madkaw Posted December 20, 2015 Share Posted December 20, 2015 (edited) All the searching I've done on these cylinder heads and I've never read a direct comparison between the 2. How much flow are you leaving on the table by not going to the larger valve? Would building a stock bore L28 with a larger cam, headers, etc.. be undermined by the smaller valve? I should add that I am staring at an MN47 head as I ask these questions. With the D shaped ports that narrow down to 31mm it makes me wonder if the larger valve is still beneficial. Edited December 20, 2015 by madkaw Quote Link to comment Share on other sites More sharing options...
madkaw Posted December 21, 2015 Author Share Posted December 21, 2015 (edited) It's a 4% increase in size, but not sure how that calculates to flow Edited December 21, 2015 by madkaw Quote Link to comment Share on other sites More sharing options...
cockerstar Posted December 21, 2015 Share Posted December 21, 2015 Area of 42mm valve = 5542 mm^2 Area of 44mm valve = 6082 mm^2 Flow is directly proportional to the size of the valve opening, so you're getting about a 9% increase with the larger valves, assuming that the valves are the limiting factor. Quote Link to comment Share on other sites More sharing options...
budgy Posted December 21, 2015 Share Posted December 21, 2015 Not sure how you can calculate overall flow from just this one number because technically the head will flow differently at all different RPM's and bigger valves are better suited to higher RPM action.But in terms of actual size of the valve, when you calculate the surface area of a circle with a radius of 22mm (44mm valve) you get 1520 square mm. A 21mm radius (42mm valve) is 1385 square mm. This ends up being about 9% bigger in terms of area not just the radius or diameter because you are dealing with a circular object. Experienced engine builders would know much better than me, but I am assuming the valve stem itself is actually restricting flow on some small level and because the stems (or the guides) are probably the same size in both scenarios the difference in flow between a 44 and 42 mm valve might be even higher. Quote Link to comment Share on other sites More sharing options...
Mack Posted December 21, 2015 Share Posted December 21, 2015 also, keep in mind, calculating the difference in diameter would give a better indication than the total area, since the middle of the valve is solid and air only flows around the perimeter of the valve face. Quote Link to comment Share on other sites More sharing options...
djwarner Posted December 21, 2015 Share Posted December 21, 2015 Mack is correct. The area of the opening when the valve is depressed in the shape of a cylinder. The area is equal to the lift times pi times the diameter. The increase in cylindrical area would be equal to 44/42 or 104.7%. The smaller valve can easily be compensated for by a slightly higher lift. Quote Link to comment Share on other sites More sharing options...
TimZ Posted December 22, 2015 Share Posted December 22, 2015 With the larger valve you may start running into shrouding issues as well, depending on your combustion chamber design. Quote Link to comment Share on other sites More sharing options...
Xnke Posted December 24, 2015 Share Posted December 24, 2015 You're already shrouded with the stock sized valve. Look at the way the valve moves into the bore, and it doesn't even start in the middle. As the valve advances down and into the bore, it gets closer to the bore wall the whole time. Going to a bigger valve can show some gains IF the port is worked to support it AND you have 89mm+ bores. Want more than that? Move the valve centerline over so as the valve opens, the valve head moves AWAY from the bore. It's not exactly trivial, but it's certainly doable. Quote Link to comment Share on other sites More sharing options...
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