Corvette Forums - Corvette Enthusiast Site

This is long and gives Background, My thoughts on if you should do it, and estimated cost

Background

"Port and Polish" is a rather old (but still applicable) term for:
porting the heads -- widening the intake and exhaust ports and passages through which the engine breathes by grinding away metal inside those passages to make them wider. Hence the engine can get more air in and exhaust out and ought to be more powerful.
Polishing refers to a subsequent step in which one smoothes the various surfaces of the intake and exhaust passages to near-mirror smoothness so that air rushes through them faster. It addes only a little value - the big contribution is the porting.

Issues about porting include: a) grinding too much metal away: there are two dangerous results. First, the head may not be strong enough, and warp, or crack, b) more common though, the head may overheat in places - metal is often there just to conduct heat away from near the combustion chamber and ports to the cooling fluid:: remove too much metal and heat cannot flow fast enough to the coolant and the combustion chambers over heat.
c) the third issue is over-porting the heads - high-horsepower engines need big wide ports at high RPM to cram lots of air and fuel inside them, but around town and at lower RPM the engine needs smaller ports - they promote good engine response at part throttle and give low end power and driveability. A head with the ports opened up to wide has no low end torque, does not respond instantly and well at low RPM when you open the throttle, gets very poor (and mean really poor) fuel economy, and has high emissions.

In the 60s through the 90s, porting and polishing was done by hand, and expensive as the dickens, and the three dangers I outlined right above very real possibilities. Every shop had a porting expert or knew one and some guys were good, some weren't. I ruined a three liter Healy engine by porting its aluminum head by hand myself: added a very real 10% at top RPM but killed torque and driveability around town, which had been the car's strong suit - I had to throw the head away and start again.

Today, porting and polishing is called "CNC" (compterized numerical control) and done by "robot" computer controlled milling machines, usually so-called "five axis" machines that are something like a gigantic robot arm dentist drilling machine that repeats, exactly, the grinding on each port, to just the right amount. Most aftermarket suppliers (SLP, Patriot, AFR) offer heads CNC'd to various levels -- for example you can or could buy patriot I, II, or III heads, each level opened up with wider ports, AFR sells two levels of head: small or big port, each available finished (finished and polished) or not, etc.

Somewhere along the line, folks porting the heads also install slight larger valves, which helps breathing, too (but requires stronger valve springs, pushrods, etc.

The effect (improvement) of porting is measured by something called flow testing. The head is tested (off the car, on a bench) in a special machine that pumps air through it and measures how much goes through an intake port, or an exhaust port. Porting does help flow. Stock LS6 heads flow, if I remember right, about 265 cfm, Patriot III heads about 320.

High flow numbers mean high HP at high RPM but can mean the the engine will stumble and not run right at lower RPM. It is a mistake to buy too much porting - it will ruin the engine. Usually, it is good to ask the supplier and follow his advice: generally the really high flow numbers (really widely ported heads) are appropriate only for stroked, big-displacement engines. I have Patriot III heads, further ported and polished by Justin engineering, on my vette. They flow 347 (about 33% better than stock) -- a truly high number (at the time I bought them, the highest available) but then the engine is stroked and bored versus stock by 22%. Even so, the car is right on the edge of having driveability problems around town in the 1000-2250 RPM range: it took a lot of ECM recalibration to make it behave right and it has a trace of "cam surge" at 2200 due to too large intake ports.

Are they right for you?

Many people with superchargers do not get ported heads. The supercharger effectively imprves breathing by forcing air into the engine, rather than, as porting does, inviting it in through lower-flow resistance.
If you have a supercharged engine, and install ported heads, the net result will be less resistance to the intake flow, and the bottom line will be: a) the power will stay about the same, b) the boost you see will go down (the supercharger is pumping the same amount of air, and it all makes its way into the engine as before, but there is less resistance to its flow now, so the pressure goes down, maybe by as much as 2 lbs, c) you can change pulley size to spin the SCr faster and bring boost up to what you had before without risk of detonation, etc. (maybe, have a shop do it carefully): now there is more air flow and more power, and assuming your fuel injectors are large enough to keep up, more power. How extreme is this effect?: the Procharger on my 'vette, pumping air through stock heads into a stock engine, would be producing about 12 lbs of boost. Right now on my car its pumping all that air through those glorious Justin-ported Patrioot III heads, but only getting to about 7 lbs.

If you do not have a SC'd engine, porting will ido little unless you have an aftermarket cam. Then it will increase power at the top but do next to nothing at the low end. Porting and a cam can open up the top end of an engine by about 33% -- equivalent to about 5-6 lbs of boost from a supercharger.
Advice: if you car is non-SC'd, call a reputable supplier and ask their advice on a "heads and cam" kit - a set of heads that have been ported enough but not too much and a cam matched to them, that will all work together. If your car is supercharged, and has a stock block, don't go over about 485 RWHP regardless, or durability will suffer - most superchargers can give this much power easily through stock heads, so save your money to buy an aftermarket built block so in the future you can add more boost and the engine can take more power.

How much does it all cost? Shop around, but the best ported heads (AFR, etc.) run about $3500 a set, other parts (gaskets, etc.) are another several hundred, and labor is about $1500.