over size valve curiosity

[datsunfish]

I have over size Z intake valves on the head in my 510 but alot of other crap going on too so not sure really what sort of enhancement they have compared to if I had stock.

I have heard of using the 210 head intakes as exhaust valves but when I test fit some with the big intakes the seats would have intersected or at least has very miniscule meat between them.. The stock intake valves on most L heads are pretty big compared to the exhaust so my main question is which would have better over all results if it was one or the other? Mainly just curious. Seems to me that larger exhaust mani/header and pipe is futile if the exhaust valves regulate the exit flow.

[datzenmike]

The limiting thing with intakes is that the air is only under atmospheric pressure or about 15 PSI. (compared to a vacuum) This means that no matter how hard a piston pulls down to draw it in, it has an upper limit. Intakes should be as large as reasonable so they do not obstruct the flow. Exhaust on the other hand even after having pushed the piston down to do some work is still at several hundreds (if not much higher) PSI and it seriously want OUT!! The exhaust valve can be much smaller and will still work fine.

 

Generally heads breath fairly well at lower speeds and good enough at highway speeds so increasing the valve size helps only the very high top end.

[RoadRace]

in my experience, valve seat detail, IE width, angle , for-aft angles are more critical than the diameter of the valve itself. guides are also critical to accuracy of airflow around the seat, IE loose guides = wobbly valve = irratic air flow around the valve and seat. take care of the seat area and you'll be in good shape.

[hang_510]

bigger valves arent going to help either, if the ports arent increased as well...

[WMR]

I have over size Z intake valves on the head in my 510 but alot of other crap going on too so not sure really what sort of enhancement they have compared to if I had stock.

I have heard of using the 210 head intakes as exhaust valves but when I test fit some with the big intakes the seats would have intersected or at least has very miniscule meat between them.. The stock intake valves on most L heads are pretty big compared to the exhaust so my main question is which would have better over all results if it was one or the other? Mainly just curious. Seems to me that larger exhaust mani/header and pipe is futile if the exhaust valves regulate the exit flow.

 

 

remember that the exhaust is 80 t0 90 % efficient even with the smaller valve. the piston is pushing the charge out and it is expanding at the same time. With a well designed header you will get a scavinging effect from the other tubes actually helping to pull the spent gasses out. read an artical on 2 stroke pipe design, it can be very enlightening.

WRM