View Full Version : The truth about ignition timing...

04-29-2003, 04:34 AM
I have recently read an article that technically makes sence, but goes against common folklore about 2stroke tuning. I wondered if anybody would like to give their opinion on the following:

It is normally stated that the retarding of the ignition at the top RPM range in the road SAPC boxes of RGVs and RSs prevents the engine producing power at high revs...

However, the engine produces most power when 'on the pipe' - this is only for a narrow RPM range (say 9,500 - 10,500), the theory is that by advancing the ignition below the powerband, and retarding it above the power band is effectively widened, as the timing changes affect the pressure pulse traveling down the pipe...


Cannon Fodder MOO
04-29-2003, 05:42 AM
retarding the timing will indeed reduce power output (based on an optimised stock bike) all other things being equal. At higher revs the ignition timing is more advanced because the motor is revolving quicker. *(futura 1000?).

timing pulses may well effect the pressure pulse but how? at top dead centre (TDC) the ports are closed. If timing is too far advanced *(should be retarded)*then the piston will pass TDC before combustion resulting in a loss of power, as the explosion occurs in a larger volume. Thus no matter how good any extra scavenging effect is as a result of changing timing the motor will not be producing optimum power. Also if timing is too far retarded *(should be advanced)*combustion will occur before TDC and you will put a rod through the block/suffer major piston failure. once again before any percieved benefits in extra scavenging.

so in short you may be able to extend your power a little at both ends of the spectrum, and the easiest way would be aftermarket pipes/chip. Remembering that increasing peak power increases the load on various components (including rings!) and should be taken into account.

Although I grew up in a garage at times I'm all thumbs when working on my vehicles. I don't get enough practice and am lazy. my pa is a mechanic and I have rebuilt various holden and BMW motors. so I'm no expert and would love to hear comments on my beliefs.

finally if you have a link to that article i'd love to read it :)

OK I've changed my mind again so I'll mark the WRONG bits :lol: thats cos I got it right and wrong in the same post. If you think my edits are wrong then for christs sake let me know :p

04-29-2003, 08:30 AM
"At higher revs the ignition timing is more advanced because the motor is revolving quicker."

Sorry, but that is not true. Two stroke timing maps provide the greatest amount of advance at low RPM. I am working on custom maps for our RSV250s right now and these bikes hold the highest advance value until right about 7,000-7,500 rpm. It starts to become less after this point and how much and where makes a BIG difference in the power the engine makes.

It is main a matter of the fuel you are using, head volume, deck height and head profile. I have found amazing differences on our bikes with head combinations, ignition maps and static advance.

Suzuki's stock ignition curves are designed to be able to be used with very low octane gas and to ensure the bikes are relatively bulletproof. If you wished to use high octane, leaded racing fuel, you would theorhetically need to lower head volume, change the head profile, and probably increase advance across the range while also holding on to it a bit longer. This is all dependent on just how much you do increase compression.

Rocky Stargel
Team Stargel Aprilia

04-29-2003, 12:02 PM
Thanks for taking some time to answer. I agree about needing to alter timing with compression, but I was particularly interested in the idea of using timing to alter the powerband. I know it does not immediately make sense, but you have to consider, retarding the timing means the explosion occurs closer in time to the exhaust port opening, and therefore the exhaust gasses are hotter when they enter the pipe. The exhaust pulses travel quicker in a hotter pipe - therefore the pipe tunes to a higer RPM than it would normally. (Advancing the timing results in a stronger mid-range.) Comments?

04-29-2003, 12:57 PM
Retarding the timing will lower combustion temps and thus exhaust temps. We run dual egt sensors and I constantly monitor exhaust temps. Most temp differences we see are not great enough to affect the tuned length of an exhaust.

Yes, you can move the powerband a bit with timing, but to do it in a productive way means that the timing should also suit your heads and fuel. To simplify things; short tracks = higher compression & more ignition advance, long tracks = lower compression and less ignition advance. If you ran lower compression heads and dialed in more advance than what was optimum for those heads, you would just lower your RPM ceiling and not gain appreciable midrange increases, either.:rolleyes:

04-29-2003, 01:05 PM
I just checked and the article I read has got it wrong! The speed of sound is slower in hotter air anyway - but that would tie in with what you said, retarding ignition = lower exhaust temp = faster pulse = shorter effective pipe length = power band shifted to higer RPM. I don't suppose you would be prepared to reveal how much cooler (or even what order of magnitude) the temp differences are. Then I could calculate the effective RPM shift from the change in the speed sound... I think you are right and the difference will be minimal - but it would be fun to do!

micah apriliaforum com
04-29-2003, 02:41 PM
I think the confusion here is the terms "retarding" and "advancing". Seems like some people are saying "retarding" for when the igintion fires earlier, aka before TDC.

Advancing the ignition is making it fire more degrees before TDC. The more advanced the ignition is, the more degrees before TDC the ignition fires.

The reason ignitions advance as the RPM increases is because given a constant combustion rate, the combustion is timed to give the best push on the piston. As the RPM increases the piston travels faster, so the charge has to fire earlier so as to give the best combustion at the correct piston position. Granted it isn't a constant combustion rate, but it is far more constant than the piston speed.

If your ignition is too advanced, you will blow holes in pistons and melt sparkplugs. This is because the combustion is occuring too soon as the piston is rising. The combustion is basically trying to push the piston backwards, creating lots of heat.

Hope this helps

Jon (the new service guy) using Micah's computer.

04-29-2003, 03:26 PM
For reference, I have an ignition map here, (from an RD but illustrates the point), all offsets are in degrees of crank rotation BTDC (before top dead centre)

RPM Advance
0-2000 17 deg
3500 27 deg
9000 17 deg
10000 9 deg
12000 7 deg

This map has advance peak at 3500 rpm, from there the ignition retards to max RPM. If combustion temp is higher with more advance then the advance below the powerband, and the (relative) retarding above the powerband would help broaden the powerband (with no loss of peak power) - It would be very interesting to know the exhaust temperature above and below the power-band to be able to work out how much effect this actually has.

Cannon Fodder MOO
04-30-2003, 01:49 AM
yep jon and two cycler are definitely correct <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/embarassed.gif ALT=":o">

I hope the interchange of retard/advance would then make my commentry correct. I didn't sit down and think about it and the reason I used the terms in reverse order was due to my fiddling with cars. the vacum advance used to retard ignition timing for the higher revs on my old heap. i remember having to disconnect it to tune the car a few times.

so yeah my mistake, i was thinking about vacum advance, and obviously the faster the piston moves the earlier the ignition needs to fire to create optimum combustion.

thanks to twocycler, and especially jon for their delicate handling of my indiscretion. :)

I won't edit my post though cos you guys already know I'm backwards ;)

04-30-2003, 03:52 AM
Actually, if you look at the ignition map, the earliest sparc firing occurs at 3500 RPM, the sparc then fires later and later until 12000 RPM when it is only 7 degrees before top dead centre.

In an interesting turn about - I have looked up some info from a reliable source, and found that a lot of information out there is at least confusing, or just plain wrong...

With regards to temperature affecting pipe timing:

C (speed of sound) = sqrt(1.4*pressure/density)
C2/C1 = sqrt(T2/T1)

this means the speed of sound changes proportionaly to the square root of the change in temperature...

Assuming average temp of exhaust gases is 400 deg C (average of 800 deg C at cylinder, and 150 deg C at silencer exit) - the seed of sound in the pipe is an average 518 m/s. So back to the original idea!! Speed of sound is faster in hotter gasses. Assuming the pipe is tuned to 10000 RPM, and we have some way to increase the temperature in the exaust header by 100 degrees, using the relation:

crank-angular-displacement = (0.012 * tuned-length * RPM) / speed-of-sound.

The pipe will actually be tuned to 11930 at the higher temperature...

This shows that an increase in header temperature of 100 deg C = 2000 RPM shift in power-band.

Presumably this is why Honda use water injection (in this cas to lower exhaust temperature and shift the power band down)... But like twocycler says the loss from adjusting the ignition timing to increase exhaust temperature probably (not definite yet) outweighs the gain from then wider power band (note that if exhaust temp is varied rapidly there will be no loss in peak power, just peak power over a wider RPM range)...

In terms of ignition timing ... the best reference I can find suggests that time-to-peak-pressure (after spark ignition) decreases with RPM (due to increasing turbulance in the intake air). However the decrease in time-to-peak pressure does not compensate fully for the inreasing RPM. To clarify if you want to keep peak pressure at 10 deg ATDC (after top dead centre) you have to advance (fire spark earlier) the ignition. By advancing the ignition more the peak pressure occurs earlier in the cycle, say at 5 deg ATDC when the combined pressure from the combustion and the fact it is occuring in a smaller volume can produce detonation.

At very high RPM (over 7000) the increase in turbulance is greater, so infact you have to retard the ignition to keep peak-pressure at 10 deg ATDC...

Okay this all makes sense, as what you want is to keep peak-pressure at 10 deg ATDC - which is where max- energy transfer occurs. This has nothing to do exhaust gas temperatue. Infact exhaust gasses should get hotter simply due to increasing RPM, as exhaust gases are released sooner after ignition. The temperature difference in exhaust gasses between say 7000 and 10000 RPM, if of the order of a 100 deg C, would widen the powerband by 2000RPM...

Something to think about...

Cannon Fodder MOO
04-30-2003, 06:59 AM
after reading your last item i'm thinking about retracting my above statement. :p

when I read it I was reminded that my education was that you advanced the timing at higher revs, ie you "bring forward" (advance the timing) as revs increase.

if knocking is a product of too much advance then too much advance is when you experience combustion before TDC...
...at least I think so!

so now I'm more confused! :lol:

i'm hoping that it's just a matter of semantics and the intent is still correct.

I think the vacum advance actually counters/retards the mechanical advance at low revs. Interesting.

methinks that advance is making the plug fire sooner but I'm still happy to say I'm wrong cos I can easily see people say that if the plug fires earlier the timing must be retarded. For now i'll stay with what pa taught me ;)

anyways I might re-read your last post on the weekend when I have more time, quite interesting. I do know that sound travels BETTER through more dense materials but travels FASTER through less dense materials. oh i think it's about 330 metres per second at 25 degrees with "normal" humididty whatever they set that at.

maybe you need a two stroke theory sub forum 8o

04-30-2003, 07:27 AM
I think what is confusing is that ignition occurs before top dead centre ... but there is then a more or less constant delay before combustion starts. From combustion start (considered to be when the pressure in the cylinder starts to be greater than just the pressure due to compression) to peak preasure is not instant - but takes a little time, at low RPM ignition occurs about 10 degrees before top dead centre, at top dead centre pressure is just a little higher that the pressure due to compression, but is rising rapidly. By 10 degrees after topdead centre the pressure in the cylinder is at its maximum, and does work by pushing the piston - until the exhaust port opens some time around 90 degrees after top dead centre.

Ignition always occurs before TDC, but that is to get peak-pressure at about 10 degress after TDC. Advancing the ignition means firing the sparc earlier in the cycle - maybe upto 30 degrees before top dead centre. I guess retarding may be a bit of a bad name, as the ignition never retards much further than the initial 10 degrees before TDC. It may be better to say that ignition advances more as RPM increases, to a maximum advance around 7000 RPM, then the advance decreases up to max RPM...

ysr star
04-30-2003, 11:17 PM
So I spent part of the day messing with the ignition of a RS50 on the dyno. It was very cool to change the static timing and see the changes it made. This was a fixed curve ignition.

At about stock spec there was good area under the curve (calculus flashback) that was nice and gradual and spaned to about 13.5K Rpm with a slow drop off.

As I advanced it by up to four degrees, the peak hp grew, but it fell off far sooner after peak power, and had less power leading up to the peak. There was less and less area under the curve as the ignition was advanced. The slope of the line was greater the further the ignition was advanced.

It was very cool to watch the changes and have a good graphical representation. If only I could change the actual ignition curve at say 500 rpm intervals, there could be power everywhere!

05-01-2003, 03:17 AM
Okay, so the theory and experiment agree (although it would be nice to see the results of a 1 or 2 degree retarding of the timing) - Basically

- Advancing produces a stronger mid range, and increases the 'hit' at the beginning of the power band, by allowing more time for peak-pressure to do work on the piston before the exhaust port opens. But this is over taken by the turbulance (also called maximum-squish-velocity) at high RPM ( > 7000 ), and too much advance will cause peak pressure to happen too early - possibly before TDC, causing detonation or worse...

- Retarding the timing at high RPM, causes heat to tranfer from the cylinder to the pipe, keeping the cylinder cooler, and making supe peak-pressure occurs long enough after TDC to preven detonation. The extra heat in the pipe causes the tuned frequency to move upward - broadening the power-band at the top end, and allowing the engine to over-rev more easily.

micah apriliaforum com
05-01-2003, 06:41 PM
In four stroke motors it is usually true that you will have to increase the number of degrees before top dead center as rpm rises. My Yamaha FZ750 made best power at 10,500 rpm with (OMG) timing at approx 47 degrees BTDC. It is not uncommon in very high rpm two stroke motors to use timing in the zero degree range as the squish band is coming into play and creating a huge amount of turbulence in the combustion chamber which allows the fire to spread very quickly consuming all available fuel and oxygen. The only thing we are shooting for with timing normally is to acheive the peak pressure in the combustion chamber at 12 to 14 degrees ATDC.....this allows for the maximum break mean effective pressure on the crank and on to the back tire. In two strokes you can "trick" the pipe into a longer period of resonance than would normally be possible by changing the temp/density of the charge not yet bled out of the expansion chamber.....another way of doing this is thermo wrap on the chamber....another is variable stinger diameters...another is water injection. With really sofisticated ignition systems which are truely programmable you can get the best of all worlds by giving best advance up to peak torque and then tapering into retard at higher rpm to keep the supercharging effect of the expansion chamber in play. The difference this makes is way bigger than I first expected. An example form my RS50 equipped with a PVL digital ignition can be seen in the following graphs. This is the same bike Jon has been working with and we are both shocked at the possibilities. In the graph The peak bhp line is at approx 27-28 BTDC (static) and the next line with the killer overrev is at about 23-24 degrees BTDC. Notice the small sacrifice that was made in the peak power run for the far greater high rpm power with only a change of timing in the 4 degree range. After the timing change the bike revved out to 14,000 rpm where it could not even pull 13,500 before. This is the advantage of owning a dyno and a PVL ignition system. That dyno was the best $30,000 I have ever spent! This 4 degree change made a difference of over 10 bhp at 13,000 rpm and 9 bhp at 13,500 rpm on a 50cc bike! There are big gains coming when I buy a RS250 in the next few months. If anyone wants to donate one I will give it back better than ever in a year!....at a susbstancial savings in partsand labor.

<img src="http://www.apriliaforum.com/micahupload/igntimingcomp.jpg" style="border:0;"/>

05-02-2003, 04:08 AM
So, what points was the 4 degree retard applied at, just 12,500 RPM and above? Presumably the advance before peak power is already as far advanced as you can safely go?

Looking at the graphs, if you have a 2000 RPM gear shift interval, you will gain significantly with the improved overrev, using 11000-13000 instead of 10000-12000.

As for thermo wrap, that wont quite do it as it will increase exhaust temperature at all RPM, reducing power below the exhaust frequency. What is needed to broaden the powerband is to lower exhaust temp below the resonant frequency (eg water injection, timing advance) and to raise it above (timing retard). I guess in reality you are not going to 'over advance' in the mid-range because of the risk of high pressure detonantion (although maximum risk of detination is at maximum advance AND max RPM, so maybe a little extra advance in the mid range, just before the pipe comes on-song may be possible)

Also I was thinking about the power valves, which adjust exhaust tuning, there are tree different tunings on an RS250, so it may be that given an ignition curve that hits 12-14 deg ATDC at all RPM you may want to advance a little just below first resonance, then retard above to first stage power valve opening, advance again below second resonance, and retard above to final stage power valve opening, from there advance to main resonance, retard after - remembering that these advances and retardings are relative to the 'normal' curve (where the normal curve aims to get peak-pressure at 12-14 deg ATDC)... This requires knowing the diffenent exhaust resonances - I suppose this could be measured by locking the powervalve into each different position and doing a dyno run...

I don't like the idea of water-injection as you have to have a water-tank ... which would run out. I was thinking whether injecting pressurised cold air could be effective in cooling exhaust temperatures - maybe running a little electric air compressor off the battery, and blowing it into the exhaust header below exhaust resonance. I don't know whether the cooling effect would be enough to make any appreciable difference. You would also need some way of measuring RPM to switch the compressor on and off... some aftermarket ECUs have additional control lines which could be used for this purpose...

I'm starting to think some appreciable gains can be made using some/all of the above... Are there any after market ECUs that can be fitted to an RS250, with power valve control? What functions are likely to be missing if I did this (speedo? Rev counter? - does the SAPC control any dash functions)... It might be worthwhile taking a look at the MCU chip in the SAPC box to see if it is a standard part - if it is I have some equipment at work that can read the program off the chip (providing it can be removed fairly easily). I might have to take the lid off my SAPC box and have a look...

Cannon Fodder MOO
05-02-2003, 07:59 AM
I'd just like to say WOW! :smokin:

what a thread. I'll be posting/emailing you guys when I learn about two stroke tuning/port mapping. My learning will be purely theoretical as i won't be doing any porting, just learning more about the maths of two strokes. I'm hot and cold so don't expect anything too soon :p

Oh and if all this is possible can you play with the ignition curve to get more midrange torque? I haven't owned one yet, only ridden one for a couple of hours but after the falco the power range does seem small :eek: . But I suppose thats why i want one :lol: it'll teach me to ride again

micah apriliaforum com
05-02-2003, 10:42 AM
With the PVL Digital ignition in the this test the only change made was to static timing. I do not yet have the programmable ignition in stock ($1200). If I had the true programmable ignition unit then I could have merged those two curves into one killer curve!

I agree totally about thermo wrap. I only mentioned it because it is so friggin common. It does have a bulk effect on resonance...especially on a long straight.

Powervalves effect cylinder timing....not pipe timing. This can have a dramatic effect as well. In the graph below (again a 50cc RS50 race engine) you will see all parameter are the same with the exception of port timing on the exhaust. The RS50 does not use powervalves. In this comparison I raised the exhaust port 3mm to shift the cylinder resonance into a range that better suited the Jolly Moto race pipe.

<img src="http://www.apriliaforum.com/micahupload/port%20timing.jpg" style="border:0;"/>

In this comparison the same motor has the same carb/jetting/pipe and ignition timing. The transfers have slightly longer duration and the exhaust port has a massive amount more duration. If I had the option use a powervalve I could keep the lower rpm horsepower and the high rpm power as well. Below 9000 rpm the non ported cylinder has a big advantage....above 9000 rpm the ported cylinder has a huge advantage.

Now onto water injection....it would be a great idea for a sprint bike and system are everywhere for use with snowmobiles and watercraft. Even very high tech systems are only about $800 with lots of options. For presssurized gas injection I would use something inert like nitrogen....air injection would surely cause jetting/mixture issues as gases in the pipe are stuffed back into the cylinder in the reflected wave. (supercharging effect). I like the idea of a high pressure gas system in theory it should work but will probably require a large volume of gas to effectively cool the gases in the pipe. Water is free and plentiful.

I know they are not currently in vouge but for powervalve actuation I would prefer something like the rotax rave pnuematic valves. They are tuneable with adjustable spring preload and take up very little space.

I have never seen a programmable ignition for the RS250 Aprilia. I know that something for another 90 degree two stroke could be adapted....probably with an output for water injection and or servo controlled power valves. PVL probably makes something along these lines. Who knows it may be possible to use a modded stock ecu (this is far from my expertise). It already has servo control outputs and other outputs for the air and fuel solenoids.

BTW: This really is the most exciting thread in a long time on the 250 board. Maybe Foxter could chime in with some experienced knowledge.

05-02-2003, 11:24 AM
Well, in theory the way to get more mid-range is to cool the exhaust,
but also remember that the powervalves come into play on the RS250, so its not that simple ... if the first stage power valve opens at 6000rpm, and second stage at 9000 rpm, with exhaust resonanance at 8000rpm, then a little extra advance from say 7000-8000 would cool the exhaust and bring the exhaust on song a bit early, then a little retard from 8000-9000 would extend the benefit of the exhaust a bit further. The exact RPMs where the changes would happen depend on when exactly the power valves open, and what frequencies the exhaust is tuned to.

05-02-2003, 11:53 AM
Actually power valves effect more than cylinder timing (effective stroke) they also effect the exhaust gas velocity, and exhaust gas temperature.

They also affect the tuning - if like the RS250 valve the second stage covers about 2mm and the third another 5mm, effectively shifting the port 7mm down the cylinder - the effect is that the port opens later and closes earlier. This looks to the exhaust as if the engine were actually travelling at higher RPM. To know whether we want to heat or cool the exhuast at any RPM all we have to do is work out whether the pulse is arriving back at the port just to early or just too late to catch the port closing... To make this clearer, if the pipe istuned to 10,000 RPM the port is open for 50% (eg at 90 deg ATDC) of the time, and we change this to 44% (100 deg ATDC) the exhaust is now tuned to 8,800 RPM, so if the valve opens at 9000RPM and changes port opening buy 10 degres, you would just catch the resonant peak before the valve opens, unfortunately ignition with 500RPM intervals would be too innacurate to retard the ignition for the 200 RPMs for which it would be of benefit.

It should be possible to work all this out theoretically - all you need is the exhaust frequency with the power-valve fully open, plus the angle (ATDC) the port opens, with each diffenent power valve position.

micah apriliaforum com
05-03-2003, 07:46 AM
When I get my RS250 in a couple of months I will start publishing dyno results for all these possibilities. Then we can compare theory with practice.

05-03-2003, 11:37 AM
Now that really would be cool!

The other thing I was thinking is that if max-squish-velocity affects timing (more max-squish-velocity = less ignition advance needed) it may be possible to alter the head shape without changing the timing to produce more power at the top end - all you really need to know is whether there is too much or too little advance on the standard ignition (you never know it might me just right - but I doubt it)

04-04-2016, 11:57 PM
So whats this mean to me on my modified low compression dish piston to high compression dpme top i think retardation would benifit me. My crank rotates clockwise so t f t f t f timing t f fire.
To the right with the pickup advances timing to the rear retards

04-05-2016, 03:52 PM
Shame this thread died on it arse.. Was hoping to find out the dyno results from old mate and have son links to best settings etc

2T Institute
04-06-2016, 08:00 AM
-Holy thread revival Batman........................some names there Keenan who spent mega bucks (Ti pipes $2500 ignition etc etc) on his banshee RS then promptly disappeared off the internet( I suspect female intervention ;) ) and Rocky Stargel who ran 2 Aprilia RSV 250 GP bikes in the USA. The thread seems about 10yrs or maybe older otherwise I would have pointed out retarding the ignition makes the exhaust gas hotter not colder and a few other things in those posts not quite right.

04-06-2016, 03:04 PM
Women intervention haha. Well I'm tip toeing around it at the moment. I started with hey babe I need $4k engine needs rebuild..got ear ache for a week and said okay. I will compromise and spend half now and half later... I have come off poor handling bikes last 2 being an fz8 I put r1 cams in and mt09. The RS is by far best handling bike I have ridden. But I do get tank slappers.. Some violent some not so... Happens every ride at least once