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Old 09-09-2022, 03:14 PM   #1
Wes7143
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Default Are My Heads Worth Porting?

Please take a look at the back of this valve seat and let me know if you would still try to port this.

Looks like Subaru missed the mark on aligning the intake valves on this head. All of the valve seats are off center by about 1 mm.

This makes for a big ridge that would require me to eat deeply into the valve seat to correct. I am new to porting and was hoping to take on this project as a minimalistic blend without changing the port diameters and shape too much.

Is this a big deal? Would you send it? Thoughts/feelings?

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Last edited by Wes7143; 09-09-2022 at 04:01 PM.
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Old 09-09-2022, 04:33 PM   #2
Jedi03
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i saw similar on my heads and am in a similar thought process...i dont think personally that it would hurt to blend it down as the valve seat is higher...but others may join in and verify!
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Old 09-09-2022, 04:35 PM   #3
motorbykemike
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looks like an intake port

are those d25?
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Old 09-09-2022, 04:41 PM   #4
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Originally Posted by motorbykemike View Post
looks like an intake port

are those d25?
Yes these are D25. Only one of the two heads is like this.
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Old 09-10-2022, 01:17 AM   #5
FuJi K
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that's a baby lip. I've seen worse! Open the throat up a little. I've blended it back like a venturi if the shift was really bad.
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Old 09-10-2022, 07:47 AM   #6
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Quote:
Originally Posted by FuJi K View Post
that's a baby lip. I've seen worse! Open the throat up a little. I've blended it back like a venturi if the shift was really bad.
Thanks for the reply. I'm seeing that people will have the machine shop do a plunge cut to open the throats up like you said to correct for this core shift.

This article has a good example. Looks like they took a pretty significant cut.

Are there any guidelines available for how much of a throat diameter increase is too much for stock size valves? If I wanted this 1mm lip to go away completely, I would have to have the throat diameter increased by 2mm.

Last edited by Wes7143; 09-10-2022 at 07:56 AM.
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Old 09-10-2022, 01:53 PM   #7
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I did some quick measurements and math on what my throat/valve ratios would be if I had the machine shop increase the throat diameters by 2mm. I included the possibility of 0.5mm and 1mm oversized valves.

My exhaust valvetrain will need the same treatment. Core shift of about 1mm there too.

The internet tells me that 85% throat/valve is ideal for a street car and 90% throat/valve is ideal for a high RPM race car. But I'm sure getting rid of these harsh valve seat lips is more important than the theoretical ideal throat/valve ratio.

Here's the nerdy math table. Grey cells are the OEM values.
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Old 09-12-2022, 05:32 AM   #8
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Do not put oversize valves in a dohc engine. The valves are to close. One miss shift and the valves will touch.
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Old 09-12-2022, 08:54 AM   #9
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Do not put oversize valves in a dohc engine. The valves are to close. One miss shift and the valves will touch.
Thanks for the info. I've also heard that +1mm valves are a little too big for the available heat dissipating surface area of the stock sized seats and can run hotter as a result.

The bigger issue here is getting rid of these valve seat lips though. I may or may not need oversized valves after opening the throats up. I would love some input from someone who has done this before and can recommend a course of action.
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Old 09-12-2022, 05:37 PM   #10
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Quote:
Originally Posted by spoolinsti05 View Post
Do not put oversize valves in a dohc engine. The valves are to close. One miss shift and the valves will touch.
fake news - theres a whole ton of rubies out there that don't have this problem , ever hear of properly matched valve springs for the usage ?
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Old 09-12-2022, 05:49 PM   #11
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Quote:
Originally Posted by Wes7143 View Post
Thanks for the misinformation.stock size valves can hit each other too if they float

I've also heard that +1mm valves are a little too big for the available heat dissipating surface area of the stock sized seats and can run hotter as a result.umm , think about it , it's more surface area when the seats are cut properly during the valve job

The bigger issue here is getting rid of these valve seat lips though. I may or may not need oversized valves after opening the throats up. I would love some input from someone who has done this before and can recommend a course of action.this would be done carefully and by hand

we spoke about all this i think
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Old 09-12-2022, 06:08 PM   #12
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Lil epoxy or weld the gap up then port.

I think 1mm on ex valve never hurt anyone
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Old 09-12-2022, 06:58 PM   #13
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Quote:
Originally Posted by benflynn View Post
Lil epoxy or weld the gap up then port.

I think 1mm on ex valve never hurt anyone
Epoxy is a little too ghetto for me. Have you welded under valve seats before?

Quote:
Originally Posted by motorbykemike View Post
we spoke about all this i think
We didn't talk about these particular heads because I just tore them down and got my first look at these seats. Plus, it's important to post here so the next person googling around for what to do when they have a head with core shift can see the discussion and solutions. There's not a lot out there that I am able to find, so let's put it in this thread.

My big problem with smoothing out this lip by hand is that it will make the seat shape all funky. This lip is 1 - 1.5 mm tall. Here is an artist's interpretation of what the seat will look like if I just blended the transition without cutting into the 45. Only 1 of the 2 heads has this problem too, so keeping flow equal will be hard.



Having the machine shop open up the throats with a plunge cut during the valve job seems to be the way that the spec miata guys solve this problem. Here is an article showing an example

I got the snippit about oversized valves running hotter from this Headgames Motorworks video.
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Old 09-17-2022, 05:55 PM   #14
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I ended up finding a better manufactured D25 head locally. Much easier than trying to work with a head with this much core shift.

For anyone in the future with a core shifted head like mine, Dave from Headgames Motorworks advised me against increasing the throat size or trying to smooth the affected area by hand. He recommended just leaving it since it would do more harm than good to the head's flow to try to fix it.
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