[Jonathan Jackson]

I just bought a 78 450 SLC and found the timing at idle with vacuum attached to be 5 ATDC. I set it to 5 BTDC and it seems to run much, much better. Even at this setting, I seem to be getting only about 20 degs of total mechanical and vacuum advance. That's kind of lame, isnt it? No wonder I see people on this list advancing their timing to 10deg or more. This engine ought to be able to take at least 35deg full advance without pinging. Does anyone know what the specs are for the initial timing ( i think its spec'ed at 0 deg BTDC), the total mechanical advance and the total vacuum advance? Or maybe just what the timing should be with full advance?

I was also curious if anyone has upgraded their ignition to something more modern like a capacitive discharge, TFI coil and bigger distributor cap. That oilcan coil has just gotta go. I make better sparks just walking across a carpet in socks.

 

[Don36]

Timing Specs

My Chilton's Tune Up Guide says the initial timing for the 450SLC should be TDC with vacuum.

Don 36

 

[Jonathan Jackson]

Yeah, thats about the only info about timing I could find anywhere. I just thought there ...

Yeah, thats about the only info about timing I could find anywhere. I just thought there might be someone who knows what the full advance spec is supposed to be. Im thinking that this thing won't ping until you get up to 35 BTDC and there is a lot of power being lost by setting TDC at idle with vacuum and using only mechanical advance to get you up to maybe 20 deg BTDC full advance.

 

[Greg in Oz]

450SLC ignition system

You have to remember that, dependent upon market, the cars of the '70s had some tough emission control requirements to comply with. Here in Australia, the potent high compression 350 and 450 engines of the early '70s (without emission controls) were strangled in the late '70s due to new design rules. This often involved ignition timing curves that gave poor performance and fuel economy but met the emission requirements.

With regard to the coil and other ignition system components, don't be deceived by appearances. It was an electronic system with a high turns ratio, high output coil. There is no good reason to "upgrade" any of it and you would attempt to do so at your peril. A CDI system would be a disaster, especially on the V8, with crossfiring and other such problems.

 

[Jonathan Jackson]

I have to disagree with your assessment of the ignition system. Aside from the points bei...

I have to disagree with your assessment of the ignition system. Aside from the points being replaced with an ignition module, this is a very primative system.
First off, no canister type coil is going to compete with the much higher output of a Thin Film E-core style. They are just much more efficient.
You can't just slap an ecore in becasue of the higher output voltages. You'd need to fit a bigger distributor cap to prevent arcing (crossfire, arcing to ground etc.) when you go from 25K Volts to 40K+ Volts on the TFI coil.
As far as the Capacitive discharge system. This system is a quantum leap over the existing module. This is a system that you could go racing with. It produces a heck of alot more spark energy. The Direct hits are nice in the way they do pulse compression to boost peak power, but they do not alter the amount of energy that is sent to the plug. They are just a way to use the available spark energy more efficiently; more or less a workaround for what you really want which is more spark energy.
Finally, I would say that the CDI could be used with the existing coil and distributor. Crossfiring is caused by exceeding the breakdown voltage of the distributor cap. As long as you use the same coil and don't upgrade to TFI, you havent increased the voltage (which is set by the turns ratio of the coil), and you shouldnt have any crossfire problems. The CDI system only messes with the supplied current pulse to the primary of the coil but it leaves the voltage at 12 Volts (actually more like 14.6 Volts)

 

[Greg in Oz]

Re: I have to disagree with your assessment of the ignition system. Aside from the points bei

I have to disagree with your assessment of the ignition system. Aside from the points being replaced with an ignition module, this is a very primative system.
First off, no canister type coil is going to compete with the much higher output of a Thin Film E-core style. They are just much more efficient.
You can't just slap an ecore in becasue of the higher output voltages. You'd need to fit a bigger distributor cap to prevent arcing (crossfire, arcing to ground etc.) when you go from 25K Volts to 40K+ Volts on the TFI coil.
As far as the Capacitive discharge system. This system is a quantum leap over the existing module. This is a system that you could go racing with. It produces a heck of alot more spark energy. The Direct hits are nice in the way they do pulse compression to boost peak power, but they do not alter the amount of energy that is sent to the plug. They are just a way to use the available spark energy more efficiently; more or less a workaround for what you really want which is more spark energy.
Finally, I would say that the CDI could be used with the existing coil and distributor. Crossfiring is caused by exceeding the breakdown voltage of the distributor cap. As long as you use the same coil and don't upgrade to TFI, you havent increased the voltage (which is set by the turns ratio of the coil), and you shouldnt have any crossfire problems. The CDI system only messes with the supplied current pulse to the primary of the coil but it leaves the voltage at 12 Volts (actually more like 14.6 Volts)

The ignition system Daimler-Benz engineers specified for fitment to large numbers of Mercedes-Benz cars of this era was entirely satisfactory then and still is now. Had the engines had much higher compression ratios and run very lean mixtures then a higher output system may have been necessary but the fact is they didn't.

Output voltage of the system is actually determined by the spark plug gaps and combustion chamber pressures. Open circuit voltages are not really relevant so long as there is sufficient voltage to work with the requirements of a particular engine. 40kV or even 25kV should never be realised in a correctly tuned engined. A larger distributor cap or indeed any other upgraded high tension component would only be necessary if spark plug gaps or combustion chamber pressures were increased, thereby increasing high tension voltage.

You appear to have misunderstood the operation of a CDI (capacitive discharge ignition) system. In this system the coil is not an inductive energy storage device but simply a transformer. The coil primary DOES NOT operate from the usual low battery voltage (and rely on a collapsing magnetic field to produce the HT in the secondary) but instead has hundreds of volts applied to it from a capacitor (the energy storage device in a CDI system). Due to the fast voltage rise times of a CDI system, crossfiring as a result of coupling between high tension cables can be a problem, particularly on engines with a large number of cylinders. CDI is best used with ignition components designed for this system and is more suited to applications such as two stroke outboard motors.

I still maintain that any Mercedes-Benz from the early 1970's onwards (ie. transistor assisted or electronic ignition) has an entirely adequate, low maintenance system. I doubt that any noticeable performance gain would be realised with the addition of any newer "high output" ignition system.

 

[AJSM]

European specs

Well, I don't understand 99% of the things you're talking about (I'm not a mechanic and I only want to drive the car...), but one thing I can tell you: if you upgrade the car to european specifications you'll get an incredible amount of power.
I belive that there are V8 (380, 350 or 450) that had around 135 hp in the US and up to 235 hp (500) in Europe... I know for sure that my 6 cylinder 280 (185 hp) has a lot more power than the american 380 V8.
So, why not try to put the engine specs as the european originals? I don't think this must be very difficult! After all the engines and the fuel injection should have been the same!

Jorge
'81 280 SLC

 

[Jonathan Jackson]

Re: Re: I have to disagree with your assessment of the ignition system. Aside from the points

The ignition system Daimler-Benz engineers specified for fitment to large numbers of Mercedes-Benz cars of this era was entirely satisfactory then and still is now. Had the engines had much higher compression ratios and run very lean mixtures then a higher output system may have been necessary but the fact is they didn't.

You might be right about the spark being adequate for low compression and richer mixtures. I would mention though, that it seems like a large number of people are using Direct Hits and experiencing improvements in performance. Sounds to me like the system isnt totally satisfactory.

Open circuit voltages are not really relevant so long as there is sufficient voltage to work with the requirements of a particular engine. 40kV or even 25kV should never be realised in a correctly tuned engined.

I'm sorry, Im just not buying that. If I attach a 40kV source to a spark gap, the voltage is initally 40kV and falls as the coils field energy is exhausted.

The reason for higher voltages is that it allows you to open the spark gap and expose more mixture to the spark and increases the likelyhood of a complete burn. I'm not just spouting this out, this is very well known to performance engine builders.

You appear to have misunderstood the operation of a CDI (capacitive discharge ignition) system. In this system the coil is not an inductive energy storage device but simply a transformer.

Does this mean I have to give back my PhD in electrical engineering

The coil primary DOES NOT operate from the usual low battery voltage (and rely on a collapsing magnetic field to produce the HT in the secondary) but instead has hundreds of volts applied to it from a capacitor (the energy storage device in a CDI system).

Not on the (MSD) system I have installed on my Jeep.

CDI is best used with ignition components designed for this system and is more suited to applications such as two stroke outboard motors.

Huh? visit the MSD website (or Accel or Jacobs)and check out their applications. They are predominantly supplying these systems to 4-strokes.

I guess I would just say, we're all entitled to our opinions and I'm not really trying to start and endless debate with you. Thanks for your replies, though.

 

[Jonathan Jackson]

Re: European specs

So, why not try to put the engine specs as the european originals? I don't think this must be very difficult! After all the engines and the fuel injection should have been the same!

Jorge
'81 280 SLC

I'll look around and see what I come up with for differences. I'm not sure if the engines are the same. They may have changed the cam for the US version. Im not sure I want to swap out cams, I'm just looking to dredge out any available power thats been wasted in the name of emissions.

 

[Greg in Oz]

US spec vs Euro (& Aus) spec 450

The US spec M117 450 was quite different to the European (and Australian) spec engines. I use MB North America workshop manuals and am aware of many differences. The Euro and Oz 450 of the early '70s was good for about 225hp. The US versions were well below this and got progressively lower throughout the '70s. In the late '70s here ours were strangled by emission controls although not as severe as those on the US spec engines. I know a late 450 here won't go close to matching the performance of my '73 350 M116 which, without any emission controls, is good for 200hp DIN (230hp SAE). It would be very difficult to upgrade the later engine as apart from issues such as ignition timing (that you initially questioned) you have to deal with compression ratio, cam profiles, fuel mixture, catalytic convertors, etc.

With regard to the issue of spark voltage, the voltage at the spark plug will increase to a level where ionisation will occur in the spark gap. A breakdown then occurs which causes the voltage to be clamped at a much lower level. This is the very reason that spark gaps are employed as a form of over-voltage protection or voltage clamp in the electrical and electronic industry. I stand by my statement that spark voltage is determined primarily by spark plug gap and combustion chamber pressure, not the (open circuit) secondary voltage of an ignition coil.