Porting
08-05-2002, 07:32 PM
#12
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Join Date: Oct 2001
Location: Portland, OR
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I like the six ports also. I have never seen any tests done but they appear to be equal in volume to the Turbo ports but with the aux ports have additional timing.
The Center Intermediate Plates on the 6 Port N/A Motors have VERY small ports.
How about this low cost bridgeport?
The Center Intermediate Plates on the 6 Port N/A Motors have VERY small ports.
How about this low cost bridgeport?
08-05-2002, 10:34 PM
#13
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Join Date: Oct 2001
Location: Portland, OR
Posts: 790
I'm sure you've all seen this (read this):
Written by Paul Yaw
The first step in porting a motor involves cutting the intake ports. The port timing is determined by the shape of the port window. I will not go into this here, as everyone is familiar with street porting and bridge porting. The airflow is optimized by shaping what I call the "bowl area" where the air makes the 90 degree turn, and then blending it into the runner as gently as possible. Years of testing have proven the correct shape, but I ruined a lot of ports to get there. The correct shape is rarely obvious. In most cases the runners need not be opened up. It is true that this will improve airflow slightly, but at the expense of velocity. If it does need to be opened up, it should only be widened on the long side of the port. Widening the short side will hurt airflow and velocity. When the ports are done, and matched front to rear, I take a final flow measurement, and this becomes the baseline. The next step is to bolt the manifold to the motor, and see how much this affects the airflow. In most cases, this will decrease the airflow by ten to twenty percent. Since our port runner, (which is the point of minimum cross sectional area) has not changed, but airflow has been reduced, we have lost velocity. When we lose airflow and velocity, we lose power at all engine speeds. The goal now is to increase the airflow of the manifold until it no longer affects the flow of the ports. In some cases the airflow will increase slightly with the completed manifold attached. If a carburetor is used it will ALWAYS reduce the airflow slightly, but that is the price we must pay for carburetion. If fuel injection is used, the throttle body should be sized appropriately so that it does not decrease airflow.
Now that the entire induction system has been flowed, we can move on to the exhaust ports. dyno testing over the years has show an optimum ratio of intake to exhaust flow, which varies depending on port timing. I would prefer to keep these ratios to myself, as I have had to build and dyno many different combinations to find out what worked best. The ports are then cut to achieve the correct airflow with the least amount of port timing. Keeping the port open for the least time possible will help with low rpm power and fuel economy. Needless to say, the exhaust port can only be optimized if the total flow of the induction system is known. While the exh. port seems simple enough, it has a huge effect on the characteristics of the motor. While it looks like it must flow efficiently (it's just a hole in the motor, with no direction changes)it has a coefficient of discharge that is no better than the intake port, which makes a sharp 90 degree bend.(Coefficient of discharge is a measure of the flow efficiency of a given orifice, with a venturi being the "perfect" shape at 100%) The reason for this is the shape of the stainless steel sleeve in the rotor housing. This sleeve creates a tremendous amount of turbulence, and absolutely KILLS airflow. There comes a point during porting, when raising the port roof actually decreases the airflow, even though the hole is bigger. As an example, by removing the sleeve and reshaping it, the flow of a stock 12A exh. will increase by 29% without even touching the port! This is a very good thing, since improvements in airflow are usually made a few percent at a time. I used the 12A as an example, but all of the ports are bad. Removing the insert on the 6-port motors actually decreases airflow. The stock port is horribly inefficient, and when a motor is street ported it gets even worse. This is most of the reason that ported motors have such poor low speed power. On a street ported 12A, the increase from reshaping the sleeve is good for almost 40%.
Efficiency is the key! Remember, anytime that we can increase flow, and velocity, we will gain power at low, and high speed. High output rotaries do not need to be peaky, hard to drive pieces of junk. My rotary pickup has a mildly bridgeported 13B, and it is regularly used for towing. I hope that I have cleared up any confusion, and would like to hear from everyone. I am sure that I could learn a lot from all of you, and I do not mind answering questions. (Assuming of course that I have the answer!)
Written by Paul Yaw
The first step in porting a motor involves cutting the intake ports. The port timing is determined by the shape of the port window. I will not go into this here, as everyone is familiar with street porting and bridge porting. The airflow is optimized by shaping what I call the "bowl area" where the air makes the 90 degree turn, and then blending it into the runner as gently as possible. Years of testing have proven the correct shape, but I ruined a lot of ports to get there. The correct shape is rarely obvious. In most cases the runners need not be opened up. It is true that this will improve airflow slightly, but at the expense of velocity. If it does need to be opened up, it should only be widened on the long side of the port. Widening the short side will hurt airflow and velocity. When the ports are done, and matched front to rear, I take a final flow measurement, and this becomes the baseline. The next step is to bolt the manifold to the motor, and see how much this affects the airflow. In most cases, this will decrease the airflow by ten to twenty percent. Since our port runner, (which is the point of minimum cross sectional area) has not changed, but airflow has been reduced, we have lost velocity. When we lose airflow and velocity, we lose power at all engine speeds. The goal now is to increase the airflow of the manifold until it no longer affects the flow of the ports. In some cases the airflow will increase slightly with the completed manifold attached. If a carburetor is used it will ALWAYS reduce the airflow slightly, but that is the price we must pay for carburetion. If fuel injection is used, the throttle body should be sized appropriately so that it does not decrease airflow.
Now that the entire induction system has been flowed, we can move on to the exhaust ports. dyno testing over the years has show an optimum ratio of intake to exhaust flow, which varies depending on port timing. I would prefer to keep these ratios to myself, as I have had to build and dyno many different combinations to find out what worked best. The ports are then cut to achieve the correct airflow with the least amount of port timing. Keeping the port open for the least time possible will help with low rpm power and fuel economy. Needless to say, the exhaust port can only be optimized if the total flow of the induction system is known. While the exh. port seems simple enough, it has a huge effect on the characteristics of the motor. While it looks like it must flow efficiently (it's just a hole in the motor, with no direction changes)it has a coefficient of discharge that is no better than the intake port, which makes a sharp 90 degree bend.(Coefficient of discharge is a measure of the flow efficiency of a given orifice, with a venturi being the "perfect" shape at 100%) The reason for this is the shape of the stainless steel sleeve in the rotor housing. This sleeve creates a tremendous amount of turbulence, and absolutely KILLS airflow. There comes a point during porting, when raising the port roof actually decreases the airflow, even though the hole is bigger. As an example, by removing the sleeve and reshaping it, the flow of a stock 12A exh. will increase by 29% without even touching the port! This is a very good thing, since improvements in airflow are usually made a few percent at a time. I used the 12A as an example, but all of the ports are bad. Removing the insert on the 6-port motors actually decreases airflow. The stock port is horribly inefficient, and when a motor is street ported it gets even worse. This is most of the reason that ported motors have such poor low speed power. On a street ported 12A, the increase from reshaping the sleeve is good for almost 40%.
Efficiency is the key! Remember, anytime that we can increase flow, and velocity, we will gain power at low, and high speed. High output rotaries do not need to be peaky, hard to drive pieces of junk. My rotary pickup has a mildly bridgeported 13B, and it is regularly used for towing. I hope that I have cleared up any confusion, and would like to hear from everyone. I am sure that I could learn a lot from all of you, and I do not mind answering questions. (Assuming of course that I have the answer!)
11-10-2003, 07:38 AM
#14
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Join Date: Nov 2002
Location: Central KY
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Originally Posted by dac' date='Aug 5 2002, 04:32 PM
How about this low cost bridgeport?
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