Oversized valves

[apostate]

Hi guys,

Can somebody explain the benefit of having oversized valves in a M20 head?

Thanks. Cheers.

[badwella]

Increases cross sectional area for volumetric gas flow in and out of the combustion chamber. Essentially makes it easier to breathe.

Increasing valve duration will increase the volume of gas flow into the combustion chamber as well.

[apostate]

And how is that, given the head is untouched?

[SkiFree]

Someone running oversized valves will not have an untouched head.

[apostate]

Someone running oversized valves will not have an untouched head.

Then the next questions come: how much of a porting is needed to run oversized valves?

And isn't it always a good idea to keep the valves as small as possible compared to the ports?

[jeffnhiscars]

Someone running oversized valves will not have an untouched head.

I think the question referred to the cam comment

[apostate]

Ok, let me put it this way: how come a bigger valve is better for actual flow compared to a smaller valve?
Unless you have such a huge port that you cannot close with a nominal valve, what's the use of an oversized one?
Has anybody done such HUGE ports that cannot be closed with a nominal valve?

[TobyB]

Can somebody explain the benefit of having oversized valves in a M20 head?

Bigger valves are not an immediate solution to any of the flow 'problems' with the stock M20.
Or most BMW engines, honestly.

They flow pretty well compared to most stuff out there, generally speaking.

t

[apostate]

Bigger valves are not an immediate solution to any of the flow 'problems' with the stock M20.
Or most BMW engines, honestly.

They flow pretty well compared to most stuff out there, generally speaking.

t

Well, that's a pretty "on the safe side" statement, thank you! Just didn't get whether you or anyone else got something new out of it.

[SLEEPYDUB]

If everything else outflow the stock valves, and you are forcing more air into the engine than the stock valves can flow, then the valves become a restriction.

Increase the valve size and you decrease your restriction. I personally think valve size increases are only necessary in super high horsepower FI systems. Stock valves have been proven to make a lot of power

Sent from my SAMSUNG-SM-G935A using Tapatalk

[apostate]

If everything else outflow the stock valves, and you are forcing more air into the engine than the stock valves can flow, then the valves become a restriction.

Increase the valve size and you decrease your restriction. I personally think valve size increases are only necessary in super high horsepower FI systems. Stock valves have been proven to make a lot of power

Sent from my SAMSUNG-SM-G935A using Tapatalk

Don't you think that bigger valves will hurt flow? How many are you out there guys that think as Sleepy?

[e30davie]

gotta think about it as a whole system, changing one component has effects on others.

Here is my understanding of the science....

Remember that a valve is an orifice between the head and the combustion chamber. Your valve opens, your piston starts going down and it starts to suck in air. As the air goes through the valve it goes from a high pressure area (intake manifold), to a much lower pressure area (the vacuum created by the piston). So the ari is experiencing a pressure drop across the valve, the air has a certain velocity through the valve, and thus a certain cylinder filling amount (your VE) . If you make the valve bigger, your velocity is reduced to allow the same amount of air in. Now idealy you are able to shove more air in due to other mods on the engine. so you can get the velocity up and essentially shove more air in than if you had a smaller valve (ie increase your VE)

BUT if your intake track does not allow more air in (ie its stock, same camshaft, same intake manifold) you have dropped the velocity over the valve, reducing the cylinder filling characteristics of the pressure drop over the valve, and you will end up with less air in the cylinder = less VE = power loss.

For Naturally aspirated engines it seems the general consensus is that the stocko valves size handle quite a bit of additional flow - especially with increased lift and duration from a different camshaft. It also seems that a proper 3 angle valve job does wonders on oem valves. But i admit i don't have experience with doing this on an m20.

[SLEEPYDUB]

Don't you think that bigger valves will hurt flow? How many are you out there guys that think as Sleepy?

Well see you can't "just" put in larger valves and expect a substantial increase. You have to tune it in, and allow the valve to be useful.

And this is when you turn the boost up. Let's say 35psi the valve becomes a restriction. You stop making power because the engine can't receive anymore air through the valve due to its size and the port around it. The valve and area around the valve creates a bottleneck, sorta. The turbo may supply the air easily, but the valve can only allow so much to enter the engine. So you increase the size of the valve, likely by 1mm each, and now you have allowed the engine the ability to receive more. Now all else equal, the velocity would decrease, so power could be loss if there is no current restriction. However, remember if it was restricted at 35psi, since it can receive more air, you should be able to give the engine more boost, say 40psi now, making more power.

At least, that's how I think it works...

[zaq123]

I wouldn't mess with valve sizes. Bigger is not better in this case. Velocity and the ability to trap air offers more benefits than volume. Anything large will kill velocity unless you have the means to at least maintain it. To develop and properly experiment with these things, one would not only need the flow bench and CFD, but also the engine dyno. A lot of time and money goes into the design of the head. As they say "it would be less of a mistake to select the wrong cylinder bore size than to select the wrong valve size"

[apostate]

Ok, let's get things down to numbers...

Stock head & Stock valves (42/36)

A. Stock port diameters (approx.):
Intake - 37 mm.
Exhaust - 31 mm.

Head porting.

B. Head porting (max.):
Intake - 39.5 mm.
Exhaust - 32.8 mm.
Head ports still closed with nominal valves.

Extreme (risky) head porting.

C. Needed porting for oversized valves (43/37):
Intake - 40.5 mm.
Exhaust - 33.5 mm.
In this case of extreme porting, there is a substantial consideration whether the casting of the head is consistent enough to bear such an operation and is highly risky.

My thoughts:
The whole idea is that a head port needs the max. porting to be closed with the least valve diameter possible to ensure safe operation. Unless the head is extremely ported (say for a Schrick 304 or even bigger as in example C), it is absolutely useless to tap the head port to the combustion chamber with a larger valve. Why?

1. It will affect actual velocity flows inside and out of the cylinder adversely because the larger valve is a bigger obstacle to the intake & exhaust flow;
2. The bigger valve diameter shall create more turbulence in the flow thus additionally affecting cylinder "breathing";
3. Bigger valves are closer to each other therefore transferring more heat from exhaust to intake.
4. Etc.

Now, would someone tell me why should one have oversized valves in a 885 head? N/A engines in consideration for avoidance of any doubt.