Larger intake valve question and cutting a new valve?
I think your head has a 25mm intake valve diameter and if you are going to a 27 that is .080 larger. To open up the throat you would use a piloted bowl hog either 75 or 80 degrees. You want to be conservative so opening should be no larger than 88 percent of the valve diameter max. The valve should have a 45 degree angle. You can backcut the intake with a 30 degree backcut, but if you do not have the equipment or know how to do it don't the improvement is not worth it. Also on high performance heads (cars) normally the throat or pushrod pinch areas are the MCSA point which is the minimum cross section which is the limiter of maximum flow. What you are accomplishing with a larger throat and valve is improving low lift flow up to the point where the MCSA limits any further flow. On these heads you have a very efficient runner that tapers down to about .5 - .6 sq inch. then a bowl that is machined in to intersect the runner with no blending whatsoever. The biggest bang for the buck it to optimize the short side and put a radius on it and then streamline the guide area. Here are some port molds that you can see how the bowl intersects the runner and how it necks down right before the bowl.
What sort of gains, if any, come with filling the back side of the bowl? It seems like it would be a big help all across the curve?
I've had mixed results with backside filling. Here is a hemi I did a while back with before and after porting molds. Low lift improved a lot because of the bigger intake valve. At low lifts The seat, valve and area right above the valve and the chamber make the most differences. I would guess the backfilling only works at higher lifts when the velocity in the port is high. On this head I raised the floor to get a bigger radius on the shortside. I was afraid to go too high on the roof as I heard you can break into the spring pocket if you go too much. This head had a very high velocity right at the bowl although it was fairly even all around the port compared to stock where it was much higher at the short turn. My cross section area was small and limited high lift flow but it still got 74CFM on my bench at 28"
ole4 the port molds are cool. I need some more heads to mess with. Need some more tools so I can do some real porting. The predator head looks coo. I will have to build one soon. Maybe doing a stroker motor. I am trying to build the motor I am working on now for low and mid range for street and trail riding and want low RPM torque but I think the next motor will be all out power and as much as possible. The first build is for a mini bike but I also want a fast gokart some day or drift trike, maybe on alcohol. A stroker predator on alcohol that spun to 8K+ would be cool. The pictures are very helpful.
I've had mixed results with backside filling. Here is a hemi I did a while back with before and after porting molds. Low lift improved a lot because of the bigger intake valve. At low lifts The seat, valve and area right above the valve and the chamber make the most differences. I would guess the backfilling only works at higher lifts when the velocity in the port is high. On this head I raised the floor to get a bigger radius on the shortside. I was afraid to go too high on the roof as I heard you can break into the spring pocket if you go too much. This head had a very high velocity right at the bowl although it was fairly even all around the port compared to stock where it was much higher at the short turn. My cross section area was small and limited high lift flow but it still got 74CFM on my bench at 28"
I had some cool porting calipers but think I left them behind during a job move. Now I too often break in to the spring seat, then fix it. Haa
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