40deg at 10% throttle and 3000 through 7000rpm is this right?
Its a 5.0L short stroke with TA heads and mech 248cam on throttlebodies.
Any advice would be appreciated.
mark
Load is throttle position.
Please explain my ignition map.
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Please explain my ignition map.
Does this map look OK?
40deg at 10% throttle and 3000 through 7000rpm is this right?
Its a 5.0L short stroke with TA heads and mech 248cam on throttlebodies.
Any advice would be appreciated.
mark
Load is throttle position.
40deg at 10% throttle and 3000 through 7000rpm is this right?
Its a 5.0L short stroke with TA heads and mech 248cam on throttlebodies.
Any advice would be appreciated.
mark
Load is throttle position.
Sidecar is your man for questions like this.
Can you respace some of the column and row values? I notice you have all 28's in the bottom row so that is effectively not being used.
If you re-jig things a bit you can have more resolution around the idle/low rpm area of the map where you are probably going to ramp the advance up the quickest. This doesn't make any difference to the ECU as it probably interpolates like Megasquirt, but just makes it easier to visualise yourself what you are asking it to do.
The higher load and RPM areas of the map need less focus so you can space these out a bit and let the ECU interpolate the larger spacing.
What % load and rpm does your engine idle at?
Can you respace some of the column and row values? I notice you have all 28's in the bottom row so that is effectively not being used.
If you re-jig things a bit you can have more resolution around the idle/low rpm area of the map where you are probably going to ramp the advance up the quickest. This doesn't make any difference to the ECU as it probably interpolates like Megasquirt, but just makes it easier to visualise yourself what you are asking it to do.
The higher load and RPM areas of the map need less focus so you can space these out a bit and let the ECU interpolate the larger spacing.
What % load and rpm does your engine idle at?
1974 Rover 3500s
1984 3.5 90
1959 2.25 series 2
1984 3.5 90
1959 2.25 series 2
Thanks Quagmire! Please be aware that I have never 'mapped' a mega jolt type of system, I run a programmable MSD system and the way that it is mapped is much more 'graphical'. Also I have a load of ideas I have put together over the years that I have formulated by experience, reading books and looking at what car manufacturers are doing, having said that I am too lazy to re-program my MSD to try these ideas out!
I have drawn a map which I will try to post up but never have any luck with attachments but below are my thoughts and musings:-
1. Clever people at places like Ricardo have worked out that any engine will produce the highest torque when the peak cylinder pressure above the piston occurs when the piston is 14-20 degree AFTER TDC. (The charge needs to be ignited well before TDC to obtain this)
2. Two main factors determine the ignition advance that the engine needs, the VE at any given RPM, and the RPM that the engine is running at. I know that there are a load of other factors such as chamber design, air fuel ratio, exhaust gas contamination, etc but I don’t want to cloud the issue!
3. Regardless of RPM when the VE is low the cylinder filling is poor, the peak pressure BEFORE the plug fires is low so the burn rate is low, this all means that the plug needs to fire sooner, in other words many degrees before TDC. Having the throttle virtually shut creates a low VE figure.
4. As RPM increases the time for the charge to completely burn and create the peak pressure at the correct crank angle reduces which means that the charge needs to be ignited earlier. However offsetting this is the fact that as the RPM increases the swirl in the combustion chamber increases and this speeds up the burn rate. This explains why the ‘all in’ figure can occur at around 3000 RPM and the ignition timing does not require any further advancing at RPM values higher than 3000 RPM.
So bearing the above in mind let’s take my engine my engine as an example, from messing about with it I know that it idles best at 800 RPM with 20 degrees of advance. Now the VE at idle is as low as the engine will ever have, that’s why it is ticking over! On my engine useable power starts at round 1200 RPM and this is where it already starts to get tricky! At 1200 RPM with the throttle just cracked open the timing will need to be around 22 degrees but what happens if I floor the throttle at 1200 RPM? The VE will go pretty high, the peak cylinder before the plug fires will be high, AND RPM is still low, this all adds up to the fact that 22 degrees may well be way too much advance, the timing probably needs to be pulled right back to 12-14 degrees. So to sum so far we have two different timing requirements just for 1200 RPM, one when the throttle is just cracked open and one when the throttle is wide open. Of course there are X number of points in-between these two extremes and thats just for one RPM figure only!
At the other end of the scale my MAP sensor comes into play and this is where if I could be bothered I could sort it out so that it is always adjusting the timing. (Currently it is on the ‘timed’ port of my carb). I run 28 degrees from around 3000 RPM all the way up to 6000 RPM as long as the throttle is wide open. If the throttle is just cracked open I run up to 44 degrees.
The way that I drew up the map that I will try to post up is by filling in all the extreme figures at either end of the map, I then just filled in all the bits in-between with figures that ‘look’ about right! Of course several days on a brake dyno that can hold the engine at a steady RPM is the correct way to do all of this stuff but I don’t happen to own one!
Like I said all of the above is just my thoughts, anyone is free to comment, take it or leave it, believe it or don’t, I’m not fussed either way!
I have drawn a map which I will try to post up but never have any luck with attachments but below are my thoughts and musings:-
1. Clever people at places like Ricardo have worked out that any engine will produce the highest torque when the peak cylinder pressure above the piston occurs when the piston is 14-20 degree AFTER TDC. (The charge needs to be ignited well before TDC to obtain this)
2. Two main factors determine the ignition advance that the engine needs, the VE at any given RPM, and the RPM that the engine is running at. I know that there are a load of other factors such as chamber design, air fuel ratio, exhaust gas contamination, etc but I don’t want to cloud the issue!
3. Regardless of RPM when the VE is low the cylinder filling is poor, the peak pressure BEFORE the plug fires is low so the burn rate is low, this all means that the plug needs to fire sooner, in other words many degrees before TDC. Having the throttle virtually shut creates a low VE figure.
4. As RPM increases the time for the charge to completely burn and create the peak pressure at the correct crank angle reduces which means that the charge needs to be ignited earlier. However offsetting this is the fact that as the RPM increases the swirl in the combustion chamber increases and this speeds up the burn rate. This explains why the ‘all in’ figure can occur at around 3000 RPM and the ignition timing does not require any further advancing at RPM values higher than 3000 RPM.
So bearing the above in mind let’s take my engine my engine as an example, from messing about with it I know that it idles best at 800 RPM with 20 degrees of advance. Now the VE at idle is as low as the engine will ever have, that’s why it is ticking over! On my engine useable power starts at round 1200 RPM and this is where it already starts to get tricky! At 1200 RPM with the throttle just cracked open the timing will need to be around 22 degrees but what happens if I floor the throttle at 1200 RPM? The VE will go pretty high, the peak cylinder before the plug fires will be high, AND RPM is still low, this all adds up to the fact that 22 degrees may well be way too much advance, the timing probably needs to be pulled right back to 12-14 degrees. So to sum so far we have two different timing requirements just for 1200 RPM, one when the throttle is just cracked open and one when the throttle is wide open. Of course there are X number of points in-between these two extremes and thats just for one RPM figure only!
At the other end of the scale my MAP sensor comes into play and this is where if I could be bothered I could sort it out so that it is always adjusting the timing. (Currently it is on the ‘timed’ port of my carb). I run 28 degrees from around 3000 RPM all the way up to 6000 RPM as long as the throttle is wide open. If the throttle is just cracked open I run up to 44 degrees.
The way that I drew up the map that I will try to post up is by filling in all the extreme figures at either end of the map, I then just filled in all the bits in-between with figures that ‘look’ about right! Of course several days on a brake dyno that can hold the engine at a steady RPM is the correct way to do all of this stuff but I don’t happen to own one!
Like I said all of the above is just my thoughts, anyone is free to comment, take it or leave it, believe it or don’t, I’m not fussed either way!
Last edited by sidecar on Thu Nov 26, 2015 8:24 pm, edited 4 times in total.
this may help visualise;
The static ignition advance curve of a legacy distributor should correlate to the 90/100 MAP (wide open throttle) across the rev range.
(I'm not sure why it's pulling timing out from 28 to 27 between 90 and 100 map. You would only start pulling timing if going over 100 (boost).
Then everything below that is essentially the same static curve + the advance from vacuum advance can.
The static ignition advance curve of a legacy distributor should correlate to the 90/100 MAP (wide open throttle) across the rev range.
(I'm not sure why it's pulling timing out from 28 to 27 between 90 and 100 map. You would only start pulling timing if going over 100 (boost).
Then everything below that is essentially the same static curve + the advance from vacuum advance can.
40 degrees at 10% throttle will be at cruise so the high figure is fine, I'd probably go higher!. Nothing wrong with that table except I'd probably go slightly higher at idle, 10 degrees sounds low for a tuned engine. JMHO
Edit to add a picture of one of my early ignition tables, as sidecar says nothing changes after the all in figure so that is my maximum.
Edit to add a picture of one of my early ignition tables, as sidecar says nothing changes after the all in figure so that is my maximum.
Big Tone
www.cobratone.co.uk
www.cobratone.co.uk
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Muscle-Manta
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Muscle-Manta
- Forum Contributor
- Posts: 343
- Joined: Tue Mar 31, 2009 11:41 am
- Location: Thatcham Berkshire
