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+--- Thread: Porting (/showthread.php?tid=4052)
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RE: Porting - Greig Smith - 22-03-2020
I did quite a lot of work on the ports of the block we used in our supercharged single seater, being mindful that less is more, I concentrated on opening out of the ports a wee bit - basically straightening the 'flow' to the valve seats by cleaning up the walls and the bump curve of the walls. The standard Austin port is quite large and opening it out too much will actually have a negative effect as it slows the speed of the fuel charge coming into the port, which then needs to be counteracted by a high lift cam to keep the valves open for longer and allow more fuel past the valve... this is a slippery slope as everything actually needs to be considered holistically, head, block, intake manifold, exhaust manifold and cam choice + follower radius...
Back to the block - the bowl and flow up to the valve seat was next, again open less & rather clean more. I used a rotary die grinder and then strips of emery cloth and my finger, the object was to remove the casting lumps & nodules of cast iron. You don't actually want a mirror polish on the intake ports as the wet fuel in the charge clings to the walls & slows down, instead you want tiny radial lines which cause tiny pressure waves and effectively keep the wet part of the fuel air charge in suspension. Basically like the inside of a fine thread nut, although not as pronounced as actual thread ridges, instead radial scratches do the job perfectly. You ideally want a mirror polish in the exhaust ports to stop carbon sticking.
The maximum gain is actually from matching the ports, both inlet & exhaust and the gasket in between - the gasket should be max 0.5mm / 2- thou smaller than the matched ports and in theory the 'squish' of clamping the bits together should fill this gap so that the port-to-manifold transition is seamless.
Another trick is to make a port divider or port separator. This is effectively a shaped flat section of 1mm / 40 thou steel that fits simultaneously into the intake port and extends into the intake manifold. Great car needs to be taken here to get a perfect fit inside the intake port thus effectively dividing the port into 2 halves. This stops the parasitic robbing of the charge from one intake port by the other due to valve overlap. To fit these wedges of steel, simply make a shallow hacksaw cut in the top & bottom of the port. The divider is then shaped to slide into these cuts. By making a step in the metal of the divider just where they exit the port, the exhaust manifold & gasket effectively secure these 2 dividers into place.
This was part way through shaping & polishing the port
Aye
Greig