Porting SOHC Neon Exhaust Manifolds


By Matt Beazer
Last Updated 9/15/03


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The Neon exhaust manifold is a small piece, designed more to have a low thermal mass (to help the catalytic convertor light off faster on cold start) than it is to flow well, especially at high RPMs. 

The design isn't bad as far as exhaust manifolds go, it's much better than the exhaust manifold for the '80s turbo cars, but worse than the header on the 2.0 Magnum used in the 2001+ Neon R/Ts or any aftermarket header.

To keep weight and thus the thermal mass down, the manifold is made out of nodular iron.  Nodular iron is a very hard alloy of cast iron, thus it's very strong for a relitively small amount of material.  The less mass you have in the exhaust manifold, the less heat it can absorb, so the more heat the cat gets to bring it to operating temperature.

The fact that it's made of nodular iron means it's very difficult to remove material while porting it.  In the process of doing the very mild porting I did (little more than portmatching it to the head and cleaning up the exhaust outlet) it took a long time to take a small amount of material off, even with a carbide porting bit on a die grinder.  At one point one of the 80 grit sanding rolls actually started smoldering and smoking from the heat buildup.  It also quickly destroys sanding rolls, so keep this in mind before you start.

The only reason I did it instead of putting a header on is that I want to keep the engine bay as stock looking as possible.   I have a warranty, and I don't want any dealership monkeys bitching about it.  So, I just wanted to port match it to the head a bit and smooth out the rough spots where the downpipe bolts up.  There isn't a lot of "meat" to allow you to change the port sizes much, but it would take a long time to do anyway, considering the hardness of the material.

This one has a slight crack in it between runners 2 and 3.  I plan on welding the crack shut when I get a chance to get my Dad to do it with his MIG welder.  Examine yours closely for cracks as well, the DOHC units tend to crack easily, and I suspect many SOHC units are cracked as well.  I think this has a lot to do with the head shield trapping heat.  The heat shield is important though due to the location of the stock airbox.

Make sure you remove the O2 sensor as well, it will make the work easier as well as keeping you from destroying the sensor with the bit.

I used the bits and sanding rolls I had from my headporting project on this; I'd not recommend spending the money to get these bits unless you plan on using them for other projects.  The sanding rolls and mandrels are cheap, but they're not nearly as effective either.

Tools Needed




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Here's the stock "downspout" for the exhaust manifold.  Off to the left and right are passages that lead to the individual runners.  The openings for the runners is relitively small, but there is very little that can be done about this due to the thin nature of the metal, and because it's a real pain in the rear to even get to the end of the runners, much less further up.  You could probably have it extrude honed, but what's the point?  It's cheaper and more effective to go to an aftermarket header.

I wanted to do more here by clearing up the casting flash you can see at the base where the runners meet, and I wanted to put a "knife edge" on the runner walls as far back as I could reach.  I also wanted to open up the mating point for the downpipe.  You can see the "ridge" in the lower right quadrant of the opening, I felt this was a little rough on flow.

I couldn't find my grinders grease, so I hosed it down with some WD-40.  I'll have to remember to clean it carefully before I put the O2 sensor in, WD40 is a quick way to kill an O2 sensor!  Keep this in mind if you use any kind of petroleum-based lubricant.  When you re-install it, coat the threads with antisieze, or you'll regret it the next time you change it!

I found out how hard the metal was while trying to put the edge on the runner walls.  I spent perhaps 5 minutes working on it to next to no effect.  It was shiny and a little thinner, that's about it.  I worked on the downpipe opening a bit, and managed to get most of the "ridge" gone, though on the left side by the O2 sensor housing I didn't grind too much for fear of messing up the O2 sensor hole threads.  When I was done, I went over it with an 80 grit sanding roll.  On an air powered die grinder, the roll started smoking.  It made the metal smoother but not by much!  Mostly it got off some of the carbon buildup.  Here's a picture of how it looked completed:


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I'd suggest looking at the larger image to see for sure what I did.  It's not very obvious!  You can see that the ridge is gone, the opening is a bit larger, and the runner openings are a bit more thin and shiny.  It probably won't do much of anything power wise, but every little bit helps until I get a header.  If you want to spend more time and effort, go ahead, there's a lot of material around the downpipe opening you can remove, it's just time consuming.  It took me a good 40 minutes to get this much done, and I'm starting to question the sharpness of my porting bit! In theory, you could cut it all back in the area that's black where the carbon buildup borders on where the downpipe doughnut mates up, as long as you didn't cut through the wall of the manifold. I would probably take an exhaust doughnut and place it where it mounts when the downpipe is bolted on, and use a marker to mark where you could widen the hole. Keep in mind the thickness of the metal of the downpipe inside the doughnut when doing so, so you don't over-cut.

The biggest change was on the runners themselves.  Here's some photos of the stock runners.


     
  
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From top left, clockwise, is ports 1, 2, 3, and 4 from left to right, positioned as if it was mounted to the head.  As you can see, 1 and 4 have a "squared off" port to one side, I assume since they were afraid to machine closer than that to the edge of the manifold.

Place the exhaust manifold gasket over the manifold itself.  Be careful to line the holes up exactly with the bolt holes in the manifold.  You might even want to place the exhaust manifold bolts through the holes in a few places to hold it in position.  If you like, use a red marker to outline the spots where the metal of the manifold pokes into the exhaust stream inside the holes in the gasket.  This will be especially true if you've already portmatched the gasket to the head (something I'd recommend doing if you've done any work on the ports).  You'll find that in places it will actually be too wide for the hole in the gasket, especially the side opposite the "squared off" section of the openings for runner #1 and #4.  Don't worry about this, what we're going for is the same size or larger than the port.  As long as it's not blocking flow, don't worry about it.

Once you've marked the spots where you want to remove material, or at least have eyeballed it sufficiently to know where to cut, take the cutting bit and take it one port at a time.  Focus on the areas where material needs to be removed, but make sure you got in circles around the port once in a while to keep it as round as possible.  At first, you'll get a cloud of carbon and rust, then you'll hit the hard stuff.  Use some lubricant regularly, and spray down the head of the bit unless you're using grinders grease to keep the bit from loading up.  Remember to work deeper into the port as well, so you don't get a ridge inside the port where you've not cut. 

When you're done with a port, place the gasket back on the manifold to be sure you've removed enough material.  If in doubt, remove a little too much.  When you install the gasket, chances are it'll move slightly when installing.  This will "burn out" part of the gasket, so no big deal there, but if the manifold isn't aligned perfectly, you might get part of the manifold suface blocking the port.  So it's safer to remove a little more than you need to, just don't go nuts with it. 

When done, go back and use the 80 grit sanding rolls on it to smooth the surface out a bit.  Don't worry too much about higher grit, I doubt that anything smoother would have much effect on the metal surface.  Then use compressed air to blow the crud out of the manifold.  Stand back when you do, a lot will come flying out.  Alternately, flush out with water, but it will rust pretty much immediately.  If you used WD40, I'd recommend soap and water to clean it out thouroghly, otherwise you might damage the O2 sensor upon installation.  Don't use petroleum based solvents to clean it out, unless you use something like brake cleaner that evaporates completely.

I also used some gasket removing pads I got from Eastwood (Kit, #46055) to clean up the mounting surface of the runner openings. 

Here's the runners in the same order as above once I was done:

  
  
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As you can see, the ports are significantly larger, especially on #1 and #4, where I mostly removed the "Squared off" edge of the runner opening.  You can also see where I tried to polish deeper in the port with sanding rolls without much luck.  This will at least keep flow smooth out of the head without significantly weakening the structure of the manifold.  It might be worth 1-2 horsepower tops, depending on how much your manifold overlapped the openings in the head.  But it's cheap power to me since I already had the tools!

On the head I used for my headporting project, there was a large "ridge" of carbon buildup around the edges of the ports, especially #1 and #4, where it had been built up by exhaust gasses hitting the protruding metal.  This had shrunk the port openings in the head as well due to the buildup, especially around the base of the port where the large "hump" is on a stock SOHC head.  This, if nothing else, will help prevent that.

Here's a final shot of the runner openings and manifold when I was done.


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Good luck, if you can afford a header and don't mind it not looking stock, go for it!  It will be a bigger gain than this, though it will likely take some exhaust modifications.


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