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MG Midget and Sprite Technical - Timing and advance

Okay just need to check if my basic assumtions about ignition timing and advance are correct

There are 2 types of advance.
-The fixed curve (either determined by weights or a programed curve in some electronic ignitions)
-the "dependant" type, as in dependant on some input about the load on the engine (tps or vacuem/boost)

The fixed curve increases with revs to a pre set maximum.
The amount of advance at a certain rpm (idealy) is the max advance possible under full load without pinking at that RPM.

The "dependant" curve changes dependant on engine load AND rpm
The "dependant" curve is more of an economy thing that dials in more advance when under light load to help with economy.
But again idealy should be the max advance possible at a certain RPM onder that load condition without pinking.

So am I then correct in saying that at idle (so no load at all) you should see the most advance added by the "dependant" curve?

Just prepping to program an ignition module with a TPS input instead of vac advance
Onno K

Thats a new one on me....dependant curve

Good luck

Prop
Prop and the Blackhole Midget

Onno,

The “dependant” curve on an old-fashioned car wit vacuum and rpm advance is as you describe.

The ideal curve as you are programming on an ECU (electronic control unit?) is different. First: what is your goal?
Top speed, max torque, at what RPM or miles per gallon?
You are working with a TPS (throttle position sensor) and an RPM sensor. What other inputs are there?

Flip
Flip Brühl

Prop
The dependant curve is the vac advance curve on a normal dissy.

Flip
It's just rpm and tps no other inputs
Verry basic more like an progamable dissy.
I went for a TPS instead of vac since my delorto and cam spec won't realy give a nice vac signal.
I can give an advance at a certain rpm and a advance related to the tps input at each rpm
Onno K

Hi Onno

The advance should be the minimum to make best power at any throttle opening. If advance used at all positions means 'just' not pinking you could be way off the mark for best power. Rolling road for foot flat stuff and loads of road testing for part throttle settings.

Peter
Peter Burgess Tuning

Onno,

You asked: "So am I then correct in saying that at idle (so no load at all) you should see the most advance added by the "dependant" curve?"

I don't have a certain answer, but here are a couple things to ponder. I am not convinced that this is correct for two reasons. One is, the vacuum presented to the vac adv is going to be different for ported vac vs manifold vac systems. The second is, do you know if an engine is making high vac at idle? I think with high RPM and a closed throttle the vac might be significantly higher than at idle.

Charley
C R Huff

Onno,

Peter Burgess programmed my programmable 123 with rpm only on his rolling road. I had a normal 123 with vacuum before. He gave the car more power, more torque, better MPG and better starting in winter even without choke. I did not have to change the SU needles. The funny thing for me was that the fuel-air ratio (as measured with a Lambda sender) was influenced by the ignition. I do not know if you get a better performance if you use TPS. I suppose it does.

Flip
Flip Brühl

I would agree with Peter - with a slight caveat. Personally I think if you have optimized the ignition at around 3000 under load, you will not be a million miles off for any other throttle setting. Further correction - on the 123 anyways - for other settings may be made by choosing a suitable curve.

Again, personally speaking, I don't believe any of this is an exact science. There are far too many other variables at play here.
Mark O

Peter
That was exactly the kind of info I was looking for!
My seat of pants will have a job with the laptop beside me for a while then.

Local RR visit is planned for end of september so should help
Onno K

Onno
There is a major difference between what Peter suggests and what you are proposing to do as far as setting maximum fixed curve advance
You mention setting just back from the point of pinging while Peter says (correctly) to set the curve at the minimum amount of advance without losing power
Example
An average A series 1275 will produce maximum power at around 31-32 max advance but some can run up to 40 or so before there is an audible ping
On the rollers the correct way is to get to a point of max. power and then pull it back to the point where power just starts to drop off and then just go back up a tiddle till max power is reclaimed---write it down
Repeat in 1000 rpm steps until you have an advance arc to build or program your new distributor

Your load dependant curve ,on the other hand can be set as you suggest just short of pinging but with zero dependant advance on anywhere near full throttle

Idle speed advance CAN be full on dependant advance but usually you will get a stronger idle without added advance at idle but feed it in as revs increase just above idle speed
8-10 deg(starting point) jump up at 1500 revs with very light throttle and then taper it off with throttle load up to about 1/2 throttle being zero and cap it at 3000/3500 revs
Doing it this way you will know that anything over 3500 revs isn't going to get advance spikes above your base curve to cause engine damage

willy
William Revit

Thanks for the tips Willy
The unit has a feature to set a max advance where it will never go over to avoid spikes.
I'll set that at 32 degrees until I get it to the RR and determine the real curve.

Always nice to learn more
It was a clear case of understanding the basic theory but not the practical reality ;)
Onno K

Onno set it at 26 max till you get to the rollers.

Peter
Peter Burgess Tuning

Im grasphing the concept here...but Im ith onno at the 32 as to my understanding

But if set to 26 then what.happens at 27

To me 26 seems lite

Prop
Prop and the Blackhole Midget

Hi Peter
I knew it was going to be a mistake to start quoting figures but--did it to get Onno to think conservative instead of aggressive with his timing
I would have thought 31-32 total was a safe figure
But, if you advise 26 then 26 it is
Better safe than sorry
Cheers
willy

Onno
I'm sure you have a grasp of what you are doing---
Could I add something here for others that might be following this thread

Total ignition timing
Total timing, being the subject here is the sum of the base timing setting plus the advance amount added by the distributor-------example
10 deg base plus 20 deg curve gives 30 deg total
I have seen people with a misunderstanding of this set the 30 into their distributor thinking that is what is meant by total advance and then set it in the engine at say 10deg base and get a dangerous 40 total

Clear as mud
willy
William Revit

Peter & Willy

Thanks and will do
Onno K

Hi Willy

Mild road stuff below 9.75:1 does take around 28-32 depending on individual engine. Hairy beasties like Onno's may well take a lot less advance. Our 'bestest' race engine only takes 21 degrees max. The greater the efficiency the less the advance needed. Mainly to do with CR and breathing capability. We have seen holes in pistons in two 1min laps if even two degrees too much!

Peter

Peter
Peter Burgess Tuning

All good
Cheers
willy
William Revit

This thread was discussed between 16/08/2015 and 20/08/2015

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