Heads: Port size and valve size question

[82SC]

Well I have a question...what makes a head flow better...it is the port size, the valve size or both?

Maybe I am confusing the terms or don't understand them correctly...

if I increase my port size do I also have to increase my valve size or are these 2 independent things...

My dilema is that I was planning on a 3.4L engine and I bought a set of 3.2L heads...

well talking to the Supertech guys...the intake port flange is thinner on the 3.2L heads as opposed to the 3.0L SC heads...which makes it hard to put an MFI port on them

But I bought the 3.2L heads because they "flow" better...and the 3.2L head's combustion chamber is supposed to be better...

can I make the SC heads as good as the 3.2L by increasing the port size? (BTW they are 82 SC heads)

anyone have advice?

[jluetjen]

The answer is.... It depends!

The simple answer is that both the valves and the ports have to be able to flow enough air to keep the engine fed. How much air is really a function of the engine size and speed. In round figures, your engine will need 3.4 liters of air every 2 turns of the crank shaft. If you are going to spin your engine to 6500 RPM, you do the math. As long as your valves and ports can deliver that amount of air, then they're adequate. If either is to small, they will limit your horsepower. As a rule, stock 911 ports seem happy pulling gas speeds of 70-100 meters per second. That is a pretty wide range. 911 valves tend to be pretty big, so I doubt that they are gating you.

In the hot rodding world, there is a lot of "black magic" to porting. Some of it is real and a lot of it is a function of the basic engine design. For example:

- Detroit's water cooled V8's often have some severe compromises in the heads. Space is required for the water jackets and push rods. The geometry of the V8 also means that the intake passage needs to change directions and the single shared carb also adds complexity. Getting those ports to flow the required amount can be really complex considering you can't poke a hole in a water jacket, you have unequal length intakes and the ports bend. Very little of that applies to a 911 - there are no water jackets or push rods. The ports are almost a straight shot into the valves.

- Detroits V8's generally have bath tub shape combustion chambers. The good news is that they have qood squash which promotes combustion. In stock form these will often shroud the valves. There is a real art to the porting of these engines since the shape of the intake port and proximity of the valve to the wall of the combustion chamber can often promote swearl in the combustion chamber. Once again, given the 911's Hemi head, a lot of this doesn't apply. The valves in a 911 are generally more then adequate. Since the ports are pretty straight, I've come to the conclusion that as long as you keep pretty close to the stock shape and finish, you can't go too far wrong. Keep in mind that Porsche got their standard 2.2S port to generate at least 210 HP in the 2.7 RS. I've checked some 2.2S heads that I have and measured the port shape and diameter moving in from the intake manifold. I got the following readings:

Manifold Face: 36 mm circular
1/2 inch in: 37 mm circular
1 inch in: 38 mm circular (the same as a 2.8 RSR BTW)
1.5 inches in: 39 mm circular
2 inches in: 40 mm circular

It doesn't seem too radical to me! Nor is their any bias in the shape, they are pretty well centered on the valve. I haven't had the chance to measure some T/E heads yet, but I'd be interested to see how different they are. You might want to do the same thing with your existing heads. Maybe someone here could do the comparable measurements on 3.6 to see what if any the differences are.

Black Magic: There are some things that can be done to the valve shape and valve seat area that can sometimes have an affect - or maybe not. As a rule a 3 ange valve job is recommended, but 911's already have that. Another thing that can be done is to reduce the width of the 45 degree angle cut which can sometimes improve flow. Some people swear by more then 3 angle valve jobs, some people say they don't work. You decide.

Does anyone some measurements on 3.6 liter heads? 82SC, what are the measurements on your heads?

[KNIGHTRACE]

The best cut on the valves is a radian cut. It flows about 7% more over the multi angle cut. not many machine shops have the tooling for this. The stock heads work great on a 3.4 liter motor. as far as putting mechanical injection on them there shorter and I would ask Henry at supertech what he wants but if you open up a 3.0 head to same as 3.2 by the way this is a $1500 job done correctly on a CNC machine. The hand porting does not work in my opinion, and also hand port jobs need to be extrude honed to match them to each other again. I have ported out big port 3.0 heads and had them extrude honed to beat 3.2 heads but not with gains for the amount of money. you could have extrude honed 3.2 heads and been better. The best set up for the money is the 3.2 head with an american valve radian cut on intake and stock exhaust valve. shape of valve more important than size. Hope this helps, William Knight