Gs 1100 ex porting

[posplayr]

Terry knows his stuff, he did my head and it is the best

[headsbikesmopars]

I have heard of it being done on a lathe,..I personnly have never tried that. I feel it best to have them back cut on a valve refacing machine. The operator can change the angle in an instant to what you desire. Thay will grind the existing angle off at 37 degrees and bring that cut right up to the top of the 45 seat on the back side of the valve itself. this cut is only about .030 wide, and looks to ' radius ' the backside of the valve. This cheap and easy mod REALLY works and is great for street engines as well as race engines. I perform this on ALL my engines whenever I have the valves out. It is ALL good and there isnt ANY down side to it. More flow is a GOOD thing. I learned a long time ago that a flow bench is an impartial judge and doesnt care to hurt your feelings,... ! My flow bench LOVES it when I back cut the valves, it tells me so.

Terry

[4cyl h2]

thanks for the info, thats just what i was looking for, im wondering about the depth or width of this cut, i have seated the valves on the seats near the outer edge of the valve and it seems like if the backcut area was to go up and almost touch the bottom edge of the lapped in area on the valve the 37 degree cut would be very wide ?

[headsbikesmopars]

Width really isnt an issue since the back cut doesnt touch anything like the 45 does. On a complete set you will have some wider than others, and it doesnt hurt anything, it is all good. when you move the seat out to the edge of the valve you can make the bowl larger and it is almost like having a slightly larger valve. The bowl should be 88% of the valve diameter. I have gone as large as 93% but those were on Pro Stock heads (auto) that are freshened after ever race. I feel there just isnt enough material under the seat area to support the seat. Bowls at 88% flow very well and is durable enough for daily use.

[GregT]

I've done the back cut both ways - lathe and valve refacer.

Valves with a larger stem diameter are easy to do in the lathe as they don't flex or chatter under the tool. Small OD stem valves are best done in the valve refacer IMO. So 8V stuff is fine in the lathe but I'd use a refacer for the 16V valves.