Thom Fitzpatrick

We're going to have my heads and cylinders machined to clean up a small amount of distortion caused by the leakage from the broken head studs. The non-broken cylinders did not exhibit any leakage. I don't have the updated pistons and cylinders.

Will the resultant increase in compression ratio negatively affect the engine? My mechanic thinks it would bump it a quarter point at the very most.

Here in California, we can only get 91 octane now. I always use 91, and never use OTC octane boosters.

Is the engine management (ie ping detection) sufficient to counter this? If not, what's the best plan of action (short of getting new P/C's)?

MelissaM

Thom,

I remember double-checking my manual (1990 C2) after we moved down from 92 to 91 octane. 91 octane (as calculated by the method used in the USA) is sufficient.

If you want me to find the blurb in the owner's manual, let me know.

-- Melissa

Kevin

Thom;

The Motronics will sense any detonation. From memory I think we are at 11.3 to 1 compression. As you mechanic states it will be a minimal increase. When you do get detonation the engine management will retard the spark. I would run some race gas on a track day 50/50 mix at a minimum. Good luck

Thom Fitzpatrick

Thanks for the feedback! For a minute there I was afraid I'd have to go with the 3.8 P/C's...

Bill Gregory

Thom,

Oh my gosh! That quarter point increase, coupled with the decline of California premium octane can only put your knock sensor into overdrive, which retards your ignition, causing poor performance - only one solution: 3.8, c'mon say it with me, three point eight, three point eight, you can do it, 3.8, 3.8, there you go, 3.8, feels better now, 3.8, 3.8 ......

J richard

Thom,
The reduced volume should not affect the compression too much to keep the DME from doing it's job. There are a few ways to deal with it if it does:

1.If you re chip the car have the chip tailored for a slightly higher compression ratio.

2. If they have to take off a lot of material from the head, you could look into using a head gasket, the old style, or there are metal base gaskets of thicker material that will push the whole cylinder out to maintain the proper compression ratio. Your mechanic should know if it will be a problem, go with his experience.

Sorry to see you get stuck with this kind of work on a supposedly "fresh" motor.

I have run re used stretch bolts before, had no problem, but I always cringed a bit when I zipped it up between gears. At the least I would replace them with factory, the raceware stuff would keep me sleeping at night...

Oh wait, forget everything I said, The milled heads will raise the compression ratio to the point that they may cause a MAJOR engine grenade, the only solution is really a 3.8 set, (and rs cams, porting and polish, slide valve intake, SS headers...)

JBH

Can someone explain to me why I can not find the part number for that cylinder head in my parts catalogue?

Bill Gregory

The numbers are:

964.104.033.04 1989, superceded to '06'
964.104.033.06 1990
964.104.033.08 1991 on

I believe Thom has the '06' cylinder heads.

Adrian

Dear Thom,
I have a question about your 3.6 set up. You have installed the DME but how has the DME been set up. Where did the DME and Harness come from and how are the country and variant code plugs set up. I am sure there is no reason for concern but if you activate the knock control unit (2 knock sensors installed) they will tell the DME you have a knock and the DME will retard the crankshaft timing by up to 6 degrees. This does have a fairly significant impact on the performance and fuel consumption.
I would be interested to hear how your DME is hooked up, which configuration is used and have you played around inside the DME. Part number of the DME would also be interesting.
Going up to 3.8 does not clear the set up problem either. The 3.8s were set up in a particular way as well for optimum performance. Example: A C03 or Californian 964 is set up differently to a C02 rest of USA version unless you have the latest DME part number,
Ciao,
Adrian
911C4

PS: Why is this all important. It may be possible to optimise the DME settings so you will avoid any loss of power and certainly avoid any further internal damage to your engine.

JBH

I still want to know how the heads got in that condition after only 3500 miles.

Thom Fitzpatrick

The DME came with the engine. I don't have #'s handy off of it, but I'd be happy to get them for you if you tell me which numbers you need. I don't even know if it's a 'california' DME or not.

I, personally, have never been inside of it. Steve Timmins installed the chip.

You say "if you activate the knock control unit" is it *not* activated by default? How would you check to see if it was activated?

As far as the country and variant codes - I haven't a clue!

Is there an online resource for this stuff?

As far as the head condition, the leak was caused by 2 broken head studs. I'd assumed the carbon buildup was normal. It's actually very light.

Adrian

Dear Thom,
Unfortunately you have run into a bit of a problem. I would like the Porsche part number off the DME. It is either going to start 911 or 964. Please give me the whole number.
Somewhere in the loom under the drivers seat or wherever you have it there should be two plastic jumper plugs. You need to find which pin numbers these wires from these plugs go to. This will tell me what country and variant set up you have.
Okay with the knock sensor. Here is a little check you can do to see if your knock sensors are working. You activate them.
Run your engine at 2500RPM. Get your timing light and line up the timing marks. Accuracy is not an issue here as long as you can see them. With the handle end of a screwdriver or a very light plastic mallet, lightly tap the knock sensor. You will see the timing advance or retard. Doesn´t matter which way because it is not really possible to predict but at least you will see them work. When you stop tapping the timing marks must realign themselves.
The onl yplace where all this data will be found in one place, is, yes you guessed it, in my book. There is almost nothing online regarding Porsche/Bosch DME systems,
Ciao,
Adrian
911C4

a3holerman

Adrain,

Been reading this post with much interist.
Two questions if I may.

First I just bought a 1991 964 cab from California. I will be shipping the car to sunny Cape Cod where I live. I noticed in one of your posts that you mentioned something about the dme for a Ca. car. Is there a difference in setup or performance for the Ca. cars of this vintage. Could one , and should one change the setup of a Ca. car if out of state ?

You mentioned your book. Where is it available. Thanks in advance for any help.

Tom

Cape Cod

Adrian

Dear Tom,
Hard you say. Depends on when your 1991 model was actually manufactured. There were more than one 1991 model unfortunately. You need to check your label under the hood. If it says C03 you have a California model if it says C02 you have a rest of the USA model. Sometime in the 1991 model year production the C03 model ended. If you have a C03 model it is basically tuned or mapped to run on slightly lower octane fuel and I believe lower quality fuel. I doubt that you would ever notice the difference. You can check your DME part number if you want to and let me know what it is.
About changing it. You can but this would need to be carried out by an expert and you will need a dyno and an exhaust/CO2 test rig as well. Probably not worth the effort unless you are going to try for performance improvements. Of course please remember to check what model it is before doing anything more. CO2 or CO3.
My book, hopefully next spring,
Ciao,
Adrian
911C4

a3holerman

Adrain,

Thank you very much. The car should arrive sometime around the end of the month via enclosed truck from Ca. Where exactally in the trunk are the stickers located ?

Again thanks for the help. The car is just a pleasure ride for my wife and I. I thought because of Ca.'s stricter emmission regs there might have been a big performance hit on those cars but I guess not.

Thanks again


Tom