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Nevdog69

How far should I advance ignition timing

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A local porshe specialist has advanced my timing beyond spec and it scares me a little.  He says its fine but if its not, he gets to rebuild my engine.  I like the performance but don’t like the idea of possible engine damage.

It is a 1978 911sc US spec.  It is the lower compression version with a ratio of 8.5:1 (They lowered the compression because they only had access to poor unleaded fuel in the US in 1978). 

I only ever use 98 Ron fuel.  

The timing has been advanced to 12 degrees BTDC at 900 RPM (The spec is for it to be only 5 BTDC so he has added another 7 degrees).   The only markings on the flywheel are Z and 5BTDC.  Therefore I can’t tell how much it is advancing at higher RPM.

I have installed SSIs and it pulls well all the way to the redline so I figure it’s not that far advanced it is fighting against itself .  I have not heard any pings under load, but then again you may not hear it that well when the engine is behind. 

Due to the low compression and high octane fuel, I think it probably is safe.  I was wondering what others experience is.

I love the setup now and don’t really want to wind back the timing, but if its gonna stop in blowing a ring or piston, I would.

Any advice is welcome.

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Hi @Nevdog69,  I'm also running a US spec. lower compression SC like yours but later model with the O2 sensor (disconnected) and vacuum ignition idle retard (disconnected). From memory I have 5 deg advance at 950rpm idle speed with vacuum retard disconnected and 33ish deg adv. at 4000rpm  I only use 98 or higher and my engine performs just fine...all 150kW of it. ;) 

The factory ignition timing spec. is written for conditions in the US ie shitty low octane fuel.  The timing  has to be tuned to avoid pre-ignition for the worse case fuel combo.  We are lucky to have better fuels.  A discussion on Pelican for a similar car running in Norway on 98 RON fuel gives a good insight to the confusion around how to tune for various fuel/engine combo's  

http://forums.pelicanparts.com/porsche-911-technical-forum/381031-please-help-me-understand-ignition-timing.html

the most informative for me is the last post on the page...

Hello Ole,

No, your car has more advanced timing. The full advance is 38 BTDC, more advance than 30 BTDC. The idle spec is retarded for emission test purposes, the high speed advance is what matters. There are many other factors involved in your comparison. The early car had 9.8:1 compression, you have 8.5:1, a difference of several octane numbers, which would allow more advance. Your car also has a larger bore, which generally means more advance is needed for peak torque. The early smaller bore engine has higher dome pistons, which reduce the amount of advance that can be used before detonation. My guess would be that the early car is more knock limited and 30 BTDC was chosen to provide a comfortable margin of safety for a mfg. The lower compression of your engine, combined with the smaller dome and better combustion shape at TDC allows more advance with a similar safety margin. Running the earlier engine on higher octane fuel with more timing would probably make more power. Running your engine on higher octane fuel and advancing the timing past 38 BTDC would probably yield little benefit. The later SC's had even larger bores, with higher compression, but were limited to 25 BTDC because of the 87 CLC fuel spec. These are the engines that really benefit from more timing with higher octane fuel.

 

Every engine has a sweet spot once all the factors are accounted for.  Consider what emissions controls at idle, fuel type, mods. to your engine. distributor type, ignition system type etc.  It gets very murky without all the info.   My best advice is talk to your mechanic and understand his reasoning for the tune you have. 

 

 

 

 

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Is that just me who can see an irony in all of that? 

CR for late cars US market 8.5:1 for poor fuel quality back in the days in the U.S. and yet we use 98 of presumably much better quality and scared to go a bit further with ignition...

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4 hours ago, Cheshire Cat said:

Is that just me who can see an irony in all of that? 

CR for late cars US market 8.5:1 for poor fuel quality back in the days in the U.S. and yet we use 98 of presumably much better quality and scared to go a bit further with ignition...

I don’t get it either.   I just know that my car runs like crap on less than 98.

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23 hours ago, Nevdog69 said:

Hi Ches Cat, I don’t get it. Are you telling to wind back the advance?

 

I`m not suggestion anything. But one of us really needs to hook up a knock sensor and check the theories... 
But I can`t see why additional 7 degrees can help on idle but can pretty much destroy your engine on WOT

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16 hours ago, Cheshire Cat said:

8I`m not suggestion anything. But one of us really needs to hook up a knock sensor and check the theories... 
But I can`t see why additional 7 degrees can help on idle but can pretty much destroy your engine on WOT

I agree a bit more data would be good, even knowing the distributor mechanical advance curve would be useful along with understanding the actual vacuum advance/retard setup on the car.

The additional 7 degrees would help all through the rev range provided it isn't over advanced at any point in the range and throttle opening combination.

I understand for maximum power a total advance of somewhere between 34 to 38 degrees would be expected for this particular engine.

Assuming the mechanical advance contributes somewhere around 15 to 20 degrees on this car, even with a static of 12 degrees, the total advance would be 27 to 32 degrees. ie safe but leaving some power on the table.  

At idle you can put heaps of advance in because the combustion is so slow but is usually pulled way back for idle quality and emission reasons by having the vacuum advance unit only operate at above idle throttle openings.

I reckon its well worth exploring further but wouldn't lose any sleep over the mechanic recommending 12 degrees static.

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5 hours ago, Peter M said:

I agree a bit more data would be good, even knowing the distributor mechanical advance curve would be useful along with understanding the actual vacuum advance/retard setup on the car.

The additional 7 degrees would help all through the rev range provided it isn't over advanced at any point in the range and throttle opening combination.

I understand for maximum power a total advance of somewhere between 34 to 38 degrees would be expected for this particular engine.

Assuming the mechanical advance contributes somewhere around 15 to 20 degrees on this car, even with a static of 12 degrees, the total advance would be 27 to 32 degrees. ie safe but leaving some power on the table.  

At idle you can put heaps of advance in because the combustion is so slow but is usually pulled way back for idle quality and emission reasons by having the vacuum advance unit only operate at above idle throttle openings.

I reckon its well worth exploring further but wouldn't lose any sleep over the mechanic recommending 12 degrees static.

shit, completely forgot it`s a mechanical stuff which I have no idea of how it works lol

 

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On 08/06/2019 at 16:56, Cheshire Cat said:

Is that just me who can see an irony in all of that? 

CR for late cars US market 8.5:1 for poor fuel quality back in the days in the U.S. and yet we use 98 of presumably much better quality and scared to go a bit further with ignition...

Probably not irony, just a sign of the times.  The '70's was a pretty grim time for the internal combustion engine with manufacturers struggling to met increasing emission standards using very primitive engine control systems.  Consequently they used retarded ignition timing and reduced compression ratios to deliberately reduce emissions (namely hydrocarbons and nitrogen oxides respectively in this example) rather than a response to "poor gas".  

I think the matter of "poor gas" isn't really supported by fact and I think is more of a result of the US changing their advertising standard for octane in 1972 that resulted in the same gasoline posting lower octane numbers on the pump. (Change from RON to AKI which is an average of RON and the much lower MON).  Hence the common mistaken belief that fuel quality dropped at this time.

Anyway, enough chitchat and history, I hope Nevdog69 sorts out his timing concerns successfully  

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On 08/06/2019 at 07:02, Nevdog69 said:

A local porshe specialist has advanced my timing beyond spec and it scares me a little.  He says its fine but if its not, he gets to rebuild my engine.  I like the performance but don’t like the idea of possible engine damage.

It is a 1978 911sc US spec.  It is the lower compression version with a ratio of 8.5:1 (They lowered the compression because they only had access to poor unleaded fuel in the US in 1978). 

I only ever use 98 Ron fuel.  

The timing has been advanced to 12 degrees BTDC at 900 RPM (The spec is for it to be only 5 BTDC so he has added another 7 degrees).   The only markings on the flywheel are Z and 5BTDC.  Therefore I can’t tell how much it is advancing at higher RPM.

I have installed SSIs and it pulls well all the way to the redline so I figure it’s not that far advanced it is fighting against itself .  I have not heard any pings under load, but then again you may not hear it that well when the engine is behind. 

Due to the low compression and high octane fuel, I think it probably is safe.  I was wondering what others experience is.

I love the setup now and don’t really want to wind back the timing, but if its gonna stop in blowing a ring or piston, I would.

Any advice is welcome.

Sounds fine to me.

Drive it and enjoy it. Feel free to bring it over if you'd like a second set of ears to listen for pinging while you drive it.

Mike

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