Oversized valves

[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

[apostate]

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

[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]

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.

[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):-)

[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?

[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

[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.

[rturbo 930]

Since you apparently knew the answer before you even asked it, why make the thread in the first place?

[e30davie]

Well this is awkward.

engines be about trade offs, thats why there is no number 1 solution. what works for someone on their home race track might not work for you on your home track.

If only i had unlimited money, I could test all the combinations.

[digger]

if you want a discussion lets have an open minded discussion, with some specifics

[digger]

you are the one that killed it, i explained why you might want them and no matter you said that absolutely no, never....

[apostate]

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.

[digger]

That's a very good advice, except it doesn't work when I disagree with the people "who know" and then I cannot avoid the "No, you're wrong" statement. Sorry.
Anyway, good day (night) everyone!
Life is bigger than a small argument/fight in here. And no one is bigger than life.
Cheers!

maybe dont ask generic questions and claim that you have all the answers already and that it is black and white scenario

[LateFan]

Then show us why you think you're right....with numbers, bench flow tests, dyno numbers, etc. We want to learn. I love engine theory threads.

Look up many many posts by digger and ForcedFirebird for just two examples - real world testing and years of experience in engine theory and building.