I am considering an 1972 911 that had the engine built to a 2.8 in the late 70's. the car is otherwise quite stock.

As far as I know the original case was used. I have copies of the receipts that lists '92mm P's & C's' which I assume are the RSR ones. Its hard for me to discern what else was done, partly because of the poor legibility of the receipts.

The car is definitely single plug. The owner claims to use regular gas... I assume 91 octane. Is a single plug 2.8 feasible? Since the car has done 100k miles since the original 2.8 build I would guess the answer is 'yes'... its just I'd always heard in the past that twin plugs are recommended for any compression ratio above 9.8:1 , and I assume that a 2.8 will have a considerably higher ratio than that.

After about 95,000 miles (the engine has 100,000 now) there is a receipt for dissasembly of right bank, machining of cylinders & cylinder heads, reset deck height. They also noted that "damage was at mating surface cylinder to cylinder head at cyl. #1" I'm not entirely clear what this suggests?

Any 2.8 info or feedback appreciated! Thanks.

 

Hi:

This is a complex question due to all the variables at work here but I'd simply offer that a 2.8 litre engine using the Mahle RSR 92mm P/C's yields a 10.8-11.4 CR using 2.4 or 2.7 heads. The original 2.8 RSR heads had much bigger chambers that gave 10.3-10.5:1.

These engines will not run safely on 91 or 92 with single ignition without detonation,.......

Mating surface damage (fretting) is just one indicator of detonation.

 

Thanks Steve!

Thats what I would have thought... about detonation being an issue. Stoddard built the engine in the late 70's- I guess twin plugging was a more expensive proposition back then?

What would one want to budget for converting an engine to twin plug?

The 100,000 miles on the single plug 2.8 worry me. I wonder if I really do want a 2.8 if I wouldn't be better off getting a 2.4 & having it done myself? Of course this car already has the expensive 2.8 P's & C's... but there is probably a chance that they would need replacing anyway!

Are compression & leakdown tests useful to figure the health of the engine in a case like this, or should I just assume that the engine will need a complete overhaul because of detonation in the past?

 

$1,500 or more depending on your DIY skills - it helps if you are a low level machinist & have a Bridgeport handy...

 

I believe that damage at the mating surfaces can also be caused by running the engine for a while with a broken head stud..

 

LOL,...The twin-ignition option was as expensive then, as it is now,....

There are several ways to do it and some run better than others. You should budget in the $ 2500 range for all the hardware & machine work.

Early 92mm cylinders had a tendency to fracture the liner between the CE ring & the bore at the top due to detonation so thats something to inspect. If the ring lands on the pistons are worn, you'll need a new set anyway (an improved version is available).

Leakdown tests show the general health of the rings and valves, but they will not tell you anything about the rod bearings and at 100K, its usually time to replace them if the motor ran rich.

If you suspect that this motor had some detonation issue, its would be more than wise to take it all apart and carefully inpect everything.