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The lean mean donut machine
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Discussion Starter #1
I've read many magazine articles and such on choosing the right port volume for a particular engine build, but I have never tested the practical application of this theory for myself. I know that the larger the port volume, the slower the velocity at lower RPM, and the better the breathing at higher RPM. Conversely, the smaller the port volume, the better the velocity at lower RPM, and the more restricted the upper RPM breathing becomes. What I would like to inquire is the DEGREE at which throttle response and torque are affected by port volume. Is it a really measurable difference from say a 170cc head to a 210cc head, or is it hardly noticeable in everyday driving? With such variance in plenum sizes for intake manifolds of each make and model, I don't see the significance a few cc's of volume could make to the overall induction system. Perhaps some of you can shed some light on this issue. If it is a significant factor, it may alter my cam/head selection in future street builds.
 

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Port volumn

Port volumn is very noticeable in an engine.
You will notice that Winsor Fords have what everyone considers soda straw exhaust ports, but you will find it hard to find a head that lends itself so well to low end torque.
Port volumn, in my estimation is more critical on the intake side than it is on the exhaust side. This is because as you mentioned, velocities in smaller runners are higher and are better for low end power where larger runners, do slow down the fuel air mixture and work better on the higher rpm band. I would ask first what kind of RPM range are you planning to turn and what is the displacement of the engine.
Plenum size in the intake also is a critical issue as well. Single plenums, are generally made for engine operation in a sustained area of over 3000 RPM, on up, where duals are designed to run from the basement up to about 6500-7000 RPM.
So if you are building a 6000 RPM engine, running a single plenum and or large volumn runner intake ports, you are going to be somewhat disappointed in your performance.
As I am fond of saying, you can always over wind a mild built engine but you cant undewind a radical one.
I am presuming you are building a conventional small block, under 400 CID, so I would recommend the 170 CC heads vs the 210 CC. Even on the larger displacements, you would have to keep the engine wound up somewhat in order to enjoy the benefits of the larger runners.
 

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Port velocity is more important than port volume. Different head designs (runner designs) will flow at varying velocities. The characteristics of your engine also dictate the velocity in the intake runners.

Check the knowledge base. It has a few good articles on the subject.
 

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Your camshaft and cubic inches play a big roll in velocity. What rpm is your camshaft designed to operate? What gear are you running? You have to design the car as a whole package, and know that what you compromise or over build, will affect the entire characteristics. If you go with larger heads, you can help the bottem end by using a well designed dual plane intake, such as the Performer RPM ( I swear that intake is designed after the intakes used on the 375 hp 396's/LS6 454's, just less bulk on the runners to keep the weight down, not port size, short runners as compared to other dual planes, but a dual plane design nonetheless), to keep the runner length longer for velocity, and improve throttle response, and you can get away with a larger carb with a dual plane. Or with smaller heads, a good single plane such as Weiand's Xcelerator and a smaller carb. Weiand's small block Xcelerator intakes come with a divider if you loose too much bottom end, to simulate a dual plane. Just take it a step further and notch it out a third of the way. They are also have a pent up bottom design, so that you do not get reverberations or pulses fed back up the carb, and

Again, it is a package deal that you are shooting for. My engine is +30 over 454, with ported and polished oval port heads, and the same Xcelerator intake. The performer rpm came off, and I put the xcelerator on. The big block intake did not come with the divider, so I made my own, notched out the center per the suggestion of a very knowledgeable person here on the board. Holley 850 vacuum secondary. The cam is a Clay Smith grind, .246 @ .050, .572 lift, 110 lsa, 108 centerline. It is pretty lopey, but I have excellent throttle response with my setup, and pulls HARD to 6200 rpm. With my 4.11 gears and 3000 stall converter, it is a blast to drive. I may back down to 3.73's, but nothing less, due ot the rpm range that my cam runs at, 3000-6500. It seems I have the best of both worlds. Again, it is a package deal, rpm range that you want to run at. If I had used rectangular port heads, I would have stuck with the performer rpm, or the GM version dual plane from GM's performance parts.

I think I gave the long version of lluciano77, but he stated it perfectly, velocity is what you are after. Hope this helps.
 

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The lean mean donut machine
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Discussion Starter #5
Thanks for the insight, guys. I currently run about a 165cc port on my ported 305 heads on my 350, and they have excellent throttle response and low end torque, especially considering my cam is the largest for the stock converter and I have 3.27 gears. I am currently grinding some corvette heads to replace these, and they started out with a 163cc intake port and 58cc chamber. I am grinding these quite heavily and expect to see 185 or 190cc ports and flow numbers near or even better than AFR heads with the redesigned ports I have created. I know I could stand to lose a bit of low end with the larger runners, but I could gain as much as 100HP upstairs. The question is what to do about the low end sogginess now that I can expect to have it: get a 383 stroker kit installed when I swap heads, get a higher RPM stall, or increase rear gears to 3.73? I use this as a daily driver so I'm debating which is the best option.

My engine is a 355 (30 over 350) with 224/230 cam, ported 305 heads, RPM air gap manifold, 11.7 compression, holley 650 carb, 700R4 transmission. I get about 400 HP out of this combo.
 

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a 2500 stall converter would be great for your setup, and you could go with the 3.73's since you have an overdrive already. The lower gear would help out with any "low end soggyness" you may or may not feel.
 

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Port volume isn't so much a determining factor by itself. The cross sectional area of a port is a bit more critical when compared to cylinder volume.

Look at it like this, A long skinny pipe and a short fat pipe may have the same volume internally but the cross section of the short fat pipe will allow more air to pass within the same given volume as compared to the long skinny pipe.

The trick is balancing the cross sectional area and volume of the port with the volume of the cylinder that needs to be filled. This mostly determined by the operating range (rpm) needed and the efficiency of the port shape. Higher rpm's will dictate the need for a larger or more efficient port, it works both ways. Larger ports, given the same cylinder volume, generally need more rpm to be used effectively.

It would stand to reason that the widest port possible would be the best port. If not for the need to mix fuel with the air and suspend it this would be true. That's where velocity comes into the equation. The air speed vs volume of air must be balanced as to provide sufficient cylinder filling capability and maintain mixture quality.

Different, swirl inducing, port shapes and combustion chamber designs aid in mixture and suspension of the mixture as well. That's a different chapter.

Larry
 

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The lean mean donut machine
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Discussion Starter #8
thanks for the insight Larry. I will have to factor that into my next port job. I've been so concerned with airflow in my port work that I neglect cross sectional area in relation to length. I instead concentrate more on cross sectional area in relation to uniformity throughout the entire port, but I will definitely have to re-think that for the practical use of the heads.
 

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port volume

i would pay very close attention to the exaust ports and polish them and port match to the headers, this is important to the flow out of the head, you seam to have the porting job going well but dont forget to keep the fuel mixture tumbeling or twisting, this keeps the atomized fuel from falling out of suspention at lower speeds. the lower gears will defenitly help with any slugish ness with the new heads. have you made sure the valves wont hit with the small chambers? good luck and hope to hear how it turns out.
 

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It would stand to reason that the widest port possible would be the best port.
Actually a taller port for the most part is better. When the port is raised it allows for a more direct shot at the valve.
 

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Porting is not just a matter of hogging out the ports. It is the total design which can only be achieved with lots of research on the flow bench. You will likely not even get close to the AFRs on flow. You will likely end up with a set of heads that dont perform very well if you are just grinding away with no experience. Best thing to do is a gasket match and bowl port, then leave them alone.

Chris
 

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The lean mean donut machine
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Discussion Starter #12
actually I do have quite a bit of experience porting heads, I was able to get 55cfm more out of BB dodge heads for my dad's engine, which is 26% over stock, and 34 cfm from my 305 heads on my car, which was the first set I ever ported. I have done a ton of research on head porting as well as aerodynamics and fluid physics, and I have studied various porting examples including heads used in the engine masters challenge. I know both from trial and error as well as from others' experience where to take material from and where to leave it alone. I am very meticulous about my work and I am not making a long shot when I say near AFR numbers. I have put quite a bit of time into crafting a new design for the heads I am currently working on, and I am not kidding when I say it sometimes takes over a month for me to complete a set.
 
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