Why don’t we see more valves per cylinder?

aw614 said:

Can you get a VW 1.8t head to rev high? It seems even though they had 5v per cylinder, they didn't rev particularly high, but it also mainly came in the 1.8t turbo cars with the smallish k03. I recall there was a 20v n/a in europe and eurosport had an n/a build, but it was stolen a few years ago, trying to find out the details about that motor.

ALL VWAG engines (that were twin cam anyway) were five valve after a certain point.  Not just the 1.8t, but also the 2.7, 2.8, and 3.0 V6s, and the 4.2 V8s.  And probably a bunch of stuff in the home market too.  Then they switched back to 4v/cyl when they started to design new engine architectures.

The 1.8t engine did not rev high because it was the base model engine.  If you wanted a fun revvy engine, you got the 2.8, which was a real fun lil' motor when you kept it over 4000 or so.  For some reason people did not seem to want to pay extra for the 2.8 when the 1.8t was available for $22,000 in an A4.

The center intake valve opening of the 5-valve Yamaha's was retarded by x-degrees of the two other intake valves to swirl the intake charge. Also, lighter valve weight vs heavier valves as to spring rates, revs and redline. I'd imagine the other mfg's of 5-valves are of similar theory. 

In reply to fasted58 :

That's a good chunk of why 5 valve is off the back.  (Is "off the back" still a thing people say, or do I need to get an onion for my belt?)  Swirl is not in vogue anymore, tumble is, and it's super effective to get tumble via 4 valve head with narrow valve angles, and straight ports with no short side radius, or even a "negative" SSR, to shoot the airflow towards the backside of the chamber instead of straight down in and around all of the curtain area.  Yes, effectively only half the valve area is being "used", but the area around the SSR is shrouded by the cylinder wall anyway, so why focus on it?

Here's a pic of my Volvo 1.9l ports that I took years back when I had the intake off, to show to OST to make him cringe

It's hard to make out but the seat actually extends into the port.

OTOH it also pulled that car to remarkably good fuel economy (could touch 42mpg, could expect 38-40), with 1990s head technology.  The goal was not big breathing RPM, the goal was fuel economy and being able to run high compression with a turbo on top.

The all time problem with poppet valves is the valves getting in the way of the air flow.    I was thinking about that BMW but had forgotten where I had seen it.

Sliding valves?  Desmodromic Valves?  Air valves?  What we need is a valve that works like a camera shutter.

codrus said:

Patientzero said:

The almighty LS only has 2 valves.

Which is why it doesn't breathe as well at high revs as 4- or 5-valve engines. :) 

10,000rpm (almost 11,000 now) is plenty for anything I'm doing.

Knurled. said:

The 1.8t engine did not rev high because it was the base model engine.  If you wanted a fun revvy engine, you got the 2.8, which was a real fun lil' motor when you kept it over 4000 or so.  For some reason people did not seem to want to pay extra for the 2.8 when the 1.8t was available for $22,000 in an A4.

It was also easy to add a $500 chip to a 1.8T and make way more power than you ever could on a 2.8.

Another factor is that VW/Audi engines were designed to be mounted longitudinally in front of the front axle, meaning they had to be short.  This reduced bore spacing and made them undersquare more than usual, which contributed to the lack of "reviness" in many of them.

5 valves seems like a bunch of extra complexity for some marginal gains on paper.  That's precisely the sort of thing that Audi loves to put in their cars (and I say this as someone who's owned at least one Audi for the last 20 years now). :)

In reply to codrus :

The irony is that same 88mm bore spacing that locked them into having to add more cylinders, actually works for them again, as one of the other things people have settled on is that, for the kind of load/speed spectrum seen in an automotive engine, an undersquare 500cc cylinder is just about perfect.  And that is exactly the corner that they painted themselves into.

Except for the ultraweird 4.2 V8, which had 92mm bore centers, but the cylinder head was built on the same 88mm bore centers as the 3.6, so the cylinders were all offcenter to the chambers to some degree.  And they made thousands (millions?) of them and they worked just fine.  Makes me laugh when people say OMG YOU CAN'T PUT AN LS HEAD ON A FORD WINDSOR THE BORE CENTERS ARE .020" DIFFERENT.  Pleeeeease, that ain't nothin'....

In reply to noddaz :

I'm pretty sure that flow bench tests have shown that poppet valves aren't a flow restriction at some amount of lift, can't recall what that lift was but it flowed the same with the valve open and in place, as the same port with no valve a'tall. 

In reply to Knurled. :

5-valve was 80's-90's tech but pretty good for the day. I merely stated what others missed or omitted.

Sorry, I didn't bring pictures. 

noddaz said:

The all time problem with poppet valves is the valves getting in the way of the air flow.    I was thinking about that BMW but had forgotten where I had seen it.

Sliding valves?  Desmodromic Valves?  Air valves?  What we need is a valve that works like a camera shutter.

FWIW, since engines of various types have been using some kind of air valve for well over 100 years now, every type of valve has been tried.  Over and over again.  For ICE's, poppet valves consistently win the competition.  

Same can be realistically said about 5 or more vs. 4 valves, since that "competition" has been happening for about 30 years now.  This one may be a closer discussion, but between cost and packaging, 4 valves consistently win.

Knurled. said:

In reply to fasted58 :

 straight ports with no short side radius,

This + shrouded outer valves + ugly squish pads which inhibit combustion.  A 4 valve has nice straight ports, relatively unshrouded valves and well defined squish pads.  A 5 valve has pretty good low lift flow relative to a 4 valve and combustion isn't too bad at low rpm, but max airflow is limited by a tortuous path for the 2 outer intake ports/shrouded valves and combustion isn't good at high rpm when there's not much time available to light things off.

Just get a proper two stroke engine....................problem solved (well other than emissions & fuel mileage)

Implicit in this is the question, "why are valves circular?"

Why not have two big valves with big, non-circular shapes that occupy all the space that's available?

Once you've decided that such a design wouldn't flow right, or maybe flow evenly, I think it's a pretty small leap from there to settling on 4 valves.

Valves rotate. 
 

The vw 20v is an interesting little pos. You can get them to rev with the right stuff but are pretty lift limited on the intake side due to geometry. The 16v fsi/tsi heads with the rockers can outflow them easily. 

Streetwiseguy said:

I think it can probably be summed up as having the law of diminishing returns applied.

How many valves in a modern F1 engine?  I'm sure if there was a benefit to five or more, they'd use them.

This.

I remember following a discussion that some F1 engineers were having a number of years ago.

The breathing problems F1 is running into now has to do with the fact that air doesn't like to travel faster than the speed of sound.

Valves are not the restriction they used to be thanks to better cam profiles and cylinder head designs.

ShawnG said:.

Valves are not the restriction they used to be thanks to better cam profiles and cylinder head designs.

F1 uses 4 valve heads because the rules say they have to.