16V Cylinder head flowbench fixture
01-27-2014, 12:09 PM
#16
Nordschleife Master
There have been posts where ported heads had a decreased flow from stock. Just because the ports are increased, doesn't mean they flow better. There is lots of info on the net on this. On an N/A engine, porting mostly results in nominal improvement. Sometimes so little that a but dyno won't notice it. Just be cautious of the expectations, that's all I'm saying.
As I wrote down earlier, knowing how much the head flows on a flowbench _and_ knowing the port dimensions is useful in picking up other components. It's also useful in forming expectations about how much power it should make in debugging stage.
01-27-2014, 12:14 PM
#17
Pro
Thread Starter
There have been posts where ported heads had a decreased flow from stock. Just because the ports are increased, doesn't mean they flow better. There is lots of info on the net on this. On an N/A engine, porting mostly results in nominal improvement. Sometimes so little that a but dyno won't notice it. Just be cautious of the expectations, that's all I'm saying.
I'm aware of a highly polished port flowing less than a rougher finish, which is specifically why there isn't a high luster polish on any of my intake ports.
I hold no specific expectations on the head flow themselves. This "new" motor will such an improvement from the stock '82 motor that you'd be hard pressed to figure out where the gains came from. Displacement is up, headwork, euro intake, ported j-pipe and throttle body, euro '78 cams, headers, standalone...blah blah blah
Having the flow numbers of my heads would be beneficial not just for me, but for the archives in general as I've documented pretty well the work that I performed on the heads.
01-27-2014, 12:41 PM
#18
Captain Obvious
Super User
Super User
I'm on my phone and the search on it sux but, there were threads on this a couple years ago.If my memory is correct, Greg Brown had some interesting posts that shead light on this topic.
01-27-2014, 12:58 PM
#19
Nordschleife Master
If you want to read about porting 2-valve and 4-valve heads, go to speedtalk.com, forums 1 and 15. Search for the likes of Erland ***, Nitro2, Stan Weiss, SchmidtMotorWorks, Hoffman900, maxracesoftware, etc.
01-27-2014, 01:36 PM
#20
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Looking through my files for a 928S (Euro) cylinder head having 45/40mm valves, at maximum valve lift the intake port flow is 210,8 CFM (12mm) and exhaust port flow 160,6 CFM (11mm). Like Mr. Brown´s flowbench mine is working at a pressure differential of 10" of water. To make comparison easier the figures have been converted to 28" of vacuum. The same head being ported with 48/40mm valves the intake flow is 256,3 CFM and the exhaust flow 227,5 CFM measured at 13,2mm of valve lift (maximum valve lift for a new sports camshaft). The intake port size 40,5mm was kept unchanged as the head was intended for a 4,7L engine. The intake flow is somewhat limited from shrouding depending of the small 97mm bore.
Åke
Åke
01-27-2014, 01:52 PM
#21
Pro
Thread Starter
I'll have to check out speedtalk later. I'm pretty familiar with smaller 2-valve engines, as I've ported numerous VW heads in the past. This was the first aluminum V8 set of heads I've done though. Have a couple VW 4 and 5 valve heads done as well. One of the 4-valve heads I built later put down 200+ wheel on a daily driven street car, I was told for AGES with the mild bolt ons that it wasn't possible...
Looking through my files for a 928S (Euro) cylinder head having 45/40mm valves, at maximum valve lift the intake port flow is 210,8 CFM (12mm) and exhaust port flow 160,6 CFM (11mm). Like Mr. Brown´s flowbench mine is working at a pressure differential of 10" of water. To make comparison easier the figures have been converted to 28" of vacuum. The same head being ported with 48/40mm valves the intake flow is 256,3 CFM and the exhaust flow 227,5 CFM measured at 13,2mm of valve lift (maximum valve lift for a new sports camshaft). The intake port size 40,5mm was kept unchanged as the head was intended for a 4,7L engine. The intake flow is somewhat limited from shrouding depending of the small 97mm bore.
Åke
Åke
01-27-2014, 05:27 PM
#22
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I wish I could agree with your statement.
And I especially wish I could agree with it regarding heads on this very forum.
Stick a set of 968 valve seats into the intake side and open up the port/ports to match the seats....proceed to measure the loss of flow, over the stock head.
Virtually every head that I've personally flowed (which had larger valves) lost flow over the stock port and valves. This includes heads from both Threshie and Devek.
These are very tricky heads to improve on. Easy to ruin.
__________________
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
01-27-2014, 10:23 PM
#23
Nordschleife Master
I was born in 1973...
I've been studying this compressible fluid flow now to the point that I am probably going to buy a flow bench this summer. See how theory works in practice. Flow anything with a hole. (Almost like in high school and college, but with a subtle difference -- but I digress...)
The theory says that a bigger hole will almost always flow more air. Now, what do I mean by "a bigger hole" in this context? It's the minimum cross-sectional area (MCSA) of the port, that's what I mean. A port with a bigger hole, i.e., larger MCSA, will almost always flow more air on a flow bench. That's my theory (not original research. I didn't come up with that on my own...)
In 2-valve head, the MCSA may or may not be at the valves -- never looked at a 928 2-valve head intake port myself, except in photos.
In the 4-valve head, based on a mold it's higher up in the port, the MCSA about there where the valve guides are and the two ports start merging.
Why might a larger intake seat and valve sometimes flow less air at full lift? Suppose it's a 4-valve head and not the 2-valve head that this thread is about. A larger valve on it's own isn't making the full lift hole bigger, since the valve wasn't the MCSA location to start with. All the larger valve and seat are doing, if no other changes are made, is increasing the diffusion angle after the MCSA.
Is that increased diffusion angle a good thing or a bad thing? Well that depends on what the diffusion angle was to start with. If it was less than optimal, then it's good. If it was larger than optimal, then it's bad.
Air flows really well out of the the hole when the diffusion angle after the hole is shallow. I've been told by someone who ports heads for living but doesn't yet know everything about air flow that he thinks the optimal included angle is 7 degrees and the maximum included angle without flow separation is 10-15 degrees. After 15 degrees, you get flow separation, and at that point abrupt termination into an open space is usually better than any diffuser.
So what might be going on with a 4-valve port in which installing larger valves loses flow is that the larger seat and valve alone make the diffusion angle after the MCSA too large and hurt the flow. If this is the problem, then the MCSA needs to be made larger and/or moved further upstream to restore the lower diffusion angle and improve flow. Whether moving upstream or makign larger is better depends on how much air one intends to flow thru that hole when the engine actually runs.
Some people have reported that smoothing the abrupt transition from the seat to the head casting proper sometimes loses flow on a bench. This might also be indicative of a too large diffusion angle. The abrupt transition can flow better than a smoothed one, if the diffusion angle is too large.
It'll be interesting to see how all this squares out with reality when I have time to get on with experiments. I am fortunate to own two large-valve 928 heads, both of which flow more than the stock head. One set based on a flow-bench test and another set based on a dyno experiment. But I didn't have anything to do with porting either head, and I can't really say I understand those heads as well as I would like to.
I've been studying this compressible fluid flow now to the point that I am probably going to buy a flow bench this summer. See how theory works in practice. Flow anything with a hole. (Almost like in high school and college, but with a subtle difference -- but I digress...)
The theory says that a bigger hole will almost always flow more air. Now, what do I mean by "a bigger hole" in this context? It's the minimum cross-sectional area (MCSA) of the port, that's what I mean. A port with a bigger hole, i.e., larger MCSA, will almost always flow more air on a flow bench. That's my theory (not original research. I didn't come up with that on my own...)
In 2-valve head, the MCSA may or may not be at the valves -- never looked at a 928 2-valve head intake port myself, except in photos.
In the 4-valve head, based on a mold it's higher up in the port, the MCSA about there where the valve guides are and the two ports start merging.
Why might a larger intake seat and valve sometimes flow less air at full lift? Suppose it's a 4-valve head and not the 2-valve head that this thread is about. A larger valve on it's own isn't making the full lift hole bigger, since the valve wasn't the MCSA location to start with. All the larger valve and seat are doing, if no other changes are made, is increasing the diffusion angle after the MCSA.
Is that increased diffusion angle a good thing or a bad thing? Well that depends on what the diffusion angle was to start with. If it was less than optimal, then it's good. If it was larger than optimal, then it's bad.
Air flows really well out of the the hole when the diffusion angle after the hole is shallow. I've been told by someone who ports heads for living but doesn't yet know everything about air flow that he thinks the optimal included angle is 7 degrees and the maximum included angle without flow separation is 10-15 degrees. After 15 degrees, you get flow separation, and at that point abrupt termination into an open space is usually better than any diffuser.
So what might be going on with a 4-valve port in which installing larger valves loses flow is that the larger seat and valve alone make the diffusion angle after the MCSA too large and hurt the flow. If this is the problem, then the MCSA needs to be made larger and/or moved further upstream to restore the lower diffusion angle and improve flow. Whether moving upstream or makign larger is better depends on how much air one intends to flow thru that hole when the engine actually runs.
Some people have reported that smoothing the abrupt transition from the seat to the head casting proper sometimes loses flow on a bench. This might also be indicative of a too large diffusion angle. The abrupt transition can flow better than a smoothed one, if the diffusion angle is too large.
It'll be interesting to see how all this squares out with reality when I have time to get on with experiments. I am fortunate to own two large-valve 928 heads, both of which flow more than the stock head. One set based on a flow-bench test and another set based on a dyno experiment. But I didn't have anything to do with porting either head, and I can't really say I understand those heads as well as I would like to.
I bought my first flow bench in 1973.
I wish I could agree with your statement.
And I especially wish I could agree with it regarding heads on this very forum.
Stick a set of 968 valve seats into the intake side and open up the port/ports to match the seats....proceed to measure the loss of flow, over the stock head.
Virtually every head that I've personally flowed (which had larger valves) lost flow over the stock port and valves. This includes heads from both Threshie and Devek.
These are very tricky heads to improve on. Easy to ruin.
I wish I could agree with your statement.
And I especially wish I could agree with it regarding heads on this very forum.
Stick a set of 968 valve seats into the intake side and open up the port/ports to match the seats....proceed to measure the loss of flow, over the stock head.
Virtually every head that I've personally flowed (which had larger valves) lost flow over the stock port and valves. This includes heads from both Threshie and Devek.
These are very tricky heads to improve on. Easy to ruin.
01-27-2014, 10:57 PM
#24
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I was born in 1973...
I've been studying this compressible fluid flow now to the point that I am probably going to buy a flow bench this summer. See how theory works in practice. Flow anything with a hole. (Almost like in high school and college, but with a subtle difference -- but I digress...)
The theory says that a bigger hole will almost always flow more air. Now, what do I mean by "a bigger hole" in this context? It's the minimum cross-sectional area (MCSA) of the port, that's what I mean. A port with a bigger hole, i.e., larger MCSA, will almost always flow more air on a flow bench. That's my theory (not original research. I didn't come up with that on my own...)
In 2-valve head, the MCSA may or may not be at the valves -- never looked at a 928 2-valve head intake port myself, except in photos.
In the 4-valve head, based on a mold it's higher up in the port, the MCSA about there where the valve guides are and the two ports start merging.
Why might a larger intake seat and valve sometimes flow less air at full lift? Suppose it's a 4-valve head and not the 2-valve head that this thread is about. A larger valve on it's own isn't making the full lift hole bigger, since the valve wasn't the MCSA location to start with. All the larger valve and seat are doing, if no other changes are made, is increasing the diffusion angle after the MCSA.
Is that increased diffusion angle a good thing or a bad thing? Well that depends on what the diffusion angle was to start with. If it was less than optimal, then it's good. If it was larger than optimal, then it's bad.
Air flows really well out of the the hole when the diffusion angle after the hole is shallow. I've been told by someone who ports heads for living but doesn't yet know everything about air flow that he thinks the optimal included angle is 7 degrees and the maximum included angle without flow separation is 10-15 degrees. After 15 degrees, you get flow separation, and at that point abrupt termination into an open space is usually better than any diffuser.
So what might be going on with a 4-valve port in which installing larger valves loses flow is that the larger seat and valve alone make the diffusion angle after the MCSA too large and hurt the flow. If this is the problem, then the MCSA needs to be made larger and/or moved further upstream to restore the lower diffusion angle and improve flow. Whether moving upstream or makign larger is better depends on how much air one intends to flow thru that hole when the engine actually runs.
Some people have reported that smoothing the abrupt transition from the seat to the head casting proper sometimes loses flow on a bench. This might also be indicative of a too large diffusion angle. The abrupt transition can flow better than a smoothed one, if the diffusion angle is too large.
It'll be interesting to see how all this squares out with reality when I have time to get on with experiments. I am fortunate to own two large-valve 928 heads, both of which flow more than the stock head. One set based on a flow-bench test and another set based on a dyno experiment. But I didn't have anything to do with porting either head, and I can't really say I understand those heads as well as I would like to.
I've been studying this compressible fluid flow now to the point that I am probably going to buy a flow bench this summer. See how theory works in practice. Flow anything with a hole. (Almost like in high school and college, but with a subtle difference -- but I digress...)
The theory says that a bigger hole will almost always flow more air. Now, what do I mean by "a bigger hole" in this context? It's the minimum cross-sectional area (MCSA) of the port, that's what I mean. A port with a bigger hole, i.e., larger MCSA, will almost always flow more air on a flow bench. That's my theory (not original research. I didn't come up with that on my own...)
In 2-valve head, the MCSA may or may not be at the valves -- never looked at a 928 2-valve head intake port myself, except in photos.
In the 4-valve head, based on a mold it's higher up in the port, the MCSA about there where the valve guides are and the two ports start merging.
Why might a larger intake seat and valve sometimes flow less air at full lift? Suppose it's a 4-valve head and not the 2-valve head that this thread is about. A larger valve on it's own isn't making the full lift hole bigger, since the valve wasn't the MCSA location to start with. All the larger valve and seat are doing, if no other changes are made, is increasing the diffusion angle after the MCSA.
Is that increased diffusion angle a good thing or a bad thing? Well that depends on what the diffusion angle was to start with. If it was less than optimal, then it's good. If it was larger than optimal, then it's bad.
Air flows really well out of the the hole when the diffusion angle after the hole is shallow. I've been told by someone who ports heads for living but doesn't yet know everything about air flow that he thinks the optimal included angle is 7 degrees and the maximum included angle without flow separation is 10-15 degrees. After 15 degrees, you get flow separation, and at that point abrupt termination into an open space is usually better than any diffuser.
So what might be going on with a 4-valve port in which installing larger valves loses flow is that the larger seat and valve alone make the diffusion angle after the MCSA too large and hurt the flow. If this is the problem, then the MCSA needs to be made larger and/or moved further upstream to restore the lower diffusion angle and improve flow. Whether moving upstream or makign larger is better depends on how much air one intends to flow thru that hole when the engine actually runs.
Some people have reported that smoothing the abrupt transition from the seat to the head casting proper sometimes loses flow on a bench. This might also be indicative of a too large diffusion angle. The abrupt transition can flow better than a smoothed one, if the diffusion angle is too large.
It'll be interesting to see how all this squares out with reality when I have time to get on with experiments. I am fortunate to own two large-valve 928 heads, both of which flow more than the stock head. One set based on a flow-bench test and another set based on a dyno experiment. But I didn't have anything to do with porting either head, and I can't really say I understand those heads as well as I would like to.
You'd be amazed at what intake port configuration flows the most amount of air, with a 968 intake valve. It's not what you would think. I've never actually dyno tested, in this configuration....it just seems so illogical.
When you get your flow bench and you start to actually work with air flow...you will understand. It's more magical than scientific.....and you are so technical and logical, it will be a whole new experience for you.
Stock up on your favorite "mind numbing" liquid medication, beforehand.
01-28-2014, 06:35 AM
#25
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Thought this was about 2-valve heads. Tuomo, the minimum cross-sectional area shall not be at the valve, a totally incorrect design. The MCSA shall be at the manifold gasket and is 2.00 sqin for a circular port of 40,5mm (see post #20). The largest cross sectional area is made at the bend about where the valve guide is protruding. For the 48mm intake valve a second venturi (43,5mm - 2.30 sqin) is made right before the valve seat. At the maximum lift of 13,2mm or .520" the valve curtain area is 3.10 sqin. I always pay much attention to the short side radius where a lot of gain especially for low intake ports can be found. I never do multi angle valve jobs, I use radiused MIRA valve seat cutters, which come in many different designs, for optimum flow. Think the MIRA system is the best valve seat cutting equipment available.
http://www.miratool.ch/
When making these 2-valve heads I did an experiment putting a 100mm (4") long extension between the air horn and the intake port.
With air horn directly to the head the flow at maximum valve lift was 256.4 CFM.
With extension having an inside diameter of 41mm between air horn and head 255.2 CFM.
With extension having an inside venturi of 36mm 253.0 CFM.
Reducing the inside diameter by 5mm from 41mm to 36mm had very little effect on the flow but do increase the the airspeed quite a lot. The question remains would the engine perform better with a smaller intake port? The 928S (Euro) heads are coming with a round 40,5mm intake port which is too large for the stock 45mm intake valve. Theoretically the port should be about 37mm to match the intake valve size best.
I have not had a flow bench as long as Mr. Brown, think I got the first bench in 1978 being one of the very first in the country. Flow benches was not well know by people over here at that time.
In contrary to Mr. Brown I cannot see any magic in head porting, it is just a matter of common sense. I did porting jobs more than ten years before having a flow bench available. Flow testing some of the old jobs I could see it was not too badly done but being able to measure the flow many small details can be checked and verified if it is a gain or loss.
Åke
When making these 2-valve heads I did an experiment putting a 100mm (4") long extension between the air horn and the intake port.
With air horn directly to the head the flow at maximum valve lift was 256.4 CFM.
With extension having an inside diameter of 41mm between air horn and head 255.2 CFM.
With extension having an inside venturi of 36mm 253.0 CFM.
Reducing the inside diameter by 5mm from 41mm to 36mm had very little effect on the flow but do increase the the airspeed quite a lot. The question remains would the engine perform better with a smaller intake port? The 928S (Euro) heads are coming with a round 40,5mm intake port which is too large for the stock 45mm intake valve. Theoretically the port should be about 37mm to match the intake valve size best.
I have not had a flow bench as long as Mr. Brown, think I got the first bench in 1978 being one of the very first in the country. Flow benches was not well know by people over here at that time.
In contrary to Mr. Brown I cannot see any magic in head porting, it is just a matter of common sense. I did porting jobs more than ten years before having a flow bench available. Flow testing some of the old jobs I could see it was not too badly done but being able to measure the flow many small details can be checked and verified if it is a gain or loss.
Åke
01-28-2014, 10:59 AM
#26
Burning Brakes
when I had the 16V heads done , by Harvey Resnick , a engine builder /racer with 30+ years exp . He used a flow bench to check his work as he progressed , but relied on very carful measurement's based on ratio of valve size and the shape of the intake port. A lot of time was spent on carefully shaping the bowel and to creating a slight swirl effect in the intake . The " MAGIC " was in his years of experience and knowledge . The intakes flowed 280 -282 cfm when he was finished , I forget what the ex flowed
01-28-2014, 11:02 AM
#27
Nordschleife Master
This email thread reminds me of the third law of Clarke: "Any sufficiently advanced technology is indistinguishable from magic." The cylinder head is clearly in the magic territory for me.
Ake -- yes, this is about 2-valve heads. As I've demonstrated to everyone, I know nothing about the 928 2-valve heads! ;-)
So my question to you is why do you think the flow was reduced so little by reducing the diameter of the inlet pipe to the head? If the gasket is 40.5mm and the inlet pipe was 36mm, that's an effective reduction of area equal to 21% for a flow reduction of about 1%. My first instinct, with all the associated caveats, is that the effective restriction point of the port is somewhere far downstream of the intake manifold flange. Otherwise, the 21% reduction in the area at the gasket would have caused a bigger flow loss, no?
Greg -- Do I understand that you have a set of those Threshie 4-valve heads that one of your customers owns? If you and the customer think they are junk, I might be interested in purchasing them for racing junk prices. It could be an interesting science project for me in the summer to measure the heads in every possible way, if they are cheap.
Ake -- yes, this is about 2-valve heads. As I've demonstrated to everyone, I know nothing about the 928 2-valve heads! ;-)
So my question to you is why do you think the flow was reduced so little by reducing the diameter of the inlet pipe to the head? If the gasket is 40.5mm and the inlet pipe was 36mm, that's an effective reduction of area equal to 21% for a flow reduction of about 1%. My first instinct, with all the associated caveats, is that the effective restriction point of the port is somewhere far downstream of the intake manifold flange. Otherwise, the 21% reduction in the area at the gasket would have caused a bigger flow loss, no?
Greg -- Do I understand that you have a set of those Threshie 4-valve heads that one of your customers owns? If you and the customer think they are junk, I might be interested in purchasing them for racing junk prices. It could be an interesting science project for me in the summer to measure the heads in every possible way, if they are cheap.
01-28-2014, 12:46 PM
#28
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I think a big part of Greg's observations has a lot to do with him flowing the heads through the stock intake. Which makes perfect sense considering 99% of the engines he's built use the stock intake. When you factor in some of Greg's other knowledge nuggets regarding the intake, mainly that no intake mod seems to improve power much at all. Seems like most everything has been thrown at the intake: welding on a larger throttle body, extensive porting, plenum spacers, extrude honing and the list goes on and on and all paths seem to lead to a barely measurable difference. This being the case, is it surprising that improvements in head flow are so hard to come by? Or that getting real flow improvements might require mods that go against conventional port logic? Lastly, what would be gained from a head that flows 500 cfm @ .100 lift, if it is bolted to the same maxed out intake? How is this unlike an Olympic runner being forced to breathe through a straw?
01-28-2014, 01:36 PM
#29
Nordschleife Master
I don't think he's flowing these heads thru the stock intake. I think he's using a fairly standard inlet trumpet. I am sure he'll tell you whether I am right or not.
If those big-valve Threshie heads are junk, I'll pay something for them just to be able to take a look at them. I am kind of curious what's happening here, even if the results is me learning what not to do.
Turning back tot he 2-valve heads that I know nothing about: There's a fair bit of info on those in two places: 951/944 forums and postings on Speedtalk.com of certain nic "928S"... ...such as http://speedtalk.com/forum/viewtopic.php?f=1&t=14840 You probably figure out pretty quickly who 928S is.
If those big-valve Threshie heads are junk, I'll pay something for them just to be able to take a look at them. I am kind of curious what's happening here, even if the results is me learning what not to do.
Turning back tot he 2-valve heads that I know nothing about: There's a fair bit of info on those in two places: 951/944 forums and postings on Speedtalk.com of certain nic "928S"... ...such as http://speedtalk.com/forum/viewtopic.php?f=1&t=14840 You probably figure out pretty quickly who 928S is.
I think a big part of Greg's observations has a lot to do with him flowing the heads through the stock intake. Which makes perfect sense considering 99% of the engines he's built use the stock intake. When you factor in some of Greg's other knowledge nuggets regarding the intake, mainly that no intake mod seems to improve power much at all. Seems like most everything has been thrown at the intake: welding on a larger throttle body, extensive porting, plenum spacers, extrude honing and the list goes on and on and all paths seem to lead to a barely measurable difference. This being the case, is it surprising that improvements in head flow are so hard to come by? Or that getting real flow improvements might require mods that go against conventional port logic? Lastly, what would be gained from a head that flows 500 cfm @ .100 lift, if it is bolted to the same maxed out intake? How is this unlike an Olympic runner being forced to breathe through a straw?
01-28-2014, 04:14 PM
#30
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Thought this was about 2-valve heads. Tuomo, the minimum cross-sectional area shall not be at the valve, a totally incorrect design. The MCSA shall be at the manifold gasket and is 2.00 sqin for a circular port of 40,5mm (see post #20). The largest cross sectional area is made at the bend about where the valve guide is protruding. For the 48mm intake valve a second venturi (43,5mm - 2.30 sqin) is made right before the valve seat. At the maximum lift of 13,2mm or .520" the valve curtain area is 3.10 sqin. I always pay much attention to the short side radius where a lot of gain especially for low intake ports can be found. I never do multi angle valve jobs, I use radiused MIRA valve seat cutters, which come in many different designs, for optimum flow. Think the MIRA system is the best valve seat cutting equipment available. Mira Centronic valve seat cutting - YouTube
http://www.miratool.ch/
When making these 2-valve heads I did an experiment putting a 100mm (4") long extension between the air horn and the intake port.
With air horn directly to the head the flow at maximum valve lift was 256.4 CFM.
With extension having an inside diameter of 41mm between air horn and head 255.2 CFM.
With extension having an inside venturi of 36mm 253.0 CFM.
Reducing the inside diameter by 5mm from 41mm to 36mm had very little effect on the flow but do increase the the airspeed quite a lot. The question remains would the engine perform better with a smaller intake port? The 928S (Euro) heads are coming with a round 40,5mm intake port which is too large for the stock 45mm intake valve. Theoretically the port should be about 37mm to match the intake valve size best.
I have not had a flow bench as long as Mr. Brown, think I got the first bench in 1978 being one of the very first in the country. Flow benches was not well know by people over here at that time.
In contrary to Mr. Brown I cannot see any magic in head porting, it is just a matter of common sense. I did porting jobs more than ten years before having a flow bench available. Flow testing some of the old jobs I could see it was not too badly done but being able to measure the flow many small details can be checked and verified if it is a gain or loss.
Åke
http://www.miratool.ch/
When making these 2-valve heads I did an experiment putting a 100mm (4") long extension between the air horn and the intake port.
With air horn directly to the head the flow at maximum valve lift was 256.4 CFM.
With extension having an inside diameter of 41mm between air horn and head 255.2 CFM.
With extension having an inside venturi of 36mm 253.0 CFM.
Reducing the inside diameter by 5mm from 41mm to 36mm had very little effect on the flow but do increase the the airspeed quite a lot. The question remains would the engine perform better with a smaller intake port? The 928S (Euro) heads are coming with a round 40,5mm intake port which is too large for the stock 45mm intake valve. Theoretically the port should be about 37mm to match the intake valve size best.
I have not had a flow bench as long as Mr. Brown, think I got the first bench in 1978 being one of the very first in the country. Flow benches was not well know by people over here at that time.
In contrary to Mr. Brown I cannot see any magic in head porting, it is just a matter of common sense. I did porting jobs more than ten years before having a flow bench available. Flow testing some of the old jobs I could see it was not too badly done but being able to measure the flow many small details can be checked and verified if it is a gain or loss.
Åke
Airflow over a surface in laminar. Generally, the smoother the surface, the bigger the layer is that tumbles and the rougher the surface (to a point), the thinner the layer is. Short, relatively straight intake ports suffer worse from surface finish than longer intake ports with bends (old school American V-8 heads).
The 2 valve 928 port is not an exception. Also easy to go backwards, depending on what is done.
