Oversized valves

[digger]

as for when you need more cross sectional area it depends how much hp you want to make.

also when you are lift limited and you need to get more curtain area then a bigger valve is one way to achieve this, because a bigger valve means a bigger seat

[digger]

read my post i edited it

[apostate]

Ok, let me try another approach...

Valve:



Port in the chamber:



Now, why would someone put an oversized valve in a port in the chamber that is already oversized but can accommodate a nominal valve?

[digger]

you need a bigger valve when you dont have enough area in the throat (inner seat diamater below the bottom cut) to make good enough VE to meet your hp target as the throat is heavily related to the valve size

also the port area relationship of the entry size to minimum CSA (near guide on 885) and throat (inner seat) area matters. someone said above it is a system that means its all related

[apostate]

So why, when you have a m20 and M5x style head with the same size bottom end (cubic inch and compression) does the 24V engine make much more power with the same cam duration?

or look at it another way to make the same power the 24V head needs way less duration than an m20. So why is this?

also think about what characteristics are important in a cylinder head and what does the engine care about (what does it need). lets assume we are talking about a high performance engine

Tell me, as per the examples of the porting I have given regarding the 885 head, at what point you need bigger valves (do not mistake bigger valves with bigger ports)?

[digger]

So why, when you have a m20 and M5x style head with the same size bottom end (cubic inch and compression) does the 24V engine make much more power with the same cam duration?

or look at it another way to make the same power the 24V head needs way less duration than an m20. So why is this?

also think about what characteristics are important in a cylinder head and what does the engine care about (what does it need). lets assume we are talking about a high performance engine

[apostate]

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.
bigger valves flow more air, that simple. Same concept as huge valve on the fire hydrant vs small valve on your kitchen sink. Would be hard to put out a fire with the kitchen sink valve on 1500 GPM water supply. At the same time it would be hard to wash dishes with hydrant valve that is able to flow 1500 gpm on 3gpm household system.

If modified 885 head on modified M20 engine can't flow enough fuel/air/exhaust to unleash the full power at a lot of rpm, people are using bigger valves

Are you serous with this example? You must be joking.

[zaq123]

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.[/QUOTE]

bigger valves flow more air, that simple. Same concept as huge valve on the fire hydrant vs small valve on your kitchen sink. Would be hard to put out a fire with the kitchen sink valve on 1500 GPM water supply. At the same time it would be hard to wash dishes with hydrant valve that is able to flow 1500 gpm on 3gpm household system.

If modified 885 head on modified M20 engine can't flow enough fuel/air/exhaust to unleash the full power at a lot of rpm, people are using bigger valves

[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.

[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"

[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...

[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.

[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?

[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]

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.