Valve modification

barncat

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doing some headwork... after lapping a valve in, is there any good reason not to grind the diameter of a valve down to close to the edge of the gray zone to increase margin thickness and put a bit of radius on the edges? this would improve flow, but not sure whether it's a good idea metallurgically.

one downside is there would be no reusing the valve 30K later...
 
doing some headwork... after lapping a valve in, is there any good reason not to grind the diameter of a valve down to close to the edge of the gray zone to increase margin thickness and put a bit of radius on the edges? this would improve flow, but not sure whether it's a good idea metallurgically.

one downside is there would be no reusing the valve 30K later...
It's best to start with an oversize valve to gain a thicker valve margin. There are no gains to radius the intake but the exhaust you'll see gain by a radius on the bottom corner edge by .030 to reduce resistance around the thicker margin.
 
I've wondered this too. Done the same thing in the past, rounding the edge, since I don't like sharp corners of any type in the flowpaths and combustion chamber. However, there are some aerodynamic reasons for having sharp departure angles leading away from a droplet-infused flowstream, like those found on the aileron trailing edges of Grumman AA5 aircraft. The Grumman engineers told me it has something to do with precip fallout, condensation on the trailing edge, and flow stability. A complex and confusing subject, and I don't have enuff rocks to throw at it...
 
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One of the oldest tricks in the racing books is to increase valve margins to enhance flow stability by reducing resistance which can be heard when flowing. The XS head responds variably to these tricks and the exhaust likes a tulip shape valve.
 
thanks for the input Jack and 2M. was out all evening... scored a couple vintage Suzuki and chrome Hodaka gas tanks in nice shape for latest builds from local bike salvage yard.

current motor work is an '83 GR650. will post a couple pics soon in another thread i started a while back. did a fair amount of porting, and both intake guides were cracked so i replaced those. do not have seat cutters but they lined up VERY close to original spec, so trying to get the seal back with lapping. it's working out, with one more touch up pass tomorrow, but there is an approx .0005" tall x .010" wide lip on intake valve face circumferences. this is splitting hairs, but i think i'll grind that .020" off the diameter right back to the gray band. that will increase the margin slightly, probably allow me to get a total seal (about 30* of arc was letting a few soap bubbles form with compressed air blown in ports), and i'll just dust off the bottom edge of the IN valves rather than radius them, based on both of your suggestions. the EX valves are fine and lapped in quickly, and already have a factory radius on bottom edges.

fluid dynamics... complicated subject but makes intuitive sense. you don't want air/fuel mix bonding to intake port walls, hence the rougher surface finish to keep the boundary layer moving. and over radiused valves could reduce some desired turbulence and dispersion as you mentioned 2M.
 
all went according to plan. intake valves are now 39.5mm instead of 40mm, margin thickness roughly doubled, and got a good seal with seats. did it offhand on the benchtop belt sander rest. gotta be very careful but since it's hard steel very little material is removed per rev... this head should breathe really well as ports are more modern design to begin with.
20160623_135604.jpg
 
40mm down to 39.5mm? That works out to a 2.5% reduction in flowpath impediment cross-sectional area.

Whatever that means.

I'll be listening northward, for any loud 'splosions...
 
my guess is that will increase gas velocity nicely in conjunction with all the porting and the 34mm VM's and planned custom exhaust (see adjacent thread just updated- nice combustion chamber shape). it's a titled bike that only set me back 300 bucks, but was filthy and needs the usual 150+ hours or so labor for my builds. looking forward to the maiden voyage but that's weeks away... i'll try not to blow it up :)
 
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