Port volumes vs port cross section area
Port volumes vs port cross section area 1. It seems that port volumes are not that great a measure of the differences in air flow potential for a given head. There are so many variables such as the length, bowl circumference, radius, etc.
2. Are our ideas about smaller ports being needed to keep the velocity up skewed from carburetor needs?
3. If we can now make the air fuel correct at any rpm does it make sense to have larger ports(with in reason) to make peak power from say 4500 to 7000 rpms and give up a 50 ft lbs at 2000 rpm. I realize this affects fuel mileage.
Do not the large ports on the ls7 support the idea that you do not need high velocity at low rpm to have a smooth running engine.
All feedback would be appreciated.
2. Are our ideas about smaller ports being needed to keep the velocity up skewed from carburetor needs?
3. If we can now make the air fuel correct at any rpm does it make sense to have larger ports(with in reason) to make peak power from say 4500 to 7000 rpms and give up a 50 ft lbs at 2000 rpm. I realize this affects fuel mileage.
Do not the large ports on the ls7 support the idea that you do not need high velocity at low rpm to have a smooth running engine.
All feedback would be appreciated.
TECH Resident
Joined: Dec 2004
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From: K-W, Ontario
Some basic comments:
#2 Port design for a fuelie head is much different than carburetor. The mass
of the intake charge is much less as fuel is injected at the valve.
#3 With the extra cubes, the port volumes are justified for the LS7 heads.
More air flow per stroke with the 427.
#2 Port design for a fuelie head is much different than carburetor. The mass
of the intake charge is much less as fuel is injected at the valve.
#3 With the extra cubes, the port volumes are justified for the LS7 heads.
More air flow per stroke with the 427.
Last edited by Adrenaline_Z; Oct 23, 2005 at 11:31 PM.
The port shape has something to do with this as well because the shape dictates the dynamic cross section of the head.... a tall skinny port like a LS1 has a very small dynamic cross section compared to it's physical dimensions.... which is one of the reasons why it drives so nicely.
Bret
Bret
Last edited by SStrokerAce; Oct 24, 2005 at 12:24 PM.
In my understanding of the subject the port volume and cross section can be a variety of values, shapes, and sizes, just as long as the velocity falls within a certain range (yes it can be too high) and you don't get very high velocities in certain areas and/or very low velocities in other areas causing fuel separation and higher break specific fuel consumption values.
TECH Regular
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There's a bunch of info re this in the tech section at the end of the Proracingsim download for the Dynomation engine simulation program under the thread "Two Must Reads...". Basically, any time you can gain more % flow increase than % port minimum area increase, the port will be more 'efficient' and will pay power dividends, as well as broadening the curve. (It's also important for the area to diminish steadily towards the minimum, rather than growing and shrinking along the way.)
That's a good way to look at it Bill. I used that theory a while a go to pick the right head for a application and it seemed to work nicely. Only problem is that the GM Vortec head was the best in that area it wasn't the consideration over a very good CNC ported head.
Bret
Bret
Teching In
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Originally Posted by DAPSUPRSLO
In my understanding of the subject the port volume and cross section can be a variety of values, shapes, and sizes, just as long as the velocity falls within a certain range (yes it can be too high) and you don't get very high velocities in certain areas and/or very low velocities in other areas causing fuel separation and higher break specific fuel consumption values.
this is very much true and if the car has to be drivable then even more so ..... if the port is too big and the airspeed drops at low speed the more will be a slug to drive and whild once the airspeed comes up if it is to small then the airspeed will be so high that not only will the fuel drop out but the air will hit the walls and back of the valve and lower the total power ..... peak power happens between .5 and .6 mach
somewhere there was a calulation for how much area for a given rpm and cid but at the moment I cannot think of where I saw it .......
the old flow bench area rule was 80% of the valve area behind the valve and a not more then a 2% included taper as you go up the intake runner to the plenum for a street/ strip motor ....... race motor can be up to 20% bigger ( race motor would be like one that works from 4000 to 8000 rpm )
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Originally Posted by caddycarlo
this is very much true and if the car has to be drivable then even more so ..... if the port is too big and the airspeed drops at low speed the more will be a slug to drive and whild once the airspeed comes up if it is to small then the airspeed will be so high that not only will the fuel drop out but the air will hit the walls and back of the valve and lower the total power ..... peak power happens between .5 and .6 mach
somewhere there was a calulation for how much area for a given rpm and cid but at the moment I cannot think of where I saw it .......
the old flow bench area rule was 80% of the valve area behind the valve and a not more then a 2% included taper as you go up the intake runner to the plenum for a street/ strip motor ....... race motor can be up to 20% bigger ( race motor would be like one that works from 4000 to 8000 rpm )
somewhere there was a calulation for how much area for a given rpm and cid but at the moment I cannot think of where I saw it .......
the old flow bench area rule was 80% of the valve area behind the valve and a not more then a 2% included taper as you go up the intake runner to the plenum for a street/ strip motor ....... race motor can be up to 20% bigger ( race motor would be like one that works from 4000 to 8000 rpm )
Another reason for not having to high of an airspeed through the port, as I read it, is that it induces a pumping loss. That is a suction force is created on the piston during the downstroke of the intake stroke which reduces power (think of it like trying to suck air through a straw, and then trying to just inhale air with no straw, obviously alot easier to just inhale the air with no straw). Obviously any force agains the natural motion of the piston not during the power stroke in a n/a engine may cause power loss.
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I think a dramatic comparison would be an LS1 port and a C5R port. They both have to perform throughout the RPM range although I would imagine the C5R is more inclined to the higher side.
On The Tree
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C5R port like any advanced Race port design has all its flow area as high as casting will permit, and no open area down low where low flow activity takes place. This is the ultimate in flow efficiency since port cross sectional area is small but placed where the least flow loss will take place turning into valve area. LS1 port is narrow so it fits between pushrods, its tall so it will have sufficient area for good power, as far as compromises go this is a good design for decent throttle response and good power.
