Inliners International Forums General Discussion Hi Performance backcut valves

I was curious if anyone had used backcut valves, such as Manley "Street Flo" or Milondon "MegaFlow".

Is there any benefit to them?

I have used them. They flow better than a standard cut valve.

MBHD

I do back and front cuts myself, not hard. 15 degrees seems to be the consensus. Some engines benefit more than others, some don't at all. Some valves have more to cut on the back side than others. The end result is more flow at lower valve openings, i.e. the air gap is greater at a certain lift. But the "ramp" you are supposed to back cut, also directs mixture to the side, and if you remove it, some combustion chambers don't like it. I have read numbers like 10% more flow, but not in every engine. That's the best case scenario.

I use a .060 x 30 degree backcut on intakes with a 45 degree face angle. Backcuts do not seem to do anything on an exhaust valve, but profiling the outer face with an entry radius does.

I use a .060 x 30 degree backcut on intakes with a 45 degree face angle. Backcuts do not seem to do anything on an exhaust valve, but profiling the outer face with an entry radius does.

What he said about outside/front radius a.k.a. front cut Interestingly, the new valves I got for my 230 already had a front cut. From what I understand, intakes benefit from back cut, and exhaust benefits from front cut.

It makes perfect sense when one considers the direction of the flow across the valve. Each modification helps "funnel" the flow past the valve.

Valve seat angle also has an effect - shallower valve angle (e.g. 30 degrees) on the intake promotes low-lift flow while a steeper angle (e.g. 45 degrees) hurts low lift flow but enhances hi lift flow numbers.

I am having trouble understanding where the "backcut"is.Is this cutting another angle on the valve on the 45 degree side towards the stem that is normally not laid back? I think that is what is being talked about but I want to be sure. Sorry if this seems like a dumb question. Jay

http://www.ws6transam.org/Backcut.jpg

http://www.austincc.edu/wkibbe/headdesign_files/image006.jpg

The latter is an extreme example of a back cut, it's not supposed to be that radical. But demonstrates the area that is cut. (Left valve is back cut, right side valve is not)

Edit: one more:



Shows the back cut (removed material in red) in relation to the port and seat area. You can see how the "ramp" would otherwise create a bottleneck between the valve and the seat. In many/most valve/seat designs I would guess?

Thank you! thats what I thought everyone was talking about. I take it then front cut on an exhaust valve is on the other side of the seat point. Seems like easy HP to me.

A guy named A.Graham Bell once wrote a performance book and I think that's where I read about verified up to 10% increased flow, or was it even 10% increased power. Not sure anymore, don't even know where my book is these days.
And yes the exhaust valve front cut is on the chamber side of the valve, usually less is cut than in a typical back cut. I don't remember the theory behind it, but I suppose a sharp 90deg edge would cause harmful swirls in gas flow or whatever, and the front cut would eliminate some of it or create a more beneficial swirl pattern that helps expel the gases past the valve.

Edit: http://www.goodreads.com/book/show/3613477-four-stroke-performance-tuning

Thanks for mentioning the A. Graham Bell book. I am going to order a copy. I have David Vizard's book, "How to Port & Flow Test Cylinder Heads". Another good reference on valve, combustion chamber, and port modifications.