Hopefully this is 'Advanced' enough, if not feel free to move it.

Someone on another board is debating with me that larger valves will decrease port velocity vs a smaller valve .My arguement is that the valve size isnt terribly critical in this aspect. That as long as the port flows well and its volume stays in check then by definition it should have good velocity. His arguement is that bigger opening from the larger valve will decrease velocity since the same amount of air is now flowing through a larger hole. Here is part of his post:

Sounds a little shortsignted to me but I dont claim to be cylinder head expert either (although he does).

Any comments?

 

I seriously doubt the 2.02 valves would come as a loss on a N/A engine. He has a valid point that cutting a straw and flaring the ends would reduce velocity, but why would you cut the straw? Not like your cutting intake ports to be shorter to decrease the velocity that way. I would think a bigger valve would let more volume through without much loss in velocity. From the experiences I hear in bigger valves they really liven up the top RPM band of the car.

Just my .02

 

I think you are more of an expert than the other guy.

Port velocity is flow divided by cross sectional area. When folks talk about port velocity they usually mean air velocity along the port not at the valve curtain area. More flow thru the same size port = more average velocity.

If you compare the velocity of the air thru the valve seat with the average velocity along the port, the velocity at the seat is much higher for low lifts (small curtain area) and gradually decreases, while the port velocity is low at low valve lifts (low flow) and increases with increasing valve lift (increasing flow).

Typically the crossover point is about .400 lift on a SBC. Above that port velocity is higher than valve seat velocity.

If the port volume (and therefore average cross sectional area) remains about the same, and a larger valve results in more flow, the average port velocity will be higher, but the valve seat velocity could be about the same or maybe lower.

It's average port velocity that counts, IMO.

The straw velocity analogy misses the point. That's a nozzle or diffuser demonstration.

That's what you get for hanging around those "other" boards.
My $.02

 

Thanks guys! Its nice to hear I wasn't totally off base.

I know, I know... I normally stay out of 'discussions' like that but this 'expert' was spewing soo much excrement from his keyboard I had to step in and say something

 

I work with three groups of high school students every Saturday evening in a program here at the local school district called "career builders". It's a great program and I'm really pushing to have other programs similiar to this one put in place at the school districts around the area.

One thing I talk about quite a bit with these young adults is the fact that math, physics and science will help you a great deal in life whether you use them everyday or not. Number one reason is that it's fun, especially if you're learning it from me cause I'm gonna tie it in with stuff most guys like, cars.

A close number 2 is... it will help you cut through the BS you read on the internet.
Joking? A little... but for all intents and purposes, it's the truth.

My case and point, if you look at the most basic of IC engine flow equations you will see (in part) your answer.

m = Cd x Ae x p0/ sq root(RT0)

***and it looks so elegant on paper, sigh***

where:
m = mass flow
Cd = discharge coefficient
Ae = Effective flow area
p0 = upstream pressure
T0 = upstream temperature

When you start working the formula, you will calculate the "effective valve area", which is defined as the "curtain area" (pi x Dv x Lv). I'm sure you've seen curtain area calculations before.
The interesting twist to this is the fact that a discharge coefficient is an integral part of the formula and rightfully so. See, the answer to your question as with a great many things is..... it depends.

It depends on how well the effective valve area (corrected using the discharge coefficient) flows.

As you can see from the basic formula, any final value greater in the top rung (Cd x Ae x p0) is going to yield a greater mass flow rate.
disclaimer: Understand that there is a great deal more to this formula but nothing that will alter this point.

Now for velocity we have the same situation. What we are looking for is a "mean inlet valve Mach index" (Ms). This formula is pretty widely used in engine development:

Ms = Sp/Si x Ap/Av x 1/Cms

where:
Sp = mean piston speed
Si = sonic velocity in the intake
Ap = piston area
Av = inlet valve area
Cms = mean inlet valve flow coefficient

See, there it is again! That's why I always tell people that the discharge coefficient is of utmost importance in these types of calculations. You ultimately need to test to find it because a test is worth a million calculations and theories. Bottom line.... it depends on how well the port, valve and seat flow for their size. People just do not seem to understand this but that's just life. A larger valve will usually increase mean inlet velocity but only if that valve is at a better level of efficiency (in comparison to the smaller one) to do so. The valve curtain area, effective valve area, whatever you want to call it, is pointless without a correction via the discharge coefficient.

Don't worry about all the math... there's quite a bit of work in actually calculating the variables involved. You just need to glean a concept here and that's not beyond the grasp of anyone, least of all you.

What board is this you've been visiting... traitor?

Take care

 

Thanks Chuck! I was hoping you would have time to drop by.

I think I get the concept Looking at the first formula its pretty obvious that the better a port/seat/valve combination flows the better its discharge coeffecient will be (all other things equal). By sending your heads off to a porter (or someone who claims to be one ) they are actually trying to improve the discharge coeffecient which in turn results in more airflow.

Now for my next question Looking at the second formula it appears that the larger the discharge coeffecient the lower the Ms? Is that correct? It seems counter-intuitive since I would assume that greater velocity would result in greater airflow (to a point) yet these two formula's seem to contradict that (unless my Algebra is getting rusty).

I know, you told me not to worry about the math but I'm nosy

As with most things worth learning this subject is considerably more complex than one might imagine. In the end it looks like my friend the expert and I were both a bit off. Oh well, as long as ya learn something I guess its 'all good'

I admit, I have strayed but I have learned my lesson! Its actually a board for a local 3rd Gen club. I only go there when I'm really bored and I've read everything here I would post a link but frankly it would be waste of time. They are a nice guys but there is so much "pseudo tech" there that passes for fact I actually think I un-learn things the more I read

 

man let me tip toe on this realizing I risk getting booed out for bordering on non-advanced but my head guy assured me that the flow #'s on the LT1 head I had ported by him was so good that I didn't WANT a bigger valve considering the cam I chose. The heads flow 265/195 @ 28"mer., @ .5000lift according to the chart he provided me. Unfortunately, I didn't pay attention to the flow @ .200 and I've since stopped speaking to him. That flow was with MY stock valves in place. I would think that he would want me to make good #s at the track since I stole the heads in an agreement to generate business for him @ the local track. The only thing he recommended was Manley swirl-polised stock sized valves. Should I have stepped up to the 2.02s? Am I missing out significanlty? He said that the Manley valves had made an 8hp difference on a past customer's vehicle.

 

If the "deal" between you and the machine shop was for him to supply parts and/or labor for free than that might answer the question for ya. He might not have wanted to come off of the $ for the new valves or the time cutting for larger valves, etc.

just a thought.

As far as the larger valves in a LT1 head, I would guess going with 2.00/1.56 valves would be worth 5-10 HP over stock valves. I might be low or high????? I would think a Hot Cam car would pick up at least 5-10 HP and a car with a cam that is above 230 at .050 should pick up 10-15 if they are turning over 6000 RPM. That is my guess.

Phil ???, Chuck ???, Bret ???, anyone have a better (or more accurate) answer?


NightTrain66

 

actually he subcontracts the machining of the heads other than the porting,cc'ing, and flow testing, so I had to pay his subcontractor full price for the valvejob.