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Old 01-08-2017, 11:46 AM   #1
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Polishing M42 intake ports?

I hear the general consensus is that porting the M42 intake ports at the head is a bad idea. The ports are already matched pretty well and it just introduces a larger chance of air/fuel flow disturbance rather than optimization.

How about polishing? I noticed, at least on the E36, the ports around the valve is polished, or at least its not as porous. Has anyone taken the effort to do this?

Just thinking of things I can do while I have the head stripped down.
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Old 01-08-2017, 06:08 PM   #2
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Most times the rough surface of the intake ports helps keep the air/fuel mixture well homogenized (mixed). Smooth shiny surfaces lend themselves well to wetting out (the fuel sticks to the smooth surface) and then comes off in bigger drops.

Larry Widmer of Endyn (and I'm sure many more) will hit the intake port with an 80 grit cartridge roll after all the porting is finished.

Exhaust is a whole 'nother story though. Shine/polish away to your heart's content.
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Old 01-08-2017, 07:29 PM   #3
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i doubt there has ever been a mass produced cylinder head that could not be improved by a good skilled head porter with the right tools and knowledge of how engines work. also for the matching of manifold to head (as its not clear if you mean the manifold ports or head ports) i wouldnt bother on a run of the mill engine. many people that have tested this albeit on other engines have even said that even if the cylinder head is a bit smaller than intake it doesn't really matter for most engines. its widely touted that the intake manifold should be smaller the head ports to create a step up in area but it appears that it depends...

but yeah don't polish anything, 80 grit isn't that rough, rough is straight off the burr....someuse a bent burr to make it even rougher and some CNC machine grooves and other fancy textures. People have lots of theories as to why it works better but its hard to know for sure the exact mechanism at play.

for the e36 my guess is they probably just machine them rather than leave as bare casting so its consistent in size and the shape is good (size and shape is the key) , its unlikely to be for surface finish that machining tends to have over as cast
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new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

Last edited by digger; 01-08-2017 at 07:42 PM.
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