Ignition curve retard

Keith Archambeau · Posted: Thu Aug 16, 2001 8:46 am Post subject: Ignition curve retard

As the rpms increase the ignition advances to a certain point. Then the curves start to retard. (less advance). Due to flame travel speed shouldn't the timing keep advancing as rpms increase? I know this causes detonation.
And helps piston ect cool. What is phiscaly happening here, MSV? and at what point(rpm) do we need to start the retarding? By how much?

Pete Muller · Posted: Thu Aug 16, 2001 8:56 am Post subject: Ignition curve retard

The ignition needs to retard because it "tricks" the exhaust pipe into thinking it's shorter than it is.

In a nutshell: there is more to be gained by keeping the exhaust system closer to being in tune... than is lost by igniting the mixture "too late".

Keith Archambeau · Posted: Thu Aug 16, 2001 11:55 am Post subject: Ignition curve retard

AaHa! Thanks Pete! Anyone care to elaborate?

Pete Muller · Posted: Thu Aug 16, 2001 8:11 pm Post subject: Ignition curve retard

Keith,

Since this topic is getting so many responses

I'll add in a bit more.

If you could remove the exhaust pipe from the equation, I suspect the 3 items which would effect spark timing would be:
- combustion chamber pressure
- squish velocity
- time (or lack of...

)

I'd say that logically (once again, with no consideration for the exhaust system), there would be a point somewhere between peak torque and peak power where the timing would be "least advanced" in time (not in degrees). It's possible that this point might be shifted up a tad in the rev range depending on the shape/design of the combustion_chamber/squish_band (because of the squish velocity increasing faster than the cylinder_pressure/filling_efficiency is dropping).

Below that point in the rev range... the timing could probably stand to be advanced (I'm still referring to the firing point in terms of time, not degrees) due to very poor filling efficiency at low revs.

I also suspect that there is a point beyond highest potential flame speed (that: "peak_torque/peak_power/increasing_squish_velocity" point) where the timing could probably be advanced again as the revs climb - though if the effects of the [increasing] squish velocity are more than the decreasing cylinder-filling efficiency... it's possible that the timing needs to be retarded the entire way. Exhaust pipe aside though... I personally doubt this is the case.

Remember though that if you fire the plug at the same point in degrees... it's already like retarding the timing as the revs increase... since the same number of degrees represents less and less time.

Now... once the exhaust pipe comes into the picture... all bets are off!

The optimum firing point now becomes some balance of all the above plus firing at a point that sends the pressure and sonic pulses down the pipe with the best amplitude for a given rpm.

Pete

[ August 17, 2001: Message edited by: Pete Muller ]

Rick Blood · Posted: Thu Aug 16, 2001 8:33 pm Post subject: Ignition curve retard

Pete,
This may be over simplified but...
The electrical pulse generated in the coil and signaled by the CDI takes the same amount of time to travel all the way to the plug and jump the gap regardless of RPM. It would seem that you would need to keep advancing the timing in order to keep up with the ever increasing piston speed but we know this is not true. Can you explain what is happening?

Pete Muller · Posted: Fri Aug 17, 2001 7:40 am Post subject: Ignition curve retard

quote:
Originally posted by Rick Blood:
Pete,
It would seem that you would need to keep advancing the timing in order to keep up with the ever increasing piston speed but we know this is not true. Can you explain what is happening?

I thought I did above.

I slightly rewrote a couple sentences that might clear it up a bit (or not

).

PM

Andy Szyszkowski · Joined: 01 Aug 2001 Posts: 149

All of this sounds somewhat familiar to some of the challenges I encountered in the early days (around 1968) of turbocharging automobiles, where the timing events were very critical as a result of changing compression pressures. Back then, we didn't have the systems of control as are available today.

Andy

Rodney Ebersole · Posted: Sat Aug 18, 2001 6:04 am Post subject: Ignition curve retard

Good stuff, I just wanted to say what I got out of it and see if I'm thinking in the right direction. A motor needs the spark timing to advance with RPM's and volume metric effiency untill it reaches it's VME then as RPM's increase over it's VME it would be best to retard with the decrease in VME much in the same curve as when VME rose from low RPM's? Or am I just lost?

lynn haddock · Posted: Sun Aug 26, 2001 5:41 am Post subject: Ignition curve retard

Hey Pete,
I think we need variable volume transfer passages, varable head pipe length and a movable/well sealing divergent cone.
This would seem to 'compliment' the really cool PI (commercially available..LOL) systems don't you think ?
A P4 will fit on the back of the seat won't it ??

Pete Muller · Posted: Sun Aug 26, 2001 8:58 am Post subject: Ignition curve retard

quote:
Originally posted by lynn haddock:
Hey Pete,
I think we need variable volume transfer passages, varable head pipe length and a movable/well sealing divergent cone.
This would seem to 'compliment' the really cool PI (commercially available..LOL) systems don't you think ?

LOL...

I had two of the above: a moveable, well-sealing divergent cone. In early 1977, I built an exhaust pipe for my McCulloch [enduro] kart that had 1½" of travel on the divergent cone, and about 4" of travel on the convergent cone. All the internal parts were hard-chromed and ground. It was very reliable.
There was one major problem: the carb could not keep up.

I needed a third hand, because I operated the front cone with my left hand and the rear with my right hand.

With some electronic fuel injection though... ... this could've been killer, as it had tremendous potential. As it was though, I had a hard time keeping ring lands on the piston.

Oh well... LOL

John Denman · Posted: Sun Aug 26, 2001 2:04 pm Post subject: Ignition curve retard

quote:
Originally posted by Keith Archambeau:
What is phiscaly happening here, MSV? and at what point(rpm) do we need to start the retarding? By how much?

The VME as identified above is where the retard begins, and well before the pipe needs to be fooled. Give or take a little (porting and other differences) it begins around 7000 RPM to retard, well before the usable range of a strong motor.

This varience is why its important to have a curve programmed to your motor configuration. The curve is especially important in the 9000-13000 RPM range. Typically above 13000 the curve returns to an advance to prevent overrevs.

A custom curve that works well in one motor may not work as well as a stock curve in another. To fully benefit from a PI takes a programmer and knowing what changes to dial in under different conditions.

Unfortunately most PI units have very expensive programmers, and no guidelines on curve optimization.