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2.4l Opel 8V CIH Competition EngineEngine Strip downSince the car is in bits, being re-sprayed at the moment, I decided it was time for an engine strip down, to check the compression ratio and check for any other problems. We have measured the compression ratio, and found it to actually be only 9.72:1. According to SP Performance, who built the head the compression ratio should have been 11:1. On removing the head, we found the combustion chamber, pistons, valves and plugs to be coated in a layer of soft sticky sooty deposit. We initially thought that the dished pistons from the Opel 2.4i CIH engine were not working properly, causing the air/fuel mixture to be combusted inefficiently. Here is the reason for this initial assumption. The CIH head has a wedge shaped combustion chamber, with a "squish" area ( flat area on piston and combustion chamber) opposite the spark plug. When the piston reaches the top of it's combustion stroke, the squish area should be between 1.0 - 1.5mm deep, causing all the air/fuel mixture to be pushed into the combustion chamber near to the spark plug. Under normal circumstances, combustion of the air/fuel mixture should not occur in the squish area. On my old 2.0L engine, the parts of the pistons and head associated with the squish area had almost no deposits after nearly 13,000 miles, and the bright metal of the piston and cylinder head were still visible in most of the squish area, which indicates good combustion efficiency. We have also found the valves to be very loose in their guides, which suggests that they were not installed correctly, or the guide material was incorrect. This was a major surprise as the engine has covered less than 1000 miles. The valves were also found to have sunk into their seats, which suggests that the valve springs were much to strong for the weight of the valve train. Back to TopCylinder Head Flow CheckI have had the SP Performance cylinder head flow bench tested by a guy called Jim Garrard. I decided to compare it against a standard 2.2L head and a Stage 4 Full Race Head for a 2.2L engine. 
SP Inlet/Exhaust - The head built for my 2.4L engine, specification above. On the inlet graph, below 0.33" (8.4mm) valve lift, it flows worse than the standard 2.2L head giving poor mid-range torque. For the exhaust, it is really terrible as you can see. At maximum flow, the exhaust can only flow 50% of the inlet flow. My new head builder, Jim Garrard, says that the exhaust must flow at least 64% of the inlet flow on a normally aspirated engine. Jim says this is the most likely the reason for my engine performing badly, because the exhaust gasses cannot escape and so contaminate each new fuel charge. JN Inlet/Exhaust - This is a stage 4 full race head built from a 2.0L head by an English Manta head tuner, called John Noble. The inlet valve diameter is 45mm (with 9mm wasted stem). I think the exhaust valve diameter is 38mm (with 9mm stem). The head uses a Kent OP254 Rally/Race cam. This head was fitted to a 2.2L bottom end and the compression was approximately 11:1. See my For Sale page. Notice that the inlet flow is less than on a 2.2L standard head, which is not unreasonable since the 2.2L head has a better inlet port design. This pretty well proves that the 2.2L head can flow better than the 2.0L head, Unfortunately using the 2.2L head means that you may have a problem getting the compression ratio high enough. 2.2L Inlet/Exhaust - Totally standard 2.2L head, absolutely no modifications. This is the standard by which I am comparing the other heads. Back to Top |
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