2512cc Opel 8V CIH Competition Engine
Short Engine Specification
You may feel that my short engine is over-specified, with respect to the requirements of a CIH engine not expected to exceed 7500RPM. However, it is my intention to change to a 16V cylinder head, or turbo-charger, in the future (with new pistons of course).
The block is from a 2.0l engine, which has been used for about 50,000 miles. This very important as the stresses put in the block during manufacturing, will have been relieved. The block has been bored and honed, with torque plate fitted, to give a bore size of 97mm. The torque plate (usually a steel plate about 30-40mm thick) puts the block under similar stresses as the head would cause, when torqued down on the block, and ensures that the bores are true and round in the completed engine. After boring the block to 97mm, we removed the torque plate, and measured the bores to be out of true by 0.0015" (0.038mm) at the top end of the bore where the head bolts are located. This small distortion may seem like very little, but is enough to allow gases to pass the rings and reduce dynamic compression and hence engine output. So, our findings suggest that if the block is bored without a torque plate, the top of the bore will become distorted when the head is torqued down.
We require at least an 11.5:1 static compression ratio for this engine, which will run Super Unleaded (97 RON) petrol. The sump pan is baffled to keep the oil where it's needed, especially during cornering on the track. A block breather will be installed, located where the original mechanical fuel pump was fitted on the carburettor versions of the 2.0S engine. This should ensure that any gas passing the rings will not pressurise the lower block and so increase windage losses.
The steel billet crankshaft was manufactured by Farndon Engineering Ltd, and has an 85mm stroke, giving an engine capacity of 2512cc, with 97mm pistons. The counter-weights on the crank have been "knife-edged", to reduce both windage losses and weight. The present weight of this crank is greater than a standard crank, due to the billet material used, but the "knife-edging" process should have reduced it's weight by about 15-20% (unfortunately I forgot to weigh it before and after).
The I-beam con rods are also manufactured by Farndon, having 55mm big ends, 22mm little ends, and are 134.1mm between centres. The pistons will therefore protrude from the bore by 0.1mm (measured and verified). We require a squish depth of 0.9mm and are using a head gasket with a thickness of 1.4mm. It was therefore necessary to skim the top of the block and timing cover by 0.4mm.
The pistons have been supplied by Risse Motorsport, and were manufactured by Omega Pistons Ltd, in the UK. The pistons are forged and made for a 97mm bore, with 31.5mm compression height, and a 5.5mm dome. The gudgeon pin diameter is 22mm. The inlet/exhaust valve cut-outs were modified to give more of a wedge shape, which should give better flow between the valves during overlap. The piston was also modified with a cut-out to allow better flame propagation from the spark plug. Both of these changes can be seen in the sketches. These changes have added about 3.0cc to the combustion chamber volume. The pistons have three rings, the upper two being Compression Rings and the lower being an Oil Control Ring. The ring gaps are shown in the sketch.
A standard head gasket has fire rings with a 97mm diameter bore. When compressed, the fire ring bore diameter reduces to between 96.3mm to 96.7mm. I will be using 97mm bores for the pistons, which would mean that the fire ring could protrude into the combustion chamber and cause a hot spot. As you would expect Risse Motorsport will be providing the new head gasket, which has an increased gasket bore size of 98mm, and a compressed height of 1.4mm. Grooves have been machined around the top of the piston bores, copper wire rings inserted into these grooves. sticking up above the surface by 0.005" (0.13mm), which will pinch the gasket fire ring in place. This is a modification much used on Ford Pinto engines in the UK, and helps to stop the head gasket fire ring from being pushed out when using high compression ratios.
The crankshaft, flywheel, clutch cover, crankshaft pulley, camshaft and camshaft pulley were dynamically balanced to race standard by Steve Smith at a company called "Vibration Free". The con rods and pistons were all be weight matched to within 0.1 gram by the same company. This balancing work cost £180 (€295) at the time, which in my opinion was very reasonable.
The oil pump flow will be increased by 30% using a set of longer pump gears, and a spacer, supplied by Risse Motorsport. The pump spacer has an integrated timing sensor bracket, which is very useful. I will also be using the cast iron pump cover (Vauxhall/Opel Part No. 3448844) from the diesel engine, which gives much better pressure control, because it has a steel pressure control shuttle not a plastic one which deforms and sticks.
The Cam Chain Tensioner has been modified to give it a manual locking mechanism. A 5mm hole was drilled in the cap, on a lathe to ensure it is centred, and was then tapped out to 6mm. An 80mm bolt and nut arrangement is used as the locking mechanism. The tensioner still contains the spring, so it can still operate as an automatic tensioner. This modification ensures that the cam chain cannot become de-tensioned, and allow the cam chain to jump teeth on the sprocket.
Upon running the engine we found that oil leaked from the 6mm bolt threads, because the oil in the tensioner is at full engine pressure. In the end I machined up a replacement locking nut from 19mm hex steel bar, and installed an o-ring to make the seal, see the sketch. The new locking nut is 10mm deep, to help reduce the chance of leakage up the bolt thread again.
Be warned, over-tightening of the manual locking mechanism, can cause early wear, and potential failure, of the cam chain.
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