Basic Ignition Table tuning question...

[ewflyer]

I've got my project running pretty well ('93 Suzuki GSF400 Bandit). The element I'm the least knowledgable/confident about is the Ignition Control.

Right now my Ignition Advance table is built with a maximum advance of 38 degrees (at light-throttle cruising) and 33 degrees (for WOT situations). I chose these numbers with the intention of staying a little toward the conservative side, hoping to avoid any chance of detonation/knock.

I did a pretty good bit of research on the ignition advance numbers of small-displacement, high revving motorcycle engines that the various Japanese manufacturers built during the late '80s and throughout the '90s. In every single case I found the maximum advance listed as anywhere from 38 degrees BTDC to 45 degrees BTDC.

Here's the question: Why would the Japanese manufacturers use such large advance numbers?

When I broaden my research out to the general scientific concepts and reasoning behind ignition advance I get something like 30 to 35 for the maximum degrees of ignition advance.

[grom_e30]

on my bike i run between 50-60 btdc on very bottom of the ignition table i got the numbers from a stock ecu file.

[ewflyer]

Wow, that's a lot of timing advance. Really, I've never heard of an engine using that amount.

What sort of motorcycle is this? Setting it that far seems like a pretty fearless move to me. Either that or a very technically well-informed move.

I'd love to figure out exactly where to set my advance without putting the engine at risk. Do you have any suggestions or recommendations of information resources? Or technique?

Here's a first question for you: What is the difference (in degrees) between your light-load lean cruising and your high-load rich full throttle?

[grom_e30]

gsxr 1000 this is a stock timing map for a later gsxr 1000

[billr]

Those "big" numbers are fairly common, I think. It all depends on the engine design and where you are in the table. For instance, I have a BBC engine that gets "best vacuum" idle at 43 BTDC. It's unstable with that, of course, but 43 is where the mixture burn seems to be best, at that particular MAP and rpm.

[ewflyer]

Thanks for the inputs everybody. I used them as a springboard into more ignition timing research and ended up re-working my motorcycle's Ignition Table.

The results felt quite good when I rode the bike today. It feels like I'm very close to having a good table. I wanted to show you what I've done so you could let me know if you think I'm on target or not. (point out any flaws in this table or other mistakes I'm making, advice appreciated)

Overall this table represents an increase in timing advance settings across the entire operating envelope (except for idle). The motorcycle reacted so positively to these increases I'm now wondering how far it needs to go? I'm afraid of the possibility of taking this too far and damaging the engine with detonation/knock but I really want to get the timing set where it truly belongs. Are the "seat of the pants feel" and "rules-of-thumb" the only methods?

Seriously, I'm flying blind here...

[grom_e30]

your table is upside down you want the lower advance numbers up top and higher advance numbers on the bottom of the table.

[ewflyer]

your table is upside down you want the lower advance numbers up top and higher advance numbers on the bottom of the table.

Well, that's a revelation... Can you help me out further, with a bit of information to explain the principle?

I thought you needed to have slightly less timing advance corresponding to wherever the AFRs are targeted to be richer, and slightly more timing advance where the AFRs are targeted to be leaner. The reasoning for this (I think) is that (within the normal operational range of Air/Fuel ratios for a motorcycle or car engine) rich mixtures burn more quickly and lean mixtures burn more slowly.

The aim of compensating for these differing burn speeds with timing advance changes is to cause the Moment of Peak Cylinder Pressure to occur at TDC which will extract the maximum energy from the combustion even.

So I set the maximum advance numbers where I have the bike cruising at part-throttle with AFRs targeted at 14.5 to 15.0 (highway cruising). And I set slightly less advance where I've got 12.5 to 13.0 for making maximum horsepower/torque.

But like I said, I'm all-theory-no-experience.

[grom_e30]

normally you would have high numbers at the bottom of the table where you would be in overrun.

[ewflyer]

normally you would have high numbers at the bottom of the table where you would be in overrun.

Okay, You mean that very bottom row, the one at 50 kPa? I was wondering what to put into that row because, as you just pointed out, the engine only goes there when I release the throttle and the engine pulls a bunch of vacuum. The engine idles at 62 kPa.

What should I put into that row? Should I just copy what's in the row above?

Thanks for the help.

Greg

[Laminar]

Here's the question: Why would the Japanese manufacturers use such large advance numbers?

Here's a possibility. In general, the combustion flame front propagates at a rate that's loosely correlated with crankshaft rotational speed. As engine speed increases, the combustion mixture becomes more turbulent and the flame front propagates across the mixture more quickly. However, this relationship isn't direct.

Put simply, to get maximum power out of the engine, the cylinder pressure under combustion should be highest when the piston is pushing its rod at the most mechanically advantageous position. If your engine is spinning very fast, you need to start the combustion event sooner so that the flame has time to propagate and increase cylinder pressure to maximum by the time your crank is in the right position to accept it.

So with RPMs pushing 12000-15000, depending on head, piston, and combustion chamber design, you may need to start combustion sooner for max power. Or at least that's how it works out in my head.

[grom_e30]

Okay, You mean that very bottom row, the one at 50 kPa? I was wondering what to put into that row because, as you just pointed out, the engine only goes there when I release the throttle and the engine pulls a bunch of vacuum. The engine idles at 62 kPa.

What should I put into that row? Should I just copy what's in the row above?

Thanks for the help.

Greg

on the 4000-13500 on the 50+60kpa rows i would copy the values in the 63kpa row that will give you a more normal looking table