So we all know the 996 and 997 were very similar vehicles. Suspension, engines, and body parts are commonly swapped between those generations. Are there any similarities with the 996/997 and 991? Of was it a complete redesign with absolutely no shared parts? Having driven a couple 991's, i know the drive and interior/exterior look appear completely different, but just thought i'd ask the Porsche experts.

 

Just found a 991 PET diagram. Porsche part numbers typically follow this format:
996 series: 996 999 999 99
997.1 series: 997 999 999 99
997.2 series: 9A1 999 999 99
991 series: 991 999 999 99 or 9A1 999 999 99 (makes it hard to distinguish)

So the first three digits show the model specific part. Often a part is not updated when going to the next model so the part is reused as-is. Other times there is a minor update so just the first 3 digits are updated. Following?

Looking through the 991 PET .pdf file there are a ton of 9A1 part numbers, so these are shared with the 997.2 or have minor revisions. There are some 997 parts, even some 996 parts. So yes parts are shared.

Reused from a 997.1 are the throttle body (997 605 116 01), exhaust pipe clamps (997 111 520 00), clutch release bearing (997 116 080 02), some front rotors and calipers depending upon options (997 351 401 01, 997 351 421 0E), side mirrors interior motor (997 624 047 01), ventilated seat fans, hood emblem depending upon year, seat parts, rear wiper motor, headlight washer nozzles, navigation dvd player, frunk tool kit, and some misc sensors (hall, temp, etc.). Looks like some brake components are the biggest shared item.

New for the 991 - flywheel/clutch, oil filler/tubing, coolant system including radiators/tubing/overflow tank, air cleaner, engine mounts, fuel tank/lines, cats/mufflers/tailpipes/PSE, tranny, just about everything in the suspension (lca's, cross member, thrust control arm, uprights, shocks/springs/struts, sway bars, tie rods), steering rack/wheel, interior and exterior body and parts obviously (shell, doors, hood, windows, dash, pillars, glove box, sunroof, cab top, rear wing, seats, etc.), most of the air conditioning system, wiring, horn, front wipers, headlights, taillights, instrument cluster, interior electronics (radio, ac control, ...). Besides the obvious body changes and their affected interior components the suspension is completely new.

The hard part is 997.2 crossed parts since these start with 9A1. Some of the 991 specific parts are different further down the part number. For example, a 997.2 replacement engine depending upon options is 9A1 100 902 XX, whereas a 991 replacement engine is similar but different 9A1 100 903 XX. There are a lot of 9A1 parts listed but each would need to be checked for an exact match against a 997.2. More often than not these are newer parts that are backwards compatible (superseded) or just plain an exact match. For example engine components (idler tensioner, serp belt, crankshaft/bearings, pistons, heads/valves, camshafts, etc.), oil pump (9A1 107 050 11) and pan, water pump, fuel injectors, intake & distributor, hydraulic pump, AC compressor, coil packs, starter/alternator. Looks like mostly engine components are interchangeable.

There are even 996 and 993 parts listed but mostly fasteners or small hardware like o-rings and grommets.

TL;DR - yes there are shared parts, brakes with the 997.1, engine components with the 997.2. But a large portion are new for the 991 beyond the obvious body style differences (ie. suspension).

 

I don't think that there are as many parts shared between the 996 and the 997 as the original post would have you believe. I don't think engines were commonly swapped between 996 and 997 cars. There are certainly some engine parts that are common, like water pumps, motor mounts, etc. Complete engine swaps are not common. I think the same is true for body parts. What body parts are commonly swapped? Some people do a facelift of the 996 to make it looks like a 997, but it is not common and it requires lots of modifications. It's not "plug and play".