Engines and Numbers and HP – Oh, My!

The 2.5L 944 engine has gone through some modifications over the years, and there are always questions about them.  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.  To to take some of the mystery away, here is a little info that can help clear up some confusion.

As we know, the 944 2.5 was derived from the 928 V8 engine, so there are many similarities.  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.  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.

Numbers

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.  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.  Anyway, the typical number that you are looking for is M44/ followed by two digits.  Obviously, the M44 refers to 944; the other two numbers describe the series.

Here is a breakdown on the numbers:

• M44/01  1982-1985, 160 hp (Rest of world)
• M44/02  1983-1985, 147 hp (USA.CA,Japan)
• M44/03  1982-1985, 160 hp (Rest of world)
• M44/04  1983-1985, 147 hp (USA.CA,Japan)
• M44/05  1985-1987, 160 hp (Rest of world)
• M44/06  1985-1987, 160 hp
• M44/07  1985-1987, 147 hp
• M44/08  1985-1987, 147 hp
• M44/09  1988, 158 hp
• M44/10  1988, 158 hp
• M44/11  1989, 162 hp, 2.7L
• M44/12  1989, 162 hp, 2.7L
• M44/40  1987, 187 hp 2.5 S
• M44/41  1989-1991, 207 hp, 3.0 S2
• M44/43 and 44  1992-1995, 968 3.0L 236 hp
• M44/50  1986 2.5T, 217 hp
• M44/51  1986-1987 2.5T, 217hp
• M44/52  1987-1991 2.5T, 246 hp

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.  The 147 hp engines have 9.7 compression, the 158 hp engines have 10.2. and the Euro-spec 160 hp engines have 10.6 compression.  The pistons have a depression in the top that is larger for the lower compression numbers.  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.  There are aftermarket piston makers who do not use the “depression” cut, instead opting for a flat-top piston.  Here is a great diagram to show the differences.

Tolerance Numbers

On the top of the block and on each piston is a number that should match each other.  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.  In the 100mm bore (2.5), here are the tolerance group numbers:

• 0 – 99.980 mm to 100.000 mm
• 1 – 99.990 mm to 100.010 mm
• 2 – 100.000 mm to 100.020 mm

You can also use these numbers to check the bore spec on the block and the piston wear.  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.  Just make sure that when you are replacing pistons between engines, the numbers match.

The #2 Rod Bearing Issue

If there is a weak spot in these engines, it is the tendency for the #2 rod bearing to fail, especially on early engines.  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.  But why is it only #2?

It seems that the 2.5 engines between 1982 and 1988 have a peculiar thing going on.  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.  As the piston goes down, the air has to go somewhere since the space that it occupies is getting smaller.  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.  This temporary air bubble/pocket decreases the amount of oil around the #2 rod bearing, and over time this can make the #2 bearing wear more, causing an eventual “first failure” – failure before the other three rod bearings.  There are places for the air to go between the other cylinders, so the only problem is between #1 and #2.  When Porsche got to the 2.7L block and bored the cylinders from 100mm to 104mm, they also created a air passage between #1 and #2, so the 2.7L and 3.0L  engines do not have this issue.  Also understand that this is not normally an issue on engines that are used on the street.  However, it is a common failure in race engines that are constantly pushed at very high rpms.

So now you know.  I am investigating fixes to the early block to create an air passage and will pass that information on to you when I get it.

Any other questions?  Email and we will see what we can do for you.