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[Marks DTM Calib]

We often get asked about why multivalve is better than twin.....and hopefuly we can illustrate this!

To start with, we have to learn a little about valve opening and port size and its relationship.

Ok, if we consider that the valve has a port feeding it (in the case of the inlet) then we can easily calculate the area of the port as this is approx:

Area = (^Pi/₄) x D²

Where D is the diameter of the valve.



Now, we need to work out the height the valve has to be lifted off its seat to create an opening (called the curtain area) with the same area as the port.

The calculation for this is:

Area = Pi x D x H

Where D is the valve diameter and H is the valve lift or opening.

If we now solve these two equations for H we get

H = 0.25 D

Which tells us that there is little point in opening the valve more than 0.25 times its diameter!

Reality of course is different to this because the above assumes a perfect disc with no valve stem, valve seat etc so in reality we find that the actual is closer to

H = 0.35 D

Lets put this into some perspective by considering an Omega petrol inlet valve with a diameter of 32mm

If we apply our little equation above then we will see that we get

H = 0.35 x 32mm

This gives - 11.2mm!

And we know that the standard 3.0 cam lift is approx 10.5mm so there is little to be gained by going to a high lift cam uness you have a larger valve and larger ports because the engine designers got it pretty spot on!

So, the multivale question now.

Lets consider we now have two valves and not one and for this lets use the standard Vx inlet multivalve engine valve size of 32mm from say a 2.0 16V and compare it to the size used on the earlier 2.0 8V power plant which was 41.8mm

	2.0 8V (20SE)	2.0 16V (X20XEV)
Valve Diameter	41.8mm	32mm
Valve Lift	10mm	10mm
Total Valve Area	1372mm	1608mm
Area per valve	1372mm	804mm
Curtain Area	1313mm	2010mm

So you can see that the available curtain area on a multivalve engine is much greater than the older 8v power plant despite the use of smaller valves. This gives better flow and increases efficiency by offering less restriction to the inlet gases (or exhaust)

One thing you might also note is that for the above large single valve on the 8v example, 10mm is actually not opening it enough to match the port size so a higher lift cam (approx 14.6mm) would be better!

The additional benefits of the multivalve setup are also:

Better valve cooling as there is more of the valve face in contact with the head when shut.

Cams are accelerating a smaller lighter valve which can help the engine rev higher

The position of the 4 valves means the spark plug can now be located centrally in the combustion chanber giving better and more complete combustion.

Combustion chamber shape can be much improved.

Its possible to get better swirl effects thanks to the valve layout.

Now I hear you all cry 'but twin valve setups are have more low down torque'.

Yes but, this is a consequence of the restricted flow which can help maintain high port gas speeds at the expense of (volumetric) efficiency and is easily overcome by the use of variable inlet systems (like the multiram on the V6!)

[Marks DTM Calib]

Now with an illustration to help with understanding this!

[cem_devecioglu]

I'll archieve all those

[Andy B]

I've never ever seen area expressed like that. I only ever learned Pi r squared (I'm not cleverenough  to do the superscript. )
I now know it's the same thing.

[Marks DTM Calib]

Next installment will be valve timing.......and a little look at what happens when you advance or retard the valve timing and the benefits of variable valve timing!

[cem_devecioglu]

After I finish the university and years passed I regret not finishing mechanical engineering (my university exam points were too much enough for that department.. how stupid I was) ..As I'm a petroleum engineer (an engineer where there's very little crude )  we take many credit lessons from this department but these were not related to engine mechanics

[Jimbob]

I'm enjoying these, but please dont let em disappear in get chat - get em in tech info!

[Marks DTM Calib]

So, to those that have read the above, do you now have a grasp as to why there is a limit on how high the valve lift can be before the gains dissapear and how a mulivalve setup is more benficial?

[CaptainZok]

So, to those that have read the above, do you now have a grasp as to why there is a limit on how high the valve lift can be before the gains dissapear and how a mulivalve setup is more benficial?

Thanks Mark. I must admit to knowing sod all about this before reading your explanation but yes I can see how valve opening is limited and how the limits are overcome by having more valves.
I'll be printing these out for Tony, I'm sure they will help his studies.  

[omegaturk]

Great,I am printing all of them.Would you tell something about tuning V6 like superchargers,chips,camshafts please?Advantages and disadvantages ?I searched forum about them but can not find enough messages.I found a few pic of supercharged v6 and messages about changing plenum and inlet camshafts with 3.0 v6.Mark your opinion about v6 tuning is important and believable for me.Thanks

[cem_devecioglu]

So, to those that have read the above, do you now have a grasp as to why there is a limit on how high the valve lift can be before the gains dissapear and how a mulivalve setup is more benficial?

yep..

[Jimbob]

always assumed more valves good, less valves bad, but never really understood why  

[Martin_1962]

Number of valves is often not the issue, there are some diabolical engines for breathing, a short stroke 2 valve engine can breath better than a same sized 4 valve long stroke.

This means that some late pushrod and SOHC engines can breath better than some more modern engines.

[Marks DTM Calib]

Number of valves is often not the issue, there are some diabolical engines for breathing, a short stroke 2 valve engine can breath better than a same sized 4 valve long stroke.

This means that some late pushrod and SOHC engines can breath better than some more modern engines.

Sadly, nothing to do with valves though....thats down to inlets etc and not related to this subject. We will cover more of that in a later topic.

[Bandit127]

So, to those that have read the above, do you now have a grasp as to why there is a limit on how high the valve lift can be before the gains dissapear and how a mulivalve setup is more benficial?

Think so.

Lift
Up to 0.35D, it's the curtain area that limits flow. Beyond that, the valve diameter is the limiter. Anything more than 0.35D is a waste as you are still limited by the valve diameter....

Multivalve
Two valves of 0.7 times the diameter gives the same area as a single valve. 0.35D means they only need the lift of 0.7 times the single valve to have the same curtain area. (0.7 = [ch8730]½)
This, coupled with the lower mass of the smaller valve means you can rev higher.
Higher revs give you more power for the same torque.

Mulitvavles also give a more effective combustion chamber layout.

How did I do?