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If 4 valves per cylinder is better than 2 valves...

S

SaltyNuts

Platinum Member
Why the heck did they stop there? Why not 6 valves? 8 valves? 100 valves? I realize at some point the additional cost might outweigh the additional benefit, but it seems there would be plenty of people willing to pay a bit extra to be able to say their car has 100 valves per cylinder! That would be dope as fuk, 100 small valves opening and closing all at the same time...

Thanks.
 
Thanks HarryLui. It just seems weird - like why, when they have known since at least the freaking 1980s, and probably earlier, that the more valves a head has the better its performance, have they just NOW, like 3ish (or more) decades later gotten around to more than 4 valves/cylinder. And then, why so modest an increase? Why doesn't someone shoot for the stars? It would be like Intel realizing they could enhance performance 3x or 4x by doing a die shrink, but waiting decades to do it... would be nuts...
 
pcgeek11 said:
Three major things:

Mechanical Complexity, Cost and Reliability. Anyone of these is a deal breaker.
Click to expand...

Thanks pcgeek:

1. It adds complexity, but did not also going from 2 valves per cylinder to 4 valves per cylinder?

2. Yes it adds cost, but did not also going from 2 valves per cylinder to 4 valves per cylinder?

3. MAYBE it would make the cylinder heads less reliable, in that there are more valves to potentially fail, but then again with smaller valves there is probably less pressure on/flow through each value so the risk of failure might decrease per valve, possible offsetting at least somewhat the reliability decrease. In any event, did not going from 2 valves per cylinder to 4 valves per cylinder not also decrease reliability?

Maybe, some how, some way, 4 valves is the "sweet spot" under all these considerations, but I content that there would be plenty of people that would pay a price premium for cylinder heads with 100 valves per cylinder and the increased performance that would go with them...
 
SaltyNuts said:
Thanks pcgeek:

1. It adds complexity, but did not also going from 2 valves per cylinder to 4 valves per cylinder?

2. Yes it adds cost, but did not also going from 2 valves per cylinder to 4 valves per cylinder?

3. MAYBE it would make the cylinder heads less reliable, in that there are more valves to potentially fail, but then again with smaller valves there is probably less pressure on/flow through each value so the risk of failure might decrease per valve, possible offsetting at least somewhat the reliability decrease. In any event, did not going from 2 valves per cylinder to 4 valves per cylinder not also decrease reliability?

Maybe, some how, some way, 4 valves is the "sweet spot" under all these considerations, but I content that there would be plenty of people that would pay a price premium for cylinder heads with 100 valves per cylinder and the increased performance that would go with them...
Click to expand...

1 & 2. - Yes but going from 2 to 4 is vastly easier than going from 4 to 8 or ten. The same goes for cost.

3. There is more to just the reliability of the valves themselves. The mechanics of positioning all the 8 valves inside the small area available in the head cylinder area. Then all these holes in the head will allow less metal to maintain the integrity of the head. Then the matter of cooling the valves themselves. There has to be coolant passages around the valve seats of all 8 valves. It isn't a simple as you want to believe.

Having four larger valves is just as good as having 8 tiny valves.
 
It's like how a 12cyl 2L can usually make more power than a 4 cyl 2L if all things are equal ... but, more moving parts and smaller bits make it less reliable and much more costly for a tiny 12Cyl to be practical.
 
SaltyNuts said:
Thanks pcgeek:

1. It adds complexity, but did not also going from 2 valves per cylinder to 4 valves per cylinder?
Click to expand...

Four valves over two because they are trying to optimize fuel/air mixture to the cylinder. Adding more valves has likely diminishing returns to the effect.

My 1986 Yamaha Fazer had 5 valves per cylinder... Think about how compact an inline four cylinder bike is and how small those valves were.
 
BurnItDwn said:
It's like how a 12cyl 2L can usually make more power than a 4 cyl 2L if all things are equal ... but, more moving parts and smaller bits make it less reliable and much more costly for a tiny 12Cyl to be practical.
Click to expand...

At some point, mechanical / friction losses from additional moving parts are likely a factor re: power as well.
 
WackyDan said:
Four valves over two because they are trying to optimize fuel/air mixture to the cylinder. Adding more valves has likely diminishing returns to the effect.

My 1986 Yamaha Fazer had 5 valves per cylinder... Think about how compact an inline four cylinder bike is and how small those valves were.
Click to expand...
Those Phazer sleds were awesome ..
Air cooled 2 stoke FTW
 
SaltyNuts said:
Thanks HarryLui. It just seems weird - like why, when they have known since at least the freaking 1980s, and probably earlier, that the more valves a head has the better its performance,
Click to expand...

There are more than one way to boost the efficiency of an engine, pun intended.

In the early 1980, both Volvo and Dodge chose turbocharger to increase their 8 valves engine performance. It wasn't until few year after production both companies start adding 16 valve heads in their engine choices.
 
general laws of physics and size constraints. I'm sure that a 100 valve per cylinder V-16 would be possible, for an engine the size of a boat? But also, why add all that complexity (and increasing chance of failure) when there are better ways to do things?
 
SaltyNuts said:
Thanks HarryLui. It just seems weird - like why, when they have known since at least the freaking 1980s, and probably earlier, that the more valves a head has the better its performance, have they just NOW, like 3ish (or more) decades later gotten around to more than 4 valves/cylinder. And then, why so modest an increase? Why doesn't someone shoot for the stars? It would be like Intel realizing they could enhance performance 3x or 4x by doing a die shrink, but waiting decades to do it... would be nuts...
Click to expand...
I'm not sure I've ever seen someone use an appeal to extremes in support of their argument in an engineering situation.

Well done. I'm actually impressed!
 
Theres 2 main benefits a 4 valve has over a 2 valve.
1) Flow area - for a given valve lift, the 2 smaller valves have more area available, which allows for opening/closing points closer to optimum for the engine use case.
2)Weight - it requires less spring pressure to control each smaller valve, and so the engine redline can now be higher than a 2v (typically)

Theres other benefits as well, and some downsides (port induced swirl vs tumble) but even going to a 5 valve (3 intake) had some weird port flow behaviour that made its mechanical complexity too much of a cost to continue (yamaha has abandoned the 5v afaik)

With 4 valves, the mechanical limits can already be shifted down to the bottom end limits - rod bore integrity, piston, oil pump rpm. Adding more valves wont change that.

Then theres the fact that engines arent just sheets with their specs on them. You've got its external size vs internal size to consider, as well as its center of mass, such as gm still using a pushrod v8 in its highest performance car (they blind tested the c5 with both a dohc and the in block cam, the lower cg of the pushrod engine had more favourable results).

Now, honda did do a v4 with 8 valves per cylinder:
https://en.wikipedia.org/wiki/Honda_NR

Theres a reason that no one has tried to make that a production engine.
 
This is similar to why power lines only use 3 phases. Increasing the number of phases increases capacity but at the cost of diminishing returns due to increased parts count, etc. I think in the case of power lines it's something like 6 phases only gets you 1.8x capacity.
 
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