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There are various ways to lower the static compression ratio in high-boost - low-compression supercharger installations. One method in common use is a deck spacer, which is installed between twin head gaskets. Normally this has round holes corresponding to the cylinder bores.
The problem with these
supercharger installations is that "squish"-induced turbulence is lost because the piston no longer approaches any part of the underside of the head closely. If, however, the holes in the deck spacer were not round, but had tabs projecting into them in places where the valves are sure to clear them, these could "pinch" little bits of charge as the piston approaches tdc.
This would restore lost "squish", or even induce "squish" that had not been possible before by utilizing the extra combustion chamber height.
Closed chambers!
http://www.eiptunin...eads/16v_05_300.jpg
[Ned_Ludd, Jan 26 2007]
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So you would have big flat metal tabs sticking into the middle of the combustion chamber? I think you'd better just design new pistons. |
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Love to, but custom pistons are beyond the means of most blower cobblers. |
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The viability of this would obviously depend on the thickness of the plate and the configuration of the original combustion chamber. I wouldn't want something sitting so "loose" in the chamber that it starts acting like a glow plug. |
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Yeah it reminds me of a story involving a good-sized framing nail somehow getting sucked into an engine and lodging into the cylinder head. Needless to say, it ran a little rough and made lots of rattly noises. |
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You might have some luck adding some kind of channels or grooves in order to create turbulence, but I'm not sure it's possible to have low compression and a quench area. |
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You can with a side-valve or ioe arrangement - see my "High-performance F-head". |
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This one I was thinking Eaton M112 running 10% over on a 1781cc VW Golf engine with a 16V head. To emulate the stock 10:1 CR, then: |
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1 + ( (10-1) x 1781 / 112 x 2 x 16.387064 ) = about 5:1 static CR |
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(You will notice the engine would be pumping about 2Πtimes as much air) |
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That translates into an extra 12mm of deck height; probably an 11mm thick spacer plate. I wouldn't have the tab projecting more than about 15mm into the bore. Indeed "tab" might not be quite accurate: "web" might be more accurate, like a sort of eyelid or floppy vesica shape outboard of the intake valves. It should be possible to give the upper face a bit of a slope and radiused edges, etc. to avoid heat-holding sharp edges. The only question is, would the area be enough to induce meaningful turbulence? |
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[Ned], my first question is how hot will this piece of metal be? Since it is heated from 5 sides, and only cooled from one, I expect that a high temperature alloy would be needed: i.e. high Chrome for corrosion resistance (maybe 310S: 1150C, but there are much better and more expensive alloys available such as UMCO50:1250C, or even some with 5% Tungsten). I will conveniently ignore the pre-ignition problem for now. |
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Too hot, probably. Indeed, the pre-ignition will probably kill this one. |
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I've been trying to find a photograph of the underside of a Golf 16V head. I think the Audi 20V has "open" chambers, but the 16V has an odd arrangement with the exhaust valves in the middle, and if that means "closed" chambers, one could simply profile the holes in the spacer to match the chamber shape. Then one could even provide channels in the top face of the spacer to circulate coolant to the region of the resulting quench area. |
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//of most blower cobblers// |
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Never heard of one of these. Generally if you can afford to fiddle with a blower minor things like new pistons or heads are a no brainer. I think the biggest issue you have is that these spacers will be of limited effectiveness due to their location so low in the chamber(very near the deck height of the Piston) Any swirl they induce will be overridden by the swirl from the rising piston face. I would think a basic swirl tabs ground into the intake would serve your purpose much better. Since noone would bother to put on a SC without doing a port and polish job there is no reason not to do it that way. |
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Also its kind of pointless as you could just dial in another 1/4 PSI on the boost and make up for any lost power(which really wouldn't be lost it would just be redistributed to a different part of the powerband) |
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Nedd, if you went ahead with the Golf engine then given appropriate information I might be interested in producing you a cast manifold in aluminium, more or less at cost? Sorry to sound like a spammer, but I make casting patterns as a hobby. |
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neat, Ive been fiddling with machining here lately(just got my tailstock chuck plate the other day so i can Drill on the lathe) |
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I have been thinking about casting as well, so i can make some custom parts for my 65 midget that I am installing an S10 4 cylinder into and my 74 Midget that is Stock. |
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The VW 16V has closed chambers (see link), which renders the whole squish tab thing redundant. There's a nice quench area next to the exhaust valves. If the deck spacer follows its profile, perhaps with a 5deg batter to the edge, I'd have the static CR I want and squish. |
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[jhomrighaus], forging custom pistons is in a whole different class to fabricating manifolds out of tubing, or modifying existing manifolds, or even sand-casting new parts. Also, "squish" from the rising piston is exactly what I was trying to induce, as per quench areas commonly designed into combustion chamber arrangements. It's a much readier source of turbulence than induction swirl. |
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I admit the squish tabs were a bad idea, but I'm convinced that among bad ideas is where good ideas grow, so it hasn't been a waste of time. |
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[david_scothern], yes, I'd be interested, if I can get hold of a head. Thanks! I'd be able to generate any drawings you'll need in electronic format. |
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This is not a short-term project, however. I'm also interested (at the risk of invoking [jhomrighaus]'s possible further scorn) in running some combination of SU carburettors on this engine - I have yet to ascertain exactly what - so I'd need a carb-to-blower manifold, too. |
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Why would running Carbs induce my scorn, I have a beautiful little pair of them on my MG and it is actually a pleasure to rebuild them(which is good considering SUs on old MGs need to be rebuilt every 7 or 8 miles) Would you really need more than 1 though with a SC? I suspect you could flow far more than you could use through even a reasonably sized carb. |
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They dont make a reduced length rod for those engines? Why not just have new rods machined? |
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No worries, Nedd - I'm not a short-term sort of guy... I guess I'd need the following: |
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- drawing of the intake manifold mounting face (ie where the ports are and how big they are, plus where the manifold is to be bolted to the head).
- similar details for supercharger
- some indication of desired runner length, minimum bend radius, and what sort of direction you'd like it all to head in. I'd advise you come up with a rough model of the engine bay and use it to work out where the super and its ducting will fit. From my point of view, putting the super above the cam cover would be ideal, as that way the manifold becomes a simple U shape, but if it's not feasible then an alternative must be found. |
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Keep in mind that, although I'm a mechanical engineer, I'm by no means a fluid dynamics expert. I can produce you a cast manifold (pattern by me, casting done by a local foundry) but I can't guarantee that, left to myself, I will come up with a beautifully efficient system. Obviously I'll try for gentle bend radii and no sharp changes in anything, but if there are some subtleties in the flow path I'm likely to miss them. |
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Incidentally, a friend locally might be interested in a twin carb conversion for an 8v golf. It's probably too far from what you're considering to be of interest, but if both of you were looking at the same project then I could make two castings from one pattern. |
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Well, [jhomrighaus], you've presumably also got a live axle under at least one of your MGs... But I'm glad to hear you appreciate the beauty of the constant-vacuum carburettor. The HS2 on my Morris Minor is pretty much set-and-forget. |
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I suspect that a single HS8 will do the job on the supercharger. |
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Thanks, David. It'll probably be a while. |
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