This chart shows the theoretical resultant static compression ratios before and after certain modifications. As there are discrepancies in the construction of engines and in official engine specifications this chart shown can only be used as a rough guide.
Gudgen / Wrist pin compatibility between 4AGE Bigport cylinder heads: The Bigport variants of the 4AGE engines use a press fit 18mm wrist pin. This is unlike all of the other A-series engine variants that use a 20mm pin. Therefore these 4AGE Bigport pistons can’t be used with 7A rods and hence why there is no quoted figure for the result.
Piston to block / head clearances with 7AFE conversions: All of the quoted compression ratios for the 7AFE bottom end are the calculated result if the piston to deck height is kept the same between the engines. Without any changes to the 7A engine block & rods and with some 4age pistons fitted the top edge of the pistons are held below the deck face, where as they would normally be level when fitted to the 4AGE. Therefore the piston to head clearance would be increased and resulting compression ratio would be lower than the calculated figures above. It is always a good practice to not have a larger piston to head clearance than the engine in it’s original form (1.2mm) as it compromises the function of the combustion chamber. The first step in working out the correct course of action is to do a test fit of the internal parts and then measure the difference between the height from the piston to block deck, this resulting figure will show how much the gap needs to be altered. This increased clearance can be corrected via a few different methods, the most common being to have the deck of the block machined down to a lower height. Another is to run a thinner head gasket, we offer TRD options that are 0.8mm thick (-0.4mm) or for when it’s to suit a 20V head a 7AFE head gasket can be used which is reported to be only 0.4mm thick (not compatible with a 16V head). Lastly there has been a report of some aftermarket 7A con-rods being 0.5mm different in length from original, this will need to be checked in advance if there is a plan to upgrade the rods at a later point. It is also of critical importance that the piston to head clearance is not too small either, various factors effect this minimum clearance but as a guide down to 0.8mm is considered safe.