The black art of porting...
Porting can be tedious, when doing it right - but well worth the effort.
The idea of having a set of practice heads is a bonus - I haven't always had the luxury of this!
The basic idea is to clean up the shape of the ports so there are no sudden angle changes or sudden port shape changes, as the intake or exhaust flow will become disturbed due to turbulence (less flow = less horsepower capability).
The trick is achieve a suitable flow performance and match this across all the ports!! (not easy without a flow bench).
However, there are some gains to be had, even without a 100% flow matching.
Basic goals:
(not in any particular order)
Remove casting dags
Remove (if possible) or reshape ridges within the ports - the valve guide bosses are a classic example of this, they nearly always have a large lump of casting jutting out into the gas flow creating a huge amount of turbulence (and a lot of sharp edges too). This is where I personally put in the most amount of effort.
Re-radius any sudden angle changes
Match the port mouth to the gasket - both manifolds and heads!
Try to create a constantly tapered port shape from the port mouth to the valve seat!! while not creating any holes into bolt holes, coolant galleries, etc... (thats where the sacrificial practice heads come in!).
Just think smooth and clean....:thumbsup:
Be aware that if you ream out the ports excessively, the low rev performance will suffer.
The intake ports should have a slightly rough finish to aid drivability (it helps manage liquid fuel movement - particularly when cold).
You can polish the exhaust ports to a mirror finish if you want to spend the time - it's actually beneficial! (reduces carbon buildup).
The intake ports are the most critical, as there is only atmospheric pressure to push the air in (14.7psi), - the exhaust gases have a large amount of residual combustion pressures to assist the flow on the exhaust side...
I have seen an online article about porting which I will try to find again, and post the location. (was done by some abrasives company selling a porting kit - they described the process in some detail with lots of pic's).
