Rebuilding 1977 Turbo engine

[930philturbo]

I bought this very original car in the early 1980s with 15,000 miles, Calif car had thermal reactors still on. Engine studs pulled at 60,000 miles, minor rebuilding then. Car has 115,000 miles now some smoking, using oil and blow back. Engine apart, excessive end gap on rings, pistons/cylinders look good. Because of mileage looking into replacing pistons with J&E 70.1 or 75.1, none on shelf 75.1 special order. Another option Mahle Motorsports 3.3 98mm piston/cly in stock will bolt right on. Also available original 3.0 65.1 piston/cly set in stock. I have been told using the higher compression 75.1 will result in much smoother engine. Also replacing exhaust with original European one. Would like to hear comments.

[Dpmulvan]

I’d go with the Mahle 98mm.

[Henry Schmidt]

Originally Posted by 930philturbo

I bought this very original car in the early 1980s with 15,000 miles, Calif car had thermal reactors still on. Engine studs pulled at 60,000 miles, minor rebuilding then. Car has 115,000 miles now some smoking, using oil and blow back. Engine apart, excessive end gap on rings, pistons/cylinders look good. Because of mileage looking into replacing pistons with J&E 70.1 or 75.1, none on shelf 75.1 special order. Another option Mahle Motorsports 3.3 98mm piston/cly in stock will bolt right on. Also available original 3.0 65.1 piston/cly set in stock. I have been told using the higher compression 75.1 will result in much smoother engine. Also replacing exhaust with original European one. Would like to hear comments.

If I'm reading this correctly, you have a 77 930 Turbo. That is a 3.0.
Those engines basically had the same crank as a 2.7.
The crank and rods were interchangeable. That means they have a 70.4 stroke and a 22mm wrist pins.
The 33 Turbo had a 74.4mm stroke and 23 mm wrist pins.
Although there are way to make all that work, it doesn't just bolt in.
In order to make compression and rod to piston compatibility you would need to offset bore the rods and install a 23mm bushing.
A custom set of rods might be the more preferable avenue but modifying them will work. We done it many times to use 3.2 Carrera pistons on a 3.0.
As for higher compression, that is a challenge. The 3.0 Turbo didn't have an inter-cooler so controlling detonation becomes a problem.
Twin plug, 8.0:1, SC cams, low boost (.4 bar) does make an inspiring combination.