Question on ignition advance for increasing RPM
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Question on ignition advance for increasing RPM
This question is directed to the RPM based advance handled by the centrifugal mechanism in the distributor
I believe I understand the concept behind this. The mixture takes time to burn and hence develop pressure in the cylinder and we want that pressure to be at it's maximum as the piston starts to travel down the bore so we have the most force for the most time (bit like getting your seat height right on a push bike)
What I'm a bit confused by is that most distributor based systems I've looked at advance up to a point in the RPM range (around 3000 RPM seems 'popular') and then stop.
My understanding is that the burn time is a constant so why does the advance not continue?
I'm a little suspicious that the reason for this is due to mechanical constraints due to the moving parts of the distributor and so moving to a computer controlled system there may be an advantage to be had by continuing the advance into higher RPM.
Has anyone explored this when moving away from a distributor?
I believe I understand the concept behind this. The mixture takes time to burn and hence develop pressure in the cylinder and we want that pressure to be at it's maximum as the piston starts to travel down the bore so we have the most force for the most time (bit like getting your seat height right on a push bike)
What I'm a bit confused by is that most distributor based systems I've looked at advance up to a point in the RPM range (around 3000 RPM seems 'popular') and then stop.
My understanding is that the burn time is a constant so why does the advance not continue?
I'm a little suspicious that the reason for this is due to mechanical constraints due to the moving parts of the distributor and so moving to a computer controlled system there may be an advantage to be had by continuing the advance into higher RPM.
Has anyone explored this when moving away from a distributor?
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Re: Question on ignition advance for increasing RPM
Because it's mechanical, there are limitations as to what can be done.
Re: Question on ignition advance for increasing RPM
Thanks Steve, I thought that was probably the case.stevieturbo wrote: ↑Wed Feb 03, 2021 7:36 pmBecause it's mechanical, there are limitations as to what can be done.
Re: Question on ignition advance for increasing RPM
Don't see why there would be a limitation on the maximum advance suddenly at a particular figure? Although any tolerances may be greater, the wider the rev range. I remember a respected tuner many years ago who had a very expensive distributor tester saying very few indeed had a curve which was spot on to the maker's figure. And spent ages selecting new springs etc which got it closer.
What you can't (easily) do with mechanical is a sudden change. With my programmable MegaSquirt, I've set it to increase the advance slightly below the normal idle speed which kicks it back up, should it try and stall. Allows a slower base idle.
To me, a major advantage is getting rid of the rotor arm and cap. Makes a big difference to reliability. And the more accurate timing pulse you get from a crank trigger. Although a dizzy in good nick can work pretty well.
What you can't (easily) do with mechanical is a sudden change. With my programmable MegaSquirt, I've set it to increase the advance slightly below the normal idle speed which kicks it back up, should it try and stall. Allows a slower base idle.
To me, a major advantage is getting rid of the rotor arm and cap. Makes a big difference to reliability. And the more accurate timing pulse you get from a crank trigger. Although a dizzy in good nick can work pretty well.
Dave
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
Re: Question on ignition advance for increasing RPM
Hi Dave, I did originally put a bit of background on my first post but decided I ddin't want to put the site readership to sleep so thought better of!DaveEFI wrote: ↑Thu Feb 04, 2021 10:16 amDon't see why there would be a limitation on the maximum advance suddenly at a particular figure? Although any tolerances may be greater, the wider the rev range. I remember a respected tuner many years ago who had a very expensive distributor tester saying very few indeed had a curve which was spot on to the maker's figure. And spent ages selecting new springs etc which got it closer.
What you can't (easily) do with mechanical is a sudden change. With my programmable MegaSquirt, I've set it to increase the advance slightly below the normal idle speed which kicks it back up, should it try and stall. Allows a slower base idle.
To me, a major advantage is getting rid of the rotor arm and cap. Makes a big difference to reliability. And the more accurate timing pulse you get from a crank trigger. Although a dizzy in good nick can work pretty well.
This is realted to my other post involving the distributorless system, I have gone for the Nodiz system.
Now that I have this (as with your MS system) I have full control over the spark timing and was just looking into some background on whether I should be looking at continuing the advance up to max' RPM, albeit probably flattening off the curve or if this "dead stop" in advance is actually what the engine wants. I suspect that guy with the distributor tester would have found that any given distributor instance (or any device relying on stone age springs, vacuum diaphragms and the like) would probably not give (exactly) the same results 2 days runing, in reality characteristics probably change measurable with engine temperature and ambient pressure.
Interesting that you mention about increasing the advance below idle RPM to give a kind of "anti-stall assitance, I was also thinking of the possible benefit of doing that. May even have been my next question!
Re: Question on ignition advance for increasing RPM
By the way Dave, I've also been looking into running the vacuum load signal from the unported plenum take off rather than the standard ported route. The reading up I've done on this indicates that running from ported (reducing advance for idle even though load is low) is done for emission purposes althogh the engine actually 'wants' higher advance. One advangage of running more advance is cooler running and hence underbonnet temperatures which will be welcome. I did also notice during my early tests with the Nodiz that the stepper motor value at idle reduced significantly from where it was with the distributor supplied sparks.
Where do you pick up your vac' for your MS system?
Where do you pick up your vac' for your MS system?
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Re: Question on ignition advance for increasing RPM
There isn't, but it's a balance between what is easy to achieve, and what will actually benefit the engine as a balance overall. And they need to do all this within the space confines of a dizzy...and ensure it works reliably and consistently, with very small weights, even smaller springs, etc etc.
Dizzy's did work, they were fairly reliable...so what they had was fine. Of course if others wanted to spend more time, money etc refining things a little better...they can, and some did. Same with most aftermarket upgrades.
As always, the aftermarket just works to different rules and requirements to OEM.
OEM might be a £50 dizzy....aftermarket with a better tailored advance curve might have been £250.
Re: Question on ignition advance for increasing RPM
MS uses the same vacuum sensor for both fuel and ignition, and has to be pure vaccum - not that provided to the dizzy vacuum unit. I think a basic dizzy does the best advance curve for power via the springs, and the vacuum advances that at light load for economy. So the provided vac signal to the dizzy does pretty well nothing with the throttle closed at idle.GDCobra wrote: ↑Thu Feb 04, 2021 3:03 pmBy the way Dave, I've also been looking into running the vacuum load signal from the unported plenum take off rather than the standard ported route. The reading up I've done on this indicates that running from ported (reducing advance for idle even though load is low) is done for emission purposes althogh the engine actually 'wants' higher advance. One advangage of running more advance is cooler running and hence underbonnet temperatures which will be welcome. I did also notice during my early tests with the Nodiz that the stepper motor value at idle reduced significantly from where it was with the distributor supplied sparks.
Where do you pick up your vac' for your MS system?
I added a second take off near the one for the fuel reg. I had to make an adaptor plate for the Bosch PWM idle valve, which goes in place of the over-run valve on my SD1, and fitted the take offs to that.
Dave
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
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Re: Question on ignition advance for increasing RPM
Some cars do, some do not.
Some cars will run vac all the time to the dizzy ( when available ), others will run a spark port which only offers vac at part throttle, but not at it idle.
Even similar models can run either or, just depends what the manufacturer thought best at the time.
Re: Question on ignition advance for increasing RPM
Cheers Dave, from the reading I’ve done it seems the practice of porting the vacuum on closed throttle and preventing advance is to improve emissions by running hotter. I’d rather avoid the higher temperatures both for underbonnet heat and engine temperature, particularly when the water pump is running slowly.DaveEFI wrote: ↑Sun Feb 07, 2021 10:00 am
MS uses the same vacuum sensor for both fuel and ignition, and has to be pure vaccum - not that provided to the dizzy vacuum unit. I think a basic dizzy does the best advance curve for power via the springs, and the vacuum advances that at light load for economy. So the provided vac signal to the dizzy does pretty well nothing with the throttle closed at idle.
I added a second take off near the one for the fuel reg. I had to make an adaptor plate for the Bosch PWM idle valve, which goes in place of the over-run valve on my SD1, and fitted the take offs to that.
I was intending to T into the vac pipe running from the plenum to the fuel pressure regulator rather than add another take off as you have done (for simplicity).
Is there any reason why that would be a bad idea.
Re: Question on ignition advance for increasing RPM
I dunno. I had those brass barbs 'in stock' but not a Y piece, so used what I had.GDCobra wrote: ↑Sun Feb 07, 2021 1:00 pmCheers Dave, from the reading I’ve done it seems the practice of porting the vacuum on closed throttle and preventing advance is to improve emissions by running hotter. I’d rather avoid the higher temperatures both for underbonnet heat and engine temperature, particularly when the water pump is running slowly.DaveEFI wrote: ↑Sun Feb 07, 2021 10:00 am
MS uses the same vacuum sensor for both fuel and ignition, and has to be pure vaccum - not that provided to the dizzy vacuum unit. I think a basic dizzy does the best advance curve for power via the springs, and the vacuum advances that at light load for economy. So the provided vac signal to the dizzy does pretty well nothing with the throttle closed at idle.
I added a second take off near the one for the fuel reg. I had to make an adaptor plate for the Bosch PWM idle valve, which goes in place of the over-run valve on my SD1, and fitted the take offs to that.
I was intending to T into the vac pipe running from the plenum to the fuel pressure regulator rather than add another take off as you have done (for simplicity).
Is there any reason why that would be a bad idea.
With programmable ignition you can set it to what you want relative to revs and vacuum. So you could match any factory vacuum advance at idle or not as you choose.
Dave
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
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- Forum Contributor
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Re: Question on ignition advance for increasing RPM
It's always nice to have dedicated lines to the plenum.....but yes it will work perfectly fine.
Re: Question on ignition advance for increasing RPM
Coming back to the original question, the advance does not need to continue to rise with revs above a certain point because the mixture in the cylinder at high revs has a greater density (Cylinders rammed full) and this burns quicker than a less dense mixture. At light load cruise the mixture is less dense and consequently the vacuum advance comes into play and increases to advance to suit this slower burning mixture.
Re: Question on ignition advance for increasing RPM
Thanks for that. Was fairly sure it wasn't down to the limitations of centrifugal advance. As regards cylinder filling, at light load, you can also run at a leaner mixture (assuming no cat.) which can take more advance, to the benefit of economy.
Dave
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
London SW
Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
Tech Edge 2Y
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- Forum Contributor
- Posts: 3979
- Joined: Sat Nov 18, 2006 6:22 pm
- Location: Northern Ireland
Re: Question on ignition advance for increasing RPM
DEVONMAN wrote: ↑Mon Feb 08, 2021 11:39 amComing back to the original question, the advance does not need to continue to rise with revs above a certain point because the mixture in the cylinder at high revs has a greater density (Cylinders rammed full) and this burns quicker than a less dense mixture. At light load cruise the mixture is less dense and consequently the vacuum advance comes into play and increases to advance to suit this slower burning mixture.
Advance/timing doesn't really "need" to do a lot of things though.
I've run cars with a welded dizzy with no timing changes at all.....doesn't mean it's optimal though.
And ultimately this is where dyno tuning comes in, to determine what makes the engine happiest.