texture of ports?

[Darcy R]

Hi Guys, I'm new here, and have been in contact with bruce to get what i need to build a 8 motor bench.
I've been using the little SF110 at the engine shop where i work part time, and thought it would be nice to have a bit more pressure drop for my future testing
The question that has been on my mind for a while is about port texture. Since CNC porting has become mainstream, we are often told that "the textured cut left by the step-over of the CNC cutting does not hurt airflow, and in many cases improves it"
Is there any truth to this, or is that just what the CNC shops are telling people to convince people that what they are doing is the only way it should be?
...and if it is true that the texture is in fact a good thing, what step-over works best, and is it better in the entire port? or only in some areas? SSR for example?

[jfholm]

On my own heads I always just leave the stiple marks from the carbide cutter. I try to get all the riges out but have found more flow and performance from the marks left by the carbide cutter. This was very apparent when I was doing Cortina heads for a Turner Ford of a friend. He was national champion a few times so I guess it works.

Now if you are fighting for thousands then there are areas in the port that need a different texture. There are some generalities but each head is different. If you have extensive time on hand and access to a dyno, race track etc. then you can find out where these thousanths can be picked up. The heads I have seen on some of the really fast guys seem to have a rough sanding cylinder finish under the seat for about 1/2" and the rest the CNC finish. SSR does not like really smooth from what I have seen.

I think the very rough CNC is not the way to go but neither is a polished surface. If you look at AFR heads they offer 3 levels of CNC finish. For all out racing they sell the smoothest of the step over. They use sharper cutters, slower feed and take less off at a time. Their street heads have the heaviest of the step over and they really whip them babies out for street CNC.

Welcome aboard.

John

[ivanhoew]

really interesting post john ,thanks .

[Darcy R]

I think the very rough CNC is not the way to go but neither is a polished surface. If you look at AFR heads they offer 3 levels of CNC finish. For all out racing they sell the smoothest of the step over. They use sharper cutters, slower feed and take less off at a time. Their street heads have the heaviest of the step over and they really whip them babies out for street CNC.

Welcome aboard.

John

Thanks for the great response John. Using the SF110 at 8 inches of water, is just not the right tool for learning those sort of things, and that's why i'm here.

Darcy

[jfholm]

I found a site that I had been trying to remember that has some great pictures. This is Darin Morgan's site and I chose the NASCAR head pictures. Now keep in mind these are high RPM heads. Higher than I am going to be doing and it makes a difference. But what I want you to notice in the pictures is the area right under the seat face for about 1/2" down. They all seem to be a carbide burr finish. Lots of pictures here. Study the shapes and contours and finishs. A lot to be learned from these pictures. A lot of pros start with a CNC'd head and then hand finish them.

John

http://www.darinmorgan.com/07sb2.htm

[SWAirflowServices]

I have seen a current trend with a few NASCAR engine programs to do a hand finished port. 60 grit flapper stick through the intake with a 60 grit cartridge roll (comes out rougher than flapper) or carbide finish for about the first inch below the seat. For the exhaust 100 grit all the way through. I do more than a few hand jobs on CNC heads these days.

Here is a CNC'ed spread port BBC I hand finished recently.

BBC Spread Port 2.jpg

[SSR]

Port finish has very little, if anything to do with port flow, it is all about how it affects the fuel which gets washed over it.
Rougher the better and it's the kind of rough which catches your fingers that you want.

[coulterracn]

This picture was taken of a head ported by Larry Meaux, notice the burr finish.

[jfholm]

That is the way I usually do mine. I have a funny story about it though. I did a set of heads for a circle racer through another friend. I left them the carbide finish. The racer told my friend that they were a piece of S%&t. My friend said well didn't you go faster than you ever have and didn't you get more horsepower then ever on the dyno? The race said yes but they look like crap! He would never have me do another set. LMAO!

John

[SWAirflowServices]

This picture was taken of a head ported by Larry Meaux, notice the burr finish.

I don't think these heads are CNC ported.