port velocity
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Joined: Sep 2002
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From: plano texas
port velocity
i dont understand why a set of heads with higher port velocity makes more low end torque than a set of larger heads with less, basically I dont understand why higher port velocity means more low end. Also how the heck does a tb make any difference in the velocity of the air once the air goes through the intake then through the heads? I also dont get why backpressure makes low end torque.
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Joined: Aug 2002
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From: Red Oak Texas
These are just my opinions/theories so I am sure this post will be bombarded with hate mail from "SMARTER" people.
The smaller port or port with higher velocity usually makes more torque ( especially at lower rpm) because it FILLS the cylinders better. A flow bench will give you specific flow #s at a constant pressure (usually 28"). The 28" flow #s are a "guestimate" at what the engine is "sucking" on the head at. If a head with a 185 cc runner volume actually produces a 28" suction with a certain motor than a head a head with a 200 cc runner volume would produce a 25???" suction on the same motor. I am just guessing at the diff in suction (28"-25") but the fact remains that the suction would be less on the larger port. To get a more usable comparison of flow #s you would need to check the larger port at a less suction (25") and the smaller port at a higher suction (28"). So a higher flowing, larger runner volume head actually flows less when you drop the depression than it would at the same suction. That lower flow/suction means less cylinder filling and less HP (especially at lower RPMs).
This is assuming that the larger volume cylinder head flows the same or better at low and mid lifts also. If the smaller cc head flows better (low-mid and high lifts) it is gonna produce more low end HP and TQ, possibly more top end HP and TQ. You need a certain intake cc to make a certain HP on a certain cubic inch at a certain RPM. The less the RPM the smaller the runner volume needed. That is why you want the most CFM (low lift, mid lift and high lift) with the smaller cc of runner volume. The air is moving faster and fills the cylinders faster and only on an engine that has a head with a "too small" runner volume does it help to increase runner volume without increasing flow. If you turn the engine enough RPM it will use the extra volume and make more HP but any time you drasticaly increase runner volume you are gonna lose some torque down low.
That is my understanding of that ?.
NightTrain66
The smaller port or port with higher velocity usually makes more torque ( especially at lower rpm) because it FILLS the cylinders better. A flow bench will give you specific flow #s at a constant pressure (usually 28"). The 28" flow #s are a "guestimate" at what the engine is "sucking" on the head at. If a head with a 185 cc runner volume actually produces a 28" suction with a certain motor than a head a head with a 200 cc runner volume would produce a 25???" suction on the same motor. I am just guessing at the diff in suction (28"-25") but the fact remains that the suction would be less on the larger port. To get a more usable comparison of flow #s you would need to check the larger port at a less suction (25") and the smaller port at a higher suction (28"). So a higher flowing, larger runner volume head actually flows less when you drop the depression than it would at the same suction. That lower flow/suction means less cylinder filling and less HP (especially at lower RPMs).
This is assuming that the larger volume cylinder head flows the same or better at low and mid lifts also. If the smaller cc head flows better (low-mid and high lifts) it is gonna produce more low end HP and TQ, possibly more top end HP and TQ. You need a certain intake cc to make a certain HP on a certain cubic inch at a certain RPM. The less the RPM the smaller the runner volume needed. That is why you want the most CFM (low lift, mid lift and high lift) with the smaller cc of runner volume. The air is moving faster and fills the cylinders faster and only on an engine that has a head with a "too small" runner volume does it help to increase runner volume without increasing flow. If you turn the engine enough RPM it will use the extra volume and make more HP but any time you drasticaly increase runner volume you are gonna lose some torque down low.
That is my understanding of that ?.
NightTrain66
West South Central Moderator / Special Guest
Joined: Dec 1998
Posts: 1,650
From: Coppell, TX USA
I hate being one of the bastard smart people but..
in reality, a smaller port flowing the same amount of air will make more power EVERYWHERE, not just at low RPM. its just more noticible there.
reason being, in simpilist terms, when air is moving faster it has more kenetic engergy, IE, it has more "momentum". So even tho the cylinder has stopped "sucking"(past BDC), the cylinder is still able to fill a little bit because of the velocity of the intake port.. thats how you get over 100% VE.
make sense?
in reality, a smaller port flowing the same amount of air will make more power EVERYWHERE, not just at low RPM. its just more noticible there.
reason being, in simpilist terms, when air is moving faster it has more kenetic engergy, IE, it has more "momentum". So even tho the cylinder has stopped "sucking"(past BDC), the cylinder is still able to fill a little bit because of the velocity of the intake port.. thats how you get over 100% VE.
make sense?
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Joined: Oct 2000
Posts: 786
From: Charlotte, NC
aye.. air has quite a bit of inertia
One of the coolest things ive ever seen... one of my buddies i do heads with runs a Spintron and does valvetrain development for one of the major mopar cup teams. He had an awesome high speed video they took through an acrylic intake in an attempt to learn more about how the pressure wave interactions could be manipulated. It was sick.. that's all i can say.. cylinders you wouldnt expect would draw off others, pulling the intake charge out of that runner etc. Watching fuel/air rush towards a valve, stop.. suspend... and reverse direction & run down another runner... amazing. There must be some sort of mathematically describeable pattern to it... but I dont know of any that's been figured out thus far
.
Musser's right.. smaller port with superior shape will make more power everywhere. Our sb2 ports are tiny compared to most others, but still flow over 380.. & they always end up making more power
But yeah.. figure you'll hit a mach index around .45 b4 aerodynamic drag is bad enough to stall it... figure out how much air you need to make however much power & find yer min crossectional... yea.. somethin like that.
One of the coolest things ive ever seen... one of my buddies i do heads with runs a Spintron and does valvetrain development for one of the major mopar cup teams. He had an awesome high speed video they took through an acrylic intake in an attempt to learn more about how the pressure wave interactions could be manipulated. It was sick.. that's all i can say.. cylinders you wouldnt expect would draw off others, pulling the intake charge out of that runner etc. Watching fuel/air rush towards a valve, stop.. suspend... and reverse direction & run down another runner... amazing. There must be some sort of mathematically describeable pattern to it... but I dont know of any that's been figured out thus far
.Musser's right.. smaller port with superior shape will make more power everywhere. Our sb2 ports are tiny compared to most others, but still flow over 380.. & they always end up making more power
But yeah.. figure you'll hit a mach index around .45 b4 aerodynamic drag is bad enough to stall it... figure out how much air you need to make however much power & find yer min crossectional... yea.. somethin like that.
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Joined: Aug 2002
Posts: 1,509
From: Red Oak Texas
I knew the smart people would chime in.
(just kiddin). hopefully the person asking the ? has a better understanding of why the high velocity port works better.
Anyone wanna answer the other ?s for him?
NightTrain66
(just kiddin). hopefully the person asking the ? has a better understanding of why the high velocity port works better.
Anyone wanna answer the other ?s for him?
NightTrain66
Registered User
Joined: Sep 1998
Posts: 98
From: Ormond Beach, FL
Backpressure simply means that the pipes are smaller, and therefore require higher velocity to get the same volume of gases out.
If you add smaller port volume to smaller exhaust volume, you get higher velocity going in and going out, and will make tons of low end torque.
The downside is that as the volume increases at higher RPMs, the restrictions due to the size become too great and you start losing power. The trick is to find a happy medium.
As far as the throttle body, when you're at the top of your RPM range, you would ideally like to see 0" of Hg in your intake. Of course, this generally doesn't happen, but let's say you're pulling 2.5" of Hg, it simply means you have a restriction in the intake tract, most often being either the TB of the MAF. If this restriction becomes too great, your volume will drop, and therefore also your velocity.
If you add smaller port volume to smaller exhaust volume, you get higher velocity going in and going out, and will make tons of low end torque.
The downside is that as the volume increases at higher RPMs, the restrictions due to the size become too great and you start losing power. The trick is to find a happy medium.
As far as the throttle body, when you're at the top of your RPM range, you would ideally like to see 0" of Hg in your intake. Of course, this generally doesn't happen, but let's say you're pulling 2.5" of Hg, it simply means you have a restriction in the intake tract, most often being either the TB of the MAF. If this restriction becomes too great, your volume will drop, and therefore also your velocity.
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