What kind of surface to leave on intake port?
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Joined: Feb 2001
Posts: 878
From: Clovis, NM
What kind of surface to leave on intake port?
The heads I'm porting out now are getting close to being done.
There stock cast iron with a fin in the middle of the intake port that I have left there but smothed it out some. It looks like the fuel injector is aimed right at it, so I'm thinking it aids in the air fuel mixing.
What finish should I leave on the surface on the intake port?
I was thinking of leaving;
# 120 grit finish or,
ribbed finish in the bowl area and maybe the port ( I keep practiceing the rib thing over and over in the bowl, I just keep grinding over it when I'm done) I'm also open to other ideas.
The heads are for a kind of hot street engine, looking to make power out to about 5200-5500rpm with about .480'' intake lift.
There stock cast iron with a fin in the middle of the intake port that I have left there but smothed it out some. It looks like the fuel injector is aimed right at it, so I'm thinking it aids in the air fuel mixing.
What finish should I leave on the surface on the intake port?
I was thinking of leaving;
# 120 grit finish or,
ribbed finish in the bowl area and maybe the port ( I keep practiceing the rib thing over and over in the bowl, I just keep grinding over it when I'm done) I'm also open to other ideas.
The heads are for a kind of hot street engine, looking to make power out to about 5200-5500rpm with about .480'' intake lift.
Registered User
Joined: Aug 2003
Posts: 735
From: BTR, Louisiana
I think the reasoning is you want the intake walls rough so that there is some 'swirl' there to minimize friction. Like "ball bearings made of air" that swirl right along side the walls, roof, and floor.
When fluid flows velocity is always highest in the middle because there's no friction there. That's what leaving them rough is supposed to help.
BUT... what if? What if you polish the snot out of them? Isn't that was they do to master ports before they program the port profiles into a CNC machine? I remember reading something about they have to polish the runners so that they can be more easily captured electronically. The paper talked about dismal results from cnc heads and how the master head flowed well to get the good results but the cnc-ed heads didn't flow as great.
What if one finished up his or her porting job and then hit it with a fine stone and shined it up like crazy? Would this necessarily hurt velocity or flow volume? Wonder if any board members have experimented with polishing one intake port and leaving another rough. Bet any responses will be only theory.
Ben T.
When fluid flows velocity is always highest in the middle because there's no friction there. That's what leaving them rough is supposed to help.
BUT... what if? What if you polish the snot out of them? Isn't that was they do to master ports before they program the port profiles into a CNC machine? I remember reading something about they have to polish the runners so that they can be more easily captured electronically. The paper talked about dismal results from cnc heads and how the master head flowed well to get the good results but the cnc-ed heads didn't flow as great.
What if one finished up his or her porting job and then hit it with a fine stone and shined it up like crazy? Would this necessarily hurt velocity or flow volume? Wonder if any board members have experimented with polishing one intake port and leaving another rough. Bet any responses will be only theory.
Ben T.
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Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by StudyTime
I think the reasoning is you want the intake walls rough so that there is some 'swirl' there to minimize friction. Like "ball bearings made of air" that swirl right along side the walls, roof, and floor.
Interesting thought. At the surface there is zero flow. As you move out a few molecules thick the flow starts slowly and then increases until it gets to "free stream" velocity. This happens well before the middle of most ports.
When fluid flows velocity is always highest in the middle because there's no friction there. That's what leaving them rough is supposed to help.
Well there is always "friction" even in the free stream because air is a viscous fluid. A rougher surface may help unstick the molecules from the surface and decrease that boundary layer thickness. It has worked on other aerodynamic surfaces. How rough is probably the question.
BUT... what if? What if you polish the snot out of them? Isn't that was they do to master ports before they program the port profiles into a CNC machine? I remember reading something about they have to polish the runners so that they can be more easily captured electronically. The paper talked about dismal results from cnc heads and how the master head flowed well to get the good results but the cnc-ed heads didn't flow as great.
My take on this is that when the surface is digitized you want a true picture of the surface so a smooth tool path can be programmed. If the digitizing probe is small, surface finish irregularities might be interpreted by the digitizing software as a varying port surface and that waviness might be duplicated but with a bigger cutter.
There is CNC porting and CNC porting. If you digitize every 1/2 inch of a port (to take a gross example) the software "smooths" the curve from each of those 1/2 inch sections. Depending on the software the end result might not really be what the master port was. If you digitize every .005, you'll probably replicate the port very accurately, tool marks and all. Because the resolution is 100 times finer, it'll take much more machine time and money to digitize, so you "tooling" cost is a lot higher, and you have to increase your customer price to pay it off. Somewhere between those limits is probably sufficient.
Similarly, the number of cutter passes used to cut the ports detrmines how much machine time is necessary. Again it relates to costs; you are buying machine time. Also, some areas of the port may take some fancy twisting of the head and small cutters taking many passes to get it right. This again is machine time which all costs money. I am not surprized to see CNC ported head prices vary a lot. The more costly ones I have seen flowed on an independent flowbench meet the manufacturer's claims.
Sometimes "hand blending" could be used in areas that otherwise could take lots of extra CNC time to properly blend to the desired shape. That's my take on why this may be done.
I'm not a CNC head machining guy. These are just my thoughts based on other CNC machning which our company does.
What if one finished up his or her porting job and then hit it with a fine stone and shined it up like crazy? Would this necessarily hurt velocity or flow volume? Wonder if any board members have experimented with polishing one intake port and leaving another rough. Bet any responses will be only theory.
I would suggest you try this on the same port, not different ports. Also measure port volume before and after polishing. You'd need to keep the metal removal to a fraction of a cc to check the effect of just the surface finish.
Ben T. [/B]
I think the reasoning is you want the intake walls rough so that there is some 'swirl' there to minimize friction. Like "ball bearings made of air" that swirl right along side the walls, roof, and floor.
Interesting thought. At the surface there is zero flow. As you move out a few molecules thick the flow starts slowly and then increases until it gets to "free stream" velocity. This happens well before the middle of most ports.
When fluid flows velocity is always highest in the middle because there's no friction there. That's what leaving them rough is supposed to help.
Well there is always "friction" even in the free stream because air is a viscous fluid. A rougher surface may help unstick the molecules from the surface and decrease that boundary layer thickness. It has worked on other aerodynamic surfaces. How rough is probably the question.
BUT... what if? What if you polish the snot out of them? Isn't that was they do to master ports before they program the port profiles into a CNC machine? I remember reading something about they have to polish the runners so that they can be more easily captured electronically. The paper talked about dismal results from cnc heads and how the master head flowed well to get the good results but the cnc-ed heads didn't flow as great.
My take on this is that when the surface is digitized you want a true picture of the surface so a smooth tool path can be programmed. If the digitizing probe is small, surface finish irregularities might be interpreted by the digitizing software as a varying port surface and that waviness might be duplicated but with a bigger cutter.
There is CNC porting and CNC porting. If you digitize every 1/2 inch of a port (to take a gross example) the software "smooths" the curve from each of those 1/2 inch sections. Depending on the software the end result might not really be what the master port was. If you digitize every .005, you'll probably replicate the port very accurately, tool marks and all. Because the resolution is 100 times finer, it'll take much more machine time and money to digitize, so you "tooling" cost is a lot higher, and you have to increase your customer price to pay it off. Somewhere between those limits is probably sufficient.
Similarly, the number of cutter passes used to cut the ports detrmines how much machine time is necessary. Again it relates to costs; you are buying machine time. Also, some areas of the port may take some fancy twisting of the head and small cutters taking many passes to get it right. This again is machine time which all costs money. I am not surprized to see CNC ported head prices vary a lot. The more costly ones I have seen flowed on an independent flowbench meet the manufacturer's claims.
Sometimes "hand blending" could be used in areas that otherwise could take lots of extra CNC time to properly blend to the desired shape. That's my take on why this may be done.
I'm not a CNC head machining guy. These are just my thoughts based on other CNC machning which our company does.
What if one finished up his or her porting job and then hit it with a fine stone and shined it up like crazy? Would this necessarily hurt velocity or flow volume? Wonder if any board members have experimented with polishing one intake port and leaving another rough. Bet any responses will be only theory.
I would suggest you try this on the same port, not different ports. Also measure port volume before and after polishing. You'd need to keep the metal removal to a fraction of a cc to check the effect of just the surface finish.
Ben T. [/B]
Last edited by OldSStroker; Dec 2, 2003 at 08:04 AM.
WOW
Thread Starter
Registered User
Joined: Feb 2001
Posts: 878
From: Clovis, NM
What surface do you think would be best that I could leave useing:
die grinder
dremel
hand tools (lots of little files of all shapes and sizes)
with:
carbide cutters
Just about every dremel grinding, sanding, buffing, polishing bit made (plus some cheap o knock offs from the 100 Yen store)
Standard abrasives port fnishing kit
foam backed sand paper 80-180 grits
regular sand paper out to 1200 grit
Untill July '04 to fnish them
die grinder
dremel
hand tools (lots of little files of all shapes and sizes)
with:
carbide cutters
Just about every dremel grinding, sanding, buffing, polishing bit made (plus some cheap o knock offs from the 100 Yen store)
Standard abrasives port fnishing kit
foam backed sand paper 80-180 grits
regular sand paper out to 1200 grit
Untill July '04 to fnish them
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Joined: Oct 2001
Posts: 1,462
From: smog zone adjacent to a great lake
Re: What kind of surface to leave on intake port?
Originally posted by oil pan 4
What finish should I leave on the surface on the intake port?
What finish should I leave on the surface on the intake port?
When fluid flows velocity is always highest in the middle because there's no friction there. That's what leaving them rough is supposed to help.
Well there is always "friction" even in the free stream because air is a viscous fluid. A rougher surface may help unstick the molecules from the surface and decrease that boundary layer thickness. It has worked on other aerodynamic surfaces. How rough is probably the question.
AFAIC, again, people are making themselves victims of overkill here. BTW, 'speaking' of [other aerodynamic surfaces], and realizing air is air, and reacts to a given surface, whatever it may be, that is exposed to an airstream, wonder whether nascar teams have thot about the effect of surface finish on the vehicles they race with. My guess is they have. Do they still have that smooth surface on the panels, or do they 'rough' the paint up with 120 grit sandpaper?
OR, do rules dictate that smooth finish?
Last edited by arnie; Dec 5, 2003 at 06:31 PM.
Registered User
Joined: Jul 2000
Posts: 520
From: MD
In porting traditionally the reason for leaving a rough surface is to prevent the fuel from hitting a smooth surface and pudding out. Sometimes you'll see a spot that looks like it was washed clean when you disassemble an engine. On more serious setups sometimes you'll see dimples punched in areas to prevent this.
The reason for smoothing an area has not so much to do with airflow but to prevent heat transfer by lessening surface area (exhaust ports) or to make customers happy/impress people.
In some cases you can use surface finish to affect how air flows through a less then optimally shaped area, for example, if for some reason you have to have a mismatch between the port and the intake or some area is actually too large, you can induce flow separation by leaving a rough spot/ramp/bump to create a turbulent pocket over the uneven area. If done right the airflow in the rest of the port will ride the surface of the pocket as if it was a smoothly carved port.
FWIW, in most cases you can get more then a smooth enough finish with a carbide bur if you have fairly steady hands and good control of the cutter. Anyplace that you don't you'll need to go over with an 80 grit or so roll to clean up a little, but not make mirror smooth. After doing it a couple of times you'll find that you'll get an idea what will smooth up faster by just switching to a tapered roll VS what will take forever if you don't get it better with a carbide. If you're going to waste your time going over a whole port with an 80grit or finer roll then save it for the exhaust ports and/or something that you're going to leave sitting on your desk unless you've got the resources to do a lot of testing to figure out where it actually gains you something and looses you something (usually we'll be talking about wet flow or something on the order of 1-3cfm).
The reason for smoothing an area has not so much to do with airflow but to prevent heat transfer by lessening surface area (exhaust ports) or to make customers happy/impress people.
In some cases you can use surface finish to affect how air flows through a less then optimally shaped area, for example, if for some reason you have to have a mismatch between the port and the intake or some area is actually too large, you can induce flow separation by leaving a rough spot/ramp/bump to create a turbulent pocket over the uneven area. If done right the airflow in the rest of the port will ride the surface of the pocket as if it was a smoothly carved port.
FWIW, in most cases you can get more then a smooth enough finish with a carbide bur if you have fairly steady hands and good control of the cutter. Anyplace that you don't you'll need to go over with an 80 grit or so roll to clean up a little, but not make mirror smooth. After doing it a couple of times you'll find that you'll get an idea what will smooth up faster by just switching to a tapered roll VS what will take forever if you don't get it better with a carbide. If you're going to waste your time going over a whole port with an 80grit or finer roll then save it for the exhaust ports and/or something that you're going to leave sitting on your desk unless you've got the resources to do a lot of testing to figure out where it actually gains you something and looses you something (usually we'll be talking about wet flow or something on the order of 1-3cfm).
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Joined: Jan 2004
Posts: 71
A local head porter I know bead blast the runners when he's finished with them. I don't have specific details about the media he's using, but he told me that it was done to dimple the surface (like a golf ball). I haven't seen flow numbers or anything, but would be interested.
Aaron
Aaron
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Joined: Jan 2000
Posts: 173
From: from the land of Justin Wilson and Huey Long!
...the theory is that fuel charge droplets are spread more evenly across the a/f mix and that the slight turbulance caused by a non-mirrored surface will allow the charge to glide past the runners. I just stopped by a bud yesterday who is building an engine for someone, and he showed me a pair of CNC-ed AFR's that actually looked too smooth to me. They were [i]beautiful[i] to look at, but actually the intake ports looked as if they were bead extruded. I commented to him that the surfaces were a little too smooth. I guess in N/A applications it makes more of a difference than in boost situations...
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