do not under any circumstances unshroud or polish the intake side of the combustion side,but poilish the exhaust entry chamber and bowl towards a venturi effect,and the exhaust port too,but keep the exhaust side as small as possible to maintain critical velocity.leave the intake rough castings alone,other than a de dag and concentrate only on the turn radius with complimented 3 angle intake valve job.you want to maximise fuel control,air speed an all lift flows without hindering swirl effect,otherwise leave the heads stock instead of playing the roulette game,:undead:as we know the efi castings are good for 485bhp+:bang:
Be Careful. The port size should Remain the same or DECREASE the closer it gets to the Valve. This is all the way from the plenum to the valve. If the Ports size increases, air speed will slow at some point, reducing torque and therefore power. If you open up the intake port too much, even if you port match the intake manifold, to the point where its size is greater that the opening of the intake manifold, you will get a power loss.
so in effect - measure the port size you want to open it up too. Measure the port size at the start of the intake runner. Make sure it is Bigger in the runner than at the head. Only then will you see benefits.
More air is good, but slowing the airflow down is very bad. There have been instances where people port matchinig manifold to head have recorded power losses. You have to be careful before starting - and very careful with your measurements. triple checking is par for the course.
That being said - if Planned Plenum runner area (cross sectional) > Planned head port area then its not too hard to do.
First get an intake manifold gasket to use as a template. Mount in place on the head. Mark out the maximum area allowed by the gasket for the port. Do the same for the Manifold. using a carbide tipped tool in a die grinder, grind away the metal between the outer edge of your mark and the open ports. feather it into the body of the runner, both in the head and into the port. Smooth out casting dags. Do not remove any material from around the valve stem support or from the back of the valve in the head. Feel free to smooth out rough spots though. You want the area at the back of the vave to be as small as possible to promote faster air flow.
Do the same with the intake manifold , but here you can remove metal to make the runner volume larger. Ideally, the entire path, from valve back to the mouth of the plenum, will gently taper out, increasing volume as you go.
port matching simply makes the Port in the head and in the manifold exactly the same in size and shape. Least interruptions to airflow that way, as well as least turbulence induced by odd shappes in the path. Do NOT polish the intake side. Polished runners/ports allow fuel to bead off on them, dropping fuel out of the air stream. This is bad. Leave them a little rough, or ideally, with a 'swirl' pattern in them.
Feel free to polish the exhaust side though. There should be stuff all unburnt fuel here (ideally) and the exhaust side is all about letting the hot gases vent as easily and smoothly as possible.
Again, remove dags, and port match if you can do so without creating any weirdness in air flow. Some people cut away the Valve guide into an arrowhead shape to maximise exhaust flow.
Sure you can do it at home it's easy, But for a mild motor all you want to do is thin down the vavle boss's and a bit of smoothing in the bowl area. There's no need to go closer to the intake manifold than that or touch the chambers (can be important but not for a first go since it will throw your compression out of whack). You wont be able to polish the ports good enough to be a problem on a first go anyway no matter how hard you try as that takes alot of skills. Straight off the grinding stone or carbide cutter will be good enough. There's pictures of mine in my thread if you want to look as some 5 litre ones, a good valve grind job is most important too. The main aim should be to increase the flow as much as you can while removing the least metal as possible and only from the restrctive parts of the port. They are in order from most restrictive to least
Beyond that doesn't matter unless you are making serious power or have an especially crapy head (not the case for the ecotec)
Hmm thanks guys, good info. I might give it a go, i'm very technically minded but when i look at pictures i'm not sure exactly what i'm looking at. I can see some of the ports in your heads greenfoam, but other pics i'm not sure what's been done. I need a before and after picture. I'll give it some thought, perhaps i'll just go with the port matching of the head to the manifold. That's fairly safe right?
Also, greenfoam, did you notice any power increase when you did your heads, or were they done at the same time as other mods?
The heads aren't on the car yet, wont be untill later in the year. I don't expect much gain as the 5 litre heads are good allready but it all adds up and I'm looking for over 200rwkw so I need alot of things to add up. If you got back a page in my thread you will see pictures of the standard ports, there's alot of difference. Port matching the heads to manifold is only a minimal gain. not worth doing really, not compared to the bowl area anyways
Ok, so you've rekindled my interest in this. I understand the principle of port/gasket matching and grinding the bosses down, but with the port matching how far down do you feather the grind?
Also, in reguards to the actual port i've got a question. Hope you don't mind but i pinched one of your photos greenfoam so i could illustrate what i mean.
So the red area is the original intake port right? The green area is what could be potentially opened up to match with the intake manifold? If that is the case, why is A the only area that is removed, why not remove B too? Also how far down do i feather this port?