Understanding engine ignition timing

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This is one of them science, geeky questions.
The Haynes manual covers 4 types of VW 1.9L Wasserboxers: DF, EY, DG and GW
All of them have 94mm Bore, 69mm Stroke. All are 8.6:1 compression ratio except the GW (7.5:1).

The Ignition Timings are as follows (ignore limits):

EY:  5Deg BTDC @ 850rpm, Vacuum Disconnected
DF:  5Deg ATDC @ 750rpm, Vac connected
DG:  5Deg BTDC @ 900rpm, Vac Disconnected

My question is:

I have a DF engine, why is mine set up After TDC with the Vac connected. Why do some engines get set up with the vacuum connected, some disconnected? Why is mine setup ATDC and not BTDC?

Without going into super technical engine jargon, can someone explain how engine manufactures come up with this setup data? Could mine not be set up BTDC with the vacuum disconnected?

I assume the induction system must come in to all of this.

My crude understanding:

The idea is for the fuel to be ignited at the right moment for maximum pressure. That pressure acting on the piston (at TDC or very close. Minimum volume, maximum pressure), the maximum force is then transmitted to the crank, generating torque. Based on my limited experience it seems most engines are set up BTDC. 

Is BTDC setup due to the speed of engine rotation: Looking at time: the piston rises, hits (eg) 5 Deg BTDC (the command) and then the time delay (milliseconds) as signal passes through the ignition system to the plug (ignition). 

Is this proportional to the time it takes the piston to travel from 5 BTDC, past TDC and then ignition on the downstroke. Ie, the timing is set to account for time delay (command to ignition) of the ignition system.

Also, if ignition was set up incorrectly and setup too soon, before the piston reaches TDC, then that would force the piston the wrong way (ie engine turns the wrong way)

Well, thats my very crude understanding of ignition. 

So why set a DF engine up 5Deg ATDC? Surely the piston will be well and truly on the downstroke travel and therefore a larger volume and therefore less pressure, so completely inefficient?

Confused as ever 

BM

Bollo*, just realised this is in the wrong forum. Could someone please move this to the T3 Tech section.

Edited by BUBBLE MEISTER - 26 Feb 12 at 21:40

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BUBBLE MEISTER Members Profile Send Private Message Find Members Posts Add to Buddy List Yardie Joined: 15 Mar 07 Location: Lincolnshire Status: Offline Points: 775	Post Options Post Reply Quote BUBBLE MEISTER Report Post Thanks(0) Quote Reply Topic: Understanding engine ignition timing Posted: 26 Feb 12 at 21:38
	This is one of them science, geeky questions. The Haynes manual covers 4 types of VW 1.9L Wasserboxers: DF, EY, DG and GW All of them have 94mm Bore, 69mm Stroke. All are 8.6:1 compression ratio except the GW (7.5:1). The Ignition Timings are as follows (ignore limits): EY: 5Deg BTDC @ 850rpm, Vacuum Disconnected DF: 5Deg ATDC @ 750rpm, Vac connected DG: 5Deg BTDC @ 900rpm, Vac Disconnected My question is: I have a DF engine, why is mine set up After TDC with the Vac connected. Why do some engines get set up with the vacuum connected, some disconnected? Why is mine setup ATDC and not BTDC? Without going into super technical engine jargon, can someone explain how engine manufactures come up with this setup data? Could mine not be set up BTDC with the vacuum disconnected? I assume the induction system must come in to all of this. My crude understanding: The idea is for the fuel to be ignited at the right moment for maximum pressure. That pressure acting on the piston (at TDC or very close. Minimum volume, maximum pressure), the maximum force is then transmitted to the crank, generating torque. Based on my limited experience it seems most engines are set up BTDC. Is BTDC setup due to the speed of engine rotation: Looking at time: the piston rises, hits (eg) 5 Deg BTDC (the command) and then the time delay (milliseconds) as signal passes through the ignition system to the plug (ignition). Is this proportional to the time it takes the piston to travel from 5 BTDC, past TDC and then ignition on the downstroke. Ie, the timing is set to account for time delay (command to ignition) of the ignition system. Also, if ignition was set up incorrectly and setup too soon, before the piston reaches TDC, then that would force the piston the wrong way (ie engine turns the wrong way) Well, thats my very crude understanding of ignition. So why set a DF engine up 5Deg ATDC? Surely the piston will be well and truly on the downstroke travel and therefore a larger volume and therefore less pressure, so completely inefficient? Confused as ever BM Bollo*, just realised this is in the wrong forum. Could someone please move this to the T3 Tech section. Edited by BUBBLE MEISTER - 26 Feb 12 at 21:40
	I have BUBBLE the '85B 1.9DF (currently a DG!) WBX Camper. 29 Yrs Young. Tis Tracie's V Bubble U.

BUBBLE MEISTER Members Profile Send Private Message Find Members Posts Add to Buddy List Yardie Joined: 15 Mar 07 Location: Lincolnshire Status: Offline Points: 775	Post Options Post Reply Quote BUBBLE MEISTER Report Post Thanks(0) Quote Reply Posted: 26 Feb 12 at 21:49
	Thanks Moderators!!
	I have BUBBLE the '85B 1.9DF (currently a DG!) WBX Camper. 29 Yrs Young. Tis Tracie's V Bubble U.

Red Westie Members Profile Send Private Message Find Members Posts Add to Buddy List Yardie Joined: 23 Feb 07 Location: United Kingdom Status: Offline Points: 567	Post Options Post Reply Quote Red Westie Report Post Thanks(0) Quote Reply Posted: 26 Feb 12 at 23:32
	For a non technical person with limited technical knowledge your question sure is well put and very technical... You pretty much have it apart from your understanding of engine speed versas ignition advance......the ignition is advanced proportional to engine speed not because of a delay in the ignition mechanism or the spark in getting to the spark plugs.... but rather a delay in the time that the fuel takes to ignite and combust. To put things into context...the pistons in an engine reving at 5,000rpm are going up and down each of the 4 cylinders more than 165 times a second. So although the 'delay' in combustion remains relatively constant as the engine increases in speed, it is necessary for the point at which the spark occurs to be advanced to compensate for the decrease in 'time' the fuel has to ignite so that the actual combustion occurs at that ideal point (as you noted..at or just after TDC) This is where things get more complicated: engine load/mixture/temperature/speed also effect the 'ideal' point at which the mixture should be ignited. For example: the weak mixture created whilst cruising at speed will need a more advanced ignition point as weaker mixture takes longer to burn. The way manufacturers dealt with these variables back in the 80's was to combine mechanical and vacuum advancement devices, both working together to deliver the best ignition point for a given engine running condition. Yours will just have a different working combination of these two devices which ultimately give similar ignition advancement at speed but have a different initial setup. Although some modern engines partly rely on these older technologies for timing control, on the whole things are much more accurately managed by ECU's (electronic control units) and a miriad of sensors relaying data hundreds of times a second to these control units. Air pressure/coolant temperature/exhaust oxygen content/throttle position/engine speed/knock to name but a few of the parameters that are monitored in order to control timing and fuel injection to give optimum performance and fuel economy from modern engine designs. The engines themselves haven't changed that much in the last two decades, rather it's the way in which they are controlled with electronics, that's where the real advancements have been. Martin Edited by Red Westie - 27 Feb 12 at 00:17
	shelly

Allanw Members Profile Send Private Message Find Members Posts Add to Buddy List Vanorak SYCOPHANTIC TRAINEE WELSHMAN Joined: 21 Mar 10 Location: New Zealand Status: Offline Points: 1211	Post Options Post Reply Quote Allanw Report Post Thanks(0) Quote Reply Posted: 27 Feb 12 at 03:50
	How the timing is set is more dependant on the distributor/ignition chages between the engines, rather than differences in the actual engines. The timing should actually be set based on the distibutor you have. So if you DF shat itself, and you put a DG engine in but with all your ingition gear, then you should really use the DF settings... generally...
	Allan :-) '84 T3 Kombi, EJ25 DOHC, apprentice Viking... Duct Tape can't fix stupid, but it can MUFFLE the sound.