Thread: Porting and Polishing Heads

What are your thoughts on porting and polishing intake manifold on a gems?

The most critical areas in the port are roughly 1/2" above and 1/4" below the seats.

Opening the ports up (dramatically increasing volumes/cross sectional area) is detrimental to torque below the torque peak.

A case in point, when the V8 Supercar category was introduced all the teams used racing heads from the US until they could get their heads (no pun intended ) around what was required.

Those heads were based on NASCAR displacements (355 or so Cu In) and revved past 9000RPM
The engines here were 5 litres and rev limited to 7500RPM

It was quickly found the US heads wouldn't pull a sailor off your sister.

A lot of metal was added to the ports, volumes substantially reduced, a lot of attention paid to the throat and creating the best venturi shape possible to suit the capacity and flows required for our engines.

Another example, I had a Formula Ford head from the UK that looked (and still looks) amazing.
Biiig ports, highly polished, no valve guide bosses at all.

We rebuilt the engine, plonked it on the dyno, ran it in and did some power runs for a base line.
Prior to the rebuild this engine was at the pointy end in the NSW State FF championship with a totally novice driver.
We were offered a head to test, guide bosses were still there but shaped/contoured, no polishing, smaller throats but lots of shaping, barely any metal at all taken out of the ports, they were almost standard in dimensions.

The bloke that ported that head supplied Gibson Motorsport (Mark Skaife in the Winfield car) and HRT in the V8's and they were both dominating at the time.

The trial head was a genuine 10hp and 10lb/ft better.

10hp on a Formula Ford with only 100-110hp to play with is insane.
Engines usually only vary by one or two. 10hp would be almost a second/lap at Eastern Ck, all other things equal.

Bigger aint always better.

A couple of other things to consider.

The critical areas are where air changes direction.
We want radius as long and smooth as possible.
Pay attention to the outside of bends, eg. the roof of exhaust ports as that's where the air gets pushed.
Lots of angles at the seat and possibly multi cut valves.
Nail head valves (and especially with undercut stems) tend to flow much better in an OHV engine than nice, tulip shaped valves.

With stock cam profiles you are looking to get as much flow as possible at small valve openings, so the seats and throat just above are critical.

Chamber shape plays a huge role too.
Bathtub chambers such as used on a modern Chev are ideal, the chamber wall becomes an extension of the seat, promoting flow.
Flat heads and big, wide open chambers are yukky, it really disturbs things badly.

In naturally aspirated engines the reason you try and keep port cross sections smallish, especially with stock or road going/torque style cams is that it keeps port velocity up and helps cylinder filling at small valve lifts and durations.

We want torque at 'normal' revs.

If you're on the race track and the revs never drop below 50000RPM, things are a bit different, but if you are off road and just off idle, or at 1500 revs and you bury the foot to climb a rock ledge you don't want the engine to fall flat on it's face as the ports are too damned big.

any pics/drawings of your ideas?

ie; "root of the exhaust port"

Originally Posted by Pedro_The_Swift

any pics/drawings of your ideas?

ie; "root of the exhaust port"

root or roof ?

Yep. Port the intake, Port and Polish the exhaust. You need some turbulence in the intake to help mix the fuel and air. You just want to get everything out with least resistance as possible in the exhaust.

I spent 25 hours on my Suzuki GTI heads. There were a few other mods but standard they made 52 - 54 hp at the wheels, mine made 105hp at the wheel.

Happy Days

Not a Land rover. Suzuki GTI.

Intake before


Intake after



Exhaust after