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Ignition timing on 83 sc

I had my distributor rebuilt last fall
The instructions l had with the rebuilt unit was to set timing at 16 to 20 btdc at idle

I set it that way and it seems to run fine except I have nothing to compare, so it may not be as fine as it should.
The Bentley manual shows timing set at 5 btdc at idle and 21+/- 3 at 6000 rpm.

Needless to say I am confused.

What do you tuners say?

It seems I also read (somewhere) that timing should be 36 btdc at 6000.

Can anyone enlighten me?

Thx
Hugh
Old 07-16-2019, 06:06 AM
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an SC is more like 25 btdc at 6000 (should be all in by 4000rpm)

5 btdc at idle. but there is a vacuum retard line in the later cars which retards timing at idle by 5...which you can easily disconnect without harm. emissions thing.
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Old 07-16-2019, 06:13 AM
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if you had yours recurved, i would aim for 25 at 6000, and let the idle be whatever it is.

the older cars had much more timing (2-2.4l), which is probably why you are confused.
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Old 07-16-2019, 06:14 AM
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The 83 US distributor pod has 2 vacuum connections. Idle spec is from +/-5 degrees
The one closest to the dist body is idle.
The other is advance
The idle vacuum is created with the butterfly closed
The high speed advance is created by air pulling into the motor trying to get air from everywhere.
Bruce
Old 07-16-2019, 07:51 AM
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Thanks guys.
So, if my timing was set at app 20 btdc at idle, does that mean I had way too much advance at higher rpms and if so, how would that affect engine performance?
Just trying to have a better understanding of how this timing thing affects engine performance.
Thx
Old 07-16-2019, 12:49 PM
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Search for posts on this topic by user psalt.

You set the 5 BTDC with the 2 vacuum hoses disconnected from the dizzy. There are 2 marks Z1 (TDC) and 5.

Rev the motor to 4k and check total timing. The vacuum will add additional timing with airflow. I want to say total timing shouldn’t exceed 28. Factory was something like 25. I’m going from memory.

You can get maybe 2-3 more timing from the factory spec at 4k rpms but you need to be running high octane gas and you need to be confident that everything is set up well - fuel pressures are in spec, no vac leaks, strong ignition. These motors were a bit detuned from the factory for emissions reasons.

The main thing is you want to avoid detonation with too much timing.

How timing affects performance...

When the spark plug fires the mixture of fuel and air, it creates what is called a flame front. The ignited mixture has to start and expand to cover the volume of the cylinder. This takes a relatively fixed amount of time.

As rpms increase, the time that the plug fires has to change to keep getting a good fuel burn. The plug has to fire earlier so that the most amount of fuel and air gets turned into energy.

If it fires too late or too early it is inefficient. You haven’t unlocked the most potential energy. Too early, there is not enough fuel and compression. Too late and not all the fuel burns before the exhaust valve opens which leads to unburned fuel in the exhaust (rich smelling exhaust).

Detonation (engine knock) means that instead of getting a nice even fuel burn, you get an instantaneous explosion. This results in little useful energy and damages the piston.

Fuel delivery has to change accordingly as well.

If you want to understand more read up on how the fuel and timing tables works in EFI. All the theory is the same, it’s just implemented differently with CIS, carbs, EFi...
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Last edited by tirwin; 07-16-2019 at 03:37 PM..
Old 07-16-2019, 03:31 PM
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Thank you Tim. I'm waiting to get a timing light to go over everything.
I'll try 25*btdc at 4000 rpm, see what it shows at idle and take it from there.
Meanwhile the car is running quite nice at whatever it is so I was probably wrong when I said the timing was 20* btdc at idle.
I'll just have to confirm what it is.
Ill keep you informed.
Old 07-17-2019, 04:37 PM
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Well, it's been a while but I was finally able to get a timing light to check things out.
As I previously said, the car seems to be running very nice so I was in no rush to make changes.

My friend that has a shop that does repairs on most German cars offered to check the timing and found 28*BTDC @3000 rpm and 35* BTDC at 4000 rpm.

He suggested that I not touch it if it's running fine.

And so I'll leave it as is.
Old 07-24-2019, 05:20 PM
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"He suggested that I not touch it if it's running fine."

Was your mechanic friend talking about your car or your bank account? That sounds like way too much advance to me.
Old 07-24-2019, 09:45 PM
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should be running fine for another couple of hours.

Spec from Porsche is 25...you can go more advanced in cooler climates, with better fuel, maybe to 28 or so...35 will damage for sure.
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Old 07-24-2019, 11:01 PM
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Thanks guys, not sure what that guy was talking about but I'll get on it and have the timing re-set to the 25* range.

What sort of damage can I expect with the amount advance I currently have?

Thanks again.
Hughc
Old 07-25-2019, 04:10 AM
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depends how you've driven it...if you had knocking or not.

maybe nothing...

severe detonation = the firing happens on the wrong side of the rod orientation (before the rod is past TDC) , so the powerstroke happens against the natural rotation of the engine...bent rod, etc...

slight knock...maybe no consequence.

It does not happen all the time, I'm not sure but i think knock mostly happens somewhere around peak torque around 4,5K rpm, full throttle...
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Old 07-25-2019, 04:33 AM
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Thanks Dave
There was / is no noticeable knocking but I will have the timing reset regardless.
I often rev beyond 4500 and certainly dont want to risk damaging the engine.

BTW, I'll be in touch with you to further discuss the installation of the A/F ratio gauge.
Old 07-25-2019, 04:48 AM
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One more question. Should the vacuum hose be connected when checking timing at 4000rpm.
Thx
Old 07-25-2019, 05:08 AM
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