924Board.org

[larso]

Regarding the boost being higher on the 79-80:

In 79-80 the turbos must have been different to get more boost or the wastegate must have been different on the euros; is it a combination of the turbo being different and the wastegate being set different to get the more boost? Just the turbo being different? I can't remember but Jon F. I think we talked about this a while back...Or is it something to do with the wastegate line maybe..smaller diameter line to get more boost?

And since they upped the boost on the euro models the ignition curve and lack of emmissions then put the icing on the cake to make it 170-180...I think that the ignition curve and emmissions stuff isn't the main biggy in the extra 30 HP, look at the percentage gain.

[larso]

BTW is higher octane fuel needed in the 931 than 92? because it won't increase HP just by putting it in your tank you must raise the boost enough for it to be beneficial so I don't think any higher than 92... in other words, unless the car really needs it, higher octane fuel is just a marketing scam for those who haven't read the books on fuel burning and power. If you are running cool enough, it actually gives you less hp if you add too much octane and are burning clean. I'm not exactly sure how much octane the 931 requires but if it is 92 at the canadian pumps (98 ron) then all those performance nuts that run to the store to buy octane boost are just decreasing their performance, BUT *sustaining* their original performance if they have super hot weather or a crappy radiator. The more octane at a constant compression, the harder it is for the engine to burn. It's like methyl hydrate-injection: it only helps the HP if you have enough reason to use it: more boost or hto running, it won't actually help you if you just pour it in the tank and your car was running ok before you poured it in.

[924 turbo]

The turbos are slightly different, but if you've seen them next to each other you can see that the difference is pretty small (the boost builds 200rpm earlier in the north american turbos). The spring in the wastegate is different on euro cars, and that's how they get more boost. You can put a different spring in your north american-spec 931 and get the same boost levels of the euro cars. Or you can just use a boost controller. Same difference. I would say the power difference between north american and euro cars is 90% boost and 10% combination of no cat/ignition curve. The digital timing/compression was only worth about 4% on north american cars. The euro cars lost some boost but gained a full point of compression, compared to a half-point in north american cars, but still gained about 4% in power.

As far as octane is concerned: Yes, higher octane doesn't add power. It simply allows you to run more boost/more aggressive timing without detonation. If you don't run more boost, you're wasting your money. I wouldn't run any less than 92 octane ((RON+MON)/2) in the 931 in hot weather, but I'm sure in cold Canadian winters, 87 should be enough. An intercooler would help a lot too. The charge temperature makes a big difference as far as detonation is concerned.

[numbers]

You can actually pump up the boost more if you have the lower compression stage one pistons, since your overall cylinder pressure will not be as high. You still will not have as much low end torque as the 8.5/1 piston engines, but you will get more high end torque and horsepower due to the increased volumn of air associated with the increased boost.

[killer931]

hey, dwak
remember that the euro 931s won´t run on unleaded gas... (would be painful for the valves)

Fredrik/Sweden

[kevrl]

Wrong :- All 931's will run on Unleaded fuel, as they all have the same hardened valve seats regardless of the sales market. I checked with Porsche UK when I first got my 931 a few years ago. It is the Octane rating of the fuel that is important for Euro 931's (at least 97 RON).

Kevin

[killer931]

well, my book says the 924 (XK/XJ)
will run on unleaded fuel.
However, the 924T (M31/03) WONT.

These are Sweden specs.
Maybe it differs from country to country,
then it all matters on which market the engine was built for, doesn´t it?

Fredrik

[kevrl]

Hmmm, this is puzzling,

The XK/XJ 3103 engine is listed as Europe and RoW (i.e. not North America) so Sweden and UK should be the same. Next paragraph is an exert from a reply by Porsche UK about using Unleaded Fuel on a 931,

"In respect to the lower octane unleaded fuel we are aware of vehicles which are currently running solely on the aforementioned without any detrimental effect to the engine. However, for optimum performance we would suggest utilising the 98 Ron super unleaded. Unfortunately for a 1980 year model vehicle there are no adjustments to the ignition timing which can be made at this time. It was not until 1981/1982 year model vehicle whereby it was possible to modify the octane loop thus enabling the vehicle to run on 95 Ron fuel."

Maybe there is a differance between the fuel grades in UK / Sweden . IN any case you are right to be cautious if there is any doubt, stick to leaded.

Kevin

[killer931]

Hi

Well, yeah our gas & engines should be
similar so then it should be no problem.
Of course, if you increase the boost level
(like dwak probably will), it´s best to
just exchange the exhaust valves & seats
before mounting it. I believe the intake
valves are no problem (correct me if i´m
wrong).

BTW, while indirectly taking about valves,
are my ball bearinged valve spring seats
(dual valve springs) an available porsche
product? I wonder because one of my engines
has ´em, the other one don´t. I want them to be similar for easy exchange without mapping.

The guy on Porsche Sweden didn´t know
but someone on the board maybe does?


Fredrik

[John H]

While the motors are the same for ROW (including Sweden) cars the ignition controls for the cars vary - see below.
The digital ignition control units for the 931 are made by Siemens and the part numbers for the various units are as follows:
Market Part number
ROW 5WK1664
Carrera GT 5WK1666
USA (incl Cali) 5WK1665
Canada 5WK1665
Japan 5WK1665
Sweden 5WK1686
Australia 5WK1686

Distributors for the 931 and the GT are the same with the Bosch Part number 0237 501 001
The control units have slightly different igintion advance sand retards hence the different boost levels and ability to run on the different octane ratings. Using a ROW or even Sewdish/Aussie spec control unit in the US or Canada would probably mean the engine would detonate unless you use 96 or 98 Octane gas.

[dwak]

OK. Update on painfully slow engine swap. So far: US 931, UK 931 engine in/out/in, UK bell housing, US clutch in but won't match UK flywheel, US flywheel in, setting up timing but timing marks appear to different?? We'll work it out but anyone noticed this difference before? The UK distributer appears to be a more modern type as well. Glad my mechanic buddy is a Porsche wizz and cheap. He says he finds 911's much easier to work on.
dwak

[Cbass]

Gasoline usually has a RON of 91-100, and a MON of 83-92. Here in north america, we average the two. 87 at the pump is really 91 RON, and 91 at the pump is really 98.

[John Brown]

The short answer is US/N.American 93 is about the same as UK/EU 97/98 RON.

The label on the inside of a US spec 80 931 calls for 87. So they were making it run on the cheapest stuff around, even then. I believe for a time everyone thought there would be no way to get higher ratings in the gas and the good stuff was going to go away.

[dwak]

OK,more differences. My '82 UK 931 engine has a crank sensor in the bell housing, no pick up coil in the distributer. The '80 and '81 US 931's are the opposite to the above. The throttle bodies are different too.
dwak
PS ?