Mystery engine

[offroader1006]

Hi guys,

I bought a work in progress 73 911T targa from a guy.

He had the motor built, but couldn't remember where or find any documentation. He simply said the displacement was increased dramatically.

The engine case serial number is 6117669 and it's stamped 911/07. Checked underneath and it's a a 3R case

This should be a 2.2 from a 1971.

The engine has new cylinders, heads, head studs, and pistons just from my view removing the upper valve covers.

I can see a 911.104.306.0R on the heads. No numbers on the cylinders are visible.

I decided to measure the displacement using motor oil and a 100cc graduated cylinder.

I took the engine to 90°on the stand. Then I found TDC on compression on cylinder 3, and filled the combustion chamber with oil, to just touching the first spark plug thread.

60cc's

I then took the engine to bottom of the stroke and filled the cylinder to the bottom of the spark plug thread's, to get the total displacement.

76mm stroke, measured crudely with a rod and ruler.

465ccs

This is without the first 60cc's.

Total is 525cc's.

This seemed very high, as the resulting total displacement would be about 2.7 liters.

I rotated the engine 180° and repeated the procedure on cylinder 6.

Combustion chamber 60cc's
Cylinder displacement 470ccs

I decided I might be leaking past the dry rings, so I oiled all the cylinders and turned the engine over a few rotations.

I rotated the engine back and repeated the procedure on cylinder 3.

Combustion chamber 60cc's

Cylinder displacement 480ccs

This would work out to 2.8 liters and about a 9:1 compression ratio.

The cylinders are definitely not leaking, as I left the last cylinder sitting full for an hour, and it did not drop away from the last thread of the spark plug hole.

I'm very confused, as I've never heard of a 2.2 case with bigger displacement than 2.5 liters.

With the measured 76mm stroke and 480cc volume, solving for the diameter of the cylinder, gives a 90mm bore. This all seems accurate.

[Trackrash]

Yes, looks like a 2,2 case. It also looks like it might have been bored for larger cylinders. If the base of the cylinder is larger than the mating surface on the case then they are larger than original, perhaps 90mm. Look at the area at my arrow. If the bottom of the cylinder is obviously larger than the seating area, then I has cylinders from a 2,7.

As far as the stroke goes, I have never heard of a '76 stroke in one of those cases, but anything is possible. That would take a custom crank and rod set. The reality is you will have to tear it down to know for sure. Really clean mag motor BTW.

[dg567]

Case looks like it was sprayed battle ship gray?

[winders]

Originally Posted by dg567

Case looks like it was sprayed battle ship gray?

It's a magnesium case.....

[dg567]

is the oil sump plate and drain plug also magnesium? no, same shade of rattle can gray IMO

[winders]

Originally Posted by dg567

is the oil sump plate and drain plug also magnesium? no, same shade of rattle can gray IMO

I've never seen paint oxidize like magnesium....but you might be right.

I guess you meant to state emphatically that the case was painted battleship gray instead of asking a question and inviting other opinions. My mistake......

[stownsen914]

The most likely stroke options for early engines are 70.4 or 66.0 mm (stock for '71 is 66 mm). Later engines have longer strokes, but those cranks don't fit the early cases. Combustion chamber volume should be closer to 70 cc.

[BURN-BROS]

Those are at least 90mm cylinders.....Do they look like they have been installed upside down to anyone?

[targa72e]

Pretty easy to make 2.7 from 2.2. Bore case for larger cylinder from 2.7 (90mm). Add crank and rods from 2.4/2.7 and you have a 2.7 engine. The cylinders look to be later 2.7 aluminum cylinders.

john

[Walt Fricke]

Could even be a 2.8 - they used the 7R case. 92mm bore. Or even a 2.9 with a 93mm bore. Maybe why the fins look a bit shallow?

[deftom]

It looks like they used etching technic on magnesium case to preserve it. That would explain the grey color.

[brighton911]

Agree with Aaron, those cylinders appear to have the short fins down.