Unshrouding the Valves and relieving the bores
In both types of head the valves are very badly shrouded, and the bigger the valve you use, the worse it is shrouded. But with a stock or very mildly modified head the shrouding is a surprisingly minor problem - the stock ports flow so badly that unshrouding the valves has a negligible effect on flow. Once the ports have been tidied up though, further gains are definitely possible through unshrouding.
At the outer side of the chamber it is impossible to get much more than about 0.1" of clearance with big valves, so the shrouding effect can never be entirely eliminated. On the port side of the chamber it's fairly easy to grind some clearance but to get the ideal means removing lots of material and therefore grinding into the water jacket is a risk. The spark plug side of the head is where most of the flow will be at higher speeds, and again a lot of material will have to be removed so be cautious and study your cut-up head carefully. Use a head gasket as a guide to how far you can take the chamber wall. Of course this makes the step or ledge at the top of the cylinder bigger but the benefits of reduced shrouding far outweigh the increased turbulence from the ledge. Ideally the wall immediately around the valve head will slope back about 45 degrees, so that the distance from the wall to the valve is always about the same as the distance the valve is off its seat. In practice you mightn't be quite able to achieve this but it's critical that you get it as good as you possibly can. If you get the chance to run tests on a flow bench you will see that even minute increases in the distance from wall to valve will improve the flow (with a good port). If you are fitting oversize valves consider using eccentric valve guides to shift the valve towards the centre of the cylinder. Even if you only shift the valve say 0.040" to 0.060" you will pick up useful gains in flow. Again though, the water jackets around the bowls may be a limiting factor.
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| Unshrouding large valves requires removing lots of material so take care to avoid making the chamber walls too thin. |
Relieving the Top of the Bore
The chamber in the head is wider than the cylinder bore, so this means we have a ledge or step on each side that the flow has to negotiate on its way in or out of the cylinder. If you lay a head gasket on a cylinder head you will see that the openings aren't perfectly round, and match the shape of the chamber. Now lay the gasket on the block deck, and you can visualise the step or ledge under the chamber. Obviously the smaller the cylinder bore, the bigger the ledge, and it's a good reason to use the biggest available bore size. Practically every hot-rodded Holden six in the country has had chamfers ground into the cylinder top in order to ease the flow around these steps. I've done it, you probably have too, but I'm not sure that it's a good idea.
Let's backtrack a bit. These ledges are actually quite common in GM engines; go back through the years and you'll find plenty of straight and vee type engines featuring chamber overhang. The funny thing is when you look at the valve sizing and spacing, the chamber doesn't have to overhang. In other words the step is intentional. Why? Well, a sharp edge like this is extremely effective in shearing wet flow into a nicely atomised mixture, and I guess this is why it was incorporated. Remember that even an injected engine will have wet flow here.
The other thing is that adding chamfers adds surprisingly little to the flow. Whack a decent head on your flow bench with a plain 3.62" adaptor and check the flow. Now grind two chamfers into the adaptor and check it again. With very good-flowing heads there is a very small improvement, but with anything less there is virtually no change. And even where the flow does improve, does it make up for the loss of mixture preparation? I don't know, I just know that I rarely bother with the chamfers anymore, certainly not with a street car anyway..
If you do decide to do this I'd be careful not to go too deep. The chamfer will definitely expose the part of the piston above the top ring to a lot of heat so I'd be wary of going more than about 3mm deep, depending of course on the top ring position.
Relocating the Head
You'll probably find that the valve will still be slightly shrouded on the spark plug side even if you grind back to the gasket. What you can do to improve this is move the head towards the manifold side of the block by about 2mm. You will need to elongate the dowel holes in the head (don't just yank the pins out because they are needed to accurately locate the gasket) and also the bolt holes. Provided you don't get carried away you won't have too many dramas getting the water holes lined up, but don't forget to check pushrod clearance through the head. The extra couple of millimetres that can be ground from the sparkplug side wall will improve flow slightly and will also help enable further gains from port work. Realistically though, the gains will be very small.