Saturday, February 19, 2011

Nissan QGXXDE Piston Dimensions and Engine Capacity

Picture below extracted from the N16 EM "Engine Mechanical" section of the service manual illustrates the different piston diameters according to grades. The stock piston is a cast piston.

The diameter of the piston will be slightly smaller than the cylinder bore to compensate for heat expansion, etc but generally cast pistons can run tighter piston to bore clearance compared to forged pistons. The stock piston is available in 0.5 to 1mm overbore diameter direct from Nissan. Go larger at your own peril!

The piston skirt or "a" dimension is as follows:
QG13 = 47mm (1.850")
QG15 = 43.5mm (1.713")
QG16 = 44.5mm (1.752")
QG18 = 45.1mm (1.7756"). Interestingly the USDM B15 QG18DE piston skirt is stated as 42.3mm (1.665")
Piston Pin Outer Diameter: 18.989mm to 19.001mm (0.7476" to 0.7481")

TCM states engine capacity for N16 1.6 as 1597cc. Here's how the engine capacity is calculated:

QG16 piston diameter = 76mm or 7.6cm
QG16 stroke = 88mm or 8.8cm
This means piston bore area = 7.6cm X 7.6cm X Pi or 3.14159 / 4 = 45.3645596 square cm
Volume of 1 cylinder therefore  is bore area X stroke = 45.3645596 X 8.8 = 399.20812448cc
As the QG16 has 4 cylinders this means total engine capacity = 399.20812448cc X 4
= 1596.83249792 rounded up to 1597cc.

You can calculate what a 0.5mm or 1mm oversized piston will give u but consider that the 4mm of larger piston on the QG18DE will only give you 1769cc.

QG18 piston diameter = 80mm or 8.0cm
QG18 stroke = 88mm or 8.8cm
This means piston bore area = 8.0cm X 8.0cm X Pi or 3.14159 / 4 = 50.26544 square cm
Volume of 1 cylinder therefore is bore area X stroke = 50.26544 X 8.8 = 442.335872cc
Overall engine capacity = 442.335872cc X 4 = 1769.343488 cc or rounded up to simply 1769cc

Now if I go 1mm overbore what would my capacity then be?

QG18 1mm overbore piston diameter = 81mm or 8.1cm
QG18 stroke = 88mm or 8.8cm
This means piston bore area = 8.1cm X 8.1cm X Pi or 3.14159 / 4 = 51.529929975 square cm
Volume of 1 cylinder therefore is bore area X stroke = 51.529929975 X 8.8 = 453.46338378cc
Overall engine capacity = 453.46338378cc X 4 = 1813.8535cc or rounded up to simply just 1814cc which means that you are still @ 1.8 litres.

So why can't I go larger than 1cm overbore? Consider the fact that there must be sufficient surface area between each cylinder for you to do that. For a modern compact 4 cylinder transverse engine there is typically little space. Especially more so the QG18 where 1mm even looks like a wee bit too much with 0.5mm a safer bet. Go too much overbore and you might actually bore into the water cooling passages or there might not be enough surface area for the head gasket to seal properly. So for those guys who like to talk about larger pistons in forums the answer is you can try going slightly larger piston but it will have little difference on the overall engine capacity. This applies for "small" capacity 4 cylinder engines. Of course if you have a 7 litre V8 its a different story. If you want more capacity your best option would be to increase the stroke via a stroker kit but do consider the fact that the QG16/18 is already an undersquare engine (stroke > than piston bore) hence further increasing the stroke will just impede the engine's ability to rev. And since a stock QG already does not like to rev this option is a no-no (at least to me).

No comments: