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1991 Honda CRX
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3,714 Posts
Discussion Starter #1
So one of my DOHC ZC heads was not salvageable (guide bore problems), so it was either scrap it or experiment. Five minutes with my sawzall and it was cut in half. I was always curious where all the coolant/oil ports were for porting and clearances.

-The green is oil for the rocker arms, blue is coolant ports, and the red are the thickness of various points.
-I was surprised how little room there is from the port and coolant port on the floor. If you go too crazy with porting it will be very close to those ports.
-I was also surprised how thick the valve seats were, since these heads have "crooked" seats there is enough thickness to smooth them out.
-The intake guide bore is a lot shorter on the intake side vs the exhaust port. It looks like quite a bit of flow can be gained from smoothing out the exhaust guide area, there is plenty of room. The intake side probably shouldn't have very much removed or it seems like the guide/valve would be weakened.







 

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()*#$(*$
93 Legend L Coupe.
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11,404 Posts
Let us seee . . .

Best exhaust ports on a D-series? Check.
Much better intake valve to port angle? Check.
Good short side radius on intake port? Double check.
Saying eff u to the rest of the D-series? Triple check.

Love this engine. Love the ports in the head. What you can't tell from those pictures, though, is the width of the ports, which when coupled with the other stuff above, makes for a very well flowing head!
 

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Ek sedan
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382 Posts
You are a boss for doing this. For whoever could use it this is really useful. Repped you.
 

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1991 Honda CRX
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3,714 Posts
Discussion Starter #9
This head couldn't be saved, if it was useable I wouldn't be doing this.

I plan to use all this information in the near future for porting. Then use the other ports to do some tests with different porting techniques.

The area around the exhaust guide can really be smoothed out, now that I know there is plenty of material there. I noticed that there is always more carbon buildup there than anywhere else. So that lead me to believe it's a dead spot.

Fixing a few areas while leaving the overall size of the ports should make a good difference even on these heads.
 

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ministry of mayhem!
93 honda integra
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5,673 Posts
anything that contributes to knowledge gets a rep from me.

thread is also stickied.
 

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1991 Honda CRX
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3,714 Posts
Discussion Starter #15
Yes porting dead spots doesn't help much, but if you see the path the exhaust has to take through the port, you see the guide area acting like a wall. It's like

I was taught any fluid likes to go in a straight line, doesn't like changing directions. But if it has to change directions it needs to be smooth and free of obstructions to maximize flow.


are you going to up-cycle this chunk of zc head into some furniture perhaps?
Don't know what I would make, but I'm gonna keep the chunk on my table and hopefully help some people understand how an engine works. Visuals always help.

Hey OP do you mind throwing up pictures of Z6 Y8 A6 Cutaways as a comparison?
Don't have any other heads I want to cut up, maybe someone has a scrap head they wanna cut up and post pictures. What I can tell you is the DOHC head is about 2mm larger in overall diameter for port size.

Thanks guys, rep'd back.
 

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()*#$(*$
93 Legend L Coupe.
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11,404 Posts
You are forgetting that the valve stem and the body of the valve guide are more influential to the flow in that area than what you are perceiving as a wall. If you grind that out, you may very well be increasing the dead spot effectively making the port smaller. I'd like to see what you do with the port, eventually.

Also, don't remove much material around the seats. The port throats are already over large, and removing too much material in an attempt to smooth it out perfectly WILL cost you power as you will very significantly reduce velocity through the ports. And don't go crazy worrying about the cast marks, too. They don't effect too much at all.
 

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1991 Honda CRX
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Discussion Starter #17
You are forgetting that the valve stem and the body of the valve guide are more influential to the flow in that area than what you are perceiving as a wall. If you grind that out, you may very well be increasing the dead spot effectively making the port smaller. I'd like to see what you do with the port, eventually.
Interesting, never thought of that. I guess the best thing would to try various things to see how flow is increased/decreased. Hopefully sometime soon I can rent a flow bench and try these things out.
 

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()*#$(*$
93 Legend L Coupe.
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11,404 Posts
Remember that velocity is the most important thing through a port, at least IMO. If I had a small enough TIG torch, I would actually fill in the upper hump of the roof of the exhaust port. I would also add material in on the intake port floor to increase the radius of the short side, though you can do that with Devcon, or, heaven forbid, JB Weld, rather easily.
 

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1991 Honda CRX
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Discussion Starter #19
Remember that velocity is the most important thing through a port, at least IMO. If I had a small enough TIG torch, I would actually fill in the upper hump of the roof of the exhaust port. I would also add material in on the intake port floor to increase the radius of the short side, though you can do that with Devcon, or, heaven forbid, JB Weld, rather easily.
I wouldn't feel safe using anything besides welding on a port, but seems like you could use some type of clay just for testing purposes. That way you can easily try different sizes in different spots.
 
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