Question on ignition advance for increasing RPM

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GDCobra
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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

DEVONMAN wrote:
Mon Feb 08, 2021 11:39 am
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.
Hummm. Not sure about that, obviously engines are complex beasts and all will have their individual characteristics but why would the need for increasing advance suddenly stop at a given RPM? A reduction in rate would be more intuitive.

Also, I can understand that cylinder filling will improve with increasing RPM due to higher charge speed but surely as RPM Increases further the cylinder will have less opportunity to completely fill the cylinder.



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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

DaveEFI wrote:
Sun Feb 07, 2021 1:22 pm
GDCobra wrote:
Sun Feb 07, 2021 1:00 pm
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.
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.
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.
I dunno. I had those brass barbs 'in stock' but not a Y piece, so used what I had.

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.
Cheers Dave, that’s what I thought, in my case Ting in will be easier, for now at least although your solution does look neat.

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Re: Question on ignition advance for increasing RPM

Post by DEVONMAN »

GDCobra wrote:
Mon Feb 08, 2021 10:30 pm
DEVONMAN wrote:
Mon Feb 08, 2021 11:39 am
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.
Hummm. Not sure about that, obviously engines are complex beasts and all will have their individual characteristics but why would the need for increasing advance suddenly stop at a given RPM? A reduction in rate would be more intuitive.

Also, I can understand that cylinder filling will improve with increasing RPM due to higher charge speed but surely as RPM Increases further the cylinder will have less opportunity to completely fill the cylinder.
I agree that the advance shouldn't stop suddenly at a given RPM and ideally should be adjusted to suit cylinder filling and mixture density, but given the limitation of a distributor, I suggest the ideal advance can only be achieved electronically and or after tests. The principle I put forward was in answer to the original question ie. that advance doesn't need to keep rising with RPM and should depend on the mixture density/temperature etc., and to add, in the case of forced induction the advance may need to be reduce.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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Re: Question on ignition advance for increasing RPM

Post by SuperV8 »

Everything I have read suggests required spark timing usually follows the torque 'curve' of the engine, which also correlates to the engine's V.E.
For an N.A. engine this does not increase all the way to the red-line.

Note - 'curve'.
You can't use springs and weights to follow a curve. So the nearest/simplest mechanical way to change your timing using springs is to increase timing upto the torque max rpm of the engine.
Dax Rush 4.6 supercharged V8 MSII

GDCobra
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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

SuperV8 wrote:
Tue Feb 09, 2021 11:30 am

Note - 'curve'.
You can't use springs and weights to follow a curve. So the nearest/simplest mechanical way to change your timing using springs is to increase timing upto the torque max rpm of the engine.
That's a good point, my understanding is that the 'harder' spring 'hangs loose' at zero advance (I remember inspecting a dizzy way back and thinking the spring was stretched!) so only the lighter spring resists advance and so advance increases quite quickly in the initial stages, then the stroger spring engages which will slow the rise, effectively giving a 'curve' as a number of more or less straight gradients.
In a way the mappable systems do the same thing BUT allow a larger number of points to be plotted so equating much closer to a curve, 16 in my case.

This is partly why I was asking the question. If there is no need for advance beyond (let's say) 3000RPM then I could set this as my penultimate column with the last column being set to maximum RPM and basically flat-lining in between. As it is it looks like I'm best to use most of the points in the early RPM ranges then thin them out into the higher RPMs.

All interesting stuff though.

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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

DEVONMAN wrote:
Tue Feb 09, 2021 11:15 am

I agree that the advance shouldn't stop suddenly at a given RPM and ideally should be adjusted to suit cylinder filling and mixture density, but given the limitation of a distributor, I suggest the ideal advance can only be achieved electronically and or after tests. The principle I put forward was in answer to the original question ie. that advance doesn't need to keep rising with RPM and should depend on the mixture density/temperature etc., and to add, in the case of forced induction the advance may need to be reduce.
H Devonman. I probably need to apologise for not being clear with the background to this in my orginal post, I did start putting more information in but then thougth it had become wordy and boring so thought I'd just boil it down to the basic question and then sort out the rest as the thread develops.

I am currently swapping put my distributor in favour of a mappable system, using NODIZ in this case. I will be getting the car onto a rolling road but that is going to be later in the year (weather needs to warm up and COVID needs to bugger off first!) so I'm currently just trying to ramp up my understanding which will hopefully help when it comes to setting it up but also because I'm just inquisitive by nature.

So just to clarify I'm not trying to make a distributor behave 'better' but looking to get best 'bang for my buck' wtih a mappable system.

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Re: Question on ignition advance for increasing RPM

Post by DEVONMAN »

GDCobra wrote:
Tue Feb 09, 2021 1:14 pm
DEVONMAN wrote:
Tue Feb 09, 2021 11:15 am

I agree that the advance shouldn't stop suddenly at a given RPM and ideally should be adjusted to suit cylinder filling and mixture density, but given the limitation of a distributor, I suggest the ideal advance can only be achieved electronically and or after tests. The principle I put forward was in answer to the original question ie. that advance doesn't need to keep rising with RPM and should depend on the mixture density/temperature etc., and to add, in the case of forced induction the advance may need to be reduce.
H Devonman. I probably need to apologise for not being clear with the background to this in my orginal post, I did start putting more information in but then thougth it had become wordy and boring so thought I'd just boil it down to the basic question and then sort out the rest as the thread develops.

I am currently swapping put my distributor in favour of a mappable system, using NODIZ in this case. I will be getting the car onto a rolling road but that is going to be later in the year (weather needs to warm up and COVID needs to bugger off first!) so I'm currently just trying to ramp up my understanding which will hopefully help when it comes to setting it up but also because I'm just inquisitive by nature.

So just to clarify I'm not trying to make a distributor behave 'better' but looking to get best 'bang for my buck' wtih a mappable system.
No need to apologise. I run three v8's and dumped the distributors in favour of megajolt a few years back. Two have twin turbos and one is NA. Although I should have set things up on a rolling road, the maps are home brew and work well. All part of the fun.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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Re: Question on ignition advance for increasing RPM

Post by SuperV8 »

GDCobra wrote:
Tue Feb 09, 2021 1:00 pm
SuperV8 wrote:
Tue Feb 09, 2021 11:30 am

Note - 'curve'.
You can't use springs and weights to follow a curve. So the nearest/simplest mechanical way to change your timing using springs is to increase timing upto the torque max rpm of the engine.
That's a good point, my understanding is that the 'harder' spring 'hangs loose' at zero advance (I remember inspecting a dizzy way back and thinking the spring was stretched!) so only the lighter spring resists advance and so advance increases quite quickly in the initial stages, then the stroger spring engages which will slow the rise, effectively giving a 'curve' as a number of more or less straight gradients.
In a way the mappable systems do the same thing BUT allow a larger number of points to be plotted so equating much closer to a curve, 16 in my case.

This is partly why I was asking the question. If there is no need for advance beyond (let's say) 3000RPM then I could set this as my penultimate column with the last column being set to maximum RPM and basically flat-lining in between. As it is it looks like I'm best to use most of the points in the early RPM ranges then thin them out into the higher RPMs.

All interesting stuff though.
You can multiple your 16 rpm sites x 16 load sites! so you have a 3D map.


Interesting reading:
https://www.autospeed.com/cms/a_109132/article
Dax Rush 4.6 supercharged V8 MSII

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Re: Question on ignition advance for increasing RPM

Post by garrycol »

stevieturbo wrote:
Sun Feb 07, 2021 11:51 am

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.
Of course vac advance or retard must not be all that relevant to a dizzy system as the mechanical system is because some cars do not even have vacuum advance or retard. My 24v 3.5 RV8 in my 101 does not have vac advance and neither does my Haflinger - neither has it so must not be all that important.

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Re: Question on ignition advance for increasing RPM

Post by DaveEFI »

IMHO, the main reason for the vaccum advance is economy. You get maximum vaccum at low throttle open positions - low engine demand. And it will stand more advance under those conditions.
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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

DEVONMAN wrote:
Tue Feb 09, 2021 2:23 pm

No need to apologise. I run three v8's and dumped the distributors in favour of megajolt a few years back. Two have twin turbos and one is NA. Although I should have set things up on a rolling road, the maps are home brew and work well. All part of the fun.
I think any mappable system must be better than a distributor, I can't seen any argument for a dizzy in favour of a mappable system with at least one coil per two cylinders.
Would love to have turbo or supercharger on mine but just can't justify the cost.

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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

SuperV8 wrote:
Tue Feb 09, 2021 4:00 pm

You can multiple your 16 rpm sites x 16 load sites! so you have a 3D map.


Interesting reading:
https://www.autospeed.com/cms/a_109132/article
Aye, that's a good point 16 * 16, that's only 256 sites in all. How hard can it be?
Thanks for the linky.

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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

garrycol wrote:
Tue Feb 09, 2021 11:17 pm

Of course vac advance or retard must not be all that relevant to a dizzy system as the mechanical system is because some cars do not even have vacuum advance or retard. My 24v 3.5 RV8 in my 101 does not have vac advance and neither does my Haflinger - neither has it so must not be all that important.
Maybe, but of course many moons ago engines didn't have any automatic advance/retard at all, left it all up to the 'operator' to move a lever. Not that I remember that of course, just seen it i books!

To put it into context I'd say the most important area is when load is higher (WoT) as this is when getting it wrong will cause most trouble but for better driveability and economy having that extra advance at lower loads is useful and as I don't like things being sub-optimal it is important to me. Perhaps it's just a CDO thing for me.

I think the other reason many people tend not to run vac' advance is because they see race engines not using it and think it must be better without stoping to think that a road engine has to work in a much broader range of conditions. Of course it also means it must be 'cooler' which may be even more important.

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Re: Question on ignition advance for increasing RPM

Post by DaveEFI »

Racing engines ain't really worried about economy.
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Re: Question on ignition advance for increasing RPM

Post by GDCobra »

DaveEFI wrote:
Wed Feb 10, 2021 6:53 pm
Racing engines ain't really worried about economy.
I don't think it's that so much, OK cost of fuel may be insignificant compared to other items but what about the 'cost' of carrying it?
The main reason race engines don't use load based advance is that they will spend very little of their time not at WoT, better not to have the complexity.

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