Oversized valves

[DMN]

Has anyone used oversized valves for their 250f, and if you have, what were the results? Second, what's the deal with Kibblewhite valves. Are they really junk? I know that they had a bad batch, but is that still the case? Thanks.

[Racer24]

if they are stainless steel they're are really durable and last long, but they are heavy, so preformance isn't top notch, personally i like the titanium valves, but the only problem is tht i have to do more valve jobs, but w/e havent done one on my new kxf...just broken in.

[DMN]

I was wondering more about the performance advantage of bigger valves.

[charles t]

an engine is nothing more than an air pump. It sucks air in and it pushes air out. In the mean time it explodes the air. simple rule, the more air you can flow through an engine (larger valves) the more power it will make. there are however some fairly important reliability factors to consider. the distance between the valves, head thickness, squish area, the list is long. my opinion is if you need more performance, spend you money on porting, valve seats (copper) cam profile (more aggressive) increase the compression, I would even increase the bore 2-4 mm before I monkeyed with larger valves. but, yes larger valves will on paper increase the flow through the engine. best bang for your buck, though will be new cams and an exhaust system, if you can afford a pro circuit head, they are amazing.

[mxnate116]

dont do it. unless your running big bore they will most likely just hurt performance and fuel consumption. when you get bigger valves(normally) they slow the velocity that the intake and exhaust flows. this is especially bad in a high revving engine such as these. if you've read the mxa reviews on the new honda 450 they actually made the exhaust valves smaller to increase performance. your money would be better spent porting the head so that the intake runners are the same or closer to the size of the valves as they are now.

[DMN]

Well, here's the deal. I've got an Athena 290 kit , ordered a set of Hotcams (stage 1), and have the head sent out for porting now. I know that the bigger valves will flow more, but I do have your concern about intake velocity. Although since I want the most power from mid to lower top end the quality of the port job might be more crutial then total flow. The head is going to be flow tested before and after to get an actual increase.

[mxnate116]

so what do the hot cams do do they increase lift or duration?? with your setup you could see a performance increase but theres no real way to tell. having it bench flow'd is def a good thing. intake velocity is a biggie but exhaust backpressure and velocity is something you should keep in mind as well. id wait to see what the shop thinks with the new flow charts...

[DMN]

They increase duration, and lift is increased (0.6mm I, 0.26mm E approx). I think that I'll stick with the stock sizes for now, and see what happens when it's time to do the valves. I'll post the flow results when the head is done.