Head Questions

Explorer

Explorer

Well-known member
CI aluminum head. I understand 3 angle valve job. Can't get a grip on "unshrouded" and "backcut" terms. Can someone put this in words even I can understand? From reading older posts, I gather that back cutting intake valve's 30 degree's will increase cfm flow. Turned 60 today and sometimes my mind works a little slow.
 
In my humble opinion, all of the "back cutting" and all for tricking out performance is money thrown in the wind. Shaving a few microns off the valve underside is not going to make any performance that you can measure.

I'd let that one go by.
 
Explorer":190j1n6g said:
CI aluminum head. I understand 3 angle valve job. Can't get a grip on "unshrouded" and "backcut" terms. Can someone put this in words even I can understand? From reading older posts, I gather that back cutting intake valve's 30 degree's will increase cfm flow. Turned 60 today and sometimes my mind works a little slow.
Click to expand...

8) back cutting the valves can improve airflow, and gain you some power, depending on how poor the airflow through the ports are, and how bad the airflow around the valve is. dont expect a lot of power though on a street engine.

unshrouding the valves will gain you more power though as you open up the area that kills airflow around the area where the valves are closest to the chamber walls. doing this in conjunction with back cutting the valves will really smooth airflow into the cylinders, even on a street engine.
 
Tony63 said
In my humble opinion, all of the "back cutting" and all for tricking out performance is money thrown in the wind. Shaving a few microns off the valve underside is not going to make any performance that you can measure.
Click to expand...

Tony you are not correct about backcutting valves.
If you don't backcut the intake valves you are leaving HP on the table.

Port flow valves are not backcut, the port flow means the valve stem under the valve head in smaller in diameter than the rest of the valve stem.

The proper way to backcut valves is after the seat is ground or cut & the 45 degrees face on the valve is dreassed, then lap in the valves using valve grinding compound or apply prussen blue to establish the valve to seat contact area on the valve.

Then place the valve in a valve grinder & set the degree setting to 30 degrees. Then proceed to cut the sharp point of the valve seat to the lower lap in line.

Thats how you backcut a valve.

If i had my lawn mower engine apart i would even backcut the valve or valves to produce more flow.

I would guess you could gain possibly 5 HP by just performing this backcutting operation. Outlaw
 
A 3 angle valve job, is actually a 3 angle grind or cut of the valve seat. IIRC the 3 seat cuts are a 65 deg top cut, a 45 deg middle cut and a 30 deg bottom cut. These 3 cuts radius the edge of the hole (valve seat entry into the bowl) for a smooth transition. The valve head is usually ground at a 44 deg cut to seat in the 45 deg seat cut. A 30 degree cut on the back side of the valve does improve flow around the valve head (by radiusing the valve around the back side). It is called back cutting the valve. Back cutting the exhaust valves are normally not done, so that the maximum amount of heat can be transferred or dissipated between the surface areas of the 44 deg valve surface and the 45 deg seat surface.
 
littlest outlaw":10qzah9t said:
Tony63 said
In my humble opinion, all of the "back cutting" and all for tricking out performance is money thrown in the wind. Shaving a few microns off the valve underside is not going to make any performance that you can measure.
Click to expand...

Tony you are not correct about backcutting valves.
If you don't backcut the intake valves you are leaving HP on the table.

Port flow valves are not backcut, the port flow means the valve stem under the valve head in smaller in diameter than the rest of the valve stem.

The proper way to backcut valves is after the seat is ground or cut & the 45 degrees face on the valve is dreassed, then lap in the valves using valve grinding compound or apply prussen blue to establish the valve to seat contact area on the valve.

Then place the valve in a valve grinder & set the degree setting to 30 degrees. Then proceed to cut the sharp point of the valve seat to the lower lap in line.

Thats how you backcut a valve.

If i had my lawn mower engine apart i would even backcut the valve or valves to produce more flow.

I would guess you could gain possibly 5 HP by just performing this backcutting operation. Outlaw
Click to expand...
When using log style heads, any gain is just that....gain! And while Tony did have a bit of truth in his statement, the gain might be negligable as far as seat of the pants increase, Outlaw is also correct in that it is worth doing, regardless of how little it can benefit. Also, most cylinder heads have a bowl that is too small for even the stock size valves that come in the heads to begin with, so increasing the bowl area can dramatically improve air flow, even if larger valves are not installed. Even these CI heads can benefit from these same concepts and techniques, if they don't already practice them.
 
Interesting! I think I have a grip on it now. I try to learn something new every chance I get. It's a shame I had to retire to have time to learn new things, too busy working for a living. Thanks for your input guy's.
 
It's like cooking. A little of this, a little of that, and while you may not taste any one particular ingredient, it all adds up. And if you leave something out it may not be so easy to go back and add it later.
 
tony1963":233zxflx said:
In my humble opinion, all of the "back cutting" and all for tricking out performance is money thrown in the wind. Shaving a few microns off the valve underside is not going to make any performance that you can measure.

I'd let that one go by.
Click to expand...

I respectfully disagree.

True, it will not be a huge increase, but this trick has been done AND MEASURED on dynos a great many times. That said, it is most useful on engines with relatively mild camshafts having short durations and low lifts. The reason for this is it helps the flow at low lifts (just off the seats) and keep in mind that we have this situation TWICE during each valve event. The net result is a tiny bit of extra "effective" duration.

That said, the big boys doing modern, extreme performance engines with radical cams and enormous amounts of lift have found this technique to be worse than useless as they have so much overlap that they actually do things to try to kill flow at low lift; such as using 50º seats and no back-cutting of the valves. Reversion can be a bad thing in those situations. You can search this topic over at www.speedtalk.com for more info.

I have done this on mild engines and will continue to do so when I believe the situation warrants it. Back-cutting is so very cheap to do that it is still worthwhile; unshrouding the valves is a bigger gain (though I would hope the new aluminum CI head would not be too cobby in this area).
Joe
 
Explorer and others I had mentioned this before and will again for the new people. If you keep an eye out for Stock Car magazine they usually have some good tech articles on increasing breathing capabilities of class restricted motors or even talk to some racers in your area if so. Alot of this stuff can be applied to our sixes and back cutting the valves has been shown to help quite a bit. I know even on the Aluminum Head some of this stuff has been tried and flowed and even more gains were noticed even with port work.
 
I'm not saying that there are no gains. I just don't see how trimming a few microns off the valve's edge does any good that you would ever notice.

As far as what gets measured, I could measure an earthquake half way around the world, but how does it effect me?
 
Several more horsepower for cheap still = several more horsepower. If ya don't want it, don't do it.
Joe
 
tony1963":bf8kw7hg said:
I'm not saying that there are no gains. I just don't see how trimming a few microns off the valve's edge does any good that you would ever notice.

As far as what gets measured, I could measure an earthquake half way around the world, but how does it effect me?
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8) airflow is funny at times. sometime something you think would have no affect, have a big affect, and other times you think this should make things much better, and in reality you go the other way. sharp edges tend to restrict airlfow more than most people realize. and it isnt just about the power gain, its about the efficiency gain.
 
Howdy All:

Very interesting!! Back-cutting the intake valves may be the cheapest step in getting a performance valve job done at a quality race shop. Check it out. Why not?

Adios, David
 
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