996 Cylinder Head Porting and Polishing

Hi guys!! So I'm in the process of rebuilding my 996 engine and have been toying with the idea of porting and polishing the cylinder heads, mainly just smoothing the port walls and removing the casting flash and smoothing the flow. Has anyone done this?? If so what were the gains like?? Any reasons why I shouldn't do this ?? Also In a Porsche factory manual I have it shows the x51 power kit mods to the standard engine , part of which shows the shortening back of the valve guides to be flush with the port walls as the standard ones stick out into the air flow in the port. I was also thinking of doing this!! Any info or previous experience with this would be appreciated!! Thanks Lee

Its very easy top do more harm to these ports than benefit with port work. The X51 port is completely different as it's bifurcation point is much higher in the port and it's runners much smaller in diameter and longer. I have found more losses with stoic displacement engines with porting, than I have gains. Its easy to do so much port work that the camshaft profiles become a mismatch to the port characteristics. A nice valve job and some bowl work is the best bang for the buck. This can gain you a solid 10% on the flow bench over stock, getting another 5% after that takes days. Here's my 4.1 R51 (CNC) cylinder head package, until we get to this level we keep ports very conservative and make more usable power all over the power band. In these pics the heads had just come out of the HAAS and hadn't been tossed onto the Serdi yet to have the seats cut or valve job done.

Wow I never considered I'd me mismatching with the cam profiles, this is getting deep! Ok so you think maybe just clean up the bowl area of casting flash to smooth the surface and a good lapping in of the valves?? Shall I just smooth the ports of rough flash also or just leave them totally alone, I did hear that on the inlet side the flash helps make the air turbulent which aids the fuel / air mixing process?? Thanks for your help!

Clean up and bowl work is all I'd do.. These heads are very good from the factory and the port flow margins are well balanced. Until displacement increases you can do a lot more harm than good. Even with added cc's you can still compromise things. We see it all the time.

Ok by clean up do you mean I can smooth off the casting flash or just clean off the carbon/dirt and leave the flashing on , I'm pretty confident I can clean up the flash without altering the shape of the ports or will simply smoothing the ports be enough to upset the balance, also what can I do to the bowl area for the best Sorry to go on!

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Lapping the valves is lawnmower rebuilding. Get a proper valve seat and valve head grind. Sometimes 3 different angles are used on the valve seat. It has been years since I did any port work -- motorcycle engines -- but what I remember at the feet of more experienced is reshaping needs a flow bench and dyno/track testing time. Just a bit of smoothing is what I'd recommend. The surface roughness causes non-laminar flow and this effectively reduces the size the port. A smoother service encourages laminar flow and thus more flow. The topology of the injector placement, the port, the angle/location/shape of valves, piston crown and combustion chamber all add to the air/fuel mixing so you are not going to make much if any improvement on this with porting unless you are really prepared to constantly check your work and be prepared to throw a head or two away if you get it wrong. So you smooth the port and rely upon the topology of the port and valves, et al, to mix the air/fuel mixture. Be sure you thoroughly clean the heads after any port work. And do the work somewhere away from your work area. Wear coveralls and a hat and remove them before walking into the shop area where you assemble the engine. The grit will fall from your clothes and contaminate every exposed surface with grit.

Macster Thanks for your thoughts , I'm gonna leave well alone, just clean up and maybe a little smoothing of the ports, will ask my local machine shop about the head/valve grind Thanks!

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It is always best to rely upon an experienced head shop for valve work. When I had car engine heads reworked I turned them over to a head shop, one that I had researched and which had a good reputation. Part of the job involved talking over the work with the guy and understanding what was gong to be done, and what the expected results would be. I forgot to mention that one area where removing metal can be of a big benefit is matching the intake manifold opening with the intake port in the head and on the other side matching the exhaust port in the head with the exhaust opening in the exhaust manifold. One doesn't want a step where the flow passes from the intake manifold into the port on the intake side and neither does one want a step where the exhaust gas flow passes from the exhaust port of the head into the exhaust manifold. Well, the head opening can be a bit larger than the intake opening in the manifold. This is not ideal, but better than the head opening being smaller than the intake opening in the manifold. And the reverse is true on the exhaust side. In other words as the flow progresses it can go from a smaller intake runner opening to a larger intake port opening and from the head it can go from a smaller exhaust port opening into a larger exhaust manifold opening -- though even this is not of course ideal. The way this was addressed was to create a template from the intake manifold for each head and from the exhaust manifold from each exhaust manifold and check how the port openings match up. To address any mismatch the idea was to remove as little metal as possible. And gentle curves are the goal. Given the flow velocity even a smooth step is going to be as bad as a sharp step. One must be very sure of the template. If it is wrong and one goes in to fix/address any steps one could make things much worse with the flow being made worse rather than made better and even worse one could compromise the head by thinning the wall between the coolant passage and the intake or exhaust ports.

No need for a template, just use the gasket as a templeta and upen up both ends to match the gasket. Its called gasket matching. But don't go deep into the head ports, only a few mm in, nothing more. Howerver, you will not feel a difference from this. I've done it a few times a long time ago and keaned not to bither with it if the engine isn't boosted. There is nothing wrong lappingnthe valves in. It was done like this in the factory too. Unless you are replacing the guides, there is no real reason to even think about a 3 angle valve job.

If you insist on lapping, use something very light, like Comet or Ajax thats highly diluted.. Lapping can really hurt wet flow.

Hi all thanks for your help and comments !! Up to now I haven't touched the intake ports at all. ( should I smooth these or leave alone) On the exhaust side I have cleaned and smoothed out the area from just behind the valve seat into the throat of the port up to where the valve guide protrudes, there was some carbon build up,rough flash and casting lines which I have smoothed but that's all, I haven't touched the port side? I have brought up the intake gasket to the head which is almost the same size so I'm not touching this, don't want to meddle and make things worst , I'm sure porsche know what they are doing!! One thing that puzzles me however is the inlet manifold side the port is quite a bit smaller than the head port , there must be at least 5mm inside the gasket port opening where as on the head port it is nearly flush or maybe a mil or so Can you give thoughts on opening this up to the same size as the head anyone?? Oh and valves , my seats look good no pitting or hammering etc, inlet valves look good so I might just tickle with some fine lapping paste just to check the seal, my exhaust ports have some slight pitting so might just have them ground if poss then tickle them in too, does that sound ok?? Thanks everyone !!

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My understanding is there are at least 2 problems with lapping the valve to the seat. One the abrasive particles can embed themselves in the softer of the two materials and this means the lapping continues as the engine runs. The other is the fit between the valve head and the valve seat doesn't want to be full contact but a partial contact biased towards the outer edge of the valve head. As the engine runs and the valve gets hot and the spring pulls the valve into the seat and combustion pressures try to push the valve through the seat the valve head deforms slightly and the valve's face makes fuller contact with the seat. Where the valve contacts the seat and how wide is the contact is determined by using dye on the valve or the seat and installing the valve in the guide and pressing on the valve and turning it to transfer the dye. Then the dye pattern is examined to make sure it falls with the engine maker's guidelines, that it is located correctly, wide enough and there are no gaps. The seat grinding stone is dressed to the proper angle along with the valve holder at the valve grinder. Then when the engine is running there is good sealing as the full width of the valve is pressed tight against the valve seat and the heat transfer from the valve head to the seat is optimum. The valves shed a lot of heat through their contact with the valve seats. If the valve starts out making full contact in a cold stationary engine when the engine is operating the edge of the valve head may actually not be in full contact with the seat. While the valve may not leak this exposes the edge of the valve head to overheating and the valve may burn.