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Compression testing guidelines
Compression testing and ratios
A compression gauge tells you what pressure it sees in the combustion chamber when the engine is being driven by the starter motor.
The first problem is that compression gauges are not precision devices nor are they intended to be. They are only good for relative measurements.
A good way to test a compression gauge is to measure the same cylinder several times over a period of time and see how the measurements change. Second, a cold engine, driven by a starter motor isn't a great approximation of an operating engine. The cam can have large effects on the numbers as can the cranking speed. This is why using a compression gauge to estimate compression ratio is likely to be inaccurate.
However, one table of CR and pressures comes with the following warning: "Just to satisfy the reader's curiosity, below is a table showing the approximate relationship between compression ratio and compression pressure at cranking speeds. However, even if a table such as this could be trusted, there is the question of gauge accuracy." (Motor's Auto Engines and Electrical Systems c.1970)
Compression ratio, calculation
Static compression ratio is the ratio of:
swept volume + combustion chamber volume to combustion chamber volume
Dynamic compression ratio is calculated in almost the same way except the volume is calculated from the point the inlet valve closes. Wilder cams lead to a reduction in dynamic compression ratio, which is why you generally need a higher static CR the wilder the cam you fit. It is difficult to quote an ideal CR figure for a given fuel type because of the volumetric efficiency of the engine in question.
The volume includes the volume of the compressed gasket, you should always include this in your calculations. Good gasket manufacturers quote this in their specs.
Compression ratio, 998cc engines
A+ engines used the CAM4180 cylinder head (giving a combustion chamber of 25.5cc), and were fitted from late 1980 (City and HL). These had a CR of 8.3:1.
From Apr 1982 flat top pistons were fitted (City E and HLE) for a quoted CR of 10.3:1.
Metros used the A+ engine, and in 1980 were quoted at 9.6:1 for the normal version and 8.3:1 for vans and fleet order.
Some people feel the 10.3:1 CR is incorrect for High Compression engines, favouring 9.6:1. You can reach a figure in the region of 10.3:1, if you leave the head gasket out of the equation.
Compression Ratios, 1275cc Metros
Here's a potentially useful fact for you. The Metro Van den Plas has basically the same engine as the MG Metro, except that most of them had the standard 1275 Metro inlet/exhaust manifold (which has the same ports as the alloy one so the only downside is the hotspot). However, the MG Metro engine's compression ratio is 10.5:1, whereas the Van Den Plas' compression ratio is 9.4:1 (also quoted as 9.6:1).
Loads of scrappies don't know this so they charge a lot
less for a VDP Metro lump than they would for an MG Metro lump.
The HIF44 carb had different characteristics on the Van
Den Plas to the MG, due to the lower CR. The coil also had different
characteristics on the Van Den Plas to the MG, and the Van Den Plas distributor
was the non electronic 59D4 even when the MG had the electronic 59DM4, until
1986 when they all went electronic.
Timing was also different on the MG compared to the Van Den Plas on early models, once again due to the lower CR.
The MG metro turbo had the different head until 86 when all the changes that were done to the turbo went to all the 1275 metros apart from the bigger first motion nose bearing and sodium cooled exhaust valves. MG metro and VDP both had the same head and cam.
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