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Back cut valves

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Guest

Guest
I also have sent my head out to complete the valve job another started. For those who send their valves out to a machinist ask him if he knows to back cut the valves.
I had this done to my 82 kat back in 82 and thee was significant power increase combines with jetting and a header. My Man says on his flow bench back cutting the valves 37 degrees increases flow 8 ro 10 cfm. A stock 1100 head flows about 80 cfm (on his bench) That makes it a 10% increase in flow.
 
I've heard of this trick. There are so many secrets the pros won't tell us concerning head porting. I'm still going to try it myself armed with the great tips from the book "performance tuning in theory & practice" by the incomparable A Graham Bell. I'll be using my favorite machine shop for things like back cutting but feel comfortable enough with this book to do the bulk of it myself.

Paul
 
This is a popular mod in the turbo mopar community, (my other money pit) the heads for those need all the help they can get.
 
you cut back from the seat edge toward the stem 37 degrees There is a lip there that needs to be removed with this cut.
You have to lap in the valve to see where the seat is on the valve so you don't get into it or the valves are junk yes..
 
There are no secrets really about head porting. There are volumes on the internet.
An average home port job would be to unshroud the valve pocket, back cut the valves and smooth the ports. All this is easily done at home with a dremel, bench grinder and a steady hand. If you are going to open the ports up a bit you have to (read MUST) leave the floor of the port alone. The floor only gets smoothed. Hogging is detrimental. and you want the entrance of the port to match the size of the valve pocket. Bigger is not better. velocity is what works and you must maintain velocity with a consistant port size throughout. Im thinking that the TSCC may have a pre determined port shape and entrance/exit angle.

I actually have quite a bit of experience on car motors but havent torn down many bike motors. I dont have the itch for long hi perf teardowns anymore. i was a freak for throwing money at Mopars in the 80's and 90's.
Im past that now and am keeping my urges in check with my 1100 now.

Anybody want to see a pic of my old 11.22 GTS? Alas, my baby has been gone awhile now.

Dont get me started.
 
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