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Back cutting Valves for improved air flow

BogusSVO

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Back cutting valves for improved air flow


This is a procedure done to help flow the air charge across the back of the valve and smooth the transition into or out of the combustion chamber.
Normally don on valves that are not fully machined or swirl polished.


Once the valve has had the face re-ground. Most common is a 44* or 45*
The valve machine is reset, and another angle is ground onto the back side of the valve towards the stem, most common is 30*

DSCF3959.jpg


The valve on the left has the basic face ground to mate back into the valve seat.
The valve on the right has had the basic seat inked,(black) and the added angle ground. (sliver)

Most OE valves are not fully symmetrical, so the width of the valve face to seat varies around the valve, the added angle will help correct this and smooth the transition of air flow across the back of the valve.
 

prove_it

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This is a cheap way to add 5-10CFM. I tested just valves back in the day with a flowbench and saw a difference. Most aftermarket valves should already have this cut in them. Generally you would do this to OEM style valves(EVO or stock valves). If your reusing your valves during a head rebuild, spend the money and do this. It makes a difference.

Just throwing that out there.
 

presterone

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brunswick maine
Bogussvo and others informative threads on this website make it easier to own a galant and generally improve my quality of living.
 

curtis

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Street Surgeon

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Question: When machining a head is it common practice to replace the valve seats and cut the new ones, or do most just re-cut the seats that are already there? Also, are there any advantages to using new seats or are there better seats?
 

BogusSVO

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No, seats are replaced as needed. The mits heads have a factory sintered metal seat, they are a good seat, but they can cut a bit odd from time to time. Most aftermarket seats are a high nickel alloy seat.

Most of the time seats will get replaced when,
1) the head has had multiable valve jobs, and the seat is low in the casting, this also leads to improper tip height and valve train geometry.

2) the seat is damaged/cracked

3) Valves are being installed that the seat insert will not accept, +2mm OS for a 4g head


Curtis, I am interested in your results.
I have been kicking around the idea of that set up.

BTW, did the machinist pay David Vizard the 7% fee? lol
 
Last edited:

prove_it

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Curtis, I'd really like to see your results. Very interesting concept.

Bogus, couldn't this set-up be done much cheaper, as you would just replace the larger valve with an 2mm OS valve and use a stock valve for the smaller? It really sounds like all that is really done is change the valve sizing. The rest is just thermo coatings. I know thermal coatings will help a lot for power, so was that 100hp gain from the valves, the coatings, or a combination? Would be nice to try just the valve idea and test that alone.
 

BogusSVO

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prove it.... From what I have come to understand it is the swirl that the Poly Quad is going for, the swirl after the valve will help atomize the fuel and give a more even dispersal of it in the combustion chamber.

The thermo coating on the valves is more for thermal expansion rates of the valve heads.

I guess I need to finish make my 85mm stand for the flow bench and play around with it and see what I can find out.....
 

mitsuturbo

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I've seen another way to get seats out... weld a smaller valve to the seat (in an aluminum head only, of course) and then whack the stem with a dead blow.
 

BogusSVO

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MitsuTurbo, If it works, it works. To me that seems like more work, than just shelling the old seat out.

The simplest way I know to remove the seat, is to slightly over heat the head in the oven, and let them fall out.
 

curtis

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Well one thing I did notice after porting the head and after the seats had been cut I need to blend in the seat on the 8 valves after they were cut. Figured as much but was a pita since I couldn't touch the seat and had the ports packed up with shop towels anyway after I was done I simply took a air blower on the air hose and shot it into the port trying to keep it as centered as I could I took my finger and just barely blocked the flow at the edge of the seat and moved it around the edge of each side and the one side did flow alot more. Packed up a pile of plumbers puddy into a VE stack and played some more. The Large valve side did flow a ton more.

We know what the car made first time but since then got a larger turbo, 3 inch IC pipes, larger throttle body, went from 1000cc injectors to 1680's and from a Hondata s300 to a Haltech plus a few more things, but he is planning on just wiring in the extra sensors and will probably make one pull on the 1000's and the hondata before plugging in the haltech and swapping injectors. Just to get some data to start the tuning process but this head I'm sure will move way more air since the factory gaskets are about 3/16 to small on every side. Porting a stainless gasket to a new size sucked the big one.


Now with this said don't grab a dremal and start cutting. I've created about 10 or 12 boat anchors over the last 25 years since I cut on my first ford head. Beginners need to take baby steps you can screw up a head in a matter of minutes, kill airflow, port into a water jacket or into a valve seat.....I've done it all. There is alot of casting errors in factory heads and have voids in the metal and thin spots where it should be thick. Antifreeze / water injection doesn't work except to steam clean one port. Read, study, read and then start just porting Ford mass air meters and play with intake manifolds to start or you be making phone calls trying to find a new cylinder head. On this Honda head I actually had to port into the oil drains and insert some aluminum, weld it in and the port it back to shape. Spent 2 days getting the two ports correct after 2 weeks of working on it. Baby steps....some say its a dark art with magic and ninja skills but actually just alot of practice, buying the correct cutters and cartridge rolls and knowing what to cut in what order. They don't give porting classes just hours, weeks and months doing it will teach you. Most people don't understand what goes into a proper done head or the time invested, we had a member here once that told me he spent over 50 hours just shaping the ports on a 1G head then machine work. He was making slave wages so he quit doing it. Don't blame him and actually figured he had more than that in them. Beware you've been warned.



When it goes to the dyno I'll post some numbers, video's etc.
 

mitsuturbo

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Quoting BogusSVO:
MitsuTurbo, If it works, it works. To me that seems like more work, than just shelling the old seat out.

The simplest way I know to remove the seat, is to slightly over heat the head in the oven, and let them fall out.



Somehow, my post wound up in the wrong thread. I must've had two tabs open with one of your posts in each while i was at work, and posted in the wrong one.

Whatever.

I've never replaced a seat, myself. I've seen them removed, but i'm not sure how they go back in if you do the weld/bang method. I'd think it would cause the hole to be a bit more sloppy and possibly allow for catastrophic damage to occur. I agree that the method you showed is the "proper" way to do things.
 

mitsuturbo

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Quoting curtis:

Now with this said don't grab a dremal and start cutting. I've created about 10 or 12 boat anchors over the last 25 years since I cut on my first ford head. Beginners need to take baby steps you can screw up a head in a matter of minutes, kill airflow, port into a water jacket or into a valve seat.....I've done it all. There is alot of casting errors in factory heads and have voids in the metal and thin spots where it should be thick. Antifreeze / water injection doesn't work except to steam clean one port......... Beware you've been warned.



When it goes to the dyno I'll post some numbers, video's etc.



I agree. Even the most skilled portwork can wind up as a dramatic failure. (BTW Dremels suck, use a pneumatic die grinder or something like i have now.. a 1/2HP Mastercarver Flexshaft tool with a bit made for nonferrous material. Anyhow, moving right along...) Years ago, as a neophyte, I once had a head "professionally" ported. It looked and performed beautifully for about a year. Then, something happened. Some porosity in the head gave way and allowed a bit of coolant into a ported exhaust port.

Long story short, it was dark and warm out, and i was nearing the end of a one hour trip with the heater off and the windows down. The car overheated so badly it lost power, sputtered, and died in a very short sequence. I'd come to find out later that the spark plug wells filled up with melted plastic as well.

I had the car towed home. Next day, it was a total bitch to pull the plugs. It'd gotten so hot that the plastic part of the boots had melted and run down round the spark plugs. I knew i was done for.
I filled the engine/radiator with water anyway and immediately noticed it dripping from my leaky flex section. Pulled the exhaust manifold and what did i see? A wet exhaust port in the manifold, for one. Lo, and behold... a PINHOLE in an exhaust port. Seriously, it was about the size of Yeti's penis, and pissing oh-so-steadily.

The job was done right. The guy i had do the port work was not at fault. The car ran great for a good long while. IMO it was a casting flaw that came to be the root cause of this issue. I wound up cutting into the water jacket in that port later on, and it took me quite a bit of digging to trace back the casting flaw, but it was apparent. About 3/8" of a somewhat winding porosity between the water jacket and exhaust port caused this head to fail after about a year after the porting had been done.

BTW, unnecessary apostrophes offend me. "Video's" would be plural, not possessive. It should be "videos". Just FYI
 

prove_it

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Sioux Falls, SD
I've found that simply sanding down the runners on a 1G is enough. The runners are so big as is that I've only reshaped the intake dividers. At wyotech I tested it out on the flowbench there. Seemed like my biggest gain were from smoothing out the ports and blending the bowl area. Some seats had a 3mm step, which is huge. You can get more from porting, but it seems to come at a cost of long term reliability. Since your thinning the metal out so much.
 
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