Head porting and flow restriction

[bodhi808]

I've been reading a very interesting little article on another website about porting cylinder heads and associated flow restrictions.
You can read it here http://www.pumaracing.co.uk/Turbo.htm
Fortunately for me it's written in a language that i can understand, but there are a couple of things that hopefully some of you guys can help me to understand.
I understand the idea behind porting the head to improve the air flow and reduce any restrictions to that flow, but at what point does this persuit go too far? If it were simply a case of making the intake as large as possible, then everyone would fit gigantic valves and intercooler pipes would be a smooth and as large as possible, which in most cases they are not.
The larger intake cross section has an influence on flow velocity of the air, much like the example in the article, blowing through a straw has a high flow velocity through the small cross section and due to the restriction you can puff your cheeks out. Without the straw, the flow restriction is so small due to the larger cross section, that you can blow without puffing cheeks out.
Back to the engine scenario, when the velocity of the air flow becomes too low, then I'm guessing that the fuel and air doesn't mix correctly, and that the intake system in general would be quite laggy (in a turbo application). I'm guessing that there are other impacts too here.
Is this correct, and if so, how is it best to assess the degree to which you would enlarge valves and intake piping?
Cheers
Chris

ETA I've just noticed that the website posted belongs to a member on here of the same name. Great site!

Edited by bodhi808 on Wednesday 2nd September 13:00

[banzai_dan]

Larger diameter intake pipes give better top end power, but sacrifice some at lower revs; and vice versa. Same goes for valves.
Maximum valve size is constrained by the shape of the cylinder head, so you can't often use valves that much bigger than standard.
As to how much to enlarge by, its a dark art that uses fluid dynamics, experiance and a bit of guesswork.

[Pumaracing]

It's very important to maintain a high gas speed in the ports because the more kinetic energy the gas has the better it rams into the cylinder, especially as the intake valve is closing when the piston is already rising up the bore and trying to push the gas back out again. Finding the optimum point between increasing flow and reducing gas speed is too complex a subject to explain in a simple posting but as a given you can say that any time you remove metal from a port and don't see an increase in flow on the flowbench then you've taken a backwards step.

As a rule of thumb most road/mild competition tuned or turbo engined heads with mild cams don't require the ports to be bigger than about 75% of the diameter of the inlet valve. For full race engines with very high lift cams this might need to be as high as 80%. On the exhaust side things need to be a bit bigger. 80% for mild engines and 85% for race ones. However it differs on every engine and only the flowbench and years of experience will give you the exact answers.

Above all the shape and angles of the valve seats and valve throats are most critical and often account for 50% of the potential power gains in just those limited areas. Just hogging metal out of the ports blindly rarely gives huge gains. Everything has to work together. In the port itself the most critical place is usually the shortside bend and the radius you shape that to.

I did used to have an article on porting heads and cutting valve seats properly on the website but everyone said it was giving far too much away to other tuners. Now that I've retired I might put it back. However if you work your way through all the other existing articles you should get at least a fair picture of what is needed to make an engine work properly and what sort of gains are possible.

Dave Baker
Puma Race Engines