Head work

[Alex]

I'm looking into getting my head ported, along with an uprated cam and Vernier pulley, at some point in the new year. And, like most things, the more I read mixed with what I've picked up over the years is confusing me.

I want to make sure this is done right. Obviously it won't now be done by PSD, so I want to make sure whoever does it is going about it the right way.

What I've always understood is that the PSD big valve heads had considerably larger exhaust valves compared to the inlet valves. I've got memories of seeing one at PSD and asking Steve, who said something along the lines of most of the porting is required on the exhaust valves.

This, to my mind, makes sense. Pressurised air in, which then combusts, means compared to a N/A engine there'll be a lot more exhaust gas to get out. I trust Steve's professional opinion on G40 engines, as I reckon there are few who know them as well as him, but having talked about this on other forums it's being questioned.

The thing which concerns me is this:
http://www.pumaracing.co.uk/Exvalve.htm

Mainly this paragraph:

The other commonly quoted situation is that of turbocharged and supercharged engines. Ask most people, many "professional" engine builders included I'm afraid, and they'll tell you forced induction engines need bigger exhaust valves at the expense of smaller inlet ones. Well after all it stands to sense dunnit mate? The inlet side has a thumping great turbo pushing the air in and the exhaust side has nothing extra to help it.

Utter bollocks I'm afraid and anyone who tells you this can be discounted as any sort of authority on how engines work and probably best avoided if you're looking for someone to spend your money with. When you turbocharge an engine you increase the cylinder pressure in direct proportion to the boost because you've crammed more air into it. This extra cylinder pressure directly increases the exhaust valve flow when the exhaust valve opens.

In effect everything stays in the same proportion as when the engine was normally aspirated. The turbo basically boosts the exhaust valve flow in exactly the same proportion as it boosts the inlet valve flow although the relationship, being one step removed, is too remote for most people to appreciate it. So forced induction engines actually follow exactly the same rules about exhaust valve size as normally aspirated ones.

I'm lost. Anyone care to shed any light on this? Is it a problem specific to G40s that they have exhaust valves which are too small?

[hayesey]

they use 2mm bigger exhaust valves. I'll be getting mine done by the bloke who does jabbasports heads and lots of others for other VW companies plus a lot of work for motorsport.  He'll only do the headwork though as in you post him a stock head and he'll port it and fit bigger valves etc.. 

[Alex]

So are the inlet ports the same size as standard?

[SamG40]

I dont think there is room on a G40 head to make the inlets bigger only bring the outlets up to the same size anyway. Pete has one he is about to flog I do believe already built up Alex. Might be worth giving him a shout.

[hayesey]

there is room for larger inlet valves and he said he has tried it but found the gains to be very small.  He said it's not worth the extra money it costs but has done it in the past for people who insist "bigger must mean better" or to whom money is no object.

[Alex]

Cheers for the heads up Sam, but I'd be getting a very, very special price on this if it goes ahead. But I'll see what happens first.

I assume there's no point getting an uprated cam without head work?

[hayesey]

I wouldn't say no point but head porting will mean you reap more benefits from a different cam profile.  The port sizes on a g40 head are a limiting factor.  I know in my car when it had it's standard head, you get to a certain speed and it feels like there is something holding you back and acceleration drops off a lot, basically it's reached the max flow the head ports can cope with and possibly limited by the standard cam too.

[Alex]

Yeah, no point is wording it a bit strong

I'm not actually sure, from what I've been offered, whether they're looking to increase the valve size at all. If it's just the inlet/exhaust ports done, but with stock valve sizes, would that offer much in the way of gains? Or do you need to have the whole lot done?

[hayesey]

yes there are still plenty of gains to be had with enlargening of the ports but keeping stock valve sizes.  Just seems that if they've got the head you might as well get them to do the valves too, although it will add on a fair bit more money.  I think pretty much the only place to get the valves from is jabbasport, the bloke I spoke to did mention the name of the company that actually makes them but I can't remember what they were called and he seemed to be under the impression that he'd have to buy through jabbasport anyway (which seemed a bit odd to be honest).

[Alex]

Money's not really a factor - I guess I'll have to see what they're game for.

[hayesey]

Can I ask who you are using?

[Alex]

Yeah, QEP.

[Dan]

I wouldn't take the Puma guide in context with the G40 head too much. The problem with these heads is the exhaust side of things, the ports are a bit of an awkward shape. Since there is little room to increase both the Inlet and Exhaust valves (unless you offset the guides) then it is worthwhile to get the exhaust the valves bigger (without making the inlets smaller). Then modifying the exhaust ports to deal with things better makes the difference.

I've had a couple of heads done in the past and the detail work in porting varies a lot. I guess you have to make your own mind up about who to use.

[Yoof]

What that 'expert' conveniently forgets is the conversion event during the combustion cycle of fuel from a liquid to a gas (rapid expansion) and obviously the large increase in volume that needs to be expelled. If you can't imagine it think of the volume required to hold 1 litre of water, and now convert that water into steam (makes around 1600 litres if my memory isn't fried).

Now that's not strictly true for the petrol liquid>gas event but hopefully you'll be able to picture it a little better.

There are several 'experts' out there, very few have the actual ability to use their own brains!

What goes in, isn't always the same as what comes out! hence the need for larger exhaust valves.

[hayesey]

yeah you can read the opinions of Internet-experts all day long and trying working out theories etc.... but at the end of the day it's well proven that bigger exhaust valves reap benefits on G40s so that's all that's worth bothering about in my opinion.