I have searched at length all existing forum threads with regard this subject and cannot find a reference to my specific query, which is,
?What controls the sealing face width for exhaust and inlet valves?? (I am advised that the target contact width is 1.5mm for inlet and 2mm for exhaust valves.)
Question 1: Is it the manner upon which the seats are cut or the manner upon which the valve faces are machined?
At first this may sound like a dumb question, (and I am sure there are some clever guys out there who will confirm this), but it is my understanding that the 3 angles on the inlet valve seat insert, (specifically the 30 and 60 degree cuts) will dictate the final valve seat contact width, as you will be simply reducing or extending the 45 degree cut face within the seat.
For the exhaust, there appears to be much differing opinion whether a 3 angled seat should be generated, and if so, how do you control/adjust the valve seat contact width at 2mm on the exhaust valves?
Question 2: Do you face the valves to achieve the desired contact width?
This question has come about because I am in the process of rebuilding a ?big valve twin cam? for which I believe the engine, other than a modified inlet cam shaft to drive an alternator, is identical to that found in Elan S130. Having recently received the valves my attention was drawn to the valve seat faces.
Please refer photo, from left to right; new exhaust valve with an approx 2mm machined seat face, new inlet with approx 1.5mm machined seat face and finally one of the original inlet valves with a very generous valve seat face of approx 3mm + (Note: I found these faces difficult to measure accurately)
Question 3: Personally, I think the new inlet valves should have a larger valve face width, or am I wrong?
Question 4: It would appear that on the Twin Cam Big Valve Heads, (Note: I reference the head ONLY), the only difference is the inlet valves which are approximately 34 thou larger in diameter ? is this true? And if so ? seriously???
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some decades ago having a larger seating area was customary, less so these days, which I understand may come from improved metalurgy of both valves and seats in order to achieve a better compromise between the functions of the assembly (heat transfer, wear, sealing - e.g. larger area gives better heat transfer but worse wear and sealing)
On my twinks I personnally prefer about 1.5mm width for the sealing area on the exhaust, a bit less on the intake (they are cooled by the fresh air). My machinist leaves the seats with tiny circles from machining, and is adamant of not lapping them flat.
Some people machine valves a bit to improve flow, but that would not be the seating area, rather a multi angle equivalent to the valve side (only on race engine for me). The valve seating area is obviously a critical part of a combustion engine, esp. if you want to make them most of it (average and transitional flow regimes are not as obvious as they may seem).
As for selling tricks and gimmicks, big this, race that... all that matters in the end is how the car runs - when it runs...
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