ideal dmc setting

ausbean

Junior member
I have a 180 Jota (full Jota spec) with a DMC ignition and wonder if anyone can tell me what curve they are using that works the best?
Hi Maurice,
My hi comp motor.
Is it a DMC2?
If so, for 32deg, I have tried 2,7 & currently on 10.
2 similar to standard.
7 & 10 a progressive rise, much the same, seat of the pants.
I have also retarded the initial timing to about 30deg, seams to be happier.
Don't no if I should be using 8 or 11 though.
Would these limit advance to about 30deg even if timing set to 32 deg.
Hope that helps a bit, probably notšŸ¤”
Phil
 

Laverdalothar

Hero member
Location
Germany
With high compression on a 1000 ccm engine, I wouldn't go beyond 30Ā° top. Main thing is you avoid the pinging. smoother running=> go with a flatter curve. Don't think you will recognize much difference...
 

Jo

Hero member
I have a 180 Jota (full Jota spec) with a DMC ignition and wonder if anyone can tell me what curve they are using that works the best?
Hi Mauricio,
AFAIK Uwe Witt recommends different curves according to tuning spec of the engine.

Here is an excerpt from his instructions (unfortunately only in German):
Witt DMC 2 p17.jpg
Witt DMC 2 p18.jpg
Witt DMC 2 p19.jpg

Generally he recommends to take advance back with higher compression engines.

Cheers,
Jo
 

sfcpiet

Administrator
Staff member
Location
Germany
Maurice,

These motors offer far too many variables to be able to recommend a certain ignition curve or timing. Max. advance depends mainly on actual compression ratio and volumetric efficiency of the engine. Worn or badly-positioned valve seats have a huge impact on both factors, as do shoddily cut valve seats. One engine to a certain spec may require completely different ignition parameters than another with the exact same basic specs.

Most motors are quite happy on 32Ā° BTDC max, the actual curve used can be selected to avoid pinking or simply rider preference. My old triple runs a twin-spark head and purrs along nicely at 27Ā° max advance. Lots of stuff in between... ;)

piet
 

iis_iis

Senior member
Location
NSW
FWIW, the published curves for DMC are not what is registered on a precision timing jig i fabricated in the late 1990's. "Broad" indicator is the most prominent feature, for the DMC printed curves.

Agree with Piet, a true 32BTDC is safe place to be, other motors ( CR head ) can go for 36DBTDC, with caution of course. True timing also includes the base reference for the ignition pickup system. IIS was based on a true TDC reference marker ( used a TDC sensor, plus other detectors for crank vector position ), located by and with the then supplied dead stop device and correct indication of TDC. This allowed for checking factory TDC and correction as required.

DMC maximum ignition firing angle has to include any base timing for cold crank start and idle advance. This becomes complicated on variable reluctor pickups, such as the oem Bosch twin pole pickup or sensor. Precision timing light and calibrated degree is a better tool for timing settings, and confirmation. Dial back timing lights also can be useful. HTH j.
 

mauricejota7604

Hero member
Thanks all in fact i set up a timing disc and strobe yesterday to establish what was really happening and ended up setting a conservative 30 deg max advance which was 11 on the rotary dial, the OEM Bosch pickups are still being used
This engine is quite an unknown although very high compression, today I will clock the cams.
 

iis_iis

Senior member
Location
NSW
Thanks all in fact i set up a timing disc and strobe yesterday to establish what was really happening and ended up setting a conservative 30 deg max advance which was 11 on the rotary dial, the OEM Bosch pickups are still being used
This engine is quite an unknown although very high compression, today I will clock the cams.
most oft the case you need more advance for a high(er) comp motor, dependent on what is defined as high comp, my reference is around the 10.5:1 or 165psi averaged wet and dry peak compression test, not easy actually measuring true compression ratio....as it may help. j.
 

Paul Marx

Hero member
Location
France
Thanks all in fact i set up a timing disc and strobe yesterday to establish what was really happening and ended up setting a conservative 30 deg max advance which was 11 on the rotary dial, the OEM Bosch pickups are still being used
This engine is quite an unknown although very high compression, today I will clock the cams.
Are you talking about compression ration or combustion chamber pressure?

Paul
 

Paul Marx

Hero member
Location
France
compression chamber pressure, which is generally related to compression ratio.
Of course it is, but one doesn't take in to account coke when one evoque's a compression ratio. One does when evoquing combustion chamber pressure which is variable with the mileage of the engine.
 

iis_iis

Senior member
Location
NSW
Are you sure you don't have that arse about Jon?
I always thought it was less advance for high compression.
Anything is possible when a pensioner is engaged, Cam, no one else here, i must be the pensioner.

Matter of flame propagation, did my best in the day to ascertain from the oil company chemists, actual rates for various cylinder fuel/air loads. Were not forthcoming.

Had to do my own work as best i could. In the heyday of iis, gathering ignition data from where ever i could ( the light aircraft engineering data i tried to put in the library is all encompassing ). V twin ducks of racing and high performance configuration ran 45DBTDC, reason for that, ( the ) higher the actual compressed cylinder load the longer the flame front has to travel, refer the aforementioned aircraft motor data. CR head SFC1000/Corsa require 36DBTDC for using the improved power available.

The iis had a Valet curve available of 25DBTC, pretty much all the Laverda triple motors here were HC of some form. The Valet curve also had a reduced rev limiter to allow the new to iis rider, what a rev limiter felt like. The 25DBTDC knocked about 30hp out of the HC engine. Not sure i can expand on this without going my usual overboard on the technical aspects and putting many into a coma... a grin here.

The twin plug heads having two initial flame fronts were a concern for me, in the day of Dougs 3c twin plug ( one of my heads as it happened was sacrificed ) best advance was 25dbtdc, Doug tried to improve on that and made the engine unridable. Pretty convincing to me, how a motor is built and how to balance carby/fuel/air load to ignition timing. Like a nomograph effect there is interaction ignition and cylinder load, you can have a same result with holding air/fuel constant and adjust timing for a given rpm or the reverse ie adjust air/fuel and hold the ignition point for a given rpm...all a bit neat when you get a handle on the interaction...better stop here...HTH j.
 

iis_iis

Senior member
Location
NSW
Just recalled being asked to provide a "retard" ignition function for turbo boosted engines, in the 90's. Car zealots were getting to 800hp on the normally aspirated ( non fuel injection, they hated FI electronics ) turbo charged configuration, the turbo lag was causing a lower rate of increase in cylinder load and the usual ignition rise of advance ( its really an ignition "retard" as there is no such function as "advance"... another of my pet hates in terms bandied around ), ie the ignition had to supply "advance" retard ( less advance ) and advance" while the rpm builtup... ie a "dip" in the curve, something their trusted distributor could not ever do without significant mechanical governance.... i declined, to my regret, they were willing to pay big dollars for me to take it on, i stuck with the Laverda family, very kind to me and life partner in the day... a true story...j
 

redax5

Administrator
Staff member
my race engines are running 13.8:1 and 13.2:1 compression ratios using 103 octane, 3.8% oxygenated race fuel
after well over 200 Dyno pulls we found that they run best at 28 degrees total advance
to work that out we took the bikes to the dyno shop where the owner gave us a dyno cell free of charge for as long as we needed it and then we just spent hours changing ignition curves and jetting, was a great day
thanks to Bevelman for giving me a hand back then, still very much appreciated and we drank so much beer together, great times

To find ideal advance for your engine go to the Dyno, do base runs and then start to dial advance out, once you start to lose horsepower, go back 2 -3 degrees and lock it in

Combustion chamber shape has an effect on the required total advance, if it is inefficient then the engine will require more advance to overcome the design restraints
The better the design of the combustion chamber, the lower the advance curve
 

iis_iis

Senior member
Location
NSW
Ah Red, few around the globe would have your experiences and resources, hard won experience is just that, has much gravitas. Sharing that experience(s) speaks of what is good in the Laverda community, a minority in a minority, my perspective. Has the effect of concentrating the resources into an elevated platform of knowledge and ongoing support.

Of the general topic for retarded ignition, not going to participate in the anomalous distortion of "advance", how can you have advance when something has not happened yet, the actual spark or multiple sparks. How ignitions work in the electronic application, you have a forward reference point before TDC and then you apply retardation of the actual spark event. IIS is not a tardis that travels back in time to make an event happen before it happens.... witness friends quietly leaving my soapbox area...a grin here.

Actual combustion cylinder fuel load, flame propagation is the main factor, molecule to molecule flame is what most of us are dealing with. Altered combustion chamber design is high end engineering requiring considerable skill and equipment, cue Redax, a team of two notables there. Life partners are vital in any engineering endeavours. Am unsure of the actual flame propagation rates for the various "fuel" soups available around the glob, actual rates are not readily available or even more poignant, actual verified rates. Make assumptions, that the rate is deflagration in nature. Others may have superior knowledge....

At least for various operating conditions, the engine is part of the scientific measuring device, it will "Pink" ( piston rattling in the bore ), it will lose HP when under efficiency, peak cylinder pressure vector point of crank/piston.... what is best ignition curve for my engine, what indeed.....j.
 
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