Revisie (Cylinderkoppen)
Above are a couple of pics of the combustion-chamber, before and after my cast-iron-dust-therapy. This is where I started, to find out which stones, sandpaperrolls etc. would best suit my needs. In the combustion-chamber there are relatively large surfaces, so you can afford to err a (little) bit, without destroying a valve seat, or a complete head. Why i polished the combustion-chamber ? Because protruding bits of metal, edges and so on can't dissipate heat as well as a smooth surface, and in this motor these bits might start to glow under high load conditions (in combination with the high compression ratio) thus causing detonation (the dreaded pinging). I think chances of that happening are very small anyway, because I'll be burning LPG mostly, with an octane rating of 104-108. Still, I want to be able to burn gas too, when neccesary (empty LPG tanks for instance). Besides, if you used one chamber for practicing, you're going to have to do the other seven as well.
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Next, I took a new intake gasket, and marked the size of the 340/360 sized ports on the heads, so I could see what needed to be ground away and smoothed out, to mate the Weiand Stealth to these heads. At the beginning of the port I took a few mm off the walls, rounding it off as I went deeper into the port. I left a few mm of flesh around the pushrod holes.
Then I started smoothing the intake ports. I began with smoothing/blending in the casting mould lines, and went on to trimming the (massive) lumps of cast-iron in which the valve guides were drilled. If you do a web query on head-porting, you'll find that everybody warns against getting the intake ports too smooth, which increases the risk of fuel-separation. But, since this motor will be be burning LPG 99 % of the time, I don't have to worry about that. My intake ports are shiny and smooth now. On to the hardest bit. If you lay the head with the combustion-chambers facing you, and feel in the intake holes, you'll find a somewhat sharp edge on the short-side radius. I tried to make this into a smooth transition from valve-seat to intake-port, with taking out as little material as possible. The turn the gasses have to make is sharp enough as it is. It only needs smoothing. Also the smoothing of the intake-valve seat to the rest of the port is a bit tricky. Here I didn't really remove a lot of material, but again tried to get the transition from the seat to the port as smooth as possible. Ideally you should make the diameter of the hole below the seat (the bowl) some 85% of the diameter of the valve-seat, in a venturi-like shape (where attainable). By now you might be wondering why I put all this effort in a couple of (junk) 318 heads, instead of Edelbrocks or 340/360 heads. Here's why: First, because this engine is going into a reasonably heavy W200, I want all the torque I can get, and, with these heads, gasspeeds at lower rpms will already be quite high, which is good for things like ram-effects, vacuum-signal and so on. Second, I thought it would be a waste to first use KB pistons to get the compression-ratio up, and then using heads with bigger combustion-chambers, which would nullify the rising of the c.r. again. |
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Above are some more intake-port pics.
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For the exhaust ports the same applies, smoothing the casting seams, removing casting flash and blending in the bowls. On the pictures above I had only done the rough grinding work, and no polishing. (...and you have absolutely no idea on how many of those Dremel sandpaper discs I saw flying by. Neither have I...)
There's differences too, in the exhausts the valve guide sits in a flat piece of cast iron, so there's little to trim there. Also, the heads were provided with air-injection ports, ending in humps of cast iron on the port floor a couple of cm from the exhaust manifold side. I ground these flat, and on the outside I tapped M6 thread into these ports, which I then plugged with M6 headless allen screws, locked with Loctite high temp silicone, and staked with a centerpunch. |
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To be able to calculate the exact compression ratio, when all is done, you need to now the volume of the combustion chamber. I did this by glueing a piece of plexi-glass to the head with a few blobs of grease. I also sealed the valves with grease, and screwed an old spark-plug into its hole. Then I used a 10cc medical syringe and a bottle of lamp-oil to fill the chamber. Unfortunately medical syringes weren't made for cc'ing heads with lamp-oil. I only needed 2 syringes per chamber. On average, I measured 69cc, which means I'll get a compression ratio of about 9.7. When I first started the engine, I also noticed this because the poor old starter now has to work very hard to rotate this engine. Compression pressure (measured after some 100-150 miles after first starting her) average is about 134 psi.
