Please help me understand ignition timing

[Ole Petter]

Can somebody please help me understand the facts of ignition timing?

I have a 1974/75 911S (9:1 CR, running 98 European octane fuel) with the data of 5 degrees at 900 rpm and 38 degrees at 6000 rpm (Waynes How to Rebuild an Modify 911 engines book). Last year I bought a Permatune Ingition CD-box and coil, and that was a big improvement over the tired Bosch unit!

But the older cars is running at 30 degrees at 6000 rpm. How is that? Is the more advance making more power, and is the later cars different because of lesser fuel?:

I am adviced to run the car at a 34-35 degrees at 6000 rpm (and that what really matters is the degrees at 6000, that idle is more of an emissions value thing, so set it up at 6000 rpm). Will this make more power? Or will I just run at higher risk of hurting my engine's longevity?

I think I understand that more advance means more power, but also more risk of detonation. I also understand that 5 ATCD is after top dead center, but how much earlier is 38 BTDC, how far does it show before it gets to 0 degrees? So if I set the ignition to 35 degrees that means more advance, and that 30 degrees is even more advance?

Please help me unmystify these things!

Many thanks, Ole Petter

[psalt]

Hello Ole,

Ideally, the ignition should be timed to produce peak cylinder pressure around 15 degrees after top dead center. Since the burn time of the fuel is relatively constant, the ignition timing needs to be advanced (sooner) as rpm increases. The amount of ignition advance and the curve is determined by the cylinder head design. On a engine with a hemi head and domed pistons (911) a large amount of advance is needed to produce peak torque. Since the shape of the combustion chamber at TDC is poor, like an orange peel, the risk of detonation is higher than on pent roof engines with compact combustion chambers. The 911 is a knock limited design and the fuel used will determine the amount of timing you can run. If you can obtain the specified fuel octane, set your timing to the factory spec. US cars were detuned with less advance because of the lower octane of the early unleaded fuels. The total advance is the more important figure and idle settings can be false due to distributor wear. If your engine does not ping at the factory spec, you can advance it a few degrees, but yes, you run the risk of detonation and the improvement in power will be very little, unlike with US cars. The probablity of detonation is highest at peak torque, when cylinder pressure and VE is highest. This is why the timing curve has a knee or kink in it.

[Walter_Middie]

Paul - that's an excellent explanation.

I also understand that 5 ATCD is after top dead center, but how much earlier is 38 BTDC, how far does it show before it gets to 0 degrees? So if I set the ignition to 35 degrees that means more advance, and that 30 degrees is even more advance?

I think there is something that Ole is missing here. The timing advances with RPM due to the advance weights in the distributor (mechanical advance). The faster the distributor turns, the more advance it automatically adds. So at idle, you might have 5 degrees after top dead center (ATDC), then as the RPM climbs, the timing will change/advance to 0 degrees, then on to a total of 38 degrees before top dead center (BTDC) - this is where the distributor stops advancing. So your total timing is 38 degrees, even though you've moved from 5 ATDC to 38 BTDC, which is a change of 43 degrees. The advance comes from both vacuum and the advance weights. At low RPM the vacuum will also move the timing from 5 ATDC to around 5 BTDC, the mechanical advance will be changing at the same time, and it is the combination of both that gets you all the way to 38 BTDC at 6000 RPM.

[Bill Verburg]

I don't have the full specs in front of me, but as I recall, T used ~30total and E,S & Carrera ~35-38total.

5 at idle w/ vac connected. The vac is used to retard idle timing. If you disconnect it timing and idle speed will both advance. At 6000 there is little or no vac. so the only the inertial mechanical advance is in effect.

[304065]

[Ole Petter]

[QUOTE=Bill Verburg;3628976]I don't have the full specs in front of me, but as I recall, T used ~30total and E,S & Carrera ~35-38total.

OK. I am starting to see the picture... What got me confused is the timing difference of the early cars to my 74/75, that the ignition was retarded from the 5 degrees BTDC to the 5 degrees ATDC of my car (due to the unleaded fuel quality in the early 70'). And then that the older cars had a 30 BTDC at 6000 and my car has a 38 BTDC at the same 6000 revs...

As mentioned here my car has a "total advance of 43 degrees" as compared to the older cars' 25 - how is that?

And if I set my car to 34 degrees BTDC at 6000 revs, then what of the advance or lack thereof at idle? I was told that the only thing that should matter to me is the ignition timing at 6000 revs... That the idle setting is more to do with emissions.

Am I gettin' it or am I worse off than at the beginning

[Bill Verburg]

John's chart shows distributor degrees, double it for crank degrees as determined w/ a timing light.

Yes, total max advance @ 6k is all thats important. At idle 0-5 is the norm.

[roadhog]

if 911s are like most cars the vacuum advance retards the timing when the engine comes under heavy load.this is when spark- knock is likely to occur.

[Bill Verburg]

Originally Posted by roadhog

if 911s are like most cars the vacuum advance retards the timing when the engine comes under heavy load.this is when spark- knock is likely to occur.

It depends on the year but for the years and models under discussion here there is vac retard at idle, part throttle.

This by the way is the oposite of what most cars do(the part throttle retard any way). For best fuel economy and throttle response most cars(pre computer control) ran vac advance.

[Ole Petter]

Why is the graph different for the Carrera, S and E versions of the 911? Are there different weights in the distributor? The ignition systems of the various cars is as far as I know just about the same?

Ole

[85carrera]

Ole,

To answer your last question, its not just that the ignition systems are different, its the shape of the combustion chamber, compression ratio, cc's, intake design, etc which differs from engine to engine. Each engine design has a different point at which spark knock (detonation) will occur. For example, there are points in the combustion chamber called "hot points" (someone correct me if I'm wrong), which can cause early detonation. That's why old cars ie...30-40s were forced to run at really low compression because any higher compression would cause a detonation due to the poor clyinder head design. (and also poor gas octane). That's why today you see cars which can run 10:1, 11:1 compression on regular octane gas- its because cylinder head design and combustion chambers have come so far.


Also, modern cars have what's called a knock sensor which allows the ECU to advance the timing to the point _right_ before the engine is about detonate. So......what happens is if you put in crap fuel your engine will make up for the lack of octane and retard the timing a bit- thus resulting in lesss power. However, on the flip side if you pay the extra penny your engine can advance the timing resulting in more power. granted a couple degrees of timing difference won't throw your head back...

Honestly, to truly understand ignition timing, retard vs advance, read a book on basic engine dynamics. It all starts with the fact the there is no real "explosion" in the combustion chamber, but rather a movement of the flame wall starting at the ignition of the spark plug moving down and hitting the piston. Detonation is where the flame ignited too soon and it hits the piston on its way up- but the piston can't go back down, because its on its rotation up! -Thus creating serious damage- holes in pistons, clyinder walls, throwing rods, etc. To get the engine at its optimal efficiency, the spark plug is ignited while the piston is on its way up, but in an effort to cause the flame to hit the top of the piston RIGHT AT THE MOMENT the piston finishes its rotation, and is ready to go back down.

Octane rating puts another twist on things. The higher octane rating the higher you can compress the fuel/air mixture before it ignites on its own, which would then result in detonation.

Hope this helps....it helped me get my mind off work for 15 minutes.

[85carrera]

oh yeah...so those different cars you see in the graph have different limits to how far you can advance the timing, so they designed the ignition system to appropriately advance the timing depending on the circumstances...

[RWebb]

right

his other question is mechanistic - how that is achieved - answer there is the wt. of the - ah - wts. and length of the fulcrum arm they operate on

[Ole Petter]

So the answer to the ignition timing differences is bigger than I thought!

The CIS cars - with domed pistons, less CR (thanks in part to the fuel crisis, lower quality fuel and the demand for lowered emissions) differ from their earlier, higher revving more powerful brethren in many ways...

This was very enlightening for me!

I didn't see the differences in the head- and piston design in connection with the ignition timing. So there is not much in it for me to change the ignition advance to be more like the earlier cars... This will not suit the my 2.7! But I can adjust the advance at 6000 revs from the recommended 38 degrees to maybe 34-35. But again, this will not amount to much power!

Thing is that by installing the Permatune ignition CD-box and coil I got a noticable power- and torque increase. That got my mind thinking that I could maximice on this advantage and reduce the advance because of my new and more powerful ignition system. Like you would do if you had a twin-spark set up on the engine, then you would reduce the advance to 24-25 degrees because of the more efficient ignition...

My point is to make my 2.7 engine as powerful and rev-happy as the CIS configuration will allow.

Ole

[psalt]

Ole wrote,

But I can adjust the advance at 6000 revs from the recommended 38 degrees to maybe 34-35. But again, this will not amount to much power!

Thing is that by installing the Permatune ignition CD-box and coil I got a noticable power- and torque increase. That got my mind thinking that I could maximice on this advantage and reduce the advance because of my new and more powerful ignition system. Like you would do if you had a twin-spark set up on the engine, then you would reduce the advance to 24-25 degrees because of the more efficient ignition...

Hello Ole,

Adjusting the timing from 38 to 34 would be retarding it, lowering the power. These numbers are BTDC (before top dead center), a larger number BTDC is earlier, advancing the timing.

The twin spark comparison is not valid. It is about burning all the fuel, not spark energy. As mentioned above, the 911 has a large combustion chamber with domed pistons that results in a long thin combustion space at TDC, that requires a lot of advance to burn the fuel from one side to the other in the time available. A twin plug engine starts the burning at two points, lowering the amount of time required and therefore max torque will occur at a lower advance. The basic mechanical curve is determined on a dyno, the ignition is advanced at different rpm until torque drops off. The US cars were spec'd for a lower octane fuel and the optimum timing curve could not be used due to detonation risk and were detuned. Porsche knew how to set a basic timing curve and you will not find much improvement changing the ignition timing unless you are using a higher octane fuel than the spec. on a detuned engine. There should be no "real" difference with a Permatune ignition, unless the old one was defective, and you are now back to "normal."

The vacuum retard was an early attempt to meet emission tests. It reduces HC emissions at idle and has the side effect of raising the engine temperature. Most engines run better without it. Vacuum advance is a fuel economy device. At part throttle, low load cruise, the mixture is leaner and needs more advance to completely burn the fuel. Vacuum advance adds more advance and can raise the efficiency 10-15%. Detonation a low load is not a problem because VE is low and cylinder pressure is low.

[Ole Petter]

Let's start again, one more time... (as the teacher in school said)

If I advance the ignition more, that means more power! Right.

So to go from 38degrees to 34 that is to retard the ignition and this gives less power.

My problem is that I thought the earlier cars had a more power-friendly ignition curve. But their curve is different. Waynes book states 5degrees ATDC at idle and 38 degrees BTDC at 6000 rpm. The 1971 S model had 30 degrees at 6000 rpm.

Why is this? My car (ex California car) has 5 degrees ATDC at 900 rpm. The older cars had more like 0 degrees at 900 rpm. So my car has a retarded ignition curve.

So then I will drive the car with the ignition set as per today. 5 degrees ATDC at 900 rpmand 38 at 6000.

If I at some time, would change my CIS for PMO's, high rise cams and high CR pistons, then I could change the ignition curve by "tuning/resetting" the distributor and so on and so on... Money rules!

Ole

[Bill Verburg]

Originally Posted by Ole Petter

Let's start again, one more time... (as the teacher in school said)Ole

If I advance the ignition more, that means more power! Right.
Sometimes, just advancing timing does not guarantee more power. Some engine configs need more than others is all. Full power spec is not subject to EPA restrictions or regulation so use the factory spec for full advance for your engine. If the spark is over-advanced you run the risk of burning a hole in a piston. When I was young and foolish that happened to me on several occasions on my racing bikes

So to go from 38degrees to 34 that is to retard the ignition and this gives less power. See above, sometimes

My problem is that I thought the earlier cars had a more power-friendly ignition curve. But their curve is different. Wayne's book states 5degrees ATDC at idle and 38 degrees BTDC at 6000 rpm. The 1971 S model had 30 degrees at 6000 rpm. again different engine configurations have different spark timing needs

Why is this? My car (ex California car) has 5 degrees ATDC at 900 rpm. The older cars had more like 0 degrees at 900 rpm. So my car has a retarded ignition curve.Idle and part throttle regimes are subject to EPA regulation, in the bad old days before 3 way cats and lambda control there were all sorts of ugly strategies used to try to reduce emissions. retarded spark, EGR and secondary air pumps were all part of the program.

[psalt]

Ole wrote,

My problem is that I thought the earlier cars had a more power-friendly ignition curve. But their curve is different. Waynes book states 5degrees ATDC at idle and 38 degrees BTDC at 6000 rpm. The 1971 S model had 30 degrees at 6000 rpm.

Why is this? My car (ex California car) has 5 degrees ATDC at 900 rpm. The older cars had more like 0 degrees at 900 rpm. So my car has a retarded ignition curve.


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.