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Xj750rh port polish

Discussion in 'XJ Technical Chat' started by Aurel, Jul 9, 2024.

  1. Aurel

    Aurel Member

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    I am rebuilding the top end of my Seca, and since I'm in there I am thinking about giving the exhaust port a good polish with high grit, and the intake a rough one with 40/60grit.

    Anyone experienced with porting and that has pics of his work? Been reading through forum but one guy that had it ported and tested on dyno (with great results) never posted the pics

    Also, anyone know why there is a lip at the left side and bottom of the exhaust port? 20240709_210153.jpg


    Thanks!
     
    Last edited: Jul 9, 2024
  2. Dave in Ireland

    Dave in Ireland Well-Known Member

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    The heads aren't cast perfectly and they get put into a machining jig for the facing surfaces, which includes the exhaust holes. There's a slight mis-match, which normally means nothing. However, many people have found on various engines it can be worth matching the flowing areas in the head and the intake or exhaust. If you're going to roughen up or smooth the internal surfaces, you might as well match the ports, too.
     
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  3. Roast644

    Roast644 Well-Known Member

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    I'm a complete dummy about porting and polishing and have never attempted it. What is the idea behind roughing the surface? Better mixing? Is there a balance between smoothing for less resistance versus improved mixing? Or is this like the surface of a golf ball where rough is actually smoother?
     
  4. Franz

    Franz Well-Known Member

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    Me too I don't know anything about it. I thought it was used more in engines used for racing. Yes mabe the fuel air mix is less turbulent like the air around the golf ball.
     
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  5. Aurel

    Aurel Member

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    So I've watched about 10 hours of videos about it, it's apparently a real thing it's usually done by proffesional shops.

    Porting is basically getting into the ports, removing metal to allow more flow, to gain HP easily, it's the most basic mod and the more money efficient if done right. The goal is the remove any sharp edge and make it easier for the air flow to go through the port, there is some knowledge to know where the gas WANTS to go, taking too much material or at the wrong place can have the opposite effect and ruin the head.

    I am not attempting it because even after watching so many tutorials, none is for my specific head, and I'm not just gonna take a gamble. Could've attempted it if we had someone here that was knowledgeable about it and had pictures of before and after.

    Polishing is usually what you can do after the porting, but can also be done on it's own. You want to have a rough finish for the intake port, so the fuel air mixture have something to grab on while still flowing efficiently in the port, if it's too smooth it can cause problems, so its advised to have if done to 60grit.

    Exhaust on the other side there's no limit at how smooth it can be, the faster the fumes out the more can come in, and apparently just having the polish done can really help the engine, and reduce carbon deposit in the port too.

    This is the result of someone that had it done on a xj750 Screenshot_20240709_060816_Brave.jpg
    I would've definitely tried with some pics of the job done...

    Anyway, I noticed I have a lot of pitting in my combusted chamber #1 and #3 I'm still learning about it but it appears it's a result of detonation, that can apparently happen when there is straight edges or already pitting in the combustion chamber creating hotspots... so I've got to remove the pitting the best I can and smooth out the combustion chamber, then try to remove the same volume of material on all different chambers... something I would've never know without finding out about porting. Yes I will lose some compression, but apparently the polishing of the chamber is still worth it to gain some power and prevent hotspots and engine detonation. I suspect my PO caused all of this problems in the engine (completely scored piston sleeves, pitting in chambers) for having the carb jetted completly wrong, and other things I noticed when taking the cams off...

    Already replaced the jugs cylinders, can't afford to get a new head so I have to try to fix somehow. Might need to add some aluminum welding into the pitting in some areas... yeah it sucks
     
    Last edited: Jul 10, 2024
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  6. Aurel

    Aurel Member

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    What a freaking mess
    20240710_155151.jpg 20240710_155155.jpg
     
  7. minimuttly

    minimuttly Member

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    Leave it alone, it won't affect your bikes performance. I've had bits of metal embedded in heads before now, you clean up as best you can, peen the lumps down with a small ball pein hammer and it's good to go.
    A 16v I did a while ago..
    Actually, I have a belief, mostly based on years of experience, that the old XJ heads have a couple of issues. The inlet and exhaust ports around the valves are restrictive, and could do with tidying up carefully. The chambers have no squish bands, so are susceptible to detonation, so limiting CR and or ign timing.
    What are the fixes? 6mm valve stems would help high lift flow, maybe 30 degree seats would help low lift flow. maybe some anti reversion grooves, plus radiusing the exhausts might reduce "camminess". Welding up the chambers to make some flat surface, together with machining of a pair of bands onthe pistons could provide the squish required. Making the ports bigger would be the last thing to do, if at all.
    So, porting and polishing? sounds simple, but it's quite an all enveloping science...
     

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    Last edited: Jul 10, 2024
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  8. Aurel

    Aurel Member

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    Yeah I started sanding down the pitting already, it's extremely hard because this head has the seats wider than the valve heads, so even with an old valve inside, I could touch the seats, and even tho I'm sure it would be OK, I don't want to.
    Just gonna clean up sharp edges I could've made and leave it there. But I do know pittings can really cause issues with hotspots and detonation down the line. Oh well, I'm just gonna leave it there and hope for the best.
     
  9. k-moe2

    k-moe2 Guest

    Unless you're riding to make money (racing, which honestly costs more than anyone ever earns) then just leave it alone.

    Port and polish refers to the intake and exhaust tracts, not the combustion chamber.
     
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  10. minimuttly

    minimuttly Member

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    You do know? How? Those pittings will quickly fill with carbon and you won't see any effects, I absolutely guarantee you.
     
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  11. Aurel

    Aurel Member

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    I was stating a different issue with my combustion chamber, it's not related to the port polish!
     
  12. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    No need to yell. I was merely replying to your initial inquiry.
     
  13. Aurel

    Aurel Member

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    I didn't mean to yell at all, sorry you took it that way, I didn't intended to...
     

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