Port Velocity, shape of ports, etc
10-25-2007, 01:07 AM
#1
Port Velocity, shape of ports, etc Every one compares flow numbers, but wouldn't it matter more how fast the air can move through the ports in to the cylinder? Isn't that where the difference comes in far as performance, I imagine you could hog a head out to flow a ton, but then I see cars with heads that don't necessarily flow as much per say on a bench but the car runs its *** off relative to the set up. Getting kinda curious because alls every one talks about is these flow numbers, but they seem kinda like dyno numbers, they don't really mean any thing. So what sets heads apart when it comes to porting? Shape of port, turn radius, velocity, doesn't the smoothness of the post also play a factor? I don't have any formal training in head porting, so excuse the post. Just trying to think like air, get in the cylinder with as much of my friends as possible, as smoothly as possible and with the least amount of obstruction as possible.
10-25-2007, 01:32 AM
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You are correct in pretty much everything you just said. I would move this post to the advanced tech section, it will get more responses and better answers.
10-25-2007, 03:04 PM
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here is pretty much all of the good info in that thread mentioned.
I figured that I would start a thread about some of the misconceptions going around about cylinder heads since most people do not know enough in this area and are going by what is force fed to them. The few people that actually KNOW about cylinder heads either do their own work or are having Larry Meaux, Air Flow Development, Trevor Johnson, etc, etc, do their heads and are dropping big $$$ to get what they want. The other 95 % of the people on this board will gain some knowledge by reading this.
The first thing that people ask is "what can you get them to flow?". This is a VERY confusing question for a few reasons.
1- flow #'s are NOT AS IMPORTANT as runner shape and measurements in key areas of the port.
2- most people inflate or lie about #'s since that is what most customers wanna see.
3- it will vary depending on what valvce size we are using and what the port NEEDS to be shaped like.
4- more CFM on a flow bench at 28" of depression will not always make more power. The engine will have the air traveling MUCH faster than this and find alot of problems with the port that a 28" flow test will not show. Flow testing at a higher depression and LOOKING for problems is a better way to design a port, even if this means shaping the port different so that it flows LESS at 28" of depression.
The next thing being thrown around is that a large runner volume kills velocity and will make the car accelerate slow. Velocity where? If you have a good flowing valve job, and the right measurement at key areas, the port will have plenty of velocity to fill the cylinders well. Velocity is not something that you want to be "highest". It is something that you want to control. The port needs to be small enough and shaped right to keep the air moving fast enough to fill the cylinders BUT it also needs to be large enough to SLOW DOWN the air and allow it to stick to the port and valve job under the conditions that it will see when the engine is operating. The higher the HP, the faster this air is moving so you need to adjust this and make certain areas larger to keep the air stable.
Runner volume is a bi product of getting the port shaped like you want it. Once you determine the valve size and venturi diameter of the valve job, get the measuerments that you want over the short side and at the pushrod pinch that allows the air to travel the speed you want it that will feed the cubic inch/RPM that you want and then get the port taper where you want it, the runner volume just "happens".
This is NOT a lesson in air flow going out to Air Flow Development, Larry Meaux, Trevor Johnson, Yelvi, etc, etc,. . . . . lol . . . . this is just a brief overview to arm the average cylinder heads consumer with some better knowledge than "what do they flow", "that runner volume is too big", etc, and put things in words so that most people understand so they will not just keep repeating the bad info going out there. It seems once this stuff is repeated several times, it becomes "fact" and then "common knowledge", even if it is wrong.
you want the air to move fast enough to "fill the cylinders well" but if it is moving too fast, it will NOT stick to the walls of the port and just go turbulent. Once it starts bouncing around, the flow drops dramatically. This means the cylinder fills with less air and makes less power.
-----------------------------------------------------------------------
Air does not wanna change its cross sectional area or make any turns so the more gradual you make the turns, the better. Making certain areas wider and larger (short side, etc) will allow the air to slow down and make the turn as opposed to "jumping" off of the port and killing flow. If it MUST change cross sectional area, you want it to remain as stable as possible so the taper needs o be slow.
Most porters know this but it is easier to remove less material and say "a big port kills TQ". This is true when talking 240-250 cc ports but a 200 cc port is NOT big on a 6200 RPM 355 engine with an LT1 intake.
------------------------------------------------------------------------
I offer $25 flow tests and I get to see TONS of heads that people bring by. This is more of a customer service than a money maker since you take 30 minutes out of your day to meet the people, disassemble the head, clean the head enough to go on the flow bench, set up valve fixture and then spend about 1 minute flow testing the head, LOL. In doing so, I get to see ALOT of heads. I spend a good bit of time looking over heads that come from cars that are REALLY flying and make good power. I measure/inspect everything I can and after a while you see they have alot in common.
They are getting certain measurements at the minimum cross sectional area, venturi diameter, valve job angles/widths, chamber shape, cross section over the short side, how far the short side is laid back, etc, etc, etc. The heads that really WORK have a different sound on the flow bench like the air is moving effortlessly through the port and no matter what depression you raise the bench to, it never goes turbulent.
Obviously you CAN get a port too big but I doubt if you could ever get an LT1 casting too big for a 6200 RPM 350 and definitely not too big for 6500-6800 RPM 350 or ANY 383. You can have it shaped wrong or the wronmg measurements in key areas that allow the head to not perform well but I doubt if you could ever get it too big.
If these are the two heads to chose from . . . .
180 cc
1.700" venturi diameter
1.800" cross sectional area
200 cc
1.800" venturi diameter
2.100" sross sectional area
If they flow the same at 28", just crank the depression up to 48" and see what happens. Use a velocity probe at the pushrod pinch, shortside and venturi/valve seat. Once you install the smaller head, the air will be moving TOO FAST to make stick to all of the walls and make the turn at the short side to go through the valve seat and into the cylinders and stay stuck to the walls.
There is ALOT more to it than that but at least this explains things a lil better to the "average" person on the board.
Obviously a 6200 RPM 350 is a mild set up and I would probably shape things a lil different than I would for the "average" customer running 230+ duration and turning 6500-6600 RPM or more cubes.
I figured that I would start a thread about some of the misconceptions going around about cylinder heads since most people do not know enough in this area and are going by what is force fed to them. The few people that actually KNOW about cylinder heads either do their own work or are having Larry Meaux, Air Flow Development, Trevor Johnson, etc, etc, do their heads and are dropping big $$$ to get what they want. The other 95 % of the people on this board will gain some knowledge by reading this.
The first thing that people ask is "what can you get them to flow?". This is a VERY confusing question for a few reasons.
1- flow #'s are NOT AS IMPORTANT as runner shape and measurements in key areas of the port.
2- most people inflate or lie about #'s since that is what most customers wanna see.
3- it will vary depending on what valvce size we are using and what the port NEEDS to be shaped like.
4- more CFM on a flow bench at 28" of depression will not always make more power. The engine will have the air traveling MUCH faster than this and find alot of problems with the port that a 28" flow test will not show. Flow testing at a higher depression and LOOKING for problems is a better way to design a port, even if this means shaping the port different so that it flows LESS at 28" of depression.
The next thing being thrown around is that a large runner volume kills velocity and will make the car accelerate slow. Velocity where? If you have a good flowing valve job, and the right measurement at key areas, the port will have plenty of velocity to fill the cylinders well. Velocity is not something that you want to be "highest". It is something that you want to control. The port needs to be small enough and shaped right to keep the air moving fast enough to fill the cylinders BUT it also needs to be large enough to SLOW DOWN the air and allow it to stick to the port and valve job under the conditions that it will see when the engine is operating. The higher the HP, the faster this air is moving so you need to adjust this and make certain areas larger to keep the air stable.
Runner volume is a bi product of getting the port shaped like you want it. Once you determine the valve size and venturi diameter of the valve job, get the measuerments that you want over the short side and at the pushrod pinch that allows the air to travel the speed you want it that will feed the cubic inch/RPM that you want and then get the port taper where you want it, the runner volume just "happens".
This is NOT a lesson in air flow going out to Air Flow Development, Larry Meaux, Trevor Johnson, Yelvi, etc, etc,. . . . . lol . . . . this is just a brief overview to arm the average cylinder heads consumer with some better knowledge than "what do they flow", "that runner volume is too big", etc, and put things in words so that most people understand so they will not just keep repeating the bad info going out there. It seems once this stuff is repeated several times, it becomes "fact" and then "common knowledge", even if it is wrong.
you want the air to move fast enough to "fill the cylinders well" but if it is moving too fast, it will NOT stick to the walls of the port and just go turbulent. Once it starts bouncing around, the flow drops dramatically. This means the cylinder fills with less air and makes less power.
-----------------------------------------------------------------------
Air does not wanna change its cross sectional area or make any turns so the more gradual you make the turns, the better. Making certain areas wider and larger (short side, etc) will allow the air to slow down and make the turn as opposed to "jumping" off of the port and killing flow. If it MUST change cross sectional area, you want it to remain as stable as possible so the taper needs o be slow.
Most porters know this but it is easier to remove less material and say "a big port kills TQ". This is true when talking 240-250 cc ports but a 200 cc port is NOT big on a 6200 RPM 355 engine with an LT1 intake.
------------------------------------------------------------------------
I offer $25 flow tests and I get to see TONS of heads that people bring by. This is more of a customer service than a money maker since you take 30 minutes out of your day to meet the people, disassemble the head, clean the head enough to go on the flow bench, set up valve fixture and then spend about 1 minute flow testing the head, LOL. In doing so, I get to see ALOT of heads. I spend a good bit of time looking over heads that come from cars that are REALLY flying and make good power. I measure/inspect everything I can and after a while you see they have alot in common.
They are getting certain measurements at the minimum cross sectional area, venturi diameter, valve job angles/widths, chamber shape, cross section over the short side, how far the short side is laid back, etc, etc, etc. The heads that really WORK have a different sound on the flow bench like the air is moving effortlessly through the port and no matter what depression you raise the bench to, it never goes turbulent.
Obviously you CAN get a port too big but I doubt if you could ever get an LT1 casting too big for a 6200 RPM 350 and definitely not too big for 6500-6800 RPM 350 or ANY 383. You can have it shaped wrong or the wronmg measurements in key areas that allow the head to not perform well but I doubt if you could ever get it too big.
If these are the two heads to chose from . . . .
180 cc
1.700" venturi diameter
1.800" cross sectional area
200 cc
1.800" venturi diameter
2.100" sross sectional area
If they flow the same at 28", just crank the depression up to 48" and see what happens. Use a velocity probe at the pushrod pinch, shortside and venturi/valve seat. Once you install the smaller head, the air will be moving TOO FAST to make stick to all of the walls and make the turn at the short side to go through the valve seat and into the cylinders and stay stuck to the walls.
There is ALOT more to it than that but at least this explains things a lil better to the "average" person on the board.
Obviously a 6200 RPM 350 is a mild set up and I would probably shape things a lil different than I would for the "average" customer running 230+ duration and turning 6500-6600 RPM or more cubes.
10-25-2007, 05:42 PM
#6
Yeh alot of old wives tales about heads are being flushed nowadays.Cam technology has also helped this.
I have a damn near 250CC intake port on a 383 LT1 with a super vic on stock electronics and opti, power peaks at 6700 rpm. It makes 400+lb ft of torque at 3000rpm through a 4L60E and a 4.11 geared 9 inch rear end and is almost flat across the board peaking at 436lb ft. It moves pretty good.
Lloyd can work some heads, he did mine.
David
I have a damn near 250CC intake port on a 383 LT1 with a super vic on stock electronics and opti, power peaks at 6700 rpm. It makes 400+lb ft of torque at 3000rpm through a 4L60E and a 4.11 geared 9 inch rear end and is almost flat across the board peaking at 436lb ft. It moves pretty good.
Lloyd can work some heads, he did mine.
David
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10-25-2007, 09:54 PM
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Every one compares flow numbers, but wouldn't it matter more how fast the air can move through the ports in to the cylinder? Isn't that where the difference comes in far as performance, I imagine you could hog a head out to flow a ton, but then I see cars with heads that don't necessarily flow as much per say on a bench but the car runs its *** off relative to the set up. Getting kinda curious because alls every one talks about is these flow numbers, but they seem kinda like dyno numbers, they don't really mean any thing. So what sets heads apart when it comes to porting? Shape of port, turn radius, velocity, doesn't the smoothness of the post also play a factor? I don't have any formal training in head porting, so excuse the post. Just trying to think like air, get in the cylinder with as much of my friends as possible, as smoothly as possible and with the least amount of obstruction as possible.
When you look at the air flowing through a port it flows in layers, the air that flows along the port walls moves the slowest. About 50 ft per second, the finish used on the runner walls should help make this layer as thin as possible.
The next layer of air flows a bit faster about 75 to 100 ft per second, then you have a third layer which is about 150 ft per second. We try and keep these three layers as thin as possible, which effectively makes the port bigger. If your finish is not correct the boundary layers will get thick and close the port down. Finish on the walls, the floor, the roof and proper shape are involved here.
The air in the middle of the port has more room to flow if the boundary layers are thin. The air in the middle (inside the boundary layers) is stacked top to bottom in three or four layers, most people think the air flowing at the top of the port is the faster air. Not so, the air going across the short side is the fastest air in the port. The top layer of air will take up about a third of the runner, from the roof to about the middle of the port. This air flows around 200 ft per second. The middle layer of air, which moves faster than the top layer, takes up about a third of the port as well. It's moving at about 250 ft per second. The bottom third of the port is split in two layers, these flow the most ft per second, anywhere between 300 and 450 ft per second, just depends on the head. All of these numbers depend on the head, I being very general here.
As stated you cannot recreate the airflow dynamically as the motor will see it. The flow bench is used as a comparing device only. As you port heads and see what makes power you tend to use those lesson learned.
The one thing that I never hear all these head porters talk about is piston location. The piston position and speed has a ton to do with filling the cylinders as well as total cfm.
As the intake valve opens the piston is moving up the bore, how important are your low lift numbers at this point? I’m not giving this away just food for thought.
Once the piston goes across TDC it helps pull the intake charge in the cylinder. This is where swirl is so important. Picture a stream of air flowing into the cylinder, if it swirls (moves in a circle going down the bore) it can fill the cylinder up with more air and fuel compared to the air flowing in more of a straight line down the side of the cylinder.
If the air moves straight down the side of the cylinder it will hit the top of piston sooner and turn back up towards the top of the cylinder. This creates a tumbling effect and does not fill the cylinder well. If the air is swirling, moving in a circle using the entire bore diameter it has move room (total volume) and will get more air and fuel in the cylinder faster.
This is the reason why a head that flow less total cfm can make more power than a head flowing greater cfm numbers. No smoke and mirrors, just putting more fuel and air in the correct place at the correct time.
It's really about filling the cylinder efficiently. While at the same time keeping the fuel atomized in the air. I personal think a head with good swirl and a better peak flow number will make more power than one with better mid lift numbers and less swirl.
Piston speed is between 2000 and 3000 ft per second, can you picture the effect this has on the air coming into the cylinder? Not to mention the effect of the exhaust scavenging the combustion chamber, which by the way is why a N/A motor can have over 100% volumeic efficiency rating.
So if you can swirl the air, maintain high flow numbers, keep the fuel atomized, and have proper exhaust scavenging you can make more HP than the next guy. It’s a lot to think about, great question.
Harold
10-25-2007, 10:56 PM
#10
TECH Fanatic
Perhaps you meant 200 and 300 fps at it's peak speed. The fastest air in the intake port is flowing quite a bit faster than the piston is moving at it's peak velocity, and a LOT faster than the average or mean P/S.
