Ignition Timing Discussion

On the topic of “blueprinting” everyone mentions the timing as one parameter that is often addressed. While probably not technically blueprinting, it is paying attention to every detail. What is not clear to me though, what does altering/adjusting timing actually do for the performance of this or any 2 stroke motor? Yes, it adjusts when the spark plug fires in relation to where the crank / piston is, but what does that do to the performance?

I haven’t raised a komit 100 since 1978, but I did pretty well. I set my timing at .060" BTDC. Most people set their timing that .085", some a little higher.

In retrospect, I think I would’ve changed a few things.

First off I’d have a air density gauge. On good days I would retard the timing some, on bad days I would advance it some. A little more octane on good days, the little less on bad days. A slightly hotter spark plug on bad days, a little colder on good days.

Remember, without an exhaust gas temperature gauge (EGT) you will not know if any of this stuff is working, so have one, EGT that is.

I would have both an air density gauge and a compression gauge, both of the highest quality. The compression gauge for obvious reasons, and the air density gauge so you’ll know what’s a good air day and a bad air day.

Every motor has an ignition advance limit, you will make more power each degree of advance you put in until you hit that limit, assuming you have enough octane to not detonate.

1 Like

Here’s one basic way of looking at it… Generally you want the peak force on the piston/crank to occur at 20deg ATDC. Ignition timing is one way of adjusting that. Of course, 20deg is a moving target because it changes with respect to time as the engine speed changes.

Up to a point, you’ll often want as much ignition advance as your setup will handle. Again, speaking very generally here. Factors that limit this are the fuel you’re using (it’s octane), compression ratio and squish area. When you hit the limit for timing… you’ll start to get pre ignition and/or detonation.

One thing to keep in mind for others reading this… Measurement of advance in distance varies with stroke and conrod length. That is to say, you can’t compare timing between engines of different stroke and/or rod length… At least not without a conversion. Other variables should be taken into consideration too, fuel, compression etc.

Timing is a pretty big and interesting topic in and of itself so I’ll break this one off.

What do you mean by ATDC? I have usually understood it to be either Before TDC or After TDC and advanced timing is BTDC. I am assuming you don’t mean 20deg After TDC.

Usually timing is adjusted for max power but where does torque fit in to this?

I mean after for peak force. The spark is fired before as you say. But the peak force on the crank itself is after top dead center.

He’s correct and does mean after top dead center but he’s taking about peak force from the Flame event or whatever the technical term is for when the full energy of the bang happens, not where the timing is set.

when the engine is spinning at 16,000 rpm the ignition event triggers before top dead and there’s still a small delay before you get to the max force from full burn of that ignition event resulting in max force happening after top dead center.

Specific to KA, the safe bet is to set timing to the max .106 before top dead center and go with it. Most groups run high enough octane gas as spec to where you aren’t going to have to worry about detonation. The two KA’s I’ve had my hands on, from 2 different builders, both had the timing set right at max advance. That said, we are fortunate to have a reputable builder here who has suggested they have focused some on the acceleration characteristics of the engine with different timing settings (pull at the end of the straight vs off the corner) and there may be advantages of running less than max advance. That’s where paying someone for blueprinting, despite just really optimized settings on modern motors, gets you the expertise they gain from hours and hours of tweaking then dyno testing.


So to put what James and Andy said simply, changing your timing changes where your power band is. Less Advance gives an earlier rise in the curve, more Advance gives a later rise in the curve. Depending on the track, tight and twisty or long and flowing, you may want to adjust your timing accordingly to make the most of your power band.

1 Like

If that is the case how is this measured or accounted for? The tool for adjusting timing seems fairly simple so how is “peak force” figured?

Ignition timing is one way of adjusting that. Of course, 20deg is a moving target because it changes with respect to time as the engine speed changes.<<

James, I am also intrigued as to what mean by the above quote. What are other ways to change “peak force”?

Its not. At least not directly. The stopwatch and/or dyno curve have the final say.

Theory vs practice basically.

The peak force is a combination of cylinder pressure and crank angle. There’s mathematics out there to work that out, but it’s beyond my ability. I still count with my fingers :joy:

1 Like

Exhaust flex length is another way to make a subtle change to your power band. You can think of it like spark timing. After the spark it take X amount of time for burn to propagate through the combustion chamber and reach maximum pressure. By changing exhaust length on a two stroke, you change how long it takes for the return pressure wave to compact the cylinder just before the exhaust port closes. Like spark timing, its a moving target. The faster the engine is turning, the shorter the window for the returning pressure wave to reach the cylinder. Unlike exhaust length, spark timing is not limited by port closure, but instead by crank position. This is how power valves get around the issue of port closure at higher rpms. Alternatively, digital ignition systems get around the rpm variation by automatically adjusting the spark trigger point based on the rpm at any given moment and can also be programmed to limit rpm. Hence the expression, Banging off the Rev Limiter.

There are some other ways of changing the Peak Force like combustion volume, combustion chamber shape and compression ratio, but these days almost all of them are fixed by rule sets. Even still, many series have fixed ignition timing rules and it looks like manufacturers are moving away from flex pipes to single piece exhaust systems. Not sure if the rules have specified thickness of exhaust gaskets yet, as that would be the only other means to change exhaust length.

We’re not too far off from spec karts, spec wheels, spec tires etc. etc.

Why even race, award trophies based on qualifying speeds, no practice, just qualify and break out the beer. Fixed fuel, fixed oil, fixed compression, fixed spark plug& and heat range!

Stop the innovators, at all costs! Take the machine out of the equation, make it more like running and jumping. We’ve got to take the machine out of “man and machine”.
Darn those innovators, they think they’re so smart, I will show them.

You’ll be horrified to know some series even have those restrictions Al.

Most people want to drive, vs tune. It’s just how the world has gone as a whole. That said, vintage karting is pretty strong, laydowns have reached some relative stability, Daytona sees some cool stuff each year in unlimited and the UAS karts on dirt still do some crazy things.

I look spec at spec karting as a different kind of innovation. I enjoy the closeness of competition in some of the spec series where a second cover 45 or more drivers in qualifying, top ten covered by two tenths.

As they slowly remove the soul from racing.

1 Like

Al, I get your point but only to a point. Why do people race as a hobby? For me its the experience of driving, competing and community. While I enjoy working with my hands trying to make the machine better, faster, etc. I also have limited knowledge and to some degree ability particularly when it comes to engines.

So does that mean the guy that does have the knowledge and ability or worse, the financial means to pay someone that has that ability, to be faster? Innovation has always been a part of racing, to me, ignition timing (even though I don’t fully understand it) is no different than adjusting axles or seat location, its a known parameter to adjust.

However, someone finding or doing something that no one else has while may not be cheating it creates an uneven playing field and for me that diminishes the hobby.

Robert, I’m not against that classes, I encourage him in fact, but they’re getting ridiculous. Except for a couple years running an open Mac, I ran nothing but spec classes. Thing is, spec in those days meant the engine. That was it.

I have nothing against a spec class for beginners, just don’t make the whole sport a spec class. I used to get excited about Indy racing, not anymore.

Just remember that everyone has a different perspective. Some prefer open racing where you can see innovations and mechanical minds at work. Some prefer more spec formats where you can see the drivers making the difference.

I happen to think Indy is some of the most exciting racing out there.

I agree in that a spec class allows one to focus more on there driving development, but disagree that it is only for beginners. In Karting, there are very few, if any completely “Spec” classes. Chassis are still open outside of rental racing and even in those as many of us know first hand, the engines are far from equal. In our local classes we currently have several classes with mixed motors. 100cc reed, TaG and Shifter all different engine options. Mostly in the younger classes you find spec engines.

Even in Indy you have different engine options, though there currently are only two. It stills leaves room for tuning and tinkering and teams are always trying to squeeze the last ounce of power out of them.

Indy racing is really cool, just not the oval tracks. Never been a huge fan of oval track layouts lol :joy:

I don’t know much about the whole ignition timing thing, I’m assuming it applies mainly to 2 stroke engines though. How do you adjust ignition timing and what exactly does it mean?

A post was split to a new topic: X30 Max Theoretical Power?