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  #31 (permalink)  
Old 01-14-2009, 08:48 PM
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Quote:
Originally Posted by ap72
And the reason AFr talks about it is because runner size is a way of comparing cylinder heads, if one of them poses less of a restriction (with all other things being equal- which the never are) then it would preform better.
If you believe that, you drank the Kool Aid. They are the ONLY one making that claim, it is their marketing.

Of course smaller displacements take smaller runners....

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  #32 (permalink)  
Old 01-14-2009, 10:07 PM
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I have always wondred why head companies never really seem to talk about these things that often. To me combustion chamber should be a bragging point for any head maker.

I can see both ideas. I mean ideally both velocity and volume would have their benifits right?

For a higher velocity and smaller flow I see more turbulence which if you had a problem with fuel atomization it may help.

However as a down side that higher turbulance would be created by more friction. Friction is never really good.

Big ports I could see being benificial as the air is right there and it should be easier to bring in more air if it is not restricted?

Is there not an optimum point for any engine also?

An interesting thing I did read too was brodix is working on oval port heads for smallblock chevy's. To me this indicates there is much more to ports than volume and velocity.

Also I don't know if it is AFR "myth" or whatever but doesn't higher velocity just move your torque curve lower down? Not that this is better as you will always get less HP and likely less torque, but it may be better for certain applications, IE a low RPM pulling motor?


I am in no way an expert in any way, just a youngun trying to learn and understand different viewpoints.
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  #33 (permalink)  
Old 01-15-2009, 05:29 AM
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Quote:
Originally Posted by ChevyThunder
I have always wondred why head companies never really seem to talk about these things that often. To me combustion chamber should be a bragging point for any head maker.
Dart for one does indirectly. They talk about wet flow development and Spintron technology, which is the same thing as talking about overall desigh.
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I can see both ideas. I mean ideally both velocity and volume would have their benifits right?
Yes, but in the scheme of things it is not the ONLY factor.

Quote:
For a higher velocity and smaller flow I see more turbulence which if you had a problem with fuel atomization it may help.

However as a down side that higher turbulance would be created by more friction. Friction is never really good.
yes, goes back to your original comment about DESIGN not just numbers
Big ports I could see being benificial as the air is right there and it should be easier to bring in more air if it is not restricted?
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Is there not an optimum point for any engine also?
Well, that's the point, yes. And the answer isn't always the same.
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An interesting thing I did read too was brodix is working on oval port heads for smallblock chevy's. To me this indicates there is much more to ports than volume and velocity.
Yes, they are, again proving it's all about design.
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Also I don't know if it is AFR "myth" or whatever but doesn't higher velocity just move your torque curve lower down? Not that this is better as you will always get less HP and likely less torque, but it may be better for certain applications, IE a low RPM pulling motor?
They like to use this term "Power Under the Curve". Bunch of crap if you ask me. AFR and some poor sucker they sucked in spent thousands of dollars on a test to spite me. At the end of the day there was a 25HP difference on a rigged test where AFR even ground the cam for the test.

In Tony's rantings he re-defined HP as to be at 6300 RPMs and not peak. Every time AFR does not shine to the standards get changed and definitions get revisited. If they lose on HP it's "power under the curve" if they loose on torque it's another excuse. They just keep redefining the world to frame the argument and guys like AP repeat it verbatim without even realizing they are doing it. It is an EXCELLENT internet marketing machine, but no more.

And to your point, I've seen specific examples where head to head the bigger port head made more torque. it's about the combination.

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I am in no way an expert in any way, just a youngun trying to learn and understand different viewpoints.
I think I speak for a number of people when I say I hope some of us here can make you look critically at manufacturer's claims and start looking at these things from a realistic perspective. Be a clear thinker, not a sheep.
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  #34 (permalink)  
Old 01-15-2009, 06:25 AM
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Thanks again for all the input. And thanks to techinspector! I really really thank you!!!

Shawn
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  #35 (permalink)  
Old 01-15-2009, 09:32 AM
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I still can't see why there would be an advantage in a larger runner if the fuel distribution and flow is equal with a smaller one.

I'm trying to make it a simple question in hopes that someone can answer just that point, but no one has. Not just in this thread, but that I've ever seen. essentially it comes down to why would you want dead space in a runner.
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Old 01-15-2009, 12:05 PM
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JSup I think that is one thing for all aftermarket parts. There needs to be more clearly defined benchmarks for comparison rather than just claims by the companies selling the things.
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  #37 (permalink)  
Old 01-15-2009, 05:37 PM
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Quote:
Originally Posted by ap72
I still can't see why there would be an advantage in a larger runner if the fuel distribution and flow is equal with a smaller one.

I'm trying to make it a simple question in hopes that someone can answer just that point, but no one has. Not just in this thread, but that I've ever seen. essentially it comes down to why would you want dead space in a runner.

The best explanation of this that I have seen is in Vizard's cylinder head book and this ties in with the .6 mach that Jsup keeps mentioning. He says that because the air in the intake port has to start and stop with the opening and closing of the intake valve it takes as much energy to overcome that inertia as the additional air flow creates. That limiting velocity is .55-.6 mach at engine temperature. I think the air gets somewhat compressed in the port hence gaining density and is harder to initiate.

In my experience with a stock headed 350 SBC from years ago you hit a point where sticking more cam in it didn't really make it pull any higher. Maybe there was something else wrong with my combination that fouled the experiment but the port limiting velocity deal made sense to me.

The larger port with the same flow will make more power because it will rev higher and not suffer from an inertia block.
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  #38 (permalink)  
Old 01-15-2009, 05:56 PM
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I understand the .6 mach thing- but what's the likely hood of seeing that on a street driven NA 327 with 220 runners- or 180 runners? I think application matters a great deal here and what works on a race track isn't necessarily what works in a street engine.
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  #39 (permalink)  
Old 01-15-2009, 06:06 PM
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Quote:
Originally Posted by ap72
I understand the .6 mach thing- but what's the likely hood of seeing that on a street driven NA 327 with 220 runners- or 180 runners? I think application matters a great deal here and what works on a race track isn't necessarily what works in a street engine.
Why do you keep going back to that, it's exactly what the AFR crowd keeps saying, again out of the handbook.

The laws of physics still apply weather it is street or strip or oval track. When building a street engine why not look at racing where billions of dollars are spent to get the reslut. Race engines have plenty in common with street engines and the same principals apply. How don't they?
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Old 01-15-2009, 06:07 PM
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Originally Posted by RustoRod
The best explanation of this that I have seen is in Vizard's cylinder head book and this ties in with the .6 mach that Jsup keeps mentioning. He says that because the air in the intake port has to start and stop with the opening and closing of the intake valve it takes as much energy to overcome that inertia as the additional air flow creates. That limiting velocity is .55-.6 mach at engine temperature. I think the air gets somewhat compressed in the port hence gaining density and is harder to initiate.

In my experience with a stock headed 350 SBC from years ago you hit a point where sticking more cam in it didn't really make it pull any higher. Maybe there was something else wrong with my combination that fouled the experiment but the port limiting velocity deal made sense to me.

The larger port with the same flow will make more power because it will rev higher and not suffer from an inertia block.
When people talk about a motor as an air pump it is an oversimplificaiton about what is going on. A head port does not continually suck air like a pump. The motor pulses air in and out, not like a pump. From post 15:

Looking at cylinder head flow on a flow bench represents simply how restrictive the intake tract is. In other words, are you breathing through a straw or a garden hose… Unfortunately for everyone that develops engine combinations; air isn’t simply sucked through the intake tract. As we have all seen at some point or another, things like the intake valve closing, pressure bleeding through the ever so slightly opened intake valve cause what is commonly referred to as reversion. This is a result of your average Joe’s putting a name on something they see and don’t quite understand. The “reversion” is actually a pulse of pressure that is part of the longitudinal wave traveling back up the intake track. You see it push back through the carburetor when the timing of the event (actually the tuning of the intake tract) is off and the pulse is passing too far up the intake tract. Now this brings us a little bit into this notion of tuning this longitudinal wave to force air into the cylinder. This can be done but physics requires that the cross sectional area, length, and velocity of the pulse, be precisely matched to achieve this affect (and it will not be throughout a broad range). Another important point is that air cannot compress (at any velocity we will find inside an intake tract) so it must speed up or slow down with changes in cross sectional area.

As far as my reference to .6 mach, it is the point where air compresses, or becomes more dense. If you can't make air more dense, you can't reach a VE of more than 100%. Once a combination gets to 100% VE, no more amount of air, or air speed, is going to help UNLESS you spend tons of money and tons of time to really work the intake tract. The goal is to get a combination that is best designed chamber for fuel atomization and fuel delivery while achieving 100% VE.
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Old 01-15-2009, 06:19 PM
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I am an example. I have a 4x4 formerly daily driver/play truck that needed heads about 7 years ago. Vortecs were cheaper than a valve job new guides valves etc. I had to buy a manifold self aligning rockers new valve covers etc. For about the same money I could of had a set of Dart Iron Eagles and used my my old Perfromer Rpm old rockers etc. So now with these small port Vortec heads I have no problems smoking the tires or getting a sled moving with a 2,800 stall converter and big low end torque but I run out of steam at 4,800 rpm and I don't have enough ground speed. If I had known then what I know now the appropriate size heads would have been on the truck.

Additionally the original poster is putting a big solid cam in a light Chevy 2 with a 4 speed and 4.56 gears. How do you think he's going to operate the vehicle ? I think he's going to test the rev limiter, I would.

How do you drive your hot rods?
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Old 01-15-2009, 06:46 PM
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As far as my reference to .6 mach, it is the point where air compresses, or becomes more dense. If you can't make air more dense, you can't reach a VE of more than 100%. Once a combination gets to 100% VE, no more amount of air, or air speed, is going to help UNLESS you spend tons of money and tons of time to really work the intake tract. The goal is to get a combination that is best designed chamber for fuel atomization and fuel delivery while achieving 100% VE.[/QUOTE]


Maybe I misunderstood. I thought the momentum of the air that was already started in the intake tract was strong enough to keep filling the cylinder while the piston was starting the compression stoke just after BDC and before the intake valve closes. Where do you need to achieve .6 mach ? In the cylinder or in the port ? And how much is that in feet per second ?
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  #43 (permalink)  
Old 01-15-2009, 06:51 PM
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Rustorod, you almost pinpointed it. I plan on turning some rpms with the motor. As a matter of fact, mid range and upper rpm hp and tq is what I was after. Cruise rpm in my car arround town in 3rd gear is about 3200, give or take a hundred rpm. With that said, when leaving a stop light, in 1st, my car is above 3 grand before I even get out of the intersection. Given that, a soft bottom end in the little cube engine is not a factor. I really want to put as much mid range tq and upper hp in the car. I believe I am on the right track here??? Right???
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Old 01-15-2009, 07:03 PM
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[QUOTE=Hacksaw]Rustorod, you almost pinpointed it. I plan on turning some rpms with the motor. As a matter of fact, mid range and upper rpm hp and tq is what I was after. Cruise rpm in my car arround town in 3rd gear is about 3200, give or take a hundred rpm. With that said, when leaving a stop light, in 1st, my car is above 3 grand before I even get out of the intersection. Given that, a soft bottom end in the little cube engine is not a factor. I really want to put as much mid range tq and upper hp in the car. I believe I am on the right track here??? Right???[/QUOte/]

I would try to get a little more compression. Maybe mill the heads run a thinner head gasket etc. but yeah that sounds like fun. The debate about ideal head size will no doubt continue even after your motor is built.
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  #45 (permalink)  
Old 01-15-2009, 07:16 PM
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Rustorod, Brodix said .006 per cc. I was unsure how far to take them down. I wanted to get to at least 9.5.1 comp. I called comp cams and described my engine and plans, techinspector nailed the cam to a t. They recomended that exact grind. Must be on the right track!
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