Port Velocity?
Thread Starter
Registered User
Joined: Jun 2002
Posts: 1,817
From: O-Town
Port Velocity?
If two heads, with different sized ports say 200cc and 215cc for sake of argument, had the same flow up until .400 then after that the larger head took off, which one would have better port velocity? Wouldnbt the smaller port have better velocity?
Thanks
Jeremy
Thanks
Jeremy
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Re: Port Velocity?
Originally posted by mastrdrver
If two heads, with different sized ports say 200cc and 215cc for sake of argument, had the same flow up until .400 then after that the larger head took off, which one would have better port velocity? Wouldnbt the smaller port have better velocity?
Thanks
Jeremy
If two heads, with different sized ports say 200cc and 215cc for sake of argument, had the same flow up until .400 then after that the larger head took off, which one would have better port velocity? Wouldnbt the smaller port have better velocity?
Thanks
Jeremy
Depends on the rpm but the port size itself is a bit misleading as it tells you nothing about "port velocity". The smallest cross section area of the runner is where peak velocity will be determined. Smallest area and the pressure differential upstream vs downstream. So, port volume doesn't really tell us much.
-Mindgame
Registered User
Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Re: Port Velocity?
Originally posted by mastrdrver
If two heads, with different sized ports say 200cc and 215cc for sake of argument, had the same flow up until .400 then after that the larger head took off, which one would have better port velocity? Wouldnbt the smaller port have better velocity?
Thanks
Jeremy
If two heads, with different sized ports say 200cc and 215cc for sake of argument, had the same flow up until .400 then after that the larger head took off, which one would have better port velocity? Wouldnbt the smaller port have better velocity?
Thanks
Jeremy
If you mean "average velocity in the port", which is how I believe many folks think of "port velocity", here's my take:
My definitions:
Average velocity = Flow divided by average cross section of the port. Let's use ft per second.
Flow is in units of cubic feet per minute (CFM) or ft^3/min. or if we divide by 60, it's in cu. ft. per second.
It comes from flow bench measurements.
Average cross section is port volume divided by port length. If port volume is given in cubic centimeters (cc) multiply by .061 to get cubic inches, so 200 cc = 12.200 cu. in. and 215 cc =13.115 cu. in. Intake port length on a LT1/LT4 is about 5.5 inches (for sake of this example). 12.200/5.5 = 2.218 sq. in. or 2.218/144 = .01540 sq. ft. (200 cc port) and .01656 sq. ft. (220 cc port)
Now, pick a flow, say 200 cfm at .400 lift.
200 cu. ft./min divided by 60 is 3.333 cu. ft. /sec. 3.333 / .01540 = 216 ft. per second (FPS), which is average velocity for 200 cc port and 201 ft. per second for the 215 cc port. Note that the ratio of velocities is the same as the inverse ratio of the volumes. (215/200 = 1.075:1)
So, at the same flow, the "average velocity" is higher for the smaller port. But wait, there's more: You said after .400 the flow of the larger head "took off". Let's say at .600 lift the 215 cc port was 300 cfm, and the 200 cc port was 270 cfm. Which has higher "average velocity" @ .600?
200 cc port has 270/60/.01540 = 292 ft. sec.
215 cc port has 300/60/.01656 = 302 ft. sec.
Larger port has higher velocity at .600, because it flows more than 1.075 times as much air as the smaller port. Would it really be that much? Perhaps not, but it's your example, and I'm sticking to it.
In other words, I don't think there is a simple answer to your question as stated. You need to look at various points or maybe just at averages. This post is long enough so I won't go there.
My $.02. Have at me.
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Sure it depends and there isn't enough information to form any kind of logical explanation as far as I'm concerned. But that's just an opinion based on my experiences.
Jon,
Have you ever measured the port volume on let's say a 220 cc Dart Pro 1 vs a 220 cc AFR?
If you do let me know cause you're going to find that the port volume is distributed differently throughout and it may very well vary the most not in the port itself, but in the bowl.
That's what I've seen of quite a few heads from various manufacturers over the years.
Ok, so we take similar port volumes, similar port trajectory, similar bowl volume... same castings. Each head is 287 ccs, head "A" has a minimum cross section area of 3.102 in^2 while head "B" has a minimum cross section area of 2.823 in^2. They also have a different degree of taper. Can we still assume the same "average port velocity"?
Not according to what I've seen in my experience with these same heads. They're going to create maximum ram effect at different rpms, but of course they're built with two different applications in mind. I'm talking sb2.2 castings here if there's still any doubt.... I've also used both of these heads and have talked with Don Losito who performed the port work on them rather extensively.
Anyways, I see some problems with deciding whether head "A" or "B" is best for a given combination purely based on port volume. Sure-fire way to end up with a dud of a race engine or street engine for that matter.
Just my .02.
-Mindgame
Jon,
Have you ever measured the port volume on let's say a 220 cc Dart Pro 1 vs a 220 cc AFR?
If you do let me know cause you're going to find that the port volume is distributed differently throughout and it may very well vary the most not in the port itself, but in the bowl.
That's what I've seen of quite a few heads from various manufacturers over the years.
Ok, so we take similar port volumes, similar port trajectory, similar bowl volume... same castings. Each head is 287 ccs, head "A" has a minimum cross section area of 3.102 in^2 while head "B" has a minimum cross section area of 2.823 in^2. They also have a different degree of taper. Can we still assume the same "average port velocity"?
Not according to what I've seen in my experience with these same heads. They're going to create maximum ram effect at different rpms, but of course they're built with two different applications in mind. I'm talking sb2.2 castings here if there's still any doubt.... I've also used both of these heads and have talked with Don Losito who performed the port work on them rather extensively.
Anyways, I see some problems with deciding whether head "A" or "B" is best for a given combination purely based on port volume. Sure-fire way to end up with a dud of a race engine or street engine for that matter.
Just my .02.
-Mindgame
Banned
Joined: Oct 2002
Posts: 6,518
On the other side of that.............
Flow numbers tell you alot. Let's take OldSStrokers average velocity thing and add something else. I'm going away from just velocity, because teh average port velocity will tell you the RPM range and the displacement along with HP level that the heads will work best in.
This is to help the normal guy. You want to compare 2 heads, (bare with me), tis is long The answer is not what you think it is in the end.
OK we have 2 heads, the same casting and different ports. Only intake ports too. ( the exhaust port can be mostly fixed thru cam anyways)
Both heads are only going to run a .600 lift cam.
Port #1.
.100 75
.200 135
.300 189
.400 231
.500 258
.600 258
185cc Runner. 5" runner,
Port # 2.
.100 64
.200 138
.300 193
.400 239
.500 266
.600 272
200 cc Runner 5" runner
Now using this equation, Average Flow ((.100-.500 cfm x 2) + .600 cfm)/(Effective Runner Diameter (Volume relative to port length))
So Port #1 is 184 average CFM (based on chucks formula, where you take twice the numbers below peak and peak flow once and divide by the total number of data points. So we are using 11 here)
The effective runner diameter is 1.70". This is the volume divided by the length giving you area and then you find the comparative diameter of a cylinder that long with that volume. As if the port was a tube.
So Port #2 has a average flow of 188 cfm & a effective diameter of 1.76"
(The effective diameter, makes the runner a comparable to runners of different lengths. The problem with this is the tuning of the longer and shorter runners will be different)
Since both ports are the same length we can just take the average flow divide it by the effecitive area and get:
Port #1: 108.23 CFM/Inch Sq
Port #2: 106.81 CFM/Inch Sq
So per cc the smaller port flowed more air. Now the problem is here, that port # is a good low RPM head, but if you throw cam at it it will make some hight rpm power. Now port #2 is bigger but it flows alot more, so you can put less cam to it, make up the TQ deficit and more compared to the smaller port and make as much or more HP because of the flow.
That's where cam's come into it. On a 350 I would rather have the 200cc port and a 214/224 cam vs the 185cc port and a 224/236 cam. I'll make more average power, because HEAD FLOW IS THE MOST IMPORTANT THING!
Now this ports too big thing can go too far......... A 215cc port flowing 290cfm peak vs a 285cc port flowing 360cfm, is most likely going to get killed on a 358 cube engine by the 215cc port on a street car in the 2500-6500 rpm range. Now if we picked a 6500-9000 RPM range I'm going with the bigger port.
Now you gotta be confused.
The answer is simple. The right Port volume and effecitive diameter along with flow is all dependant on alot of things.
First the RPM range you are looking at, in my mind you need a enough head that it doesn't have too much velocity at the high rpm's of it's useable range that it becomes restrictive. When you have a 185cc head trying to spin 7500rpm then it need duration because it needs flow below the max lift due to the fact that the velocity is too high, so you add duration. I would rather run less duration, the same lobes area and have the head not have too much velocity at high lifts so I can open that puppy up a ton. This will also add TQ below HP peak, which means I get to peak faster, therefore I win. On top of that the RPM range can be increased with the same cam when you have more volume or effetive diameter. So RPM is a big part of this, just like it is for cams. Think about that alot and you'll see why on the street I like less cam and more port.
Second, the HP level. If I am pushing 450hp vs 600hp I want more port volume or effective diameter per HP. So that 185cc head will make a good truck head but a ****ty drag head. Because with RPM comes HP. Even a blown head or N2O head will get in the way of itself if it's too small. Granted the port flows more air or mixture because you are putting more in the system artificially, but it still needs the abilty to flow all of that without the port being too small.
Third, cubes and port. Cubes eat port volume and duration, so a 434 small block needs a BIG HEAD, not just a 220 Motown, but a 265cc 18 deg head. A 305 (if it's the same RPM range, which is a waste BTW) needs a 185cc port at the most.
Either way you can have a 434cube SBC make 700hp and a 305cube SBC make 700hp. One will be going alot less RPM than the other, on the other hand they should have about the same piston speed. But that gets into TQ per Cube (or L) but that's a whole different game, but it explains why a 358cube (5.87L) Winston Cup engine makes about the same TQ/L at peak HP as a 18,000 RPM 3.0L F1 engine. It's piston speed.
Anyways I'm sidetracked.
The mid lift numbers make so much of a difference it's silly. I've looked at a 340cfm head vs a 360cfm head and the 340cfm head made the same HP with 20 lbs ft more TQ at peak than the 360cfm head just because the .300 and .400 cfm numbers were so much better, that's crazy. It's more about were the flow is, bowl volume, port trajectory, valve size, throught area etc in the end just comes down to flow and where it is. You can have too much flow @ some lifts and too little flow in some too. (obviously) The key is where the flow is in the lfit curve. If you ever just play with a flow curve, adding here subtracting here (which you can't do in the real world unless you are the man) you see how what helps and hurts where. I'm not going to let the cat out of the bag, but you can find a good place to go with this.
Who would think that a head with 20 more CFM at peak would make the same HP but get killed at TQ peak that much? All with the same runner volume.......
Anyways, I'm pooped.
Bret
Flow numbers tell you alot. Let's take OldSStrokers average velocity thing and add something else. I'm going away from just velocity, because teh average port velocity will tell you the RPM range and the displacement along with HP level that the heads will work best in.
This is to help the normal guy. You want to compare 2 heads, (bare with me), tis is long The answer is not what you think it is in the end.
OK we have 2 heads, the same casting and different ports. Only intake ports too. ( the exhaust port can be mostly fixed thru cam anyways)
Both heads are only going to run a .600 lift cam.
Port #1.
.100 75
.200 135
.300 189
.400 231
.500 258
.600 258
185cc Runner. 5" runner,
Port # 2.
.100 64
.200 138
.300 193
.400 239
.500 266
.600 272
200 cc Runner 5" runner
Now using this equation, Average Flow ((.100-.500 cfm x 2) + .600 cfm)/(Effective Runner Diameter (Volume relative to port length))
So Port #1 is 184 average CFM (based on chucks formula, where you take twice the numbers below peak and peak flow once and divide by the total number of data points. So we are using 11 here)
The effective runner diameter is 1.70". This is the volume divided by the length giving you area and then you find the comparative diameter of a cylinder that long with that volume. As if the port was a tube.
So Port #2 has a average flow of 188 cfm & a effective diameter of 1.76"
(The effective diameter, makes the runner a comparable to runners of different lengths. The problem with this is the tuning of the longer and shorter runners will be different)
Since both ports are the same length we can just take the average flow divide it by the effecitive area and get:
Port #1: 108.23 CFM/Inch Sq
Port #2: 106.81 CFM/Inch Sq
So per cc the smaller port flowed more air. Now the problem is here, that port # is a good low RPM head, but if you throw cam at it it will make some hight rpm power. Now port #2 is bigger but it flows alot more, so you can put less cam to it, make up the TQ deficit and more compared to the smaller port and make as much or more HP because of the flow.
That's where cam's come into it. On a 350 I would rather have the 200cc port and a 214/224 cam vs the 185cc port and a 224/236 cam. I'll make more average power, because HEAD FLOW IS THE MOST IMPORTANT THING!
Now this ports too big thing can go too far......... A 215cc port flowing 290cfm peak vs a 285cc port flowing 360cfm, is most likely going to get killed on a 358 cube engine by the 215cc port on a street car in the 2500-6500 rpm range. Now if we picked a 6500-9000 RPM range I'm going with the bigger port.
Now you gotta be confused.
The answer is simple. The right Port volume and effecitive diameter along with flow is all dependant on alot of things.
First the RPM range you are looking at, in my mind you need a enough head that it doesn't have too much velocity at the high rpm's of it's useable range that it becomes restrictive. When you have a 185cc head trying to spin 7500rpm then it need duration because it needs flow below the max lift due to the fact that the velocity is too high, so you add duration. I would rather run less duration, the same lobes area and have the head not have too much velocity at high lifts so I can open that puppy up a ton. This will also add TQ below HP peak, which means I get to peak faster, therefore I win. On top of that the RPM range can be increased with the same cam when you have more volume or effetive diameter. So RPM is a big part of this, just like it is for cams. Think about that alot and you'll see why on the street I like less cam and more port.
Second, the HP level. If I am pushing 450hp vs 600hp I want more port volume or effective diameter per HP. So that 185cc head will make a good truck head but a ****ty drag head. Because with RPM comes HP. Even a blown head or N2O head will get in the way of itself if it's too small. Granted the port flows more air or mixture because you are putting more in the system artificially, but it still needs the abilty to flow all of that without the port being too small.
Third, cubes and port. Cubes eat port volume and duration, so a 434 small block needs a BIG HEAD, not just a 220 Motown, but a 265cc 18 deg head. A 305 (if it's the same RPM range, which is a waste BTW) needs a 185cc port at the most.
Either way you can have a 434cube SBC make 700hp and a 305cube SBC make 700hp. One will be going alot less RPM than the other, on the other hand they should have about the same piston speed. But that gets into TQ per Cube (or L) but that's a whole different game, but it explains why a 358cube (5.87L) Winston Cup engine makes about the same TQ/L at peak HP as a 18,000 RPM 3.0L F1 engine. It's piston speed.
Anyways I'm sidetracked.
The mid lift numbers make so much of a difference it's silly. I've looked at a 340cfm head vs a 360cfm head and the 340cfm head made the same HP with 20 lbs ft more TQ at peak than the 360cfm head just because the .300 and .400 cfm numbers were so much better, that's crazy. It's more about were the flow is, bowl volume, port trajectory, valve size, throught area etc in the end just comes down to flow and where it is. You can have too much flow @ some lifts and too little flow in some too. (obviously) The key is where the flow is in the lfit curve. If you ever just play with a flow curve, adding here subtracting here (which you can't do in the real world unless you are the man) you see how what helps and hurts where. I'm not going to let the cat out of the bag, but you can find a good place to go with this.
Who would think that a head with 20 more CFM at peak would make the same HP but get killed at TQ peak that much? All with the same runner volume.......
Anyways, I'm pooped.
Bret
Registered User
Joined: Jul 1999
Posts: 128
From: Tx
Originally posted by mastrdrver
So, is a good indication of good average port velocity of a larger cc head if it can keep up with a small cc port below say .4 lift, or am I wrong on this idea?
So, is a good indication of good average port velocity of a larger cc head if it can keep up with a small cc port below say .4 lift, or am I wrong on this idea?
Velocity in the context of this question is simple... smaller port & larger port, smaller port has same steady-state flow as larger port, smaller port obviously has a higher average velocity in comparison. What Mindgame is referring to is limiting port velocity or Mach index. Very important but another subject matter.
I understand we're skimming a subject here but my concern in port development is port form and developing a port with a uniform velocity gradient. I worry about the actual port last, seat first.
Take care
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Originally posted by Mr. Horsepower
What Mindgame is referring to is limiting port velocity or Mach index.
What Mindgame is referring to is limiting port velocity or Mach index.
I think I'm leaning a bit more to high-lift port flow than average but that brings about another question. If we're talking low lift flow at and around the valve then isn't the area around the valve/seat/port-directly-under-the-seat playing a bigger part in this than anything else?
Then again this might just drive this off-subject which I apologize for in advance but I think it is relevent to the discussion. Cause for me, there just isn't much in port volume vs flow. Too many variables left unanswered in breaking it down that simple.
-Mindgame
Banned
Joined: Oct 2002
Posts: 6,518
Originally posted by Mindgame
Cause for me, there just isn't much in port volume vs flow. Too many variables left unanswered in breaking it down that simple.
-Mindgame
Cause for me, there just isn't much in port volume vs flow. Too many variables left unanswered in breaking it down that simple.
-Mindgame
I guess in a very basic way I was talking Mach index too.
I look at heads given what I am given. You give me a flow table and a port size I can get pretty dam close to the right head choice there. The flow curve says alot about the quality of the head.
Bret
Registered User
Joined: Jul 1999
Posts: 128
From: Tx
Two very similar cylinder heads will perform very differently even with identical flow curves and port volumes. Port volume is not necessarily indicative of Mach index, although that is a common misconception.
A 375 cid competition drag race engine we developed last year for instance, SB2 block, SB2.2 heads, single 4 barrel carb, production cast intake. We built and tested a number of cylinder heads but two in particular demonstrate my point. Two very similar heads of the same casting, port volume & flow-curve near identical, but one head is off peak power some 7%.
I'm not giving anything away but the difference here is not evident in the data I've presented. This is a very highly developed race engine making well over 2.5 hp/cid. I've worked with a number of engines on this level and much the same can be said of every one.
At RMRE we developed probably better than a dozen heads for Bruce Allen's Pro Stock engine during my time there. If you were privy to a "flow chart" for each of those cylinder heads vs the port volume you would be extremely hard pressed to draw conclusions close to what actual testing shown. In short, some of this lot were off better than 120 hp from our most "optimal" designs. Much the same in regards to Christian's powerplants. Of course we are talking high end race engine development but the same is true of the 1.5-2.0 hp street engine builds most here are interested in, just on a smaller percentage scale. On the competition level, I could honestly care less if a head flowed anything below .400" lift, producing an under-head port shape that refuses to flow in the opposite direction and sizing the minimum cross-section area to produce optimal ram effects is paramount. At .6-.900" of lift, the valve is simply getting out of the way of the port.
Choosing a cylinder head from a catalog is great, and it is important to know how to make intelligent decisions. That will put anyone well ahead of the game.
That goal in mind, I've tried to explain a few basics, given a few formulas for comparison and such. These will take you a good ways but it is important to realize that "optimal ports" for an application are developed, not "picked".
Mindgame,
On the seat and under-seat. Keep in mind that port velocity and seat velocity are inversely proportional. Slow port velocity, high seat velocity and vice versa. I'll try to cover more on this my next trip through.
No one has it all figured out..... and that is reassuring.
Happy hunting.
A 375 cid competition drag race engine we developed last year for instance, SB2 block, SB2.2 heads, single 4 barrel carb, production cast intake. We built and tested a number of cylinder heads but two in particular demonstrate my point. Two very similar heads of the same casting, port volume & flow-curve near identical, but one head is off peak power some 7%.
I'm not giving anything away but the difference here is not evident in the data I've presented. This is a very highly developed race engine making well over 2.5 hp/cid. I've worked with a number of engines on this level and much the same can be said of every one.
At RMRE we developed probably better than a dozen heads for Bruce Allen's Pro Stock engine during my time there. If you were privy to a "flow chart" for each of those cylinder heads vs the port volume you would be extremely hard pressed to draw conclusions close to what actual testing shown. In short, some of this lot were off better than 120 hp from our most "optimal" designs. Much the same in regards to Christian's powerplants. Of course we are talking high end race engine development but the same is true of the 1.5-2.0 hp street engine builds most here are interested in, just on a smaller percentage scale. On the competition level, I could honestly care less if a head flowed anything below .400" lift, producing an under-head port shape that refuses to flow in the opposite direction and sizing the minimum cross-section area to produce optimal ram effects is paramount. At .6-.900" of lift, the valve is simply getting out of the way of the port.
Choosing a cylinder head from a catalog is great, and it is important to know how to make intelligent decisions. That will put anyone well ahead of the game.
That goal in mind, I've tried to explain a few basics, given a few formulas for comparison and such. These will take you a good ways but it is important to realize that "optimal ports" for an application are developed, not "picked".
Mindgame,
On the seat and under-seat. Keep in mind that port velocity and seat velocity are inversely proportional. Slow port velocity, high seat velocity and vice versa. I'll try to cover more on this my next trip through.
No one has it all figured out..... and that is reassuring.
Happy hunting.
Last edited by Mr. Horsepower; Apr 6, 2003 at 12:37 AM.
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Originally posted by Mr. Horsepower
On the competition level, I could honestly care less if a head flowed anything below .400" lift, producing an under-head port shape that refuses to flow in the opposite direction and sizing the minimum cross-section area to produce optimal ram effects is paramount. At .6-.900" of lift, the valve is simply getting out of the way of the port.
On the competition level, I could honestly care less if a head flowed anything below .400" lift, producing an under-head port shape that refuses to flow in the opposite direction and sizing the minimum cross-section area to produce optimal ram effects is paramount. At .6-.900" of lift, the valve is simply getting out of the way of the port.
At low lift, the section area is determined by the valve lift and that "valve curtain area" as I've heard it called a few times in your other replies. At a high lift range, theres some point where the valve curtain area isn't the smallest cross section in the flowpath any more so the port comes into play. This is where my point about varying cross section areas in ports of the same cylinder head with the same volume. Obviously, and I'm glad to see this confirmed in your post, a cylinder head with near identical volume and flow rate but varying cross section areas are gonna perform very differently. Your point well taken.
Mindgame,
On the seat and under-seat. Keep in mind that port velocity and seat velocity are inversely proportional. Slow port velocity, high seat velocity and vice versa. I'll try to cover more on this my next trip through.
No one has it all figured out..... and that is reassuring.
Happy hunting.
On the seat and under-seat. Keep in mind that port velocity and seat velocity are inversely proportional. Slow port velocity, high seat velocity and vice versa. I'll try to cover more on this my next trip through.
No one has it all figured out..... and that is reassuring.
Happy hunting.
-Mindgame
Banned
Joined: Oct 2002
Posts: 6,518
Originally posted by Mr. Horsepower
On the competition level, I could honestly care less if a head flowed anything below .400" lift, producing an under-head port shape that refuses to flow in the opposite direction and sizing the minimum cross-section area to produce optimal ram effects is paramount. At .6-.900" of lift, the valve is simply getting out of the way of the port.
I would agree with that if I was working on a Pro Stock Type question with up to or over 1.000" of valve lift. When you constantly work in the under .700 and mostly under .600 lift range then you have to look at that area. On the other hand mid lift on a .700 lift cam is .350 vs .500 on a 1.000" cam. The lift number seems to be a generalization, where we should really be talking about the mid lift for that particular engine.
Choosing a cylinder head from a catalog is great, and it is important to know how to make intelligent decisions. That will put anyone well ahead of the game.
That goal in mind, I've tried to explain a few basics, given a few formulas for comparison and such. These will take you a good ways but it is important to realize that "optimal ports" for an application are developed, not "picked".
Very true, this comes back to the budget again. If I can work with a head that is $2100 vs a hand ported race port that was developed for that engine being 1.5 to 3 times that much, here is another case of blowing the budget. Once there is more money there it's an obvious place to spend it though. One thing we find here is that 99% of the guys need help in the "picking" category and not the "development" category. I fully understand where you are coming from though.
On the seat and under-seat. Keep in mind that port velocity and seat velocity are inversely proportional. Slow port velocity, high seat velocity and vice versa. I'll try to cover more on this my next trip through.
yeah, I'd like to hear more on this too.
No one has it all figured out..... and that is reassuring.
On the competition level, I could honestly care less if a head flowed anything below .400" lift, producing an under-head port shape that refuses to flow in the opposite direction and sizing the minimum cross-section area to produce optimal ram effects is paramount. At .6-.900" of lift, the valve is simply getting out of the way of the port.
I would agree with that if I was working on a Pro Stock Type question with up to or over 1.000" of valve lift. When you constantly work in the under .700 and mostly under .600 lift range then you have to look at that area. On the other hand mid lift on a .700 lift cam is .350 vs .500 on a 1.000" cam. The lift number seems to be a generalization, where we should really be talking about the mid lift for that particular engine.
Choosing a cylinder head from a catalog is great, and it is important to know how to make intelligent decisions. That will put anyone well ahead of the game.
That goal in mind, I've tried to explain a few basics, given a few formulas for comparison and such. These will take you a good ways but it is important to realize that "optimal ports" for an application are developed, not "picked".
Very true, this comes back to the budget again. If I can work with a head that is $2100 vs a hand ported race port that was developed for that engine being 1.5 to 3 times that much, here is another case of blowing the budget. Once there is more money there it's an obvious place to spend it though. One thing we find here is that 99% of the guys need help in the "picking" category and not the "development" category. I fully understand where you are coming from though.
On the seat and under-seat. Keep in mind that port velocity and seat velocity are inversely proportional. Slow port velocity, high seat velocity and vice versa. I'll try to cover more on this my next trip through.
yeah, I'd like to hear more on this too.
No one has it all figured out..... and that is reassuring.
I agree with that. It's scary to think of the things you don't know.
Bret
Thread
Thread Starter
Forum
Replies
Last Post