Oversized valves

[zaq123]

This all is nonsense. The thread was supposed to bring a discussion for a specific issue.

I am sure that the people that understand the material content here, have benefited one way or another and this is good enough for me.

I will say no more.

sir, I understand how this can be mind boggling.....I only suggest that you get your head out of the "head" and start looking at the whole system, go read up on some basics (start with Bernoulli's Principle) and go from there.
There is a difference between a discussion and an argument and yes, it can be a thin line difference so tread lightly.

Once more......Forget about the head, it is the SYSTEM ( intake --engine--exhaust).
Your theories is not the engine, Flow bench is not the engine, CFD (computational fluid dynamics) is not the engine.....dyno for a given Spec is where engines are born.

Yes there are heads with huge valves and small ports or vise versa... all have their purpose.

[digger]

after having 8 schooners at the local i thought it might be a good to to understand is why velocity is important, with the intake tract the main reason people harp on about velocity is the ramming effect.

the ramming effect is using the velocity of the column of air in the intake tract to keep filling the cylinder after BDC, obviously the piston is moving back up after BDC so its quite handy if you can continue to fill the the cylinder until as late as possible to maximize how much mass is trapped inside the cylinder by the time the valve closes.

the fundamental relationship is you need pressure in the port to remain high to appose the rising pressure in the cylinder. it turns out the relationship between the pressure to fill the cylinder is a function of velocity squared. when you look at basic derivations it is due the conservation of energy converting the kinetic energy of the flow to a pressure energy. it is not an effect of inertia as some claim.

why is this important for the discussion, it is because it should be clear the velocity of importance is that in the inlet tract (port plus inlet runner) not just the local velocity in the seat area.

if you fit a larger valve and seat but do not address the remainder of the port you arent going to have a dramatic effect on the port velocity as the area upstream of the valve remains unchanged with just a valve change, it doesn't mean it is a good nor bad idea. normally when people talk about killing velocity they change the overall port area such that it is excessive for best ramming. using a too short length of runner also reduces the effectiveness of the ramming effect the pressure waves arent timed correctly (the best length changes with the rpm you want it to work at). it seems to be that overly short length is more detrimental than a being abit too big in a diameter

[LateFan]

its quite handy if you can continue to fill the the cylinder until as late as possible to maximize how much mass is trapped inside the cylinder by the time the valve closes.

Good explanation.

A couple of questions -

What is the relationship between kinetic energy and inertia?

Why do longer runners help increase torque? And is that at all rpms or a certain band depending on length?

[jeffnhiscars]

I'd say kinetic energy is a result of motion or changes in motion whereas inertia is the resistance to changes in motion...ie a body at rest tends to stay at rest etc etc

In other words, kinetic energy is what over comes inertia. If it wont move (too much inertia), push harder (more kinetic energy please):-)

[apostate]

Sticking around here just a bit more only due to Digger...
@ Digger - yes, your schooner analysis is almost on spot. Let's assume that porting (head and manifold) is done properly and the valve seat internal diameter is set to say 40.5 mm. Why would one want to close this throat with a 43 mm. valve vs a 42 mm. instead? How would this affect ramming effect and air trapping into the cylinder? Furthermore, imagine that you close the mentioned valve seat with even bigger valves like 44 or 45 mm.? Good idea?

Another overlooked issue with 885 head is the combustion chamber shape. It is such for a reason. Port (and valve) angles are such for a reason. Head port shape and angle in/out and length/height position (inlet&exhaust) are further variables when talking of a SYSTEM as some folks (not mentioning them) very much like to address it. Head port angle is crucial to velocity and ramming. Stock M20B25 manifold is done not as a piece of art (though it seems like one) but to address all these issues with the 885 head design. It wasn't easy or cheap to produce it back then in the 70's.

Air-pocket traps are crucial too (kids, do not port-match your stock manifolds to a stock head at home). Overlapping affects ramming too. Especially when you have a good designed free-flowing exhaust system (lol, again talking about a SYSTEM, sorry).

Ovesized valves over restricted internal diameter valve seats? I don't thing so.

Cheers.

[LateFan]

Well, the seat diameter changes with the valve diameter, no? You don't increase valve size without machining out the chamber and pressing in a larger seat. If not, of course the larger valve is pointless.

Jeff - thank you. My son is studying physics and we got talking about this so I could wrap my slow brain around it.

Inertia is connected to Newton's first law. It takes force to get a mass moving, it takes force to stop a moving mass. In the case of our engine, the force is difference in pressure, correct?
Kinetic energy = 1/2 mass x velocity squared, measured in joules.
Momentum (P) = mass x velocity.


EDIT - digger should be in bed at this point - it's 3 in the morning.

[apostate]

http://www.stangtv.com/tech-stories/engine/ferrea-helps-explains-valve-flow-dynamics/

[TobyB]

(kids, do not port-match your stock manifolds to a stock head at home)

oops.

I'm jus' trina rap my hed rount this.

t

[jeffnhiscars]

Well, the seat diameter changes with the valve diameter, no? You don't increase valve size without machining out the chamber and pressing in a larger seat. If not, of course the larger valve is pointless.

Jeff - thank you. My son is studying physics and we got talking about this so I could wrap my slow brain around it.

Inertia is connected to Newton's first law. It takes force to get a mass moving, it takes force to stop a moving mass. In the case of our engine, the force is difference in pressure, correct?
Kinetic energy = 1/2 mass x velocity squared, measured in joules.
Momentum (P) = mass x velocity.


EDIT - digger should be in bed at this point - it's 3 in the morning.

I wrote that to help get my own mind around it :)

[LateFan]

Ha! I'm trina wrap my head around how to grind a 45mm valve to fit a 40mm seat.

I did learn some things in the article posted above.

"In fact, the area approximately ½-inch above and below the intake valve is the most influential factor of the entire inlet flow path. Maintaining good airspeed past the rapidly opening and closing valves is the engine builder’s top priority."

"Steady-state airflow past an open valve (on a flow bench) is one crucial measure, but starting and stopping that column of air (and fuel droplets) many times per second is not conducive to the smooth transfer of the fuel/air medium from the induction system."

ForcedFirebird talks about this, that a flow bench only gets you so far.

Also, I didn't realized the "tulip" shape was different on intake and exhaust valve stems for flow. Trina catch up.

It doesn't seem like our 885 head has the same problem with deep shrouded valves as that American head shown...


(MM 885 head)


...although I understand now where if the valve is too close to the cylinder wall, you're not getting full flow all the way around the valve opening.

[digger]

Sticking around here just a bit more only due to Digger...
@ Digger - yes, your schooner analysis is almost on spot. Let's assume that porting (head and manifold) is done properly and the valve seat internal diameter is set to say 40.5 mm. Why would one want to close this throat with a 43 mm. valve vs a 42 mm. instead? How would this affect ramming effect and air trapping into the cylinder? Furthermore, imagine that you close the mentioned valve seat with even bigger valves like 44 or 45 mm.? Good idea?

Another overlooked issue with 885 head is the combustion chamber shape. It is such for a reason. Port (and valve) angles are such for a reason. Head port shape and angle in/out and length/height position (inlet&exhaust) are further variables when talking of a SYSTEM as some folks (not mentioning them) very much like to address it. Head port angle is crucial to velocity and ramming. Stock M20B25 manifold is done not as a piece of art (though it seems like one) but to address all these issues with the 885 head design. It wasn't easy or cheap to produce it back then in the 70's.

Air-pocket traps are crucial too (kids, do not port-match your stock manifolds to a stock head at home). Overlapping affects ramming too. Especially when you have a good designed free-flowing exhaust system (lol, again talking about a SYSTEM, sorry).

Ovesized valves over restricted internal diameter valve seats? I don't thing so.

Cheers.

the issue if you use a seat throat of 40.5mm and 42mm valve you can not transition from the parallel throat to the 45degree seat very well as there isnt any metal to put a bottom cut (s) of 60 or 65 or 70 or whatever by the time you cut a seat wide enough to be durable and transfer the heat. so it wont be a very efficient design

there is a range of typical throat vs valve size often between 0.85 and 0.92 is "normal" range. your 40.5/42 = 96.5% i doubt you will find anyone using or advocating this with an oldschool OE port configuration with 45* seat.

Some more modern stuff use 92%, but some people use 55 or 55* seats to so horses for courses.....but often people like 88-90% its not any sort of gospel but going too far one way or another doesn't seem to be successful as im told sometimes the engine runs better despite the flowbench numbers being slightly worse. this is where you need dyno data not just flow bench data

the OE 885 head is 36.7/42= 87%. On a modern 4V engine you probably could use a higher % than an old 2V as the port is much more straight and downdraft IDK.

the other thing you are not considering is the curtain area. The curtain area dictates flow upto lift of 0.25D in the ideal theoretical sense even though it is not so clear and distinct. So even despite having a minimum cross section upstream of the valve the bigger valve still helps at the low to medium lift ranges. it isnt until higher lift that the port, inner seat throat and minimum CSA start to dictate flow

the bigger valve has a bigger curtain area, its only a linear relationship but it is there. A 4V head has way more curtain area than a 2V engine and it this is part of the reason a 4V engine needs less duration than 2V engine for the same hp. Less duration is always better if you can meet your hp goal.

often people use bore size as a means to size the valve, some rules say for a hemi a max of 53% of bore is good for maximum effort.

[digger]

Ha! I'm trina wrap my head around how to grind a 45mm valve to fit a 40mm seat.

I did learn some things in the article posted above.

"In fact, the area approximately ½-inch above and below the intake valve is the most influential factor of the entire inlet flow path. Maintaining good airspeed past the rapidly opening and closing valves is the engine builder’s top priority."

"Steady-state airflow past an open valve (on a flow bench) is one crucial measure, but starting and stopping that column of air (and fuel droplets) many times per second is not conducive to the smooth transfer of the fuel/air medium from the induction system."

ForcedFirebird talks about this, that a flow bench only gets you so far.

Also, I didn't realized the "tulip" shape was different on intake and exhaust valve stems for flow. Trina catch up.

It doesn't seem like our 885 head has the same problem with deep shrouded valves as that American head shown...
[ATTACH]114664[/ATTACH]

(MM 885 head)
[ATTACH]114665[/ATTACH]

...although I understand now where if the valve is too close to the cylinder wall, you're not getting full flow all the way around the valve opening.

the isnt many problems with the m20 heads and shrouding they are semi hemi, the valve goes away from the bore walls as it lifts it is only 8mm offset laterally (minimum shrouding when = 0 true hemi are like this) . as an example there is a 2.8L m20 running with 45.5mm valve and 85mm bore. this head was flow tested with various size bore adapters on the flow bench to check for shrouding and flow stability. some of the best pushrod stuff like shown uses canted valves to reduce shrouding the m20 doesnt need too much work in this area compared to others

[apostate]

the issue if you use a seat throat of 40.5mm and 42mm valve you can not transition from the parallel throat to the 45degree seat very well as there isnt any metal to put a bottom cut (s) of 60 or 65 or 70 or whatever by the time you cut a seat wide enough to be durable and transfer the heat. so it wont be a very efficient design

That's why I earlier stated that 40.5 mm throat is a risky venture with a 42 mm valve. 39.5 mm throat is not. Refer to page 1.

there is a range of typical throat vs valve size often between 0.85 and 0.92 is "normal" range. your 40.5/42 = 96.5% i doubt you will find anyone using or advocating this with an oldschool OE port configuration with 45* seat.

Same comment as above.

Some more modern stuff use 92%, but some people use 55 or 55* seats to so horses for courses.....but often people like 88-90% its not any sort of gospel but going too far one way or another doesn't seem to be successful as im told sometimes the engine runs better despite the flowbench numbers being slightly worse.

the OE 885 head is 36.7/42= 87%. On a modern 4V engine you probably could use a higher % than an old 2V as the port is much more straight and downdraft IDK.

I disagree. On a 2V 885 head you can go way up to 93-94% with no reliability issues if you keep the seat main angle at 45 degrees. Race designs can go down to 30 degrees but this is not considered reliable for street applications. OEM % is too conservative for obvious reasons - valve & seat alloys of inferior quality used for mass production.

the other thing you are not considering is the curtain area. The curtain area dictates flow upto lift of 0.25D in the ideal theoretical sense even though it is not so clear and distinct. So even despite having a minimum cross section upstream of the valve the bigger valve still helps at the low to medium lift ranges. it isnt until higher lift that the port, inner seat throat and minimum CSA start to dictate flow

Curtain area is calculated based on the internal diameter of the seat. Valve diameter is referred to as a function of that ID of the seat (a generally accepted coefficient). When you put an oversized valve over a certain ID of the valve seat, all those considerations change considerably for worse.

the bigger valve has a bigger curtain area, its only a linear relationship but it is there. A 4V head has way more curtain area than a 2V engine and it this is part of the reason a 4V engine needs less duration than 2V engine for the same hp. Less duration is always better if you can meet your hp goal.

Wrong. The curtain area is a function of the valve seat internal diameter and the valve lift. And no, it is not a linear relationship.

often people use bore size as a means to size the valve, some rules say for a hemi a max of 53% of bore is good for maximum effort.

What about stroke?

[digger]

Good explanation.

A couple of questions -

What is the relationship between kinetic energy and inertia?

Why do longer runners help increase torque? And is that at all rpms or a certain band depending on length?

inertia is akin to mass

kinetic enegy is 1/2mV^2

So there is a component of inertia in the energy. But if you use an inertia explanation it doesn't allow you to get the area velocity effect. If it was inertia a big port would do the same job since the bigger port has more inertia the bigger it gets

organ pipe theory, the lower harmonics (longer lengths) tend to produce slightly stronger pulses (less losses i guess).

[digger]

That's why I earlier stated that 40.5 mm throat is a risky venture with a 42 mm valve. 39.5 mm throat is not. Refer to page 1.


Same comment as above.


I disagree. On a 2V 885 head you can go way up to 93-94% with no reliability issues if you keep the seat main angle at 45 degrees. Race designs can go down to 30 degrees but this is not considered reliable for street applications. OEM % is too conservative for obvious reasons - valve & seat alloys of inferior quality used for mass production.


Curtain area is calculated based on the internal diameter of the seat. Valve diameter is referred to as a function of that ID of the seat (a generally accepted coefficient). When you put an oversized valve over a certain ID of the valve seat, all those considerations change considerably for worse.


Wrong. The curtain area is a function of the valve seat internal diameter and the valve lift. And no, it is not a linear relationship.


What about stroke?

great we are getting somewhere so you agree that if you want to use 40.5mm (wherever that number came from) you would use atleast 43mm which is a larger valve than 42mm. hence oversize valves are good and you answered your own question.

that is your opinion, but you haven't shown us where you or someone else has used this and proven it is better than something else. go out and test your theory an show us the result

there are various definitions of curtain area, none of them are actually correct when you do some CFD and see how the air moves and see the trajectory and how it isnt the same all the way around the valve and how it changes at every lift. its next to impossible to define a universal relationship to cover every combination of seat angle, width, chamber type as it all comes into to it. Simplifying it it makes life easier, some use the middle of the seat, some use the ID some use OD, whatever they are simply empirical relationships rules of thumb to make things simple. if you read the work of blair you would know this as this is what he says. the valve and seat sizes have a relationship between them so it doesnt matter how you define the curtain but it shows that at lower lifts the curtain (however you define it) governs flow obviously the top cut(s) angle and width, seat angle and width and bottom cut(s) angles and their with all come into it. the throat size affects what you can do with the angles, how wide they can be. too large throat in comparison to the valve limits how well you can manipulate the air through the hole so you need to balance it carefully

the modern trend for race designs is 50 or 55 degree seats if 45* isnt used, 30* was used years ago, few use it these days. even david vizard rarely seems to use it despite discussing it alot in his books