Oversized valves

[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]

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.

[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]

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]

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]

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]

read my post i edited it

[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

[apostate]

read my post i edited it

Thanks, that's also in my example on page 1, did you read it?
Do you know someone that did such an extreme porting which required oversized valves? In fact, even such an extreme porting of the throats can be closed with nominal valves but it is not considered reliable for a street engine.

My point here is that oversized valves are a total nonsense unless the head porting is driven to its extreme limits. And even if this is the case, nominal valves can still be used, but for reliability's sake oversized valves are recommended, but only for this reason alone.

In any other case/reason oversized valves will hurt performance.

[zaq123]

Hi guys,

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

Thanks. Cheers.

My point here is that oversized valves are a total nonsense unless the head porting is driven to its extreme limits. And even if this is the case, nominal valves can still be used, but for reliability's sake oversized valves are recommended, but only for this reason alone.

In any other case/reason oversized valves will hurt performance.

Looks like you answered your own question. Go re-read my example with a kitchen faucet , let it slowly sink in and just let this thread die

[apostate]

go re-read my example with a kitchen faucet , let it slowly sink in and just let this thread die

No, I won't do neither of those. This thread is important but may be not to you, sorry.
Waiting for Digger's input instead.

[digger]

One thing that happens when you use a bigger valve and throat is that you are able to achieve a better shape on the short turn radius which is pretty bad on the stock head. i will dig out a flow sheet Rama did with a +1 ST O/S valve with about 2min work and you can see the vastly improved flow characteristics even though it was just a quick test. justing fitting the valve made no difference to flow until the port was blended properly

Also since there is a limit on the minimum area you can achieve before you break through the walls near the guide, so a bigger valve helps slow the air before the cylinder which is a good thing rather than dumping the flow into the big volume. it works like a venturi which is the best way to get highest mass flow through a fixed size hole (apart from a snail)

- Local guy did a 45.5mm valve on a 2.8L. made over 330whp with enormous cam (~320+* )

- Same guy did another head made 305whp 2.8L (dont know the valve size) (cam ~316*)

- Rama (e21JPS) did a 44mm valve engine with a 2.9L made over 290whp (smaller cam than other two ~305*)

the head waiting to go onto my new engine is 45mm valves (done by guy who did first two examples) but unlike above mine is a street engine but 3.3L but much less duration (292*) that’s why it needs a bigger port and valve, just too many cubes so even at 6000rpm its well and truly maxed out a stock head….

As for oversized valves hurting, they hurt the wallet but if the valve isn’t shrouded (not usually problem on hemi/semi hemi) provided the valve job, port shape and areas are done well then you won’t be disappointed with a small increase on even a mild stroker. At the same time if you aren’t revving the engine hard enough with everything else in the engine working in sync then you are not going to use the additional area. With stock port areas you can make ~250bhp based on the superflow equations.

On the flow bench you wont see big gains with oversized valves, more incremental gains, but the flow bench only tells you so much and the engine is not a flow bench. The engine cares about pressure differentials and geometry (cross section areas, lengths, shapes, lift) and time

[apostate]

...

Also since there is a limit on the minimum area you can achieve before you break through the walls near the guide, so a bigger valve helps slow the air before the cylinder which is a good thing rather than dumping the flow into the big volume. it works like a venturi which is the best way to get highest mass flow through a fixed size hole (apart from a snail)
...

No. A bigger valve that kills velocity is never a good thing.
Whilst for the engines you give as examples, they are irrelevant. Because you do not have the same engines with the same specs and only the valves different to compare with, yeah?

Edit: The throat porting has its limitations, as I mentioned above (see page 1). Whatever valves you put over 42/36 will only hurt flow. 1 mm oversize both for intake and exhaust is a good choice for reliability with extreme porting. Everything above this sizes is purely optics for forum threads. 885 head ports in general have their limitations and all else over is an overkill that can only hurt performance. Because there is no sense that you tap a "small hole" with a "big cork". You'd rather tap a hole with the smallest cork possible.

[zaq123]

un freaking believable...

No. A bigger valve that kills velocity is never a good thing.
Whilst for the engines you give as examples, they are irrelevant. Because you do not have the same engines with the same specs and only the valves different to compare with, yeah?

Edit: The throat porting has its limitations, as I mentioned above (see page 1). Whatever valves you put over 42/36 will only hurt flow. 1 mm oversize both for intake and exhaust is a good choice for reliability with extreme porting. Everything above this sizes is purely optics for forum threads. 885 head ports in general have their limitations and all else over is an overkill that can only hurt performance. Because there is no sense that you tap a "small hole" with a "big cork". You'd rather tap a hole with the smallest cork possible.

[apostate]

un freaking believable...

Ignorance is always misleading.