lightning

Hi,

In cars manufactured for the model year 1986 there was a difference in compression on the US spec Carrera 3.2 and the Europe spec Carrera 3.2. The US version (930/21) had 9.5:1 and the Europe spec (930/20) had 10.3:1. Of course the US version also had catalyst, O2 sensor and a different DME setup. But what was the hardware reason for the lower compression, was it different pistons, crank or what?

Does anyone really know?

sschmerg

I've asked that question on this board before, and I don't know if I ever really got an answer. There must be something different!

Whatever that difference was they eventually changed it. Starting in MY 1987 3.2's for all markets had the same compression ratio. The only differences were that the RoW models used a pre-muffler instead of a cat, and they had different DME programming. Changing the chip and exhaust should bring a U.S. car up to the RoW HP figures.

-Sean

lightning

Dear Sean,

You are right, as from -87 they have the same compression but a different DME setup.
Since I have a US car and living in Sweden I have removed the cat and replaced it with a cat bypass, but I think I would have been better off with a premuffler though. I've kept the O2 sensor and replaced the chip with a new one from Steve W. The car runs excellent except for a little rich mixture and I have no problems with keeping up wit the European cars.

Never the less I have been curious on what hardware that is different and I’m anxious to find this out.

/Anders

Kurt V

The Euro engines had different pistons.

Richard Pankhurst

I just went through a bit of research about this topic in preparation for F-class club racing and here are my findings so far:

There are four types of 911 3.2 liter street engines made between 84 & 89

Series CR Year
930/20 – 10.3:1 84 - 89
930/21 – 9.5:1 84 - 86
930/25 – 9.5:1 87 - 89
930/26 – 10.3:1 85 - 88

The term Euro is not really a stable term over the years from what I can tell. In some model years ROW high compression cars were not uniformly supplied across Europe. Low compression it seems was an option in most Euro countries.

I have been told that the actual compression ratios of all the engines are 0.2 - 0.4 lower than spec.

Porsche makes two thicknesses of cylinder base gasket. You can raise the compression of a lower compression engine by 0.2 by using the thinner gasket if you originally have the thicker gasket installed

You can also reduce the compression ratio of a high compression engine by about 0.3 – 0.4 by adding the thicker cylinder base gasket or two thin ones. I don't know the limit or actual use of this practice in production but mathematicallly two thick base gaskets could reduce compression 0.8 of a point

Valve reseating reduces compression by about 0.1 to 0.2 depending on the amount of material removed

No one has been able to tell me definitively if Mahle high compression and low compression pistons are really different in any way.

KS pistons and cylinders have a bad reputation for failures on the track – the engine on my car has been rebuilt because of one piston failure and I was told numerous war stories

Always happy to get better data - some of this information seems to be a combination of folklore and the black art or engine building

I would still love to find out if the high compression and low compression Mahle pistons exist and more about this base gasket story.

Any takers?

Russ L.

many of the "myths" around compression ratios, cats and such have more to do with the fuel that that part of the world is/ was using @ the time of design... many eastern european countries[ from what I've been told still were using leaded fuel in the 80's ... this would kill a "cat" VERY quickly but would enable the designer to raise the comp. ratio/ h.p.
Gray market vehicles that came in from europe went under the "knife" with modifications to the bodies...and their exhaust/ injection systems as well to meet the U.S. d.o.t. standards. I own an '84 "euro" 911 myself and have been in the auto trade for 25 years ... there very well cuold be other reasons for those changes as well... but I'm sure that this had something to do with it.

lightning

Dear Richard,

Even the 930/26 engine had the comp set to 9.5:1. The only engine that had 10.3:1 compression was the 930/20 and that engine was only inserted in the non-catalyst models in 84-86.

The correct list is like follows,

930/20 – 10.3:1 84 - 86
930/21 – 9.5:1 84 - 86
930/25 – 9.5:1 87 - 89
930/26 – 9.5:1 87 - 89

Now, back to my question. Does anyone really know (and I'm not looking for answers like "I think it is") the hardware reason for the difference in compression.


Best regards
Anders

lightning

Refering to the Porsche software catalog the pistons, cylinder, cylinder head, camshaft, rods and crankshaft is/was the same for the engines 930/20,930/21,930/26 manafactured in -86. Ther e is though two types of cylinder base gaskets, one 0,25 mm and one owersize 0,50 mm.

/A

Nathan M

The difference is the compression height on the pistons ie the distance from the gudgeon pin to the crown. I know this because I'm just converting my US spec to Euro, and have examples of both pistons.

lightning

Great answer Nathan,

At last someone who really knows. Is it only the pistons or do you have to change anything else, such as rods etc.?

Best regards
Anders

Nathan M

It's only the piston - you don't need to change anything else. I've also checked in the Mahle catalogue this morning, and there's actually 1.3mm difference between the compression height of the Euro and US.

Nathan