Author Topic: Head work  (Read 7035 times)

Offline Alex

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Re: Head work
« Reply #15 on: December 23, 2008, 11:14:48 am »
Well yeah, this is true, but I've wound up learning almost everything I know from the internet. Nearly nine years on forums has been quite an education - though not always for the best. :D

And it's almost always done me well to follow the guidance of people like you guys who know a shit load about the car I drive. To be honest, it's helped me guide mechanics and experts a few times, and these are people who supposedly know cars.

I'm still a little fuzzy though. It stands to reason, as far as I'm concerned, that post combustion you'd want to get rid of more gas than comes in. Yet, from what I now understand, exhaust valves are always smaller than inlet valves. So Frode's measuring an inlet valve on this, right?


Offline Yoof

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Re: Head work
« Reply #16 on: December 23, 2008, 05:49:21 pm »
Don't get too hung up with regards to stock sizes. Often an OEM will only 'make do' with what they can to create the power needed. Also cam specification has a large influence on the sizes of valves, namely lobe seperation angle and overlap- but lift and duration are worth being aware of.

On a supercharged car (as stock) the inlet duration of the cam will be far less than the exhaust duration (typically 30' or so) so for any one given cycle you're opening the inlet valve for 'x' less milliseconds. G40s also where one of very few cars to use sodium filled exhaust valves, mainly for heat dissipation (the sodium turns to liquid inside the hollow valve and splashes upto the valve steam/ tip and radiates heat through the guide- which with polo ones isn't always a good thing either!)

Anyway, back to my point. The exhaust valves may well be smaller than the inlet valves on stock heads, but remember the original product design spec for a PY block would have been 113bhp- for as cheap as possible. Not 170bhp+ with only performance in mind  ;D


Offline hayesey

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Re: Head work
« Reply #17 on: December 23, 2008, 07:13:14 pm »
and don't forget that VW didn't design the head for a G40, they just used the existing NA designed head off the 1.3 CL, GT etc... put sodium filled exhaust valves in and a new cam design.  No doubt they found it produced the power they wanted along with lots of low down torque and didn't see any need to improve on the design as it worked well enough for the requirements of the car.  That doesn't mean it's any good when you want to get more power out of it,esp at higher revs and speeds.