Advancing or retarding?

[Nick Salmon]

True or false?

To advance the ignition is to make the spark happen earlier relative to TDC.

To retard the ignition is to make the spark happen later relative to TDC.



The answers will lead to a further question...

[Reckless Rat]

Correct. This assumes the steering centre controls and the advance control rod from the base of the column to the distributor are correctly fitted.

[Nick Salmon]

Jolly good. Thanks Reckless. Now the next question...

True or false?

To avoid kickback when starting a manual ignition engine on the handle, the spark should occur later than it normally would do when the engine is running.

[Reckless Rat]

Correct again.

[Nick Salmon]

Thanks again Reckless. Here we reach the crux of my questions...


In A7 handbooks it refers to starting the engine with 'Ignition - almost fully advanced'.


But I have always referred to 'retarding' the ignition to start the engine. So does my 1937 copy of the 'Manual of Driving and Maintenance for Mechanical Vehicles. And numerous other references. Nowhere else do I find the instruction to have the ignition advanced.

Are the terms 'advanced' and 'retarded' somehow interchangeable or am I missing a difference in definition somewhere?

[Reckless Rat]

If you like a broken wrist or thumb then by all means crank your car over with the ignition fully advanced, but the ideal way is to retard it. What you do manually on a Seven is what a later auto advance distributor does for you. The static timing allows for a retarded spark for starting but as engine/distributor speed increases the bob weights advance the ignition to give the correct setting when running.

If you think about it, there is a finite time involved for the spark to occur and the fuel to burn in the cylinder. The faster the engine goes the earlier the spark has to occur in order for the "burn" to be at its best immediately at or just after TDC in order to force the piston down the bore. Too late and you don't get the maximum downward force, too early and it tries to work against itself.

The Lucas DK auto advance distributor gives about 8° full advance (from static) which is fine for a road engine.

[Chris KC]

I'm not sure which handbook you are referring to Nick, thus unsure of context, but I agree your (and Reckless's) analysis.

Auto-advance distributors are retarded when static (for ease of starting) and advance the ignition (i.e. the spark occurs earlier) as the engine speeds up (for performance). The basic point of ignition advance is that it takes a finite time for the burn to develop, so the spark has to occur earlier when the engine is running fast otherwise the 'bang' will occur too late to be of optimal value.

I'm not too familiar with the peculiarities of manual advance though, so perhaps others will confirm I'm not overlooking something.


<edit> ha ha, our posts overlapped - just goes to show great minds think alike Reckless!

[Nick Salmon]

I totally agree and have always followed that procedure with any manual ignition system.

So why did Austin refer to having the ignition 'almost fully advanced' when starting? That's the bit I don't understand.

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All the manual ignition Austin 7 handbooks I have ever seen say the same thing. Here for example is 352 from 1927.


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[Howard Wright]

Hi Nick

Interesting! I’ve read the Austin manual and it seems from the later section that the Austin recommendation was to keep the control lever in more or less the same place all the time and retard it only if the engine “rumbles” (Austin’s words not mine).

I routinely retard the ignition to start and advance only once on the move and with engine revs above about 1500.  I also retard the ignition when descending a hill on overrun otherwise I get some popping and the occasional backfire.

Cheers

Howard

[Chris KC]

Thinking about it, I'm not sure where said handbook tells you to set the static timing in the first place, but early auto-advance cars only had 3 degrees of ignition advance anyway so 'almost fully advanced' was in fact probably still not very advanced...