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450 ignition timing.

The ignition timing on the 450 has always intrigued me, virtually one cylinder from a 900, but it uses 10* less than the 900 cylinder. The combustion chamber is the same, yet it takes less time from spark, to max pressure, at about 18* after TDC. I have worked with both engines and the only difference that I can see is the shorter rod length of the 450. This speeds up the combustion chamber ‘volume per degree’, changes over TDC. It seems that static 0*, total adv 26* remains regardless of the tune.

As some of the members have changed the stroke and rod length, I am wondering if they have found that the timing changes, or stays the same?

____ If you bother to try a search, you should find that this issue has already been covered once or twice before...
The zer0-degree static-timing was chosen to prevent kick-backs, (and not best for performance).


Dukaddy-DUKEs,
-Bob

by DewCatTea-Bob » Sat Apr 06, 2013 1:32 am

____ If you bother to try a search, you should find that this issue has already been covered once or twice before...
The zer0-degree static-timing was chosen to prevent kick-backs, (and not best for performance).

None of the posts there talked about, why the 450 cylinder takes 10* less, than the 900 cylinder, to build maximun pressure.
Has to be a reason.

" None of the posts there talked about, why the 450 cylinder takes 10* less, than the 900 cylinder, to build maximun pressure. "

____ Ohh, I believe you're right, I don't think that particular aspect has ever been mentioned-of in OUR forum, (but perhaps in some L-twin forum). _ You're previous over-use of the word "it" allowed for me to mis-comprehend what you had actually meant.
That which you actually DO mean to discuss seems to be of quite worthy interest... So can you elaborate on it enough to bring the rest of us up-to-speed on all the directly-related particulars ?


Hopeful-Cheers,
-Bob

Sorry if I confused you, my bad. What I am trying to find a reason for is.
The 450 and the 900 cylinders, and combustion chambers, are virtually the same, so the combustion phase should take the same amount of time from ignition, to maximum pressure which should occur about 18* ATDC.
The 900 fires at 36* BTDC, with max pressure at 18* ATDC, a total of 54 degrees for the burn.
The 450 fires at 26* BTDC, with max pressure at 18* ATDC, a total of 44 degrees, to burn the same amount of gas, 10* faster.

The burn time depends on pressure, temp, turbulence, and mixture strength, so the only condition that I can see affecting the shorter burn time, is the length of the shorter con rod used in the 450. The shorter rod has the combustion chamber volume at a smaller volume, over the degrees that the burn takes, than the longer rod, so the burn takes a shorter time, due to the higher pressure during the burn.

What I would like to find out is if the change of stroke, or the change of rod length, that Nigel and Eldert have used, has changed the ignition timing, or does it stay at 26/28*.

Hello Harvey, more things of interest. I hope the people that have used longer rods will comment.
I use a Honda 141mm rod but have always set the timing by feel and ear.
Maybe 1mm isn't going to make a difference.
Graeme

Hi,
I did do various calculations to try and give me a starting point for the ignition timing on my stroked engines, but I can't remember what the figure was off the top of my head. However It is immaterial as I then set the timing on the dyno. What I found was the engine ran fine on 28 degrees and advancing it to 32 degrees was marginally better. Any further advance didn't improve things, I have found all of my race 450's run best between 28 to 31 degrees but to be fair between these figures there wasn't much in it. When we tried running twin plug we even backed off to 26 degrees or less, we didn't find any real advantage in twin plugs on the 450. This timing goes for all the engines I did with Strokes of 75mm 77.5mm 80mm and 81mm all with 140mm rods. One thing that is overlooked by many is the function of the squish and the importance of a nice piston shape (as a balance with C.R.). I run a 0.8mm squish and keep the piston crown smooth with no sharp edges.

I don't know why the twins should need more advance, maybe it's the longer rod or maybe they run a larger squish. Maybe you could back off the ignition timing, I have had customers suggest this, but I've not done any dyno testing on bevel twins to confirm either way. Maybe it's all just all about large numbers, I've seen 42mm carbs on 250 racers, it's a Macho Italian thing

Regards Nigel

Thanks Graeme and Nigel for the replys. I guess I will just put it in the "to be discorvered basket".
One more thing for me to learn.
Cheers.

Harvey and all,

Had a look a look at my records and with the longer stroke my calculations indicated approximately 1 degree retard should place the piston in the same place relative to TDC for an 80mm stroke compared with the standard 75mm stroke. So no great change of any significance compared to all the other small variables that could effect ignition timing. Back to my too many variables, suck and see theory!

Concerning rod length 250/350 narrowcases engines have 125mm rods and 250/350 widecase engines have 135mm rods. Widecase 250/350 engines are given the same ignition timing at 33 to 36 degrees with strokes of 57.8 and 75mm respectively. Also narrowcase 250 Monza's and 350 Sebrings are specified at 5 to 8 degrees static with 28 degrees of advance , so 33 to 36 degrees full advance (sound familiar?). So various combinations of variations of 17.2mm in stroke length and 10mm changes in rod length here, are being specified with the same ignition timing, so more for you to think about!

Before anyone mentions narrowcase Mach 1's and Mark 3's needing 38 to 41 degrees of ignition timing. There may be some reason why they may have as standard required/require that timing, although I have no experience of standard stock road machines on a dyno to confirm. All I can say is I have never needed to use more than 36 degrees on any of my race 250's, with whatever cam, higher piston crowns etc. Piston shape, squish and fuel all probably playing a part.

One thing that is apparent is that after the factory moved to a squish "semi hemi" design with the 250, the ignition timing was reduced compared to the 175/200's.

Regards Nigel

LaceyDucati wrote:All I can say is I have never needed to use more than 36 degrees on any of my race 250's, with whatever cam, higher piston crowns etc.

____ I then presume that all your race-250s were with battery-powered type ignition-systems ?


" Before anyone mentions narrowcase Mach 1's and Mark 3's needing 38 to 41 degrees of ignition timing. "

____ The 'Mach-I' had battery-powered ignition and-also thus the same 5 to 8 & 33 to 36 ign.timing.
It was the 250-Motocross/n-c.Scrambler engine-model that shared the 20 to 23 & 38 to 41 ign.timing with the n-c.Mark-three models.



" There may be some reason why they may have as standard required/require that timing, "

____ Besides their differing AAUs with the 10-degrees difference in advance-range,, their AAU cam-lobes are also quite different because, while the battery-powered ign.system must first saturate it's ign.coil before the ign.points open and thus-then instantly cause a resulting flux-field COLLAPSE (so as to then-IMMEDIATELY create an ign.spark)... the battery-less magneto-type energy-transfer ign.system on-the-other-hand,, relies on a single pulse of alt.power to force the sudden creation of a flux-field (somewhat less abruptly) into the ign.coil, after the ign.points open.
__ While an ign.coil's established flux-field must be capable of 'collapsing' at a slightly quicker rate than a flux-field can be 'built-up' by a pulse of alt.power, (due-to 'counter-electromotive force'),, I would-not have expected that such a very-MINOR time-difference in the rate of (spark-producing) flux-field change, could possibly be actually equal to as much as 5-degrees worth of crankshaft-rotation/progression-time (@ 3k-RPM),, but evidently that very minor/infinitesimal difference in time-rate must be enough to account for how far the crankshaft is capable of progressing-forthward within, (during the difference in time that it takes for those two differing types of ign.systems to finally begin the creation of their ign.sparks [once the points have broken-open their circuit, (@ 3k-RPM)] ).
__ I can't think of any other trait-factors (as straight-forwardly logical), which could account for the need of the two ign.systems' differing ign.timing-settings.

____ Anyhow, despite the above deduction-reasoning,, it still none-the-less seems fairly kind-of doubtful that the 900's ign.system type could at-all possibly be fully accountable for the difference of 5 to 8 degrees in recommended ign.timing-settings (compared to the 450's). _ And that's naturally fairly understandable since most-all of us pretty-much ASSUME that once the ign.triggering-process has been initiated, the ign.spark then-next follows right-after pretty-much near the speed-of-light, (in which such case, it would then seem that the crankshaft certainly wouldn't get any chance to move even a tiny-fraction of a single degree even at red-line RPMs). _ But in reality, it rather seems that such presumed function-speed must-NOT really be so very-quick to actually occur ! ...
__ I can next do the-math to figure whether or not such differences (in factory-recommended ign.timing-settings & slightly-differing ign.spark response-times of the varied ign.system-types) could at-all possibly fall-into the numerical ball-park closely-enough to possibly account for the resulted ign.timing range-variances (in factory-recommended ign.timing-settings for differing ign.system-types). ...
__ Since max.advance is supposed to be reached near 3000-RPM, that's the RPM which I'll always be in reference-to,, and while that amount of engine-revs may seem like a rather low RPM, it ought-to be realized that that's (somewhat amazingly) actually equal to 50-RPM/TDCs per second ! _ Which is also equal to 18,000-degrees of rotation per second.
__ In this more deeply-scrutinized instance-evaluation, I'll have to make reference to 'degrees' per fractions of a 'second' of time (rather than the more commonly-used 'RPM'), so I now ought-to point-out that a 'mS' is 1/1000th of a second ! _ So that now brings it down to 18-degrees of crankshaft-rotation per mS (at the 3000-RPM).
And another very-relevant & important fact is that, depending on the type of ignition-system,, the life-duration of an ign.spark itself, lasts anywhere from .8mS to 2.4mS,, which at 3000-RPM -(max.advance) is equal to a whopping 14 to 43 degrees of crankshaft-rotation ! _ (Very-much unlike what we notice when we're setting the static-timing and see the spark completed in merely-just 1-degree of degree-wheel rotation ! _ And understandably so, since the crankshaft is then being rotated relatively extremely-SLOWLY, [whilst the ign.spark still goes-through it's regular time-span at the same/normal rate as always !].)
So therefore it actually ought-not be so unimaginable for the 'types' of ign.systems to vary enough (in spark-production delay) to fully account for such relatively small differences in recommended ign.timing-settings (even-though 5 to 8 degrees difference SEEMS like a LOT) ! _ As it's fairly reasonable that the duration-time it takes to initiate the creation of a spark (from when the ign.points open or the ign.triggering process has begun), could reasonably range (proportionally about) from .05 to .3mS, (which is equal to about 1 to 6 degrees of crankshaft-rotation [at 3000-RPM] !). _ (So that makes the outer-EXTREMES in time-duration [to finally produce the initial-START of a spark], to be no lower than 2% and no higher than 38% of the amount of time that a spark's life-duration lasts, [depending on ign.system-type].) _ And since an average spark-duration is 1.6mS, it's then fairly fair to figure that the time-duration it takes to finally begin the initiating-start of the production of an ign.spark, could certainly vary (in extremes) from .09 to .7 mS (averaged, for the opposing-extremes),, which is actually equivalent to 1.6 to 12.6 degrees of crankshaft-rotation (at 3k-RPM),, which makes a possible (although likely not probable) DIFFERENCE of as much as 11-degrees (between the extremes of spark-delay, of differing ign.system-types) ! _ So-therefore, (as it turns-out),, it's not unreasonable at all to possibly conclude that the submitted questionable-differences can all fall well within the same 'ball-park' (and so thus quite-possibly definitely account for the questioned-discrepancy) !
__ Thus it's now obvious (to myself at-least) that Ducati had realized & figured that the inherit-delay of spark-creation with their magneto-type energy-transfer ign.system (with respect to their battery-powered ign.system), required more max.ignition-advance lead-time,, in order for the ign.sparks (of both ign.types) to actually occur pretty-much at the very-same piston-location (at max.advance).
____ SO, I'm now thinking that the actual ANSWER to the question as to WHY the 900 & 450 ign.timing-settings are not factory-set the very-same, must have to-do with the differing delay-time it takes for their respective ign.sparks to actually COME-INTO existence (once triggered), (since the 900 doesn't share the same points-type of ign.system as the 450 employs).
__ So it now seems quite justifiable to investigate the likely different ign.spark-delay rise-times of the two differing ign.systems, before moving-on to consider any other possible cause-reasons for the difference in factory-recommended ign.timing-settings.

____ Bottom-line... When it's actually considered how far a crankshaft can really travel through it's rotation (at 3000-RPM), during the relatively extremely-SHORT time which it takes for a spark to get started, from one type of ign.system to another,, it's then obviously POSSIBLE for such differences to account for the variances between recommended ign.timing-settings !


Dukaddy-DUKEs,
DCT/DCT-Bob