Valve Size (Dave Vizard)

[hollywood63]

I was reading a article that Dave Vizard wrote and ran across this section on his take on intake valve size.

"The only time small rather than maximum size valves fall into consideration is for a production line engine that has to make a power figure that the marketing department consider to be what is needed to fit customer requirements in that market section. If the market calls for 100 hp and 120 lb’s-ft then, after the engine designers have achieved this, they go on to look at other aspects. We are not in that business â€" we are looking for the best bang-for-the-buck for the cubes we are dealing with. That being the case the biggest valve possible are ultimately the best."

Agree...................... Disagree???
Just looking for some healthy conversation  Being that winter has set in.   

[Deye76]

Not a head porter but some believe that for velocity, a smaller intake valve (1.940") is optimal. I kind of tried to generate some dicussion in this area, in a thread a week ago, didn't go very far. Guy's like WFOLarry responded and are very helpful. Others who have expertise are Don, aka Puhb1, Ed E. Sachs, K.O.C., R&R ,Palmer, & Atwood. Some of them should eventually see this and respond. It will be a great learning experience when they do.

[tomp]

Bigger valves win because they flow more at lower cam lifts so you get ultimate flow when the lifters are on the ramps of the cams and not just a the top of the lobes.

[hrdtail78]

Exhaust valve size will play a role in picking intake valve size.  The optinum number of 60% comes into play.  If I had a 2.5 intake and a 1.5 ex. it would be a no go.

[Don D]

Vizard is a very smart man. The book The theory and practice of cylinder head modification is my bible. That said I would like to read the whole context of the discussion before commenting about bigger is better theory
There are cases for larger valves but this is so complex there is no generic answer. How could there be. Just a few variables and there are SO MANY more effect the final outcome and the value or not.

Year of the head casting
Valve type and design (besides head diameter)
TB
Throat size
Bowl Size
Port inlet size
Intake VS Exhaust flow ratio
Pipe
Camshaft events

Besides all of this and more what about the build? The more generic of them don't need this much head and in best case scenerio are a waste of money and worst case a power and torque drain. Most builds peak at <6500rpm and <90%VE how much flow does your motor need?

Wouldn't it be a better scheme to leverage velocity by providing adequate flow potential to feed the motors demand (remember big air pump) Intake, also on the intake side maximize the port so that there are no vortexes and the air speed is high, now on the exhaust side provide the proper % flow ratio based on the cam used and keep the valve size small so that during overlap we can leverage scavenging using exhaust velocity to raise VEs and add cylinder fill and reduce pump losses? There are not many exhaust pipes that achieve scavenging but the best do. On the Intake side we have none of the pulse or inertia tuning potential without a lot of exotic gear that is hard to package and not on the agenda for most builders. High exhaust velocity, proper Intake VS Exhaust flow, and enough total flow potential to feed the demand is our best shot. As always all associated gear has to cooperate together or the theory of constraints kicks in to rear it's ugly head. These "happy" motors sound differant I am sure you have heard the differance..
Need more? Add cubic inches.

[ederdelyi]

>>Vizard is a very smart man. The book The theory and practice of cylinder head modification is my bible. That said I would like to read the whole context of the discussion before commenting about bigger is better theory
There are cases for larger valves but this is so complex there is no generic answer. How could there be. Just a few variables and there are SO MANY more effect the final outcome and the value or not.<<

That sums it up pretty well, note the highlighted portion in particular.

>>On the Intake side we have none of the pulse or inertia tuning potential without a lot of exotic gear that is hard to package and not on the agenda for most builders<<

That is very true. Automotive applications or MC applications  that are not constrained by "heritage" or basic design limitations can and do make use of techniques that allow larger valves (or more of them) while still maintaining good low speed power and driveability.

While this sounds like a "cop out", the answer to the question of larger/smaller valves is ... "it depends" The real question is "What is the target one is trying to hit and what combination of components will best achieve that goal?" There is no easy answer and no substitute for careful engineering and execution. If it were easy then everyone would have the ideal motor that suits their every need and all "recipe" motors would produce exactly the same results every time ... and we all know that is not the case.

[Hillside Motorcycle]

Thumb of rule, is the intake valve is 1/2 the size of the bore, or thereabouts, and exhaust valve is 75-80% of it.
Not cut into a piece of Italian Marble, but it is, and has been, a good guideline, for quite some time.

[ederdelyi]

Thumb of rule, is the intake valve is 1/2 the size of the bore, or thereabouts, and exhaust valve is 75-80% of it.
Not cut into a piece of Italian Marble, but it is, and has been, a good guideline, for quite some time.

But whose thumb is the reference? :>) Many will argue the point and there are lots of ways to "calcumate" the "optimum". But one man's optimum may be the other's minimum and ... aw, crap just go with the biggest/most you can stuff in the chamber and use VVT ... problem solved :>)

[PanHeadRed]

the biggest valve possible are ultimately the best."...Agree...................... Disagree???
Just looking for some healthy conversation

I hope I am not making a mistake jumping in here.

1st don't confuse valve size with port size, a bigger valve often times allows for a bigger port, often times a bigger port is not needed, often times flow gains can be made with a bigger valve and the same size port.

So yes I would agree about the valve.

2nd, don't confuse valve size with port size.

3rd, don't confuse valve size with port size.

[wfolarry]

4th Bigger isn't always better

[ederdelyi]

>>I hope I am not making a mistake jumping in here<<

Naw, c'mon in, the waters fine, just watch out fer de sharks, dey be kinda hongry :>)

While we are all in front of the keyboard and picking the lint out of our respective belly buttons ...

Which came first? The chicken or se ... err, egg?

[Admiral Akbar]

"Which came first? The chicken or se ... err, egg?"

That's easy,, dinosaurs were layin' eggs way before chickens were even on this planet... Ever see any evidence of a prehistoric chicken? 

The RPH list is good including what Larry says. While 3 is the best, I wave my doubts about 2.. 

Max

[PanHeadRed]

Larry, port or valve?  

IMO a discussion like this requires definitions for the terms "maximum" and "biggest"

Maximum, are we talking physical size or usefullness?

[PanHeadRed]

Max, take the values for 1, 2, & 3 and average them, then divide by .7 if your dealing with 06' up.

I can see I am going to have to put some coffee on.

[ederdelyi]

>>Ever see any evidence of a prehistoric chicken?<<

Only the one that's been in the back of my freezer since? At least I think it's a chicken! It could be your rubber one, come to think of it.

Coffee? A cup might not be the "optimum", may be the "minimum", and taking it in by IV may be the "maximum". What's the rule of thumb for coffee?

[PanHeadRed]

99-05' or 06' -up coffee?

[ederdelyi]

"old school" coffee, please.

Ohhh my, we are gonna get ourselves wacked, I feel it coming :>)

[PanHeadRed]

ed, I have a question about A/F ratios regarding decaff popping.

[Don D]

Just got my first cup. Proud to say it came out of an almost extinct "percolator"
read the whole article for a better perspective of valve sizes as it relates to a SBC 383"
Vizards perspective at least.

http://www.gofastnews.com/board/technical-articles/1247-porting-school-8-optimal-port-areas.html

[Admiral Akbar]

Damed good stuff there Don..

http://www.gofastnews.com/board/technical-articles/1247-porting-school-8-optimal-port-areas.html

Thanks for posting the link..
Max.

[ederdelyi]

Don,
Thanks for the link. As I suspected, it makes more sense when you consider his perspective and the application. Bottom line is that one should not just pick a valve size out of the air without considering all the factors, especially if you have no way of testing the results of doing so.

Oh yeah, it's worth repeating:


>>1st don't confuse valve size with port size, a bigger valve often times allows for a bigger port, often times a bigger port is not needed, often times flow gains can be made with a bigger valve and the same size port.

So yes I would agree about the valve.

2nd, don't confuse valve size with port size.

3rd, don't confuse valve size with port size.<<

EDIT: Forgot to add that the entire induction/exhaust system needs to be optimized to achieve the desired results. You can have the "best" heads in the world with "perfect" specs and still end up with crap if the supporting components can't keep up.


>>I have a question about A/F ratios regarding decaff popping<<

The more open your coffee maker is, the more likely it is to pop on decel, Especially if it's a percolator type and not equipped with a "coasting enrichner" circuit. While somewhat annoying, the popping is not harmful as long as the "openness" of the coffee maker is the root cause. Decaf, on the other hand, is known to cause reduced "power", fewer cups per pound, increased water retention, and lean mixtures. I don't recommend decaf unless you have a coffee maker with the "Flex" option, and even then the results will be less than optimal, IMO.

[KingofCubes]

Here is Vizard's genius "The first and most important factor is that if we are to make the most torque from a given compression ratio over a specified rpm band then the valves needed are the largest that can be installed without incurring a mechanical problem."

[hollywood63]

Here is all of the articles in the series. I will need to break it down because its to large to post as one file.


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[hollywood63]

section2

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[hollywood63]

section 3

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