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Cylinder Heads - What Matters Most?

 
Old 05-25-2019, 10:50 AM
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Default Cylinder Heads - What Matters Most?

When designing, developing, and buying cylinder heads, what is most important to look at?

For me, I look at port efficiency. Flow in cfm per intake runner cc. For example, my 220s on my old 346 flowed 320, 320/220=1.45. That is the base number I use to compare heads.

The thing that muddies it is if you make a runner longer or shorter you change volume just as much as if you change csa, so I can see where csa might be more important than volume.

With stock castings, I tend to think the valve job is more important than most of the CNC work when you send the heads out.

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Old 05-25-2019, 11:05 AM
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Port volume is a number that matters ONLY when comparing between like-design heads. CSA does matter, of course, as that determines how efficiently the port flows the CFM that it is capable of. All the numbers matter. Smokey has to not obsess over CSA so much. It's only ONE of the important numbers.
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Old 05-25-2019, 11:05 AM
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That is pretty well written. I agree on the stocker vj increase. I also think it's a big deal to know valve placement in the bore. Nobody really publishes that unless they are bragging they "moved the valve centerlines" and even then, there is never a quantifiable answer unless you are a head shop and have every brand available and can measure it on a bore plate. Same goes for intake air speed. I think AFR I quantifies that some, but I have seen anyone else do it. Valve drop, valve length, bowl depth, etc. There a ton of variables that affect performance that don't really show up in the flow numbers, but make real world power.
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Old 05-25-2019, 11:07 AM
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Alright! This is gettin' GOOD.....
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Old 05-25-2019, 11:07 AM
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I think also flow area under the curve that is utilized by your camshaft. The charts that show 410 cfm at like .900 lift is great but not if your cam is only .650. I think flow numbers for guys with a .650 lift cam need to focus on the flow at the .200-.600 area. I would also agree about port size versus flow but at a flow that the motor would actually use based on cam lift.
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Old 05-25-2019, 11:13 AM
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Yeah. I think there are still secrets out there.

Like how about how the shape of the combustion chamber changes flow characteristics at the valve?

Valve angle and placement I think are huge. Heard of guys using offset pins on ls7 heads to deshroud the intake valve which I think is a pretty cool idea.

Problem with valve angle and placement changes is the need for cu$tom rocker$.

Then there is the whole rectangle vs cathedral argument. Many think there is some magic to the shape. Cathedral shape was a work around. Predated offset rockers. The intake pushrod location limited how wide the port could be so they made it taller. Enter offset rockers, and the pushrod can move. Voila! Rectangle ports.
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Old 05-25-2019, 11:56 AM
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For shape, just look at any pro stock engine. They have refined that to death and then some. I think it's natural a cathedral port shape will have more air speed as the air is fastest along the port wall, and by design, the cathedral has long flat, straight port wall w/ a reasonable port volume. That being said, I think the overall shape has it's limitations and is why GM moved on and I am also for my builds. There needs to be some kinda formula correlating CSA, port volume, air speed, valve diameter, valve angle vs. engine displacement. I think once you have that, you'll be able to sorta fairly compare heads. As far cylinder head flow being a science, it's not even close. It's a black art, at best, and I think even the very best guys will guesstimate on design and "try" something. CAD design certainly brought it out of the stoneage's, but as long as it's a wet flow design, it'll always be a black art. The new LT motor w/ direct injection is going to be a huge game changer, long term, IMO.
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Old 05-25-2019, 12:06 PM
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That is sort of the opposite of what I thought. In laminar flow the center moves fastest and near the surface moves the slowest.
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Old 05-25-2019, 12:13 PM
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Originally Posted by Double06 View Post
I think also flow area under the curve that is utilized by your camshaft. The charts that show 410 cfm at like .900 lift is great but not if your cam is only .650. I think flow numbers for guys with a .650 lift cam need to focus on the flow at the .200-.600 area. I would also agree about port size versus flow but at a flow that the motor would actually use based on cam lift.
That 410CFM at .900 means you will need a camshaft and rockers that will support that lift.
There might only be a 5 or 10 cfm difference from .700 400cfm.
Most high lift engines on the street anyway, only go up to the mid .700 as this may be the sweet spot especially on LS3 and LS7 heads.
I don't know any of the other technical details the other guys here are discussing so don't mind me.
One thing I know is that the square port design is much better for high RPM power and torque and work even better on large bores over 4 inches.
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Old 05-25-2019, 12:47 PM
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Peak flow numbers sell heads. #fact

All benches don't flow the same. #fact

People think a larger runner is always better. #fact

People think a bigger valve will make more power. #fact

People overlook coefficient and discharge. #fact

Just a couple quick facts. I love facts.
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Old 05-25-2019, 12:59 PM
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I've read that on a ported 243 head the difference between the 2.00 valve and a 2.02 valve is so small as to not matter.

As port volume grows velocity suffers(generally speaking)

225 port on a 346 isn't too big.

Area under the curve is overlooked.

Peak dyno numbers are overrated.
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Old 05-25-2019, 01:03 PM
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Area under the curve is where its at.... now you giving away one of my secrets
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Old 05-25-2019, 01:05 PM
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Define area under the curve -- not being a dick.
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Old 05-25-2019, 01:07 PM
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Originally Posted by Darth_V8r View Post
Define area under the curve -- not being a dick.
The flow numbers at .200 to .500 lift is overlooked. All people pay attention to is that HIGHEST peak number.
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Old 05-25-2019, 01:09 PM
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Originally Posted by wannafbody

As port volume grows velocity suffers(generally speaking).
Given a constant CID I agree. Put a 265 port on a 346 it is lazy. Put a 265 port on a 427, it will have crazy velocity and tremendous momentum for cylinder full even while the piston is in its way up
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Old 05-25-2019, 01:11 PM
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Originally Posted by Tuskyz28
The flow numbers at .200 to .500 lift is overlooked. All people pay attention to is that HIGHEST peak number.
Ah. Sometimes people throw that phrase around and actually mean the torque curve.

I think in PatG thread he and Tony worked to maximize .200-.500 even if it meant sacrificing .600+ lift flows.
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Old 05-25-2019, 01:11 PM
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Originally Posted by Darth_V8r View Post
Given a constant CID I agree. Put a 265 port on a 346 it is lazy. Put a 265 port on a 427, it will have crazy velocity and tremendous momentum for cylinder full even while the piston is in its way up
Im running a LS3 head on my LS2 Darth, do you think its to much head for my motor ?
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Old 05-25-2019, 01:12 PM
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I think what Tusky means by area under the curve is if for example, you have a cam w/ .600 lift, then the max potential flow will be limited to .600 lift or less. The area is the space in between, from 0 to .600. If you head peaks at 350cfm @ .700 lift, you'll never know it because the valve does not get there. If you have 320cfm @ .600 lift, that is your theoretical maximum attainable flow. The area refers to the total flow at the various points in between from 0-.600 , thereby giving your total potential cylinder fill from open to close as the valve will pass all point between 0 and .600 twice in a rotation.

I want to see some hard data on intake flow, plenum volume, port taper and everything else for intakes. That whole market is shrouded in secrecy making it a pain in the *** to pick an intake for an application.
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Old 05-25-2019, 01:14 PM
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Originally Posted by Darth_V8r View Post
Ah. Sometimes people throw that phrase around and actually mean the torque curve.

I think in PatG thread he and Tony worked to maximize .200-.500 even if it meant sacrificing .600+ lift flows.

Didn't see the thread but i believe you. Another LS Legend told me this in person about 3 years ago.
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Old 05-25-2019, 01:18 PM
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Originally Posted by Tuskyz28
Im running a LS3 head on my LS2 Darth, do you think its to much head for my motor ?
I dont think it is too much head as long a you can down for it. If you overcam it will be less forgiving. Or need to rev higher to compensate. But something like 224/236 with an IVC around 43 will be quite strong midrange and still rev to 6800
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