time today.
What struck me most about it was the way the power is delivered. Comparing it to our own 1.4 Zetec-SE engined Fiesta which actually has a slightly better power to weight ratio (6 bhp down, but 150 kg lighter), the midrange punch of the eight valve engine makes it feel much more responsive in most situations. If you wring both engines to their redlines, the multivalve unit definitely feels happier, but in normal driving, I think the 8 valve acquits itself slightly better.
Don't get me wrong, I am a big fan of the Fiesta's Zetec-SE engine, but it is a "typical" multivalve engine. If you use all the revs it goes amazingly well for what it is, but it does feel quite flat under 3,000 RPM. The StreetKa's 8 valve Duratec unit pulls eagerly from pretty much the floor, and feels much more responsive, even though on paper the car is slightly slower.
This got me thinking. I recall a similar experience driving both 8 valve and 16 valve version of the Peugeot 306 XSi on the same day a few years ago. While the 16 valve was definitely the more powerful, and on paper had a modest performance advantage, it was the 8 valve that felt much more responsive and arguably felt quicker at "normal to mildly enthusiastic" engine speeds, and seemed to do more useful work over much more of the rev range than the 16 valve unit.
I realise that marketing spiel introduced the four valve per cylinder engine as a "must have" and sold it as a mark of advanced technology, but I think, based on admittedly limited experience of two types of car from two manufacturers, that the two valve design, however "obsolete" has a lot going for it. Four valves give us more power, but seem to stick it up the top of the rev range at the expense of torque lower down. Is this just chasing headline BHP figures at the expense of driveability?
What do you think?
Cheers
DP
PS - The StreetKa as a whole I thought was a pleasant surprise. A lot of fun to drive despite modest performance, and handles and steers beautifully. Too "girly" for me, but I really quite liked it.
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I agree, wife's 8v 1.6 Clio is very responsive, pulls well from low revs and with 90bhp is quite nippy. A 1.4 16v ( 95bhp) feels a lot slower and a 1.6 16v (110bhp) is a lot less punchy at low revs. The 16vs are a little bit crisper above about 4500 rpm though.
It is largely to do with ratio of the cross sectional area of the inlet v the cyl volume, a larger inlet are offers slower gas speds at lower revs hence less torque. Variable length manifolds as per some Vauxhall Ecotecs do well to boost the low to mid range torque on 16v engines, as do some VVT systems, twin choke carbs, CV carbs etc. Not an issue on forced induction engines.
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I suspect you are right - power at 5000 -6000 rpm is irrelevant to everyday sensible use. The reason people like diesels is not that they will do anything that a petrol engine won;t do, in fact they have a very limited useful range which is eclipsed by a good petrol engine, but that they produce more power at 'normal' revs and therefore feel more powerful. Ditto 2 valve/cylinder petrols compared with their multivalve equivalents.
Years ago I had a 2 litre 8 valve Carlton that felt as if it would pull the sie off a house with 'only' 115bhp. The 16 valve Omega that replaced it, rated at 130bhp, felt lethargic by comparison yet I expect it had a lower 0-60 time.
I have been told that multivalve engine development was originally driven b y emissions requirements rather than driveability.
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DP,
What are the power and torque figures (and the corresponding engine speeds) for each engine. Do they have comparable compression ratios? What do the valve timing diagrams look like?
I think the disparity in mid-range performance is partly a feature of how each engine is tuned - particularly the valve timing. Knowing that the 8 valve can't deliver at high rpm, its valve timing will be optimised for mid range, while the 16v will be optimised for higher engine speed.
I suspect that comparing an 8 valve engine with a multi-valve v.v.t. engine, you will more likely see that the torque curve of the v.v.t. engine envelopes the 8 valve engine torque curve entirely (probably being idetical curves in the mid range and below). i.e., for a v.v.t multi-valve engine, you get both the decent mid range response, and the high power offered by the higher total allowable mass flow rate through the extra valve area.
Number_Cruncher
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I suspect that comparing an 8 valve engine with a multi-valve v.v.t. engine, you will more likely see that the torque curve of the v.v.t. engine envelopes the 8 valve engine torque curve entirely (probably being idetical curves in the mid range and below).
I would say generally not NC, the VVT only in part compensates for the larger inlet area which means slower gas speeds at lower revs hence less torque. An exception perhaps being the Honda VTEC system on the VFR800 which keeps one inlet valve closed totally at low engine speeds
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Mmm, but gas speed isn't vital - especially not for fuel injected engines. If the intake air also has to carry the fuel, then yes, you need to keep it moving fast, or else the fuel ends up on the manifold/port walls.
It's the total mass of air that enters during the period that the valve is open that matters in terms of how much torque can be produced. So, if theres a larger area, and a slower gas speed, it's still possible to get the same mass of air through the port.
The main point I am trying to make is that in general, people comparing 8v engines and 16v engines aren't really comparing like with like, because they are tuned differently, for different purposes, and, as such, they suit different driving styles.
Number_Cruncher
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It's the total mass of air that enters during the period that the valve is open that matters in terms of how much torque can be produced. So, if theres a larger area, and a slower gas speed, it's still possible to get the same mass of air through the port.
Yes it is the total amount of air that matters however that is not achived at low engine speeds simply by ever larger valves because there are other factors to consider, gas speed for one. There is an optimum gas speed for any given combination of cylinder volume, bore & stroke ratio, valve timing, engine speed. Too high a gas speed is a result of the cross sectional area of the intake being too small, in such circumstances the volume of air that can enter the cylinder is restricted. Too low a gas speed is a result of the cross sectional area of the intake being too large, in such circumstances the air virtually stops between cycles needing to be acclerated again thus it has little momentum and optimised momentum (without being restrictive) of the air provides slightly higher than atmospheric pressure at the point the inlet valve(s) closes, also low air speed as it enters the cylinder means little or no swirl is acheived and swirl helps to ensure that an even mixture of fuel and air is spread through out the cylinder.
The ratio between gas speed and engine speed is always a compromise because the cross sectional are of the intake is not infintely variable (though VVT systems that effect valve lift as well as duration enable quite a wide adjustment to be made) hence this links with the "What's torque?" thread in as much as it helps to explain why an engine produces it's max torque at a particular engine speed, the cross sectional area of the intake and the gas speed are both factors that ensure that each individual cycle of each cylinder is most effective at a particular engine speed, the engine speed at which max torque is produced.
The main point I am trying to make is that in general, people comparing 8v engines and 16v engines aren't really comparing like with like, because they are tuned differently, for different purposes, and, as such, they suit different driving styles.
I agree.
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DP,you should try a Sportka-that has the Streetka engine(1.6)but only the weight of a normal Ka.That is a really nice motor.On the power/torque front,years ago I was involved in producing engines for rally cars.Roger Clark(shows my age) and team needed three engines-one for each team car and a spare.These were prepared and despatched.Normally, the power curves would accompany the engines and the most powerful would be fitted to Roger's car.On this occasion,the curves were mislaid and Roger selected his engine by having all the motors fitted to cars and driving all three.When the curves were found,he had picked the lowest performing engine on power but it was the best on mid-range torque.
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4-cyl 16V engine more efficient, but he efficiencies only come in at high engine revs where the effectiveness of head design and valve orifices/timing, etc really pay off - or not, as the case may be. Mk 2 8V Golf GTI almost as usable as the 16V in 'normal' driving, but rev it and the 16V really flew.
(Is this basically the same as NC's and Fromage's gas speed stuff?)
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Dont disgree with techincal stuff posted
but on a more down to earth aproach. A lot of engines have been made 16v for reasons of economy (greater coverage of combustion chamber etc) rather than perfomance per se.
A lot of it has been made into marketing mumbo jumbo too to move the cars.
But at end of day if you want the most power and are prepared to tune it accordingly its the multi valve engine to win anytime. But normal driving its the 8v engine that is more likely to deliver the bottom end punch needed that will make the car feel (and be ) more responsive..
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Even so there's little to match the glorious howl of a Honda VVVT engine after 5,000rpm or so....:-)
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What\'s for you won\'t pass you by
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VTEC...
IMHO the best all-round Honda engines were the 'intermediate' 12 valvers of the 1980s - smooth, very economical and with incredible pulling power. The 2 litre Aerodeck was a sports car that did over 40mpg and the old 1500 Shuttle I had a few years ago would go up round the 1 in 10 hairpin behind my house in fourth gear without strain as well as returning 50mpg with everyday driving.
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I wouldn't get too hung up on 8V vs 16V.
Main point about 16V engines is that they give better flow rate at high rpm - give more valve open area in a modern pent-roof combustion chamber. They also have the added benefit of promoting 'tumble swirl' which gives better mixture homogeneity.
The effect of valve configuration and timing is calculated by by dividing the rate of change of cylinder volume by the instantaneous minimum valve flow area - this gives you the 'pseudo flow velocity' for the valve(s). If you then plot the pfv through a cylinder's cycle you get two peaks for each of the exhaust and inlet valve flow rates and you find that pfv's are low early in the exhaust stroke and late in the intake stroke - these are the points where exhaust blowdown and ram air effects in the intake are most important. Ram air effects (as employed in 'dual ram' type induction systems) are actually much more significant than whether an engine is 2 or 4 valves percylinder - as are matters such as valve lift and overlap. 4-valve designs are simply a good way of getting a lot of valve area into a given head area - with the bonus of tumble swirl.
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I think thats the technical description of what i said earlier?
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My Ford Ka has the old endura-e 8v engine which can be found in the fiesta as well.
Being only a 60bhp but with a light body it has it can pull away pretty swiftly from low down in the range, but when it gets to the top end it can feel breathless eventually.
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Its not what you drive, its how you drive it! :-)
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With 4 (or 5) valves per cylinder the spark plug is located closer to the centre of the cylinder and has a more efficient burn and so the emissions are cleaner. ;o)
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It seems that 5-valves are going out of fashion. VAG have dropped theirs and Yamaha have gone back to 4 valves for their latest YZF-R1 bike. I suspect that emissions are the problem. It would be interesting to try an oval piston 8 valve Honda NR750 to see what torque characteristics it had. I bet it sounded nice.........
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Yamaha's justification for dropping the 5 valve head is that modern materials enable the 2 inlet valves required by a 4 valve head to be lighter than previously thus the benefits of the 3 smaller / lighter inlet valves in a 5 valve head are negated. Likewise the M1 MotoGP bike uses 4 valves though the 5 valve head lives on the Fazer 1000 (cracking bike!) that uses a version of the 2006 R1 motor.
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The 5 valve head also lives on in the TDM 900 - a low revving engine by bike standards
On the performance front, I would guess that a 4 valve head could always be tuned to out-perform a 2 valve head - they tend to have better efficiency.
However, when choosing a car for my wife, an 8v SOHC Clio appealed with (possibly) easier maintenance - she doesn't rev engines enough to get the benefit of a multivalve design.
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Ref Yamaha's 5 valve designs - when they introduced these they did it on the basis that it's not valve area that is important, it's valve circumference. Three smaller valves had a bigger circumference than two larger ones.
Seems to me like this is marketing speak following engineering - namely, find a good reason for anything the engineers choose to do.
V
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As the gas doesn't actually flow through the whole area of the valve, I find the circumference argument much more reasonable than arguments based on valve area. Although, of course, it's difficult to increase circumference without increasing area!
Of course, any material seen by the customer is heavily filtered and adulterated by the marketing department - I can't imagine any motor company being daft enough to allow its engineers to communicate directly to the punters (they might tell some uncomfortable truths!).
Number_Cruncher
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As the gas doesn't actually flow through the whole area of the valve, >>
As you know NC it is a very complex matter with numerous factors to account for not least of which is the valve lift, i.e. two engines can have valves of the same size though one may have greater lift thus increasing the effective cross sectional area of the intake (or exhaust for that matter.
I find the circumference argument much more reasonable thanarguments based on valve area. Although, of course, it's difficult to increase circumference without increasing area!
It is actually quite easy, simply by reducing the valve lift, to be clear I am referring to the cross sectional area of the intake and not the size of the whole in the head that the valve fills when closed, they are two different things.
Of course, any material seen by the customer is heavily filtered and adulterated by the marketing department>>
Yamaha's justification for 5 valve heads has been lightness of each component also 5 valves of the same size offers a 2:3 ratio between exhaust and intake valve size which is a ideal ratio and allows the largest proportion of the head area to be used as valve area based on a single plug design.
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Some seriously useful information here, thanks for all the replies.
I suspected I was oversimplifying things by referring to valve quantity, but not to this extent!! ;-)
Cheers
DP
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By the way, scuse my spellin, whole = hole etc!
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Having spent some time in the past with several (diesel) engine designers, the advantages of a 4 valve as opposed to a 2 valve layout have been outlined to me as follows:
4 (or 5) valves will give a better "fit factor" in the combustion chamber roof, enabling higher gas flows to be achieved at a defined valve lift. 5 valve layouts can compromise injector (and presumably spark plug) positioning and are generally not considered to be worth the mechanical complexity on a large diesel. My colleagues were geneally less impressed with the gas flow argument than other factors (see below) pointing out that increasing boost pressure is an inexpensive way of achieving a higher gas flow.
The advantage of a 4 valve head which featured most strongly has been touched upon above - valve seat length. A crucial factor in achieving efficient cylinder charging and scavenging is the rate of change of oriface size - which is in direct proportion to total valve seat length. Variable valve timing systems generally yield better results with a high rate of change of oriface size - achievable on a 2 valve layout only with "brick wall" cams, which would require "PD" oils to lubricate them. Large diesels are built to last a very long time, so highly stressed valve gear is not on the agenda - a 4 valve head and turbocharger is an iestablished ndustry standard (eg Rolls Royce Eagle) for most automotive applications.
659.
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