WOT Ignition Timing general question

[scarceller]

I have been working on 84-89 Carrera chips for some time now and I have a simple question about Ignition Timing at WOT:

I always thought that the reason to advance ignition timing as RPMs increase is to simply allow ample time for flame front to fully reach other side of cylinder at about the same time that the cyl reaches TDC. If this is true then one must fire the plug earlier and earlier as RPMs increase.

But as I look at WOT Ignition maps in the 84-89 cars I see they seem to advance timing following this practice to about 4000RPMs but after this all chips I've seen tend to stop advancing the timing or in some cases even begin to retard. Why is this? I would have thought that you'd need to continue the advancing as RPMs increase even in the 4000-7000RPM range.

Can anyone help me understand this?

And what about earlier cars (non-motronic) what do they do?

Thanks.

[scarceller]

Bump,

Maybe a picture will be of help:


Why do the factory chips stop advancing timing at about 3500RPMs? This seems counter intuitive? I would think you would need to continue to advance timing somewhat as RPMs rise from 3500-7000? Any one know why they advance timing only to about 3500RPM?

Thanks.

[Trog]

The only thing I can think of is to encourage you to change gears at 3500RPM in an effort to improve fuel mileage?

[911pcars]

Maybe because their ignition advance charts are scaled with a mixture of engine crank degrees and distributor rpm. (dist. = 1/2 crank speed). 3450 dist. rpm is 6900 eng. rpm, and so forth?

The factory charts are similar, but they usually stick with one (dist.) or the other (crank) in both respects (speed vs degrees advance).

Sherwood

[longhornchris04]

I know that part of the issue is that as engine RPMs rise, you get better turbulent mixing of the intake. This accelerates the burn rate, reducing the need for timing advance.

Additionally, there may be issues with dynamic compression rising at high RPMs, where additional timing advance would lead to pinging - but that's just an educated guess.

[scarceller]

I'm NOT thinking of altering this timing as I'm sure the factory did it's homework, I'm just curious why they seems to stop advancing the timing at about 3500RPM.

How about the earlier non-motronic cars what do they do?

Thanks for the replies so far.

[longhornchris04]

Typically mechanical distributors had mechanical advance via centrifugal (RPM) and vacuum (load). The centrifugal was linear up to a point, then "all-in" and flattened out.

I know there were some more complicated centrifugal setups that used multiple springs to give non-linear, or overlapping lines. Even with these, there is an "all-in" point where the advance stops, usually between 3.5K-4.5K

Electronic ignitions, especially the early ones, simply used tables to mimic the the dizzy curves. Later, they started making more complicated maps.

[911st]

Interesting.

[RoninLB]

just a wild guess


the fuel mix is able to be adjusted leaner than a more primitive fuel system which would in itself enable more rapid flame travel.. thus negating the greater elapsed time necessary than a more rich fuel mix for complete combustion... in so much that there is such a thing as complete combustion

[911st]

I hope you get a solid answer.

Just looking at the piloted curve it just dose not look natural and seems suspect.

It looks like advance may have been more close to optimum in the range most use every day. Then it was pulled back for a margin of safety.

I could understand it better if it was a more natural curve that got pulled back some near torque peak where the motor is reaching its peak efficiency and cylinder pressures are there greatest. Or after 5k where the AFM starts to loose its fidelity to build in a margin of safety.

Plus, 24 deg just seems conservative to begin with.

Still have no idea unless it is to reduce risk.

[911st]

Found a couple of points so far.

- Usually full advance will have occurred by 3500rpm.

- Ideal point of power occurs just before the point of pre detonation.

- A rich mixture of say 12.5/1 requires less timing than a lean mix of say 14/1.

- As cylinder head design progresses, total advance necessary has decreased.

- Under light load, leaner mix burns faster, rich slower.

- Correctly timed motors will create peak cylinder pressures around 12-15 deg ATDC.

- If peak cylinder pressure is reached later than about 12-15 ATDC energy is wasted and dispersed through exhaust energy heat.

From: http://www.gnetworks.com/v4files/barrygrant/Ign%20timingwithimages.pdf

[911st]

Camshaft with more duration, Improved cylinder filling at higher speed, worse at low speed - - Less advance at high speed, more at low speed.

Improved exhaust efficiency or lowered backpressure - The presence of residual exhaust gas in the cylinder retards the flame front - Less advance required when exhaust extraction effect is working, ie, higher up in the rev range

See: http://www.jcna.com/library/tech/tech0013.html

Increased engine temperature affects final charge temperature - Less advance required as engine temperature increases.

[911st]

From Rensports web site"

"Porsche's hemispherical air-cooled heads are well known for poor swirl characteristics and the bigger bore engines are quite detonation prone at 9.5:1 and above."

My thought is higher at higher RPM's swirl increases for better mixing and less ignition is needed.

Still, the few articles I could find leads one to believe our motors should be running more advance than 24 deg.

[911st]

Another quote from Rennsport:

"...When the timing is over-advanced, combustion temperatures and cylinder pressures skyrocket..."

[Flieger]

The distributor shaft rotates at 1/2 crankshaft speed and most advance maps show distributor shaft speed. Therefore, 3500 distributor shaft rpm equals 7000 crankshaft rpm.

[T77911S]

read this article. very interesting and very good information.

i think this is the site, i can not get to it at work.

http://www.clubwrx.net/forums/tuning-electronic-engine-management/14426-engine-basics-detonation-pre-ignition.html


what i have come to understand. more advance is not a good thing. it is needed to make up for bad head/piston design. the more you advacne ignition, the sooner it fires while still on the compression stroke, which actually robs HP. eventually advance gets to be too much and increases temps so much that detonation occurs. unlike idle, advence increases temps. with increased temps and pressure from hi rpm, timing may need to be reduced, like in turbo's, more boost, less timing.

[scarceller]

Originally Posted by 911st

Still, the few articles I could find leads one to believe our motors should be running more advance than 24 deg.

Should have mentioned this: Those timings I posted are from the twin plug 964, I just posted them to give you the general idea of what the timing curve looks like. For the single plug 87 3.2L the curve looks similar but max timing is still only 26deg BTDC at 4000 RPM. In the 87 car they advance to about 25deg at 4000RPM after this it's all in up to redline.

[scarceller]

I guess the best way to set timing would be on a load dyno and at the given RPM set timing for max lean torque then back off 10% or so, I think this is how the pro tuners (MoTec) folks do it.

[911st]

Yes, every motor design makes for different ideals. Dialing in on a dyno is the best way to go. Still, some leaps of faith will be needed to deal with real world conditions like temperature changes, fuel, weather...

I guess that is what pulling back from max points allows for to a degree.

As to the curve posted, then that is not only a twin plug but a motor that runs compression that is pretty extreme for street fuel requiring less advance.

[scarceller]

Just for reference: even in the 84-89 3.2L the factory most aggressive WOT Ignition was 26 degrees and this was in the 89 Carrera 3.2L at 4000RPMs.

Originally Posted by 911st

Yes, every motor design makes for different ideals. Dialing in on a dyno is the best way to go. Still, some leaps of faith will be needed to deal with real world conditions like temperature changes, fuel, weather...

I guess that is what pulling back from max points allows for to a degree.

As to the curve posted, then that is not only a twin plug but a motor that runs compression that is pretty extreme for street fuel requiring less advance.