Bini Supercharger Conversion (A perspective)
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Bini Supercharger Conversion.

As I am bored, I will put my views down and then you lot can see what you reckon.

Blower.

The blower as fitted to the Bini is an Eaton/Magnuson 4th generation blower. These guys are at the forefront of rootes blower technology and their series 4 is about as good as Rootes blowers get. Sadly that is about 60%(max) Adiabatic Efficiency (Eaton). Comparing poorly to a Screw Blower or Turbo at about 80% (Haynes). Which means when you load it excessively, the unit re-circulates excess air and heats the charge excessively. So, for 12 PSI, the temperature rise is 80 deg C (over ambient). This tails off exponentially and is about 60 at 10, etc, etc. So, the only thing that Bini blowers have got going for them is that they are very cheap. The intake housing is a bad shape and inconveniently large, the unit is gigantic, when placed next to the A Series, it is pretty mind boggling. But for a low boost unit to just augment an engine with already good characteristics, it does it a treat. Which is why the unit figures in low boost applications. It may be practical to use an intercooler to cool the charge slightly, but a boost increase will be non-linear and so the returns don?t justify the means. However, it will allow a higher compression ratio to be used, which will reveal as much power as possible. This looks like being achieved in the biggest possible capacity running as little residual boost as possible, but getting the most air through the intake and out of the car exhaust. This has to be done with particular attention to gas flow, as much as any high performance engine although the characteristics are peculiar to a supercharged engine.

Configurations.

There are only 2 serious options available to the supercharger conversion chap. The Shorrock style ?Above the inlet manifold? is the best idea. The easiest to belt up, is probably the best space for it but there is no way in God?s earth (I can see) it would be possible to pipe up for a intercooler (Without a big bonnet bulge). As the engine is a small capacity job (for the blower) and I am trying to get as much out of it as possible, I will consider the alternative. Having it sited at the front, where the alternator would go, but a bit higher up. Before you think otherwise, there is insufficient space (with the pipe work/mounts/oil filter, to mount it with the rotors stacked (ie:- Vertical) so the unit must be mounted ?sunny side up? blowing downwards into a fabricated collector. You know if you have it the right place as the unit will foul the slam panel near the bonnet catch, it will be necessary to tip the engine back a tad on special mounts. Once this is achieved, it must be belted up and mounted (which is a lot more tricky than it looks) then it can be piped into a suitable intercooler (Rover TD looks like my favourite choice, although the ?Bar and Pipe? design is not the most efficient) then blown through a conventional metro turbo set up. The difference is that a conventional exhaust manifold will be required.

Cam/Head/breathing stuff.

This is where I talk bollocks but it makes sense to me. What is the go for N/A engines, isn?t the same for Turbo engines and again different to supercharged engines.

As the Vizard book says (I had to quote that), increasing the cam duration allows the engine to behave in a form relative to it?s N/A cousins. Whilst this is true and I hear a 276 is great and ultimately a 286 is expected to give great results, I think this is largely bollocks. Whilst it might work the inherent differences must mean that the cam for a N/A engine has insufficient exhaust duration. So, this chokes it up a bit and might sod up the low RPM running. Besides, I?m not investing in a steel bottom end, so I am not going to rev its nuts off anyway. So, the point is at low RPM, the overlap will allow blow by and kill potential power, just as the converse will choke the cam off with not a bit of overlap. Bypassing a load of complicated maths bollocks, about an extra 15deg on the exhaust is a good idea, going to 20 with more wild cams. So, not wanting to go for a special grind, I looked at the Avonbar Ph2 cam, which has the added exhaust duration and lift. (Off the top of my head, it is pretty much a 266 with added exhaust duration of about 15deg).

So, having a look further at the figures for the cam, I see that the exhaust has really high lift, which is a good thing as well. I am now going to go on about the bit of valve choice. Since dP between the inlet and chamber is going to be a lot more than N/A, it follows that the inlet can flow less, as it doesn?t need to, hence less lift, less important inlet valve sizes but the contrary for the exhaust. It will be producing loads of exhaust gas (Try a 2.0 N/A Engine) and so needs big flow. It doesn?t need any more overlap to start interfering with the bottom end, so a bigger valve is more important than a greater rocker ratio.

Roughly, 1.3->1.5 rockers ups area for flow by about 15% more.
Going from a 29mm->32mm exhaust valve adds about 10% more area for flow and acts 10% quicker whilst leaving the overlap untouched. 1.5 rockers increase the significance of the present overlap. Maybe a good idea, but I think not. Obviously, the exhaust system will have to flow for the bigger capacity and I personally would choose a big bore LCB as well as a 2? system.

So, looking at the head. From a load of bollocks and personal hunching, I have decided that 12PSI intercooled, with a 8:1 CR should allow me a sensible margin with 98 RON fuel. That is with a 1380, offset bored, as the extra capacity should release much more useful ?work? than an overblown 1293. This appears to be a good move from what I have read elsewhere. Regarding the rev limit of about 6500, it may be possible to gain more power by stroking to 84mm (1430cc) as the lever action should free up a significant amount of torque.

The problem is, how to get the chamber out to the right size. Looking at my casting, and a few others I have, I think that squish matching a head to a piston, with a circular chamber milled out to about 28-30cc, radius being the most important thing. The required gap would then be filled with a matched ?Decompression Spacer? gasket, of the required thickness to drop the compression ratio. To 8:1 (I think the area required is about 15cc after 4cc taken off for 2 gaskets). The exhaust ports would be enlarged and the bosses cut back ?enough? and the exhaust ports controlled vortex bollocks eliminated and the thing totally matched.

There is very little to read about supercharging an A Series, cam choice, etc. The shit above is entirely fabricated by me, outlines what I believe to be the right way of getting the most out of the set up. It might make a useful read for someone planning to do a conversion by themselves. However, it is a lot more difficult than it looks. Much more and a kit would be pretty much a good idea.

But, that would be too easy.

Cleaned up and ready to be put back together now (as above)

Stu (off to the pub)

Edited by Hedgemonkey on 26th Dec, 2004.

Bugger off, I'm getting there.