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Back Cut Valves
https://slantsix.org/forum/viewtopic.php?t=27169		 Page 1 of 2
Author:  polara pat [ Wed Feb 13, 2008 4:05 pm ]
Post subject:  Back Cut Valves
We have acquired an older valve and seat cutting machine and I have been practicing with a bunch of old big block valves that we had lying around. (before I destroy anything good) These valves get the standard 45* cut then I've just been reading up on the benefit of the 30* back cut. My question is how much is too much? (in .001) In order to get the full advantage and still leave enough face for seating on the valve. Like I said, I'm in practice mode so don't be judgmental.

Thanks, Pat.
Author:  polara pat [ Thu Feb 14, 2008 9:14 am ]
Post subject: 
Anyone?
Author:  Doc [ Thu Feb 14, 2008 10:25 am ]
Post subject: 
Looks like most members here don't grind their own valves... :wink: :?

I am jealous... I have stones to grind valve seats but I don't have the valve facing machine. I either buy new valves or talk my way onto someone elses machine.

We "back-cut" a lot of valves, just to take-off the small step you find on many valves, right where the back of the head radius turns into the seat angle. I find that it is easier to do this work on a small lathe I have, rather then on a valve facing machine.
http://www.dutra.org/doug/draft-webpage ... /pics1.jpg

For seat widths, you can get away with a pretty narrow seat on the intake valve, .030 to .040 for a race engine not expected to go 100K + miles.
Exhaust valves need wider seats, .060 to .080 is about right.

In general, the more angles you can get, leading up to the actual sealing seat, the better. Newer technology can now produce a smooth radius leading up to the seat and this helps flow.

You hear a lot about using 30 degree seat angles instead of the 45 but I have no data to support this, when used on a SL6 head.
DD
Author:  Charrlie_S [ Thu Feb 14, 2008 1:06 pm ]
Post subject: 
Quote:
Looks like most members here don't grind their own valves... :wink: :?

I am jealous... I have stones to grind valve seats but I don't have the valve facing machine. I either buy new valves or talk my way onto someone elses machine.


I have had a seat grinder for many years, but could not find a refacer, for an affordable price. Picking up a really nice Black and Decker, tomorrow ($275)
Author:  argentina-slantsixer [ Thu Feb 14, 2008 1:42 pm ]
Post subject: 
I do the backcutting on the lathe too... I use indexable carbide tools isntead of grinding stones, say 400 rpm spindle speed and cutting oil. Then I have them valves surface hardened... just in case. Also I mirror buff the back of the exhaust valves. Gonna take some pictures soon if I can find some valve around...

Oh! also I try to even out as much as I can on the combustion chamber side of the valve's heads... it's amazing how different they are and how much that hurts the chances of getting them chambers to CC nice and even.
Author:  polara pat [ Thu Feb 14, 2008 2:19 pm ]
Post subject: 
That's what I'm talkin' about. That's all great input. Like I said, this machine is older but seems true, I'll try and take photos but I won't be out to the shop for a week. We traded it to an old Doukabour for a 350 we had collecting dust under the bench. I haven't used the seat cutting portion yet because I was getting new guides put in and figured I would leave it up to my machinist. Keep the input coming, I'll be a self appointed expert in no time.
Author:  emsvitil [ Thu Feb 14, 2008 4:00 pm ]
Post subject:  30 degree seat
I know that a 30 degree seat was used on some Pontiac V8s on the intake side.

The idea is at low lift the valve opening is slightly bigger than it would be with a 45 degree seat......... Generally more for torque motors (which the pontiacs are), so I think it should work with slants too.


Warning math:
30 degree vs 45 degree seat, .100 inch valve lift.
45 degree seat: .0707" valve face to valve seat
30 degree seat: .0866" valve face to valve seat
Author:  runvs_826 [ Thu Feb 14, 2008 4:06 pm ]
Post subject: 
Could I get some more info on this? I looked up some pictures of what the black and decker seta grinder and resurfacer looks like, but what do they do again? I've read about backcutting valves it's suppose to be beneficial, but in opinion seems to be more dependent of choice. Thanks for your time, I think I know to do these things but not necessary why or how.
Author:  polara pat [ Thu Feb 14, 2008 9:57 pm ]
Post subject:  Re: 30 degree seat
Quote:
I know that a 30 degree seat was used on some Pontiac V8s on the intake side.

The idea is at low lift the valve opening is slightly bigger than it would be with a 45 degree seat......... Generally more for torque motors (which the pontiacs are), so I think it should work with slants too.


Warning math:
30 degree vs 45 degree seat, .100 inch valve lift.
45 degree seat: .0707" valve face to valve seat
30 degree seat: .0866" valve face to valve seat
No, the seat still gets the three angle job but instead of the straight 45* on the valve cut you also add 30* where it blends towards the stem. There's a lot of articles floating around about the procedure but I'm too full of Martinis to search them out.
Author:  emsvitil [ Thu Feb 14, 2008 9:59 pm ]
Post subject:  Re: 30 degree seat
Quote:
Quote:
I know that a 30 degree seat was used on some Pontiac V8s on the intake side.

The idea is at low lift the valve opening is slightly bigger than it would be with a 45 degree seat......... Generally more for torque motors (which the pontiacs are), so I think it should work with slants too.


Warning math:
30 degree vs 45 degree seat, .100 inch valve lift.
45 degree seat: .0707" valve face to valve seat
30 degree seat: .0866" valve face to valve seat
No, the seat still gets the three angle job but instead of the straight 45* on the valve cut you also add 30* where it blends towards the stem. There's a lot of articles floating around about the procedure but I'm too full of Martinis to search them out.
I know, Doc mentioned a 30degree seat...........
Author:  Doc [ Fri Feb 15, 2008 9:11 am ]
Post subject: 
Yes... a 30 degree "back-cut" is different then a 30 degree sealing seat but I figured I would get you thinking about seat angles and widths.
Now that you have the equipment to do the valve face angles you will see that once you set-up the facing machine and start grinding the back-side of the valve head at 30 degrees, it becomes a simple matter of when you stop grinding that will deturman if that angle is a back-cut or a new sealing seat surface.

As noted above, the new 30 degree seat angle "opens the window" a little more at low lifts to help flow.
See this article for some good photos and info on doing a valve job. The article also shows the equipment we are talking about.

A 30 degree intake seat is something I hope to try on a SL6 but it will need to be done with oversize intake valves because you do not want to sink the valve any deeper into the head, as you grind-in the new 30 degree seat angle.

If I had a valve facing machine sitting in my garage, I most likely would have tried this by now.
Seeing that new O/S intake valves come out of the wrapping with a nice 45 degree seat face on them and ready to go, makes it all to easy to just grind open the head's seats and keep the 45 degree seat angle.

One of these days I will get a set of used O/S SL6 intake valves that need refacing and those will get 30 degree seat angles ground onto them. Once I have those in hand, I will give this idea a try for a "street performance" SL6 engine.
DD
Author:  LUCKY13 [ Fri Feb 15, 2008 11:03 am ]
Post subject: 
The 30* "seat" is a real talked about subject. It seems that some are totally against it and others feel it has promise in the right app. Low lift flow is its main purpose and that seems to be leaning toward a bad thing if giving up high lift flow to achive it. From what I have learned anytime you can give up low flow for high flow it is a plus, even with small cams (orcourse that is reletive).



Go to Speed Talk Dot com and search the subject and you can learn more about it and read descusions on it. Also there is some links to articals about it wihtin this search. You need to look in the advanced engine section though & you may need to be a member to do that. SPEED TALK is also a good place to learn about valve seat & face angle and margins and how each effect the air flow. I will warn you though be ready to read for a long time (not hours but months).



Jess
Author:  Charrlie_S [ Fri Feb 15, 2008 2:10 pm ]
Post subject: 
Many years a go, I bought a Cordoba with a 360 2v. It had a bad cam. While replacing the cam (stock TRW replacement) I did a valve job that
used the 30 degree seats on both intake and exhaust. I had a trip milage computer, and used that to tune the timing curve and carb, for the best milage. I got that car up to 28 on a hyway cruise. Around town, could never get beter then 13 mpg. I don't know how much the 30 degree seats helped, but it was better then any other 360 in it's weight range on the hyway.
Author:  Dart270 [ Fri Feb 15, 2008 2:14 pm ]
Post subject: 
On the head that Mike J did for me, he used 30 deg intake seats (not just a backcut). These give killer low lift flow and with a 0.460" cam, this thing makes good power even with 8.8:1 comp and a junkyard shortblock - about 210 HP, which was good for 14.90s in Project V with 3.00 rear gears. High lift flow topped out at about 185 cfm around 0.400" lift and sustained to 0.600". I had this set up for a turbo motor, and hope to get that together this year.

Lou
Author:  Charrlie_S [ Sat Feb 23, 2008 8:36 pm ]
Post subject: 
Quote:




I have had a seat grinder for many years, but could not find a refacer, for an affordable price. Picking up a really nice Black and Decker, tomorrow ($275)
Boy did I fall into a deal. Went to pick up the valve grinder, last Sunday. Just went through a box of "extras", he gave me. A Snap-On Seat grinder, with pilots, mandrels, stones (most new), and stone dresser. Valve spring hight micrometer. Valve spring pressure tester. Super heavy duty "C" clamp spring compresor. A bunch of carb adapter/spacers (new), and a Craftsman bench grinder. Christmass in Feb.
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