Setting advance on an 83 SC: final word ?

[Deschodt]

I've just reached the end of the internet (on this board anyway) about timing issues on an SC...

From what I gathered, there are 2 schools of thought, though not all that far apart in results, just different in *how* you get to the magic number... Correct me if I am wrong !

Everybody seems to agree that 25 BTDC at high RPM (the factory settings) is super conservative and set that way by porsche to account for 87 octane, higher engine compression in 83, and emissions crap.

Then there are 2 theories...

1) One recommends that I run essentially factory settings (5 BDTC, netting 25 at higher RPM, give or take a little), but that I leave the retard vacuum line off the distributor and plug it, thereby decreasing the excessive retardation at idle (an emission gimmick) that makes the engine run hot... you *can* run more total advance because this 25 degree setting is designed to be safe but that can only be determined by a pro "testing" your car on a dyno for detonation and backing off a few degrees... So, no hard and fast rule.

2) Others, including some fairly famous mechanics on this forum, generally recommend setting advance to 32-35 at high RPM and letting the idle fall where it may - generally higher: like 10 to 12 degrees. I did that 10K miles ago, I think I ended up at 32 - since I could hear it ping at 35D under WOT and backed it off a bit - and my idle was 10D BTDC with the retard line off - logically 20 degrees less than 5000 RPM which is the range of an SC distributor. My car ran well enough (it did not self destruct, at any rate) and importantly, it ran much cooler than before.

I can see where approch #1 is safer as you may not always hear pinging!

Since, as of last night I am back on factory settings (5D at idle, vac off) - and unfortunately running way hotter - I will simply disconnect and plug my retard line and see what happens. The car should run cooler at low RPM and be super safe at high RPMs. Tonight I'll measure high RPM advance with and without the advance vacuum, for giggles, because while I know that I have good retard vacuum, I never measured the advance vacuum (more debate there, some say there is zero vacuum at high RPM)....

If the car still runs too hot that way (and I suspect it might, but I'd be happy to be proven wrong), it'll be fairly obvious to me and I'll go back to the 30-32 degree settings which I've run for over a year with no *apparent* damage, and which netted me cooler temps.

At this point, knock on wood, she's got 185K miles and if she blows, she blows (doubtful since I put 15K miles last year with that same advanced timing)...Oh, I run 93 exclusively, obviously.

If someone sees something flagrantly wrong with the above, let me know before I blow up my engine !

[psalt]

Tonight I'll measure high RPM advance with and without the advance vacuum, for giggles, because while I know that I have good retard vacuum, I never measured the advance vacuum (more debate there, some say there is zero vacuum at high RPM)....


Greg,

The specs for your engine are with both vacuum lines disconnected and plugged. The vacuum advance is a load based system, not rpm based like the mechanical advance. The vacuum advance will distort the timing measurement when you are blipping the throttle at rest by varying amounts each time. However in practice, the engine is under load at high rpm and the advance you want to focus on is solely the mechanical advance. In other words, if you leave the vacuum connected the actual advance under load, when the possibility of detonation exists, will be different. The amount of vacuum at WOT under load is very small, 1 or 2 inches, below the threshold of the vacuum advance, and if you can measure it accurately, it can tell you how restrictive the intake system is and how much can be gained by reducing the restriction. Have you looked at the spark plugs closely since you advanced the timing ?

[mca]

Thanks for taking the time to dig into this topic.

FWIW I have my retard line pulled and plugged - bumped up idle rpms accordingly. Mine is set to 5 BTDC at idle and reaches around 26 BTDC at 6000 rpms (actually reaches 26 at around 4000 but doesn't seem to increase after that).

If you don't mind, try to get a reading at 4000 rmps too. I'd like to know if you are also reaching full advance at this point.

[Deschodt]

Will do.. this has been a semi-mysterious topic for a long time now, it annoys me ;-)

Thank Paul for all your advice (as option #1) ;-)

no, I have not looked at my spark plugs, I didn't know you could tell anything from spark plugs since we run modern fuel ! What should I expect ? Or are you saying I should replace them on principle ?

[psalt]

Greg,

Personally, on a hemi headed engine like a 911 or a BMW M10, I run as much advance as I can on a road car with pump fuel. I am comfortable with my skills for setting the timing. Unfortunately, you can't send this skill in an email, and over advancing your timing can do damage. If you have a lighted magnifier and look closely at the insulator on a spark plug of an engine with high speed detonation, you will see tiny specs of aluminum. When you take the engine apart, you will see piston crown errosion as the source. Mild pinging is OK in a road car that spends 85% of the time below 35% load, but another issue on a track car. The main thing to understand is there are many variables and anyone who says they can give you the correct number over the phone is talking nonsense.

[Deschodt]

Funny you'd mention the 911 engine and the M10...
Here are mine:



Although the Bimmer's ignition is now all electro-optical and completely fire and forget ;-)

I wish there was an easy way to detect mild pinging, a device of some kind. Is there ? I will pull a plug and look...
Anyway I'm starting with your recommendation and will bump up advance slowly if temps remain higher than they used to be ;-) I may piss off my neighbors tonight, 6000 rpm on a 911 is loud ! Thanks !

[psalt]

Although the Bimmer's ignition is now all electro-optical and completely fire and forget ;-)

Greg,

The M10 is a 54 degree VIA hemi, very similar to the 911, but with much better cooling. What kind of total advance are you running ? I have several 2002's and the stock distributors run out to +40 BTDC, and add vacuum advance on top of that.

The best instrument for measuring spark knock is a pressure sensor in a special spark plug, but there is no consumer version. Piezoelectric knock sensors are next, but they usually have to be tuned to the engine by experiment. An amplified mic and then your ear in last place. Heat is a major factor and Porsche was very conservative on the cat equipped SC, but 10 degrees of extra timing would be pushing it pretty far in my experience. It does not make sense to me that you could run that much extra timing on an overheating engine.

[Bill Verburg]

Originally Posted by Deschodt

I've just reached the end of the internet (on this board anyway) about timing issues on an SC...

From what I gathered, there are 2 schools of thought, though not all that far apart in results, just different in *how* you get to the magic number... Correct me if I am wrong !

Everybody seems to agree that 25 BTDC at high RPM (the factory settings) is super conservative and set that way by porsche to account for 87 octane, higher engine compression in 83, and emissions crap.

Then there are 2 theories...

1) One recommends that I run essentially factory settings (5 BDTC, netting 25 at higher RPM, give or take a little), but that I leave the retard vacuum line off the distributor and plug it, thereby decreasing the excessive retardation at idle (an emission gimmick) that makes the engine run hot... you *can* run more total advance because this 25 degree setting is designed to be safe but that can only be determined by a pro "testing" your car on a dyno for detonation and backing off a few degrees... So, no hard and fast rule.

2) Others, including some fairly famous mechanics on this forum, generally recommend setting advance to 32-35 at high RPM and letting the idle fall where it may - generally higher: like 10 to 12 degrees. I did that 10K miles ago, I think I ended up at 32 - since I could hear it ping at 35D under WOT and backed it off a bit - and my idle was 10D BTDC with the retard line off - logically 20 degrees less than 5000 RPM which is the range of an SC distributor. My car ran well enough (it did not self destruct, at any rate) and importantly, it ran much cooler than before.

I can see where approch #1 is safer as you may not always hear pinging!

Since, as of last night I am back on factory settings (5D at idle, vac off) - and unfortunately running way hotter - I will simply disconnect and plug my retard line and see what happens. The car should run cooler at low RPM and be super safe at high RPMs. Tonight I'll measure high RPM advance with and without the advance vacuum, for giggles, because while I know that I have good retard vacuum, I never measured the advance vacuum (more debate there, some say there is zero vacuum at high RPM)....

If the car still runs too hot that way (and I suspect it might, but I'd be happy to be proven wrong), it'll be fairly obvious to me and I'll go back to the 30-32 degree settings which I've run for over a year with no *apparent* damage, and which netted me cooler temps.

At this point, knock on wood, she's got 185K miles and if she blows, she blows (doubtful since I put 15K miles last year with that same advanced timing)...Oh, I run 93 exclusively, obviously.

If someone sees something flagrantly wrong with the above, let me know before I blow up my engine !

If you are going to deviate from factory specs, especially the high rpm spec i'd want it done on a dyno, and monitor exhaust gas temps/ gas content while the testing was being done.

[Deschodt]

Okay - test #1...

Drove home tonight with retard line disconnected and plugged, reset idle back down to 900....
Unfortunately my commute was rather colder and more crowded tonight than this morning... Still, I saw lower temps overall (yes this is not scientific, but I know my car very well after 14 years and it was a touch cooler than usual: 90-95C max)

Got home, timed it with all vacuum hoses off and plugged...

900= 5BTDC
2000= 10
3000= 15
4000= 20
5000= 20
6000= 20, maybe 22, everything moves more at high RPM...

Seemed low, but advance vacuum was off...

I reconnected the advance line only, and I forget the numbers now, dammit... They were a little higher overall...by 5 I think...

In doubt, I bumped the timing a little to 8 at idle... Reconnected all lines, and got about 25 degrees at 5000-6000 RPM. So far so good....

Gonna have to reenlist my helper tomorrow and recheck with advance line on, 6000 RPM in the evening scares the neighbors !

[Bill Verburg]

Keep in mind that at wot there is zero manifold vacuum. The purpose of vacuum advance wrt distributor spark timing is to ignite the lean part throttle mixtures used for fuel economy.

[Deschodt]

Even with me advancing it a little, I won't see more than 25 degrees... I doubt that is detonation region... I also wonder where the 30+ proponents are hiding ;-) Anybody ?

I probably should leave it bone stock but I saw such lower temps with more advance, it's not funny !

[Bill Verburg]

Originally Posted by Deschodt

Even with me advancing it a little, I won't see more than 25 degrees... I doubt that is detonation region... I also wonder where the 30+ proponents are hiding ;-) Anybody ?

I probably should leave it bone stock but I saw such lower temps with more advance, it's not funny !

The total advance that you want will depend on cr, combustion chamber design, rpm, temp, sparkplug heat, load , mixture etc.

spec for SCs is 25* above 4k rpm

you can experiment beyond that but pre-ignition is very hard to hear

[Deschodt]

One thing that's been bugging me is how the vacuum lines affect the results, and the only adjustment numbers you find are w/ vacuum lines disconnected and plugged ! So if you get not enough (or somehow too much) vacuum, you could be off specs and never know it.

I've done the factory thing....
- I've disconnected the lines and plugged them
- Reset my idle to 900-950
- Set idle at 5 BTDC +/- 1, call it 6 on my car
- Observed that at whatever RPM, advance NEVER goes above 20 degrees. Manual says 21 +/-3 so we're covered... maybe a little lazy, but within the range !

Now what nobody seems to do: I've reconnected the advance vacuum only (left the retard line plugged) and remeasured: I got almost 30 degrees at 6000. So vacuum advance = +10, on my car! Is that the mythical 30 that is recommended by others ? Is that expected ?

Now in theory I should reconnect the retard line and re-raise my idle. But I'm sticking with Paul's idea here for a while and leaving it off.... I tested the retard vac line effect and it seems to only retard at low RPM, no effect at higher revs. Like it should, I think !

[Bill Verburg]

Originally Posted by Deschodt

One thing that's been bugging me is how the vacuum lines affect the results, and the only adjustment numbers you find are w/ vacuum lines disconnected and plugged ! So if you get not enough (or somehow too much) vacuum, you could be off specs and never know it.

I've done the factory thing....
- I've disconnected the lines and plugged them
- Reset my idle to 900-950
- Set idle at 5 BTDC +/- 1, call it 6 on my car
- Observed that at whatever RPM, advance NEVER goes above 20 degrees. Manual says 21 +/-3 so we're covered... maybe a little lazy, but within the range !

Now what nobody seems to do: I've reconnected the advance vacuum only (left the retard line plugged) and remeasured: I got almost 30 degrees at 6000. So vacuum advance = +10, on my car! Is that the mythical 30 that is recommended by others ? Is that expected ?

Now in theory I should reconnect the retard line and re-raise my idle. But I'm sticking with Paul's idea here for a while and leaving it off.... I tested the retard vac line effect and it seems to only retard at low RPM, no effect at higher revs. Like it should, I think !

vacuum has 2 effects, idle retard is used solely to reduce emissions and can be eliminated though it shouldn't make any difference to full power levels due to the fact that at wot there is zero manifold vacuum.

part throttle vacuum advance is used to enhance fuel economy, it does do harm, is mostly a positve thing and should be retained whereever possible on a street engine.

here's a graph that shows the effects of spark advance on combustion chamber pressures


curve 1 is what you want, as you advance full throttle timing you approach curve 2 which is where preignition is occurring, yes you are getting more power but at the expense of engine longevity

[mca]

I was under the impression that the advance vacuum would have a negligible affect at 6k rpms.

[Bill Verburg]

Originally Posted by mca

I was under the impression that the advance vacuum would have a negligible affect at 6k rpms.

neither advance nor retard obtained from manifold vacuum exists at wot.

at wot the manifold pressure drops to atmospheric pressure less intake restriction + any scavenge effect from cams, which is non existent w/ any of the 80s & 90s smog cams

[fumanchu]

Why is Deschodt's timing changing from 20 or so to 30 degrees when vacuum advance is hooked up. Vacuum advance is'nt suppoded to be in the picture at WOT. From what he is saying, it is. Maybe I need to re read.

Is there a load difference when parked in the garage vs. on the road @ 6000 rpm affecting the way the vacuum advance is working?

FWIW, I run timing at 25 full advance(in by 4000 rpm) retard disconnected also. Probably a bit rich also to give some saftey as I dont know how good my CIS is at 6000 rpm.

[911st]

Not an expert. As Bill says, vac retard is an emissions thing to create heat and vac advance is a good thing to increase efficiency at mild load.

A low compression 911 can take -40 or more advance w very low load on cruse. However, it must be backed off quickly w load. It is difficult to get w a weights & springs dist. A vac advance pot can get one closer but I understand most 911's did not have them. They also should be attached to a ported vacuum that shows vac above idle.

I would look to the 2.2 & 2.4's for a hit as to how much advance that can be taking. I think some were timed in the low 30's but that may or may not have been w the Vac Advance hooked up.

Exhaust temps are good to look at. If timing is not advanced to a more ideal level, it creates more heat in the exhaust. Porsche seems to have done this, like may makers, to improve emission and during start up to help fire the cat.

However, the best way is to check things on a dyno and probably at TQ peak as that is where things are most sensitive.

Further, it should be noted that environmental conditions effect how much timing one can take. Also, ones AFR becomes a variable. If a motor is lean will not take as much timing, rich more. Thus set you AFRs first and allow room for changes in environmental factors as early cars do not have any provision to pull things back.

Here is some ignition maps I stole from 911chips.com for a stock 3.2 that might be a reference to.

Note -40 deg at light load.


This is a WOT map. Note no more than about -25. This mightt be conservative but a 3.2 has some compensating features built in where it can pull timing back w heat.



Not sure if this helps.

[Bill Verburg]

Originally Posted by fumanchu

Why is Deschodt's timing changing from 20 or so to 30 degrees when vacuum advance is hooked up. Vacuum advance is'nt suppoded to be in the picture at WOT. From what he is saying, it is. Maybe I need to re read.

Is there a load difference when parked in the garage vs. on the road @ 6000 rpm affecting the way the vacuum advance is working?

FWIW, I run timing at 25 full advance(in by 4000 rpm) retard disconnected also. Probably a bit rich also to give some saftey as I dont know how good my CIS is at 6000 rpm.

It takes time for things to come to equilibrium, so when shop tuning the vacuum is disconnected and plugged so that you don't have to hold the throttle wot w/ out a load. zinging it to 6k in the shop is not an experience which is desireable to extended.

[911st]

Originally Posted by Bill Verburg

...
here's a graph that shows the effects of spark advance on combustion chamber pressures


curve 1 is what you want, as you advance full throttle timing you approach curve 2 which is where preignition is occurring, yes you are getting more power but at the expense of engine longevity

That is is way cool diagram. I have never seen such a thing.

Is Zb pre-ignition or detonation? I am thinking Zb is detonation.

It looks like the combination of early ignition that builds heat and pressure is kind of multiplied by an increase in pressure from the piston being closer to TDC until where the mix auto ignites or flash's instead of burning in an orderly manner.

I would suspect pre-ignition would show a spike in pressure before TDC but not sure.

I think I read that we want peak pressure at about 14 deg after TDC for peak HP. That would be Za it looks like.

With Zc the mix is still burning and adding pressure after the other two times but the piston retraction is such that the the piston dose not see as much peak pressure. This is also the condition where more exhaust heat is created as the gasses are still expanding when the exhaust cycle is started.

Can near ideal timing be measured from EGT's. That is is it the low point between detonation and late ignition? Or maybe we can start w late timing and start advancing until temps start to taper as they get cooler?

I still have a lot to learn.