ignition advance question

[dave9469]

I've just fitted fully mapable electronic ignition to my TR4a. I've got some standard figures in there that I've reduce slightly until I fully understand the issues with advancing the timing.

 

What I already know,

-too much advancement can cause pinking. This is reduced with higher octane fuel and I think higher compression ratio.

-advancing the timing improves performance, but to what degree I'm not sure.

-A standard dizzy uses weights to give speed advancement & vacuum to advance the engine under high load (throttle wide open). This system gives an advancement figure for a given RPM and vacuum so there is nothing stopping it have perfect ignition timing.

 

what I'd like to know is what is the standard advancement of a standard engine and what the maximum should be? and what dictates this figure?

Can the engine be damaged by having too much advance.

how much advancement should there be with engine speed and how much should there be with vacuum.

The system take vacuum from the engine side of the carbs so with closed throttle it's about 200mBar abs and 1000mBar wide open.

 

What affect does advance have on fuel economy? with the throttle closed (using the vacuum) the advance could be set to any value.

[Rhodri]

A complicated subject, but Dave Andrews knows a lot of the answers, http://hometown.aol.com/dvandrews/ems/htm.

 

 

Sorry, can't get the link to work, you will have to type in Dave Andrews in their search box

Edited July 16, 2008 by Rhodri

[johnny250]

That's interesting Dave....i've just done similar (see other posts 'distibutorless ignition')What system are you using?. I have some ignition maps for the 6 cylinder I can send you, I don't know how the 4 compares...anyone?

I also have an excel spreadsheet for calculating figures from kind gent in USA...that might be of use too?

 

this is timing Advance Data from BL Svc Manual for Lucas pt no 41202 pg 86.35.00 sheet 3

 

Centrifugal Advance

RPM 500 750 900 1700 3000 5000 6000

Degrees Crankshaft Advance Min 0 0 0 8 12 18 18

Degrees Crankshaft Advance Max 0 0 2 12 16 22 22

 

Vacumn Advance

in. HG 2.5 3 4 6 8 15

Degrees Crankshaft Advance Min None 0 0 5 12 14

Degrees Crankshaft Advance Max None 1 5 14 18 18

 

4 degrees ATDC ignition timing at idle for emissions

12 degrees BTDC static timing (points open at 12 degrees)

800 to 850 RPM at idle

 

 

Rhodris' link works here:

http://hometown.aol.com/dvandrews/timing.htm

 

there are lots of 4 cylinder maps around (Megajolt website)...not many for the 6 though!

 

Regards

john

Edited July 16, 2008 by johnny250

[marvmul]

I've just fitted fully mapable electronic ignition to my TR4a. I've got some standard figures in there that I've reduce slightly until I fully understand the issues with advancing the timing.

 

What I already know,

-too much advancement can cause pinking. This is reduced with higher octane fuel and I think higher compression ratio.

-advancing the timing improves performance, but to what degree I'm not sure.

-A standard dizzy uses weights to give speed advancement & vacuum to advance the engine under high load (throttle wide open). This system gives an advancement figure for a given RPM and vacuum so there is nothing stopping it have perfect ignition timing.

 

what I'd like to know is what is the standard advancement of a standard engine and what the maximum should be? and what dictates this figure?

Can the engine be damaged by having too much advance.

how much advancement should there be with engine speed and how much should there be with vacuum.

The system take vacuum from the engine side of the carbs so with closed throttle it's about 200mBar abs and 1000mBar wide open.

 

What affect does advance have on fuel economy? with the throttle closed (using the vacuum) the advance could be set to any value.

May I correct a couple of things to prevent confusion?

-pinking is reduced with lower compression ratio, not higher.

-the maximum total ignition advance (static + distirbutor weights) on these engines is about 28 to 30° (crankshaft). More advance is dangerous for high speed knocking (which you cannot hear and can wreck an engine by melting a hole in a piston) and will rather impair the performance a bit. Engines with low compression ratio can stand some more advance with benefit : maximum total advance of about 32°

-vacuum advance is useful when cruising at constant speed with small trottle openings : the cylinders are only filled partially and then the combustion is slower and must start earlier : more advance needed. Sometimes vacuum RETARD is used at full trottle to prevent pinking.

Surely the engine can be damaged by too much advance : the knocking is easily noticed when accelerating from low rpm : this knocking disappears from about 3800 rpm onwards and is not very dangerous when occuring only now and then. High speed knocking is more likely in a high tuned engine, when full trottle at very high rpm is maintained during more than 5 or 10 seconds : in these circumstances, the T° is very high and knocking can make a hole in the piston crown in seconds.

-at the jet orifice (wherein the needle is moving up and down together with the carb piston), the vacuum is function of the speed of the passing air and the speed and vacuum are held constant by the moving carb piston by adapting the surface of the venturi . The needle is tapered : the fuel enters the venturi via the jet orifice : the net surface of the orifice is the surface of the jet bore minus the surface of the needle (this is the surface of the needle if it would be cut at the height of the jet orifice) : the net surface of the orifice is held proportional with the venturi surface in this way to keep the air/fuel mixture about constant.