Question about Oversize Valves

[NightRyderNy]

Was curious about what the advantage to having oversized valves was? Was also curious about what would be have to be done to the engine in order to accomodate the oversized valves (machining process etc.) What are the pros and cons to the oversized valves and any other information. Thanx in advance.

[Ezekial]

Was curious about what the advantage to having oversized valves was? Was also curious about what would be have to be done to the engine in order to accomodate the oversized valves (machining process etc.) What are the pros and cons to the oversized valves and any other information. Thanx in advance.

more air. so you can run more fuel. bigger bang i'm sure you know the rest.

but then you need to calculate the port area to see if there is sufficient gains to be had from increasing valve size.

machining of the new valves, existing seats, and combustion chamber

in multi valve pent roof cylinder heads, there isnt alot of room. between the valves. some people manage to squeeze in 1 mm bigger valves.

[Q45tech]

With DOHC and 4 valves, the valve sizes are already for the most part too large diameter for low rpm operation especially when you have an expert make the paths PERFECT and flow matched.

NASCAR engine builders devote a few DAYS per valve in optimizing the flow at low lift conditions.

Few people want an $8,000 .........$16,000 [16v V8] valve clean up job.

If you tulip the intake stem you can gain almost as much for a lot less dinero.

[95lstegman]

i was going to say the same thing. you can oversize them, but the 5-7% gain in area/flow you'd see from 1mm oversize valves would probably be killed many times over in the losses associated with the flow from those valves running into each other and causing more turbulence than they already do. you could carefully deshroud the valve seats in the combustion chambers, but i don't think even the best deshrouding would negate the losses of simply having the valves too close together and to the edges of the cylinder wall and combustion chamber edges.

now, using stock-sized valves, but made of stainless steel, can be very good for engines that will see a lot of heat stress, be it high HP or endurance type racing. but still, a little grinding of the back of the valve face by experienced hands can create significant gains, although then you'd have to make sure each valve is installed in just the right orientation.

[7thGear]

this is a long read, but well worth it

http://g-speed.com/pbh/cylinder-head-tech.html