Distributor timing with higher compression pistons in an SC
 

I am rebuilding my '79 SC's engine and I put in later SC pistons, which have 9.3 compression instead of the original 8.5. Now I am wondering what I need to change in the car to optimize performance with these pistons.

The first thing that comes to mind is distributor advance. I believe that the later SCs had less advance to prevent pre-ignition due to higher compression. Will I have a problem running the early distributor, and what are the recommendations on dealing with the ignition timing in my situation? I'd prefer avoiding having to buy a later dizzy as I'm low on funds.

I welcome any recommendations for other aspects of the engine such as fuel delivery, etc.

Thank you for reading.

Hello Brotok,

You probably won't need a new distributor. Your distributor only has 4 more degrees of advance than the later model and you can easily retard the initial if it is a problem. The timing specs for these models was retarded because of the 87CLC fuel spec, and the higher temps of the cat and vacuum retard. Your fuel octane, fuel system choice, exhaust choice, and running temperature will all have an effect on how much timing you can run. If you distributor is original, you will probably have to have it rebuilt and recurved after the engine is running and can be tested.

Brotok:
I love the Bosch 0 237 306 001 ('78-'79) with vac advance .
Keep the distributor; it's a very good one for your engine with 9.3 CR.
You can service it with an easy DIY.

http://forums.pelicanparts.com/porsche-911-technical-forum/333640-distributor-service-clean-lube-real-easy-without-removing-pinion-gear.html

You can do it. :)
Make sure the Magnetic Pulse Generator (805) moves freely so the vac advance works correctly.

When I build my SS 3.2 with 9.8 CR, I was very concerned about detonation.
With the kind help from Steve Weiner (Rennsport Systems), Henry Schmidt (Supertec) and JW learned a lot about the issue.
Steve Weiner, and others, told me that the factors for the distributor curve are CR, octane and air temps.
If your distributor has no play in the bushings, and the weights and springs are good, it should work fine. Just check the curve through various RPM's with a Stroboscope or have it checked in a shop.
Here are some numbers for my engine from the experts:

Answer from Steve at Rennsport:
Hi Gunter,
Those figures look good to me.

The engine can make more HP with total timing at 35-36 degrees, but 92
octane won't support that in hot weather. 30-33 is fine for the fuel you are using.

Answer from Henry Schmidt at Supertec:

What you want is 3-5 degrees static and no vacuum advance at idle, then 3-5 degrees additional, immediately after the throttle opens and it should be there all the way to full throttle.
Taking into account your increased bore and compression I would set my
engine at 30-32 degrees full advance with the vacuum line connected.
I would hope for 5-7 degrees at idle. +5 directly off idle, then a
steady advance to 4800 rpm where I would hope for around 26-28 degrees
then an additional 2-4 degrees (30-32 degrees total) in the remaining
1600 RPM to 6400. Henry Schmidt.

Look at those numbers! Wow! 30-32 total with vac line connected.
The engine has now 15k plus and I am very grateful for the advise and help from people who have so much experience.
In Canada we get 94 octane but I suspect that you may only get 91 in the US?
If you can get 92 or higher and have generally cool air temps, I would set it at 28-30 max, with vac advance connected, and see how it runs; I think you'll love it.

What plugs are you using? Bosch W5DC? Or?
Cheers.

Great info!
But how much more total advance can one use with 99 octane, compared to 92 octane...?

I just bought a MSD kit, with adjustable timing, and I also have 9,8 CR with 95 mm pistons. (3 liter)

I have used the stock '79 dizzy for now, but I would like to adjust the curve to gain even more "drive-ability" and performance...

otto

Otto:
A Euro distributor should have a very nice curve provided it has been serviced and all components work.
You can do an easy DIY with the thread I've mentioned.
To be safe, check the curve at various RPM's or have it checked by a shop; it should have a total 30+ deg with vac advance connected.

I don't dare to recommend anything higher than 33 deg full advance; I just don't have the experience.
Some people use high octane fuel, advance the timing until it pings (Under load?) and then back off but I have not tried this. Pinging (Detonation) is hard to hear with the general noise from the aircooled engine.

Be on the safe side. :)

Hello Otto,

Those are US CLC octane figures. US octane is measured differently than in Norway. Your 99 RON fuel is the equivalent of 93-94 CLC octane fuel. The correct timing spec for your model is 24-31 BTDC with vacuum disconnected and plugged with 98 RON fuel.

Thanks guys!
Also got another question:
Any suggestions to what plugs to use for my setup with MSD?
(3 liter, 9,8 CR, 964 cams)

Hello Otto,

Since you have raised your compression 1.3 points, I would use the later SC spec WR3CC. I prefer the normal plugs to the platnium or multiple electrode. If you want to get any benefit from an MSD ignition you need to open up the plug gap. I use 0.045", and MSD advises that you can use up to 0.055". When Porsche raised the compression to 9.8 on the later SC, it changed the max advance to "not above 25 BTDC" measured with the vacuum lines disconnected and plugged with 98 RON fuel. You cannot accurately measure max advance with vacuum connected. The engine may make more power above this warning, but you should be listening for detonation around peak torque rpm and inspecting the plugs if you chose to exceed it.

the WR3 may be a little cold, check them in a 100 or so miles, if the porcelain is grey or black, or you are fowling plugs, go 1 range hotter. i think the NGK equal is BPR9, they are cheaper and easier, for me, to find. i think it is always better to start with a cold plug and work your way up to the correct heat range.
i ran W4's in my stock 2.7, although i never fowled one, they came out grayish, the W5's look real good, well, NGK BPR7's.
+1 on the plain copper plugs.

Yep, copper plugs.
It's my understanding that the heat values with Bosch vs. NGK run opposite meaning the Bosch W5DC is a colder plug than W6 while the NGK BP8ES is colder than BP7.

Phew, that can get confusing.
Porsche recommends no lube on the threads for reasons of conductivity but there are mixed opinions out there about this.
I do put a light coat of Anti-seize because I don't want any damage to the threads; the case is just too expensive.
Suit yourself re: lube.
I apply correct torque and blow out the cavities before taking the plugs out to prevent any dirt from falling into the cylinders.
With my 9.8 CR, I tried W5 after consulting with JW, and others, and the suggestion was to check them after a hard run; they shouldn't look too white, a light brownish color is preferred.

I found that a slightly hotter plug was in order and the suggestion was W6 or an NGK equivalent.
Agree with working your way up from a colder plug to a hotter one.
Are you a spirited driver? High revs?
Try a W6DC or NGK BP6ES or BP7ES with the appropriate gap for MSD and let us know what you find.

And, of course, check the distributor curve. :)