No life (a.k.a. DattoMaster)  
Joined:
2008/10/10 22:02
From Melbourne Australia (and likely under the car)
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If you could find a gx head, they are the ones to mod, with arguably the best flow potential, but they are difficult to find and relatively expensive. There's a couple of heads for the a14/15 that are only modestly less in full flow potential (but you won't be porting either to the max anyway, so it's not as big a deal) but you want to look specifically for the closed chamber head version. This will give you more compression from the get go, but additionally the closed chamber, which results in decent quench areas, will allow you to run higher compression ratios than you could on an open chamber head, all else being equal, so it is surely the go.
As to increasing the comp via block/head machining, ideally you'd want to dummy assemble the engine and see how far up the pistons come at TDC, and if they sit below the deck get some careful measurements done, and the block can be decked to the point that the pistons are dead flush with the block deck. Theoretically with a small bore like thia and short enough stroke, and a combo in general that won't see astronomical rpms (that cam will rev out, but not quite full race territory) with all those factors, you could probably get away with the pistons sitting as much as 5 thou above the deck. But for a streeter you don't need to push it so close to the edge where piston to head contact is only just being avoided. The higher the piston is in the block at tdc (but again, don't go above deck height) the more it closes up that quench gap, and that leads to the highest possible comp ratio being safe with a given fuel. The further you drop the piston, the bigger the quench distance, and the less effective it is at what it does (which is mostly to improve mixture motion and hence lead to quicker cleaner more consistent combustion, so that the whole lot burns early, and max power is extracted, and since the burn is over sooner, it doesn't continue to heat the exhaust valve up as much on it's way out the open ex valve and out to the exhaust, and all that = best possible power (even if only by 2-5% at best) and most resistance to detonation.
SO based on that, the first thing is to meaure and machine the block deck. Then CC the head and do the math and calculate the exact comp ratio and work on it from there as to whether or not the comp ratio can be safely (and beneficially) raised, and then and only then do it by machining teh head. Obviously don't go too quick, better to need to machining cuts on teh head, to creep up on the required CR, as it's a lot easier to machine 3-5 more thou than it is to work up a band-ad fitx if you take it too far with the first cut. There are options if that were to happen (head is machined too far) but none of which are ideal
The only issue I see potentially is the fact that if you run the std a12 head, for whatever reason, then to run the dellortos on it, you'll need an intake manifold to suit the round port heads, around 3-400, then you'd need an oval port suitable intake, for another severaql hundred bucks so your costs go up to per
severe with the round port head, so you're lookin ok, Aside from the manifold costs, with teh changes in flow rates due to the head swap, it'll mean you'd have to recalibrate the dellortos to suit, again a doubling of the effort and expense. An alternative might be to considerleaving teh dellortos off it and ideally only fitting and tuning then once that last head swap is done. just an idea.
Posted on: 2014/6/3 2:20
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