Standard Ignition - mapping.
Discussion
Updated formulea: 5/8/15
For those who are interested I have set out below the spark Advance calculations for the Standard Lucas Distributor.
I have been swapping between 14CUX and Megasquirt trying to fix a 'shunting" problem (See topic below "Shunting") and will commence a bottoms up re-tune starting with the ignition. Since the car performs well with the dizzy, I will use this to Map the spark curve.
The key measurements from a Lucas plot are as follows:
Centrifugal:
Rpm Advance
800. . . .0 degrees
2000. . . .10
2700. . . .16
Vacuum:
Hg Advance
5". . . .0
17". . . .20
In the case of vacuum I am assuming linearity. From these points I have derived the following formulea (simple y=mx+c)
Std Distributor:-
WOT Advance = (X BTDC) + 16(centrifugal)
eg. 10 + 16 = 26 degrees
Centrifugal Advance = [(8.42 x rpm)/1000] - 6.74
Vacuum Advance = (1.67 x "Hg) - 8.33
where "Hg = inches of Mercury
Vac Advance = (0.505 x Kpa) - 8.33
E.g.
Assume Cruise Advance occurs at 1750rpm, @ 15"Hg(50Kpa) and BTDC = 10 degrees
Cruise Advance = 10 + [(8.42 x 1750)/1000]-6.74 + [(1.67 x 15)-8.33]
= 10 + 8 + 13.3
= 31 degrees (+/-?)
I will need to run my Vac guage under CUX and note the ported and manifold pressures (Should be the same with throttle open).
I can then compare these with my MS datalogs showing the Vacuum (MAP-Kpa)at various load/rpm's points? Noteing that the Kpa as a vacuum is the inverse %. ? (ie. WOT = 100Kpa and atmosphere (29.92 "Hg)is 0Kpa.
It is interesting to note that under heavy load it is essentially just BTDC + (0-16)Degrees and rpm based to 2700 rpm. Ie WOT = 26 degrees after 2700.
For those who are interested I have set out below the spark Advance calculations for the Standard Lucas Distributor.
I have been swapping between 14CUX and Megasquirt trying to fix a 'shunting" problem (See topic below "Shunting") and will commence a bottoms up re-tune starting with the ignition. Since the car performs well with the dizzy, I will use this to Map the spark curve.
The key measurements from a Lucas plot are as follows:
Centrifugal:
Rpm Advance
800. . . .0 degrees
2000. . . .10
2700. . . .16
Vacuum:
Hg Advance
5". . . .0
17". . . .20
In the case of vacuum I am assuming linearity. From these points I have derived the following formulea (simple y=mx+c)
Std Distributor:-
WOT Advance = (X BTDC) + 16(centrifugal)
eg. 10 + 16 = 26 degrees
Centrifugal Advance = [(8.42 x rpm)/1000] - 6.74
Vacuum Advance = (1.67 x "Hg) - 8.33
where "Hg = inches of Mercury
Vac Advance = (0.505 x Kpa) - 8.33
E.g.
Assume Cruise Advance occurs at 1750rpm, @ 15"Hg(50Kpa) and BTDC = 10 degrees
Cruise Advance = 10 + [(8.42 x 1750)/1000]-6.74 + [(1.67 x 15)-8.33]
= 10 + 8 + 13.3
= 31 degrees (+/-?)
I will need to run my Vac guage under CUX and note the ported and manifold pressures (Should be the same with throttle open).
I can then compare these with my MS datalogs showing the Vacuum (MAP-Kpa)at various load/rpm's points? Noteing that the Kpa as a vacuum is the inverse %. ? (ie. WOT = 100Kpa and atmosphere (29.92 "Hg)is 0Kpa.
It is interesting to note that under heavy load it is essentially just BTDC + (0-16)Degrees and rpm based to 2700 rpm. Ie WOT = 26 degrees after 2700.
Edited by DaveG on Tuesday 4th August 06:27
Edited by DaveG on Wednesday 5th August 14:38
Edited by DaveG on Wednesday 5th August 14:41
Edited by DaveG on Thursday 6th August 13:31
Thats useful stuff- wish I had it for the 3.9 engine as Ive got mapped ignition, but no map at the moment. The bigger engine will have a different advance curve due to the bigger combustion chamber and speed of the flame front. I just need a session on the rollers to set the timing up for peak torque and be done with it.
Hi Mark. The guy that gave me the curve plots was a sales/service/repairer of all distributors. He charged me $20 and took 10 mins or so to test mine, which is standard for the 500. He could not look up TVR in his manual (a document about 3 ins thick), but he had all the Rover / Lucas stats. Assuming your's is a 3.9 Rover and you have the dizzy ID you should be able to get what you want from a specialist?
The engine was stock Range Rover, but Ive now changed the cam and ported the heads so the torque and peak power are now very different, (Similar to the TVR set up) so the stock bob weights and springs wont have the correct advance curve any more. I can get the stock advance curve that it currently runs without too much bother, but an example on a TVR 3.9 would be useful to compare, as I think TVR did change the advance springs to lighter ones which is normal practice when an engine is tuned. The current Amethyst mapped ignition box is simply sitting inline with the dizzy at the moment with 0' advance, as the dizzy weights are not locked yet. Ive done this as Ive had bad experience with the set up, (poor triggering) so I want to be sure the electronics is working as is before I lock the weights and go fully mapped.
Great post OP. I found this is an interesting read: http://www.teglerizer.com/mgstuff/lucastuning.pdf
The discussion on springs, primary and secondary, and their impact on the advance curve is fascinating. Interesting to read that you think TVR put lighter springs in Mark, I'll have to check that out. I'd be really useful to get some advance curve graphs for the various 4.x and 500 TVR engines.
Also I've contacted the Distributor Doctor, he offers a set up service to create a bespoke advance curve, but it's not cheap. For a few hundred pounds more I could get a 123 Ignition jobbie, but once the optimum curve is calculated and set will I really need all that programmability?
The discussion on springs, primary and secondary, and their impact on the advance curve is fascinating. Interesting to read that you think TVR put lighter springs in Mark, I'll have to check that out. I'd be really useful to get some advance curve graphs for the various 4.x and 500 TVR engines.
Also I've contacted the Distributor Doctor, he offers a set up service to create a bespoke advance curve, but it's not cheap. For a few hundred pounds more I could get a 123 Ignition jobbie, but once the optimum curve is calculated and set will I really need all that programmability?
I think it depends on what you're trying to achieve. With the dizzy - the advance is proportional to RPM only, not RPM and load.
For cruise, you may want to go leaner and have a very high advance to cope with the slow burn at 20% throttle and 3000rpm, for maximum fuel economy.
Full throttle you may want more advance at 2000rpm than you can get with the dizzy without compromising the rest of the curve.
If you're a sad b
d like me, you might want to pull all the timing from off throttle to make an exhaust symphony on overrun...
I'm sure the 14cux and the dizzy do a decent job in general - but there are a few things that if you are interested enough you can change to suit yourself better. I posted before and removing advance to cure shunting without needing a rich mixture... I couldn't do that without gutting the top end power if I only had an RPM based ignition curve.
How often you change the above depends on the level of refinement you want, or if you want to modify and get the very best power at every stage. Power is all in the ignition timing, even exactly the right AFR won't help you any if you're not punching the piston down the bore at exactly the right time.
For cruise, you may want to go leaner and have a very high advance to cope with the slow burn at 20% throttle and 3000rpm, for maximum fuel economy.
Full throttle you may want more advance at 2000rpm than you can get with the dizzy without compromising the rest of the curve.
If you're a sad b
d like me, you might want to pull all the timing from off throttle to make an exhaust symphony on overrun...I'm sure the 14cux and the dizzy do a decent job in general - but there are a few things that if you are interested enough you can change to suit yourself better. I posted before and removing advance to cure shunting without needing a rich mixture... I couldn't do that without gutting the top end power if I only had an RPM based ignition curve.
How often you change the above depends on the level of refinement you want, or if you want to modify and get the very best power at every stage. Power is all in the ignition timing, even exactly the right AFR won't help you any if you're not punching the piston down the bore at exactly the right time.
davep said:
Great post OP. I found this is an interesting read: http://www.teglerizer.com/mgstuff/lucastuning.pdf
Also I've contacted the Distributor Doctor, he offers a set up service to create a bespoke advance curve, but it's not cheap. For a few hundred pounds more I could get a 123 Ignition jobbie, but once the optimum curve is calculated and set will I really need all that programmability?
dont waste your money with the 123 tune ....... either keep it as is or go full aftermarket with an ecu.Also I've contacted the Distributor Doctor, he offers a set up service to create a bespoke advance curve, but it's not cheap. For a few hundred pounds more I could get a 123 Ignition jobbie, but once the optimum curve is calculated and set will I really need all that programmability?
if i would not have had triple 45 dcoe webbers on my datsun would not have gone the 123 tune to keep the
classic look....
Thats a "ported" take off- as I understand it it shuts vacuum completely to the dizzy when the throttle plate is closed, and this prevents vacuum advance being applied at tick over. I know it caused complete mayhem to my ignition timing when I put a new take off in the plenum body that was not ported when I was trying to get the Amethyst to work.
Mark is right. The ported take-off completely shuts off the vacuum. I did a few tests today on CUX, looking at both ported and manifold (plenum). I was surprised at just how quickly the vacuum comes in on take-off. It rises quite quickly to 15" and sits happily between 15" and 18" normal driving and light throttle. Remember that 17" gives 20 degrees advance. This implies normal driving and cruising requires around 34 degree of advance (at 1500rpm and 10 BTDC) according to the calcs!! Or 38 at 2000rpm with 16" of vac. Even light acceleration does not drop the vac below 15", you have to hit the throttle hard to get the vac off.
My experimental remap under MS will look a bit different to what I have been using.
My experimental remap under MS will look a bit different to what I have been using.
This would be a good chance to try and get some differences between the Range Rover and TVR methinks- Ive got Range Rover dizzy weights at present so I should be able to ge the advance curve with the help of an Ignition timing light with a calibrated dial. In terms of weights the shape can be altered to change the peak allowable ignition timing- but far as I know its pretty much 36' for the RV8- something to do with the type of combustion chamber shape and speed of the flame front- so I dont think it would need to be changed.
The idea is the first spring takes place if the advance at low RPM, then the stiffer spring starts to come into play as the centrifugal forces increase on the second weight- mind you its all adjustable mechanically, so Id think two weak springs will give you a steep advance curve that flattens off quite quickly. Ive now had a chance to plot the stock Range Rover 3.9 dizzy- I was surprised how quickly it reaches peak advance, and little advance there was in total- maybe Ive a stuck weight(?) I hear figures of 36' bounced around for max peak advance -but this is no where near- at no more than 26' total- but I guess this is for a 2 tonne Vehicle running on stock unleaded that never goes beyond 5400 RPM. Please take the vacuum advance as guesswork at the moment- I have no way of measuring the vacuum.
Anyone out there that has a similar plot for a TVR 3.9 Id be very grateful to compare to this or any comments please:
Anyone out there that has a similar plot for a TVR 3.9 Id be very grateful to compare to this or any comments please:
blitzracing said:
Anyone out there that has a similar plot for a TVR 3.9 Id be very grateful to compare to this or any comments please:
Interesting, and quite revealing when compared with what you might expect to dial in with a programmable system.It seems the dizzy is running at least 5 degrees less advance than what would be considered optimal in the shunting zone on petrol if you had the benefit and flexibility of mappable spark.
An additional useless factoid being that on LPG I'll actually run a huge 12 degrees more advance than the distributor delivers in the 1500 - 2000rpm window.
LPG weirdness aside there's still quite a gap between what I'm running on petrol in that difficult area and what the distributor seems able to give.
Perhaps a 123Tune or Aldon Amethyst holds the cost effective answer to adding the necessary advance to smooth things out?
No idea about the dizzy question - sorry. LPG isn't all that weird. It's just different to petrol. 
Getting back to the OP's original question (maybe from the previous thread), I was never really happy with the dizzy on my Griff once I started changing things on the engine. Maybe this was because it was getting worn out. I used the new ECU's logging features to work out what the dizzy was doing and then constructed an absolute pressure vs RPM timing map for the mapped ignition. (I did this about 8 years ago, so memory is a little faded as to exactly what results I got) I then refined this timing table on the road with det cans and on a couple of rolling roads. The result is shown below:
Some notes - the cam in my Griff is an H404. The chart represents the timing table for my previous engine (so standard 5 litre capacity, standard TVR bottom end and heads), but the engine was fitted with the mk 1 version of my twin plenum intake system, designed to reduce charge robbing and pressure drops at WOT. The engine was really well behaved in this configuration and covered a lot of miles back then. It is interesting to compare this with the dizzy timings from the first post. It seems I empirically derived something quite similar.
Moving on a few years, I have built a new engine after a liner issue (plus had many work and family commitments to stop me playing with cars at all) and I need to repeat some of the above process. The new engine is much smoother and it seems much more tolerant of timing at the top end and much less fussy about timing and mixture settings at town type speeds. I am guessing I might have "fixed" some underlying feature with the original engine design. I know not what since just about everything is different...
On a maybe related note, my Elise started to shunt at low speed, much like a small TVR. This has been diagnosed as a dirty fuel filter. I await the proof of this, but pass it on for what it's worth.
Getting back to the OP's original question (maybe from the previous thread), I was never really happy with the dizzy on my Griff once I started changing things on the engine. Maybe this was because it was getting worn out. I used the new ECU's logging features to work out what the dizzy was doing and then constructed an absolute pressure vs RPM timing map for the mapped ignition. (I did this about 8 years ago, so memory is a little faded as to exactly what results I got) I then refined this timing table on the road with det cans and on a couple of rolling roads. The result is shown below:
Some notes - the cam in my Griff is an H404. The chart represents the timing table for my previous engine (so standard 5 litre capacity, standard TVR bottom end and heads), but the engine was fitted with the mk 1 version of my twin plenum intake system, designed to reduce charge robbing and pressure drops at WOT. The engine was really well behaved in this configuration and covered a lot of miles back then. It is interesting to compare this with the dizzy timings from the first post. It seems I empirically derived something quite similar.
Moving on a few years, I have built a new engine after a liner issue (plus had many work and family commitments to stop me playing with cars at all) and I need to repeat some of the above process. The new engine is much smoother and it seems much more tolerant of timing at the top end and much less fussy about timing and mixture settings at town type speeds. I am guessing I might have "fixed" some underlying feature with the original engine design. I know not what since just about everything is different...
On a maybe related note, my Elise started to shunt at low speed, much like a small TVR. This has been diagnosed as a dirty fuel filter. I await the proof of this, but pass it on for what it's worth.
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