Hi guys
What grit finish are you guys using on downdraught 16v heads?
On my ducatis 80 or even 120 looks nice but looks are not everything and lately i have been using 60 in inletports and 120 in the exausts.
I have found that a fine finish hardly picks up flow on some heads. I would like the best fuel atomosation.
thank you.
Port surface texture
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superbike
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SirYun
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On my own heads (which are straight valve 90 deg ports) i just leave them 60 grade stone finished..to be honest i first make them pretty ( because i makes me feel better) ..then grind in a rough texture for the port runner and use a 80 grit sanding roll for the last bit of the bowl right upto the seat. For exhausts i use a 120 grit finish. but when i examine a head after some time running i always looks the same.. slight carbon build up. i very much doubt the surface finish does anything for exhausts.
Joost M. Riphagen
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Guy Croft
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Inlets:
The smaller the inlet port the finer it needs but I can say with some authority that on, say, 8V Fiat type TCs going finer than 80 gr makes no difference to power, and on 16v XE Vauxhalls 120 gr. I have not determined any difference in flow on 8v or 16v heads between 80 and 120 grit but - to be honest - 80 grit looks a bit messy on 16v heads, and I like going to 120 because, ignoring functionality, appearance is important. No-one wants to take delivery of something 'rough looking'.
as far as flow is concerned the surface roughness to an extent determines the Reynolds (Re) number in the port. It can be used to calculate the place where flow goes from laminar to fully turbulent, but you'd have to know a very skilled aerodynamicist with access to top-of-the-range cfd and automotive exp to tell you what the absolute optimum finish in inlet ports should be. Certainly - I believe - no call for 'polished', because that can upset the laminar boundary layer (region about 0.2mm thick where the air is actually static).
Ex ports:
Same guidance applies although no-one really knows how the gas behaves in terms of turbulence, mostly aerodynamicists model ex ports as if they were flowing air.
As far as the old 'carrot' of polishing to delay the formation of carbon, this is a nonsense because any port will carbon-up in seconds, whatever the finish.
If any erudite reader - versed in aerodynamics and its application to engines - cares to add - feel free.
GC
The smaller the inlet port the finer it needs but I can say with some authority that on, say, 8V Fiat type TCs going finer than 80 gr makes no difference to power, and on 16v XE Vauxhalls 120 gr. I have not determined any difference in flow on 8v or 16v heads between 80 and 120 grit but - to be honest - 80 grit looks a bit messy on 16v heads, and I like going to 120 because, ignoring functionality, appearance is important. No-one wants to take delivery of something 'rough looking'.
as far as flow is concerned the surface roughness to an extent determines the Reynolds (Re) number in the port. It can be used to calculate the place where flow goes from laminar to fully turbulent, but you'd have to know a very skilled aerodynamicist with access to top-of-the-range cfd and automotive exp to tell you what the absolute optimum finish in inlet ports should be. Certainly - I believe - no call for 'polished', because that can upset the laminar boundary layer (region about 0.2mm thick where the air is actually static).
Ex ports:
Same guidance applies although no-one really knows how the gas behaves in terms of turbulence, mostly aerodynamicists model ex ports as if they were flowing air.
As far as the old 'carrot' of polishing to delay the formation of carbon, this is a nonsense because any port will carbon-up in seconds, whatever the finish.
If any erudite reader - versed in aerodynamics and its application to engines - cares to add - feel free.
GC
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