73.5 pistons
 

I am rebuilding a stock 73.5 engine, which was only in production less than 6 months with CIS. Need to replace pistons and cylinders. Not available. CIS engines,(73.5-83) have the unique dome to accommodate the CIS.
I called EBS as they have helped me in the past with some NOS stuff on the back shelf.
Nothing in this case but they suggested J & E could make a high dome wedge shape piston which would work. I don't know if I agree. I remember reading about the CIS injection system back when it came into play in the early to mid seventies on all the German cars. Article stated how important it was the new "swirl" piston design was to accommodate lower compression and CIS. stock comp. on these 911/91 engines was 8.0:1, thoughts?

73.5 pistons
 

Try to contact the Mahle factory and ask if they can help.


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Why not rebuild it as a 2.7. I imagine EBS has those in stock.

Customer wants it stock. Then you would have to change the fuel injection system and more. These engine only have 30mm intake ports. So to go 2.7 you would be changing alot.

No you don’t. All 74-77 2.7 were CiS, except RoW Carrera. You don’t have to mess with the injection. The 30 vs 32 mm head ports is negligible and on a non-performance rebuild not something you would need to fiddle with. We’ve done it for clients a number of times. However it does require machining the case for the bore change.

Externally the engine will look 100% stock. Nobody will be able to tell visually, and the car owner will only know because of the extra 20hp under his accelerator foot.

But if he wants it 100% stock, he’s the customer.

I’ve used conventional dome pistons with CIS. I couldn’t tell any difference in the way they ran. And u can keep it 2.4

Gordon,
Have you done a bunch of those type jobs ? What displacement were they.
These pistons have the highest "hump" I have seen compared to a 2.7 or 3 liter.
They must have designed the special swirl type pistons for a reason.
SSF has 84mm set for a E, that is almost flat with a small valve pockets and the correct 8.0 :1 compression, listing says for MFI and carbs.

Rick we put 9.5:1 JE's in the Carrera 3.0 rebuild we did many (many!) years ago. No adverse effects, car ran great, made tons of power on the dyno (200hp at the wheels). Putting JE's in CIS engines is pretty common. Never heard of a problem with combustion.

i should have a good used set somewhere. They came out of a 68K mile car. i replaced them
only because I could buy a new set for less than $400 at the time ('87).

I’m in the same boat, looking for 73.5 pistons and cylinders. My car runs but can/t stop the blue smoke … tired engine. I have the 7R mag case so I’m trying to keep it stock (case already bored .25 with .25 oversized rod bearings). I want to keep it stock 2.4l. I guess I’ll try the manufacturer route and see if a small run of these can happen. There has to be more out there somewhere.

cis
 

The only reason for the CIS piston shape was for the EPA, and a lack of understanding of combustion at that time
I use all kinds of shaped pistons with CIS injection
Ian