Calculating Ignition Advance...

Is there a way to quick estimate a safe range for advancing the timing (other than w/dyno)?

I've always used the advance to highest rpm/knock, then back off like 3-4 degrees for safety, then ride/check, adjust.

Were basically just increasing peak cylinder pressure, and hopefully restoring the proper torque curve position in worn out/older engines.(Still fine engines, just lower static compression than new...)

Can you use a static compression number and guesstimate what additional advance would bring that up to a more productive dynamic compression? Closer to new stock/slightly better?

Would save me some shop time...

Stumper?

:laugh:

Yo! What the...

Not a lot of people interested in tech topics

I don't think your idea of advancing the timing to restore performance will work.
The purpose of advancing the timing is to allow time for the charge to burn before the exhaust valve opens. And it seems to me that somewhere near 35 degrees BTDC is about as far as you can advance it without killing the power by going into the negative torque range. Negative torque is created by igniting the charge while the piston is still on the compression stroke, at a point that allows combustion pressure rise prior to TDC. In other words the expanding gases are pushing down on the piston while the crank is pushing up on the piston, causing a serious loss of power.

I'm interested in hearing about your experiences with worn out motors and timing. I have to go take care of a friends horses why he's out of town, but I'll be back on tonight.

By the way, if you want to increase dynamic compression, close the intake valves a little earlier.

steell wrote: Not a lot of people interested in tech topics

Yeah I know...

I don't think your idea of advancing the timing to restore performance will work.

Ah, but it does! Except it has the effect of transfering peak torque back slightly across the curve (moves left), to a lower rpm, and thus decreases max HP on a dyno. Its like a give and take thing. I can make the bike seem smoother/more powerful on take off, but it takes a notch out of max rpm performance.

The purpose of advancing the timing is to allow time for the charge to burn before the exhaust valve opens.

...right... take advantage of the most cylinder pressure that can be created, pushing down in the best window of travel/duration.

And it seems to me that somewhere near 35 degrees BTDC is about as far as you can advance it without killing the power by going into the negative torque range.

The J motors stock have 40 advance after 3.5k rpm, but seem to perform best with about 38... just my testing.

Negative torque is created by igniting the charge while the piston is still on the compression stroke, at a point that allows combustion pressure rise prior to TDC.

Yesy, cylinder pressure will rise crazy +heat! Been there... knock bang! Detonation.

In other words the expanding gases are pushing down on the piston while the crank is pushing up on the piston, causing a serious loss of power.

Right... peak power ideally should be place directly in center of best window/duration.

I'm interested in hearing about your experiences with worn out motors and timing. I have to go take care of a friends horses why he's out of town, but I'll be back on tonight.

Cool. :laugh:

By the way, if you want to increase dynamic compression, close the intake valves a little earlier.

Right on, how earlier? What kind of compression can you make before it starts to hurt output?

Thanks!

The optimum spark timing will depend on the rate of the flame development and propagation, and the length of the flame travel path across the combustion chamber. These depend on the engine design and operating conditions, and properties of the fuel, air and burned gas mixture.
MBT is the Maximum Brake Torque produced:

Cylinder pressure VS. Crank angle.



With a lower compression engine, the flamefront acts more like line #3/#5 in this pic, so we advance the timing, and move the graph to the left slightly to make the (#3 or #5) line, increase slightly in height (pressure), and move left under/into line #2 territory, thus restoring origional (hopefully) performance as closely as possible - far from detonation/pre-ignition.

Cheers!

Ah, I was thinking hp and not torque (and I should know better)

Sort of on topic:

The newer bikes run incredible compression ratios on regular gas due to the combustion chamber design, it's simply amazing. My 91 zx11 runs 11:1 on regular gas, and the new 600's run near 13:1 on pump gas (maybe premium though). Can you imagine any KZ motor running 12:1 compression on regular gas?
Tremendous strides in understanding combustion technology have taken place over the last 20 years.

As a practical matter, most people would think that a bike with more torque at lower rpm was more powerful than a bike with upper rpm hp.

How does the "advance till spark knock and retard 5 degrees" look on the dyno? Maybe that's all you need to do?

Messing with the cam timing could be a losing proposition, as the first thing you need to do is slot the cam sprockets.

This topic is of particular interest to me as I've converted the ZX11 to Coil On Plug with four individual coils with a MS2 driving them sequentially. The MS3 will do individual cylinder timing, imagine how crazy that could drive you :woohoo:

Adding initial advance my help the bottom end power, then limit the total advance by shortening the curve. Take 30 degrees of movement and remove 5 degrees if you have advanced 5. This will keep the total at the suggested 38, and no top end ill effect. Getting to the total advance quicker can really help as well on a modified motor, lots of overlap.

i see how u loose potential due to compression loss from age
.i understand why it gets better with less timing (few degrees of advance, smaller #), but ,do u suppose that there was
too much to begin with ? like the J , came with 40 but runs better at 38 ?
Maybe ma Kaw used your theory to begin with. find max advance then back it off 4 or so for safety or longevity or for poor gas????. now u have room to add 2 and still be happy ??
I like the idea of adjusting the cam timing but it is more labor some and again, u will lose a bit up top, but that may not matter on a daily runner..
U would only want to change it a couple degrees.
If u are going to take the cams out maybe an upgrade would be more noticeable, but more costly.

Right on. All good info here, and I still cant calculate advance :laugh:

Taking a real mathematical approach I started to come up with some rediculous formula, but it was huge and started to take up a few pages... not even worth it!

I suppose I'll stick to the tried and true method of old. Seems to work just fine.

Steell, that must be one bad ass bike, and yes it would probly drive me nuts.

I dont think I'll be slotting my cams any time soon, I would probly just drop in a new/adjustable pair.

As I can get some more info/talk to some racers, I will post more...

Thanks to all for the replies!

Motor Head wrote: Adding initial advance my help the bottom end power, then limit the total advance by shortening the curve. Take 30 degrees of movement and remove 5 degrees if you have advanced 5. This will keep the total at the suggested 38, and no top end ill effect. Getting to the total advance quicker can really help as well on a modified motor, lots of overlap.

Yes, this is one of several things I've done before, and it does work quite well.