SC Ignition Timing Question

[arbita1]

I'm bored on Xmas eve and looking stuff up. I'm going to be removing my distributor and taking it apart to clean and test it out as the vacuum pod is not functioning. So it got me thinking about timing.

My car is an 83SC with the 9.3:1 cr, everything else is stock, except I have SSI'S. I use 93 octane fuel. So based on that do I stick with the factory spec of 19-25 BTDC @ 6,000 RPM. Or can I use the ROW spec of 25 BTDC @ 4,000 RPM? Or something else? I ask this since I have SSI's and use thhigher octane. And I thought I read that the US factory spec is very conservative.

I searched but couldnt find anything specific to my question.

[Bob Kontak]

Merry Christmas.

Good question. I read a post within the last month where I think psalt was talking about being able to increase distributor advance with the use of higher octane fuel but I do not think he mentioned a number.

[Vereeken]

We do not have 93 octane in our country. The lowest is 98. My 83 SC -bone stock- runs 28 degrees. I have had it up to 32 before. I have never heard any pinging nor did I see any sign of overheating.
My AF at exhaust shows 14-ish at idle so I am running leaner then you would expect on an SC but I do get low AF at WOT (low twelves even 11).

I run stock Vacuum advance.

This is my situation over the last 6 years. Addmittingly not many Kms (about 40k kms')

Michel

[brads911sc]

98 european is about the same as 92 Us. I dont recall the actual conversion but someone can chime in with it.

probably the best upgrade you can do on your SC for performance is to delete the vacuum advance, have the distributor recurved with mechanical advance where you are all in at 3k RPM. then you will maximize the curve of the stock SC cam. I did this to mine bfore upgrading my cam and was running 32 advance with 93 octane all in at 3k. It really woke up the car and I had no detonation issues.

[brads911sc]

call Barry Hershon. He knows more on this subject that you will ever forget...

[Gogar]

I think a SC distributor is advanced as far as it will go by 4000 RPM, so if you set it for 25 at 4000 rpm, it will be 25 at 6000 rpm as well, so in effect the ROW spec and the US spec are the same.

Definitely search for psalt and SC distributor though; I enjoy reading his posts about this kind of stuff.

[Vereeken]

I can always be corrected but 92 Octane in US is 92 in Western Europe.

I believe just like us you are quoting RON octane numbers. Not MON.

So we have 98 RON (ex. shell Super V) en 95 RON.

I do not think that you quoted me 92 MON that would be roughly the equivalent of 100 RON.

Sure we have 92 Octane but not in Belgium, France Germany etc...

I have to add Octane booster to my fuel if I want to go travel in Eastern Europe as they have 92 RON Octane.

The 98 RON gives you better mileage but it is ofset by the higher cost.

Michel

[psalt]

I can always be corrected but 92 Octane in US is 92 in Western Europe.


Michel,

No, that is incorrect. In the US we have used a different octane rating system since the 1970's , CLC , or the average MON/RON. Since MON and RON are usually about 10 points apart, the CLC pump rating in the US is about 5 points lower than the RON in Europe. There are other differences in the fuel, we have dozens of different specs by state and season.

The ROW SC and the US SC with lambda CIS are different engines and the ignition timing is different for several reasons. The ROW engine has higher compression, bigger ports, bigger runners, different cam timing, a higher rpm torque peak, a higher redline, a different exhaust with no cat, and the fuel spec was 98 RON. They are not comparable.



Matt,

The 83 US SC fuel spec in the owners manual is 87 CLC. There was little or no high octane unleaded back then and the US mandated unleaded regular. Porsche chose to detune the US SC by dropping the compression slightly and seriously retarding the ignition timing. The optimum timing for a two valve, wide VIA hemi engine is close to the mid 30's BTDC, a lot more than the low 20's, but the 911 is a knock limited design and you cannot run the ideal timing with pump fuel. How much timing your engine can handle cannot be found in an internet post. There are a lot of variables, engine temp, air temp, compression seal, oil dilution etc. The ideal timing can only be found by experiment and you need to be comfortable with the signs of detonation. The probability of pinging is highest at the peak torque rpm, 4200, when VE is highest. The narrow runner US SC engine is tuned for low rpm torque and the main reason a carbed engine can run more advance is the change in intake manifold harmonics. A stock SC with good compression and ring seal, no intake leaks and a 13 AFR under load mixture can usually run a few degrees more total timing on the road with higher octane fuel. A few more if you disconnect the hot oil breather line, use a vented catch can, and get the hot oil vapor out of the intake air. You will probably not get beyond 30 BTDC with the small runner intake. You need to remember that these engines ran hot and lean from the factory and there was a unusually high incident of piston land breakage due to detonation when these cars where new. If you do not have experience tuning engines, my advice is to find someone who does if you are trying to go up to the edge.

[arbita1]

Thanks Paul for your insight. I have a few more questions now.

1. Im pretty sure my owners manual indicates to use 91 Ron minimum. However, I have seen you mention the 87 Ron quite a few times. Can you elaborate on that?

2. I feel having someone tune my engine at this point in time might be overkill (for me). Is there a safe recommendation you can make based on my above info, or would that be the factory settings.

3. Lastly, in part of item 2 above, Gogar made a comment that us and row spec is essentially the same. This seems to make senses when I read his explanation. Do you concur? Would the difference in the us spec be because the timing is set at idle rather than full advance? This would seem to me that it would make more Spence to set it per the row method so that you are making sure you are getting full advance by 4,000 rpm. Is this correct?

[psalt]

I have a few more questions now.

1. Im pretty sure my owners manual indicates to use 91 Ron minimum. However, I have seen you mention the 87 Ron quite a few times. Can you elaborate on that?

No, I mentioned 87 CLC, which is the pump fuel spec'd on pages 75, 78 and 83 of the US owner's manual. Unleaded US "Regular" fuel, that is roughly equivalent to 91 RON. The number on the pump at a gas station in the US is not RON. The US and Europe use different forms of octane numbers at the pump. Europe uses RON and the US uses the average of RON and MON, called CLC at the time, now called AKI. They are not the same rating.



2. I feel having someone tune my engine at this point in time might be overkill (for me). Is there a safe recommendation you can make based on my above info, or would that be the factory settings.

No, no one can tell you what is safe for your engine over the internet. If your engine runs hot, is worn, burns oil and is set lean, it can have a pinging problem at the factory setting. I have driven many US SC's that ping at the factory setting, even on US premium fuel.


3. Lastly, in part of item 2 above, Gogar made a comment that us and row spec is essentially the same. This seems to make senses when I read his explanation. Do you concur? Would the difference in the us spec be because the timing is set at idle rather than full advance? This would seem to me that it would make more Spence to set it per the row method so that you are making sure you are getting full advance by 4,000 rpm. Is this correct?


No, this is incorrect, the ROW distributor has a different curve, there is too much advance early. This is why BA states the ROW distributor is suitable for a carb conversion and the US distributor is not. The timing specs are different, the ROW is 25 BTDC max at 4000 rpm, 26+/- 2 at 6000 rpm, and the US is 21 +/- 3 or 19-25 both at 6000 rpm. Most US SC's have less than 20 at 4000 rpm.

The ROW ignition curve is set up for 98 RON fuel, seven RON points is a very large difference. If you want to know how your engine runs with more advance, my advice would be to run the fuel tank down and put in 5 gallons of race fuel and advance the timing in 2 degree increments.

It is always a good idea to check the high speed timing after setting the idle spec and most manuals recommend it.

[Lorenfb]

"You will probably not get beyond 30 BTDC with the small runner intake. You need to remember that these engines ran hot and lean from the factory and there was a unusually high incident of piston land breakage due to detonation when these cars where new."

Isn't interesting how 911SC timing discussions go into such detail and express concern
about 'pushing' ignition timing beyond the factory spec, whereas 911 3.2 owners have
little to no concern about using the so-called 'performance' chips that 'push' timing
into the high 30s or low 40s, given that both engines (USA) are basically the same
(3.2 longer stroke & higher compression). And it's actually easier to change the 911SC
timing than installing a 3.2 chip, i.e. a 13mm wrench and a timing light.

Bottom line: The typical 3.2 owner is obviously super naive and easily gets sucked into
the 'performance' hype with little to no concern about octane.