So its frowned on to polish intake ports for fuel suspension reasons, etc. Can/should exhaust ports be polished? Any downside? We are talking CNC ported Alum heads.

I know a very good HP machine shop in SO CA that polished the ports on a set of Dart BB Chevy CNC ported heads, they flowed the heads before and after, the customer didn't like the LOOKS(ridges) of the CNC finish, there was no measurable difference or gains in the ports by polishing the lines away If it is a street car and your concern is exhaust port carbon buildup go for it, if it is a drag race car don't waste your time I had a set of Eddy RPM that I had Jeff at Modern Cylinder Heads CNC port, I raced it a lot and drove it on the street some on Oregon pump gas, no noticable carbon build up on them the last time I had the heads off

How about heat retention? Say in a road race motor with extended run times and limited airflow? Woould seem a lot less exhaust heat would be absorbed for little/no benefit.

I know nothing about road racing or heat retention based off of polishing the exhaust ports, sorry On further thinking the ridges left by the CNC program probally have more surface area than if you polished them off and made the sides straight, shiny and bright That's probally one of the things that needs to be tested at at the race track using back to back testing, AKA A,B,A tests

I bought my cnc eddy heads for Jeff at MCH. I wanted to pay extra to polish the exhaust ports old shool style. He told me he would not do it, and to save my money. "The ridges keep the air in suspension and the exhaust port is already too large", and that is a quote.

Since that conversation, I don't bother with over polishing any exhuast ports that I work on. I use my dremel and a particular small stone to nicely cut a similar finish and pattern in the bowls and runner. I still polish valves.

Now on the other hand, I hear that some Nascar teams further rework cnc heads by hand. I guess it is all application.

I still polish the exhaust ports... the piston is
pushing out the burnt gasses so I try to reduce any
possible restriction and it helps the draft pulse on
the over lap.... JMO

The ridges keep the air in suspension

That pretty much takes him off the "ask" list.

what i usually tell people,, you want a surface that has enough texture to create the thinnest boundary layer possible,, if the surface finish is so coarse that it creates a larger boundary layer, you reduce the funcyional cross section of the port

polishing out minor ridges from a typical cnc'd head wont do anything for you,, polishing the chamber to reduce surface area,,, wont do anything for you,, on a cast chamber there is a great deal of surface area,, and polishing gains are very minimal..

guys make too big of a deal about this....

id guess if you had an 800 hp motor with cnc'd heads,,, if you polished the ports and chambers,, maybe it would hit 802hp

this is just a myth that people would "think" work,, like lightweight parts rev up faster going down track,,, they all do,, the gains are just soooo very minimal......