A numbers exercise . . . . . .midget,As promised the valve diameter and area calculations. And yes, I was a bit surprised. The traditional thinking is: "A four valve HAS to be better than a two valve!!"
Really? Some number juggling:
BMC 'A' series:Bore diameter: 2.815"
Intake valve diameter: 1 x 1.475"
Intake valve area: 1.7087 sq. in.
Int valve dia/bore dia: 52.4%
Int valve area/bore area: 27.46%
Exhaust valve diameter: 1 x 1.150"
Exhaust valve area: 1.0387 sq. in.
Ex valve dia/bore dia: 40.9%
Ex valve area/bore area: 16.69%
Rover 'K' series:Bore diameter: 75.5mm (2.972")
These are the stock sizes. REC has slightly oversize valves available.
Intake valve diameter: 2 x 1.089" (27.67mm)
Intake valve area: 1.863 sq. in.
an 8.3% increase over the BMCInt valve dia/bore dia: 73.3%
Int valve area/bore area: 26.86%
a 2.2% decrease over the BMCExhaust valve diameter: 2 x 0.949" (24.10mm)
Exhaust valve area: 1.415 sq. in.
a 26.6% increase over the BMCEx valve dia/bore dia: 63.9%
Ex valve area/bore area: 20.40%
an 18.2% increase over the BMCREC 21-4N Stainless Valves
REC #739
Intake valve diameter: 2 x 1.161" (29.5mm)
Intake valve area: 2.1173 sq. in.
a 19.3% increase over the BMCInt valve dia/bore dia: 78.1%
Int valve area/bore area: 30.52%
a 10.0% increase over the BMCREC #740
Exhaust valve diameter: 2 x 1.023" (26.0mm)
Exhaust valve area: 1.644 sq. in.
a 36.8% increase over the BMCEx valve dia/bore dia: 68.8%
Ex valve area/bore area: 23.70%
a 29.6% increase over the BMCI did not use any comparison percentages for valve dia/bore dia between the two engine types. Although the comparisons are valid two valve to two valve, and four valve to four valve, my own opinion is that it is not a valid comparison two valve to four valve. I use valve area/bore area as a valid comparison.
REC also has 21-4N valves in the stock diameters, and the shapes are probably better for flow.
It is possible that the increase from the larger REC exhaust valve might not be necessary, but the head needs to be flowed before this can be determined. Flow increases on the order of percentages that are seen here, would allow a "softer" exhaust cam to be run without penalty.
The best choice could be to fit the larger, aftermarket valves, even if they need to be cut down slightly on diameter to fit the existing valve seats. The larger valves would also allow "reshaping" of the valve seats (and possibly the valve seat angle) to a "radius" style seat form. JMO at this point, I still need to flow the head and analyze those numbers . . . . . . .
The takeaway here is that it is easy to be fooled into thinking: "Well, this just has to be better." It's why you should always, "Run the numbers". Increases in the number of valves might be negated by the increase in bore size. As it turns out, limited valve size and the resultant reduced flow available is a well known problem with the 'K' series. It probably is not going to be an issue at one liter of displacement, but is always better to find out ahead of time . . . . . .
And yes, before somebody chimes in, the first comparison here is between a race developed head, (the BMC), and the stock Rover head, not really a "fair" comparison. But, once again, we are trying to make a silk purse out of a sow's ear . . . . . . . . . on a budget . . . . . . . .
The budget for refreshments needs to be increased . . . . . . . . . . . . . .
JMO
Fordboy
edit: Checked my math and I had to do some edits . . . . . . Rechecked my math after prying my head out of my a** and corrected a second time!! I gotta write a spreadsheet analysis program for this. These hand calculations and number transpositions are driving me nuts . . .