Flycut porsche 944 piston12/17/2023 ![]() Unfortunately, the design of the block does not have a place for the air to go – it just goes down into the oil, then over to the area under the #2 piston and rod – which is going up. As the piston goes down, the air has to go somewhere since the space that it occupies is getting smaller. Visualize this – when the #1 piston is in its down stroke, there is an air pocket between the bottom of the piston and the oil in the pan. It seems that the 2.5 engines between 19 have a peculiar thing going on. For our race engines, we dutifully drop the oil pan each year and roll in new rod bearings to ensure that we don’t wreck the engine due to a failure. If there is a weak spot in these engines, it is the tendency for the #2 rod bearing to fail, especially on early engines. Just make sure that when you are replacing pistons between engines, the numbers match. While it may not seem like a lot, these tolerances will make a difference on how well your engine runs – and how long it will last. You can also use these numbers to check the bore spec on the block and the piston wear. In the 100mm bore (2.5), here are the tolerance group numbers: This is a bore tolerance group, either “0,” “1” or “2.” (By the way, the only way to check the block is to remove the cylinder head…) What these numbers mean is that the cylinder and its corresponding piston are in the same machining tolerance level. On the top of the block and on each piston is a number that should match each other. Here is a great diagram to show the differences. There are aftermarket piston makers who do not use the “depression” cut, instead opting for a flat-top piston. Rule of thumb is that if the round depression is the size of a silver dollar, it is a 9.7 piston size of a quarter, it is a 10.2 piston and the size of a dime, a 10.6 piston. The pistons have a depression in the top that is larger for the lower compression numbers. and the Euro-spec 160 hp engines have 10.6 compression. The 147 hp engines have 9.7 compression, the 158 hp engines have 10.2. In the normally aspirated single cam engines, the difference in power is mostly found in the compression ratio, which is determined by the piston design. Obviously, the M44 refers to 944 the other two numbers describe the series. ![]() Anyway, the typical number that you are looking for is M44/ followed by two digits. I say “most” because I have come across a few that do not have numbers stamped on them, but these appeared to be early engines – maybe factory replacements, who knows. Most 944 engines have a number stamped on a flat spot of the block on the back, near the firewall, where the block meets the bell housing. Single cam and dual cam heads were also introduced, but the bottom end of the engine stayed pretty much the same throughout the production run. ![]() That said, the displacement stayed at 2.5 from its inception in the 1983 944 up until the introduction of the 2.7L in 1989 and the 3.0L in 1990. To to take some of the mystery away, here is a little info that can help clear up some confusion.Īs we know, the 944 2.5 was derived from the 928 V8 engine, so there are many similarities. Many of our cars have had engine swaps over the years, so there is a real possibility that the engine in your car is not the original, and may not be the original spec year for your car. The 2.5L 944 engine has gone through some modifications over the years, and there are always questions about them. ![]()
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