Ignition timing setting

[Bill Spruill]

The Tunning Glossary has the following: Ignition Advance/Retard - The advancing or retarding (in crank degrees) of ignition spark relative to the piston location in the cylinder. In performance applications, the goal is to set ignition timing such that peak cylinder pressure occurs at 16-18 degrees after top dead center (TDC).

Is this an error? I am rather sure that the ignition timing should be before top dead center (BTDC).

On that point, I am wondering about general parameters for retarding ignition for various RPMs. I have 2ltr., 4cyl., highly modded, Mitsu.(4g63T), pushing 300HP, 7K RPM, 20-23lbs boost. I now have ign. timing at -30*. I am going to dyno. and am wondering about setting the timing closer to TDC. Any suggestions? I can set timing for any 250RPM segment. Should I do each one under load? What would be a good base point by increment over the RPM range? Thanks for response. Bill Spruill

[Andereck]

Its not a misprint. Its talking about peak pressure, which takes some time to build after the spark event. Peak pressure needs to occur within a fairly narrow range to make the best torque. Stroke length and rod length determine where the most leverage will occur for maximun torque. After this maximum mechanical advantage position of the crankshaft combustion pressure looses it affect pretty rapidly. Depending on your effective compresstion ratio with boost you will want to move your spark advance around from stock to obtain the best torque on the dyno. More effective compression should speed up the combustion process and reduce the advance needs of the engine. Of course the reason why doesn't matter so much..you should play with your timing if you are fortunate enough to spend some time on the dyno. Timing changes can also affect your fuel needs too. Many times retarding the timing will call for less fuel, the opposite effect is true as well. Don't be afraid to make changes. Just try to make one change at a time if you can afford it. Dyno time is expensive.

If I assume you have a 9 to 1 motor with 23 lbs of boost you are in the 14.5:1 compressio range. You must be running very good gas, like C12 or C14. Max timing might end up around 16 or so under full boost. Maybe a bit more. MAP based timing is going to be most effective for you as the cylinder pressure will be higherst at lower/midrange rpm when the boost comes on. I would start with best unboosted timing, which depending on your engine may be 32 -36 degrees - only guessing and retard it with boost. 1 deg per psi might be a place to start.

[candymanjl]

the motor is spinning so quickly, especially at higher rpms, that even though the spark happens before TDC, the flame won't start pushing down on the piston until much later.

timing really does need to be mapped at different loads and rpms, too advanced and you won't gain power, you might even loose power, plus it's more stress on the motor. retarded and you'll loose power and your EGT's will go up. I'm not sure what you're using to tune with, but get your a/f ratio's tuned first, then start to play with the timing. really rich a/f ratios cause a slower burn and effect your ignition timing, so get one set before you mess with the other.

[gassr]

Originally Posted by Andereck

Depending on your effective compresstion ratio with boost you will want to move your spark advance around from stock to obtain the best torque on the dyno.... Timing changes can also affect your fuel needs too. Many times retarding the timing will call for less fuel, the opposite effect is true as well.

Originally Posted by candymanjl

... retarded and you'll loose power and your EGT's will go up. I'm not sure what you're using to tune with, but get your a/f ratio's tuned first, then start to play with the timing. really rich a/f ratios cause a slower burn and effect your ignition timing, so get one set before you mess with the other.

I view timing and AFR as going hand in hand. IOW, not just adjust timing after AFR adjustments, but AFR after timing adjustments as well.

Let's say your are not on a dyno, to adjust timing to best torque. AFAIC, second choice would be to monitor EGT, going for the lowest reading under power.

But let's say we are fuel octane 'challenged'. Now we need to retard more than optimum/desired. EGT will also be higher than desired. What changes in AFR can be expected when timing is retarded, especially more retard than best torque would require? Technically, if changes to AFR are observed, wouldn't these be considered artificial? How would one differentiate actual AFR from an artificial reading?

[klatinn]

Hi,

You should not see changes to indicated AFR UNLESS the ignition is retarded to a point where the burn is not completed at the time of exh. valve opening. The resulting (burning) rest gas can mix with existing exh. gas, resulting in flame quench and leftover O2 (and other stuff like HCs and CO).

Regards,
Klaus

[gassr]

With WBO2 sensors actually measuring the partial pressure of oxygen in the exhaust, would this have more of an effect on the AFR displayed, or is it primarily due to the actual increase of O2 in the exhaust stream, due to incomplete combustion? I take it, they are one and the same. Thanks, Klaus.

[legend4life]

Originally Posted by klatinn

Hi,

You should not see changes to indicated AFR UNLESS the ignition is retarded to a point where the burn is not completed at the time of exh. valve opening. The resulting (burning) rest gas can mix with existing exh. gas, resulting in flame quench and leftover O2 (and other stuff like HCs and CO).

Regards,
Klaus

With advancement and retarding of timing changing power output and therefore fuel usage to make a certain power, wouldn't it also affect AFR? If you set your AFR to say 12.0 at WOT, then tune timing and advance it until MBT and pick up power, won't the AFR lean out?

What's the best sequence or method of going back and forth to tune both AFR and timing achieve optimum settings for both?

[klatinn]

Hi

Boy, are you resurrecting an old thread .

No, timing affects power output, but not fuel usage. Why would it. You are only changing the time when you light the fuel in the process, not how much you are using. For example, if you short out your ignition system and tow the car with ECU running and engine rotated at decent RPM and WOT (in gear), you would use the same fuel as if the engine produced any power. The fuel would just come out unused in the tailpipe.
Timing effects how much of the energy in your fuel is converted to useful work instead of heat into the cooling system or exhaust.
Even in the best case with a spark ignited engine you are only getting about 25-28% of the energy contained in the fuel as work at the crankshaft.
At your typical part throttle operation it's more like 12-15%.

The usual way is to tune AFR first. Then tune timing. Then modify AFR in specific areas to possibly get a little more. Then adjust timing in those areas again to MBT.

Regards,
Klaus

[legend4life]

Originally Posted by klatinn

Hi

Boy, are you resurrecting an old thread .

No, timing affects power output, but not fuel usage. Why would it. You are only changing the time when you light the fuel in the process, not how much you are using.

The usual way is to tune AFR first. Then tune timing. Then modify AFR in specific areas to possibly get a little more. Then adjust timing in those areas again to MBT.

Regards,
Klaus

Guess it shows you that I searched .

I had been thinking about it like when you have a tuned AFR, do some modifications to increase airflow and therefore power, your AFR will lean out. But I guess it is not because you are making more power, it is because there is physically more air, leaning out the air to fuel ratio. Your explanation makes sense though and I understand.

"Modify AFR in specific areas to possibly get a little more"

Could you please elaborate on this, which specific areas and get a little more what?

Thanks!

[klatinn]

Hi,

This is only doable on a load dyno and with time to play. You move the AFRs around a bit to see if you get more torque in a specific cell.
For example some comb. chambers like to produce max torque at 13.x AFR at some load number. But let's say you can get there only with race gas. Playing around with the AFRs and timing at that area you get the best compromise between torque and octane when running pump gas. In the higher RPM areas, where you have lower VE, it might be possible to lean out more to the optimum for the chamber as the octane requirements change with rising RPM as well, depending on the chamber design. This of course assumes you are not cooling system limited.

Regards,
Klaus

[Bill Spruill]

I hope this is not too basic. I understand that the tuning cycle goes AFR and then Ignition Timing. I have an SDS eng. mgnt with fuel enrichment separately set for load. So, the normal AFR is set for varying RPM's only, and then I should set timing, right? After these parameters are satisfactorily met, should I then move to valve overlap (intake/exhaust advance-retard) combinations? (I have DOHC with adj. cam gears.) Thanks

[klatinn]

Hi,

No, you start with the cam timing with convervative AFR and ign. timing. Changing cam timing changes the VE curve of the engine. AFR and ign. tuning depend on that. Always change the most independent variable first.
Best cam timing depends on your goals and the intake and exhaust system of the engine. This changes VE
Fuel flow is relative to VE and load.
So adjust AFR next.
Ign. timing depends on VE, AFR and load.
So do that last.
Then go back and forth to fine-tune AFR and ign. timing.

Regards,
Klaus

[Bill Spruill]

Big THANKS.

[legend4life]

Is it worth it to get adjustable cam gears to play with cam timing on a high hp turbo SOHC motor?

When you tune cam timing first, then afr, then ign timing, then afr again, then ign timing again..........should you do this whole process just for one cell all at once and then move on to the next cell and do it all there again?

Or should you tune the entire map for cam timing first, then the entire map for afr, then the entire map for ign, then the entire map for afr again, then the entire map for ign again?

When you tune all of these you are basically just trying to get the maximum torque reading, or MBT, limited by knock right? What's the best sure way to know you are getting slight knock to back off without having reached peak torque?

Thanks Klaus, you are awesome!

[klatinn]

Hi,

No, with a turbo SOHC engine adjustable cam gears are not needed Unless you are experimentin with different cams). Cam timing changes how the engine breathes and can maximize that breathing by taking advantage of natural resonances in the intake and exhaust. Not applicable on a turbo. With compressed air (turbo) you can make a corpse breathe. There is some advantage of adjustable cams on a DOHC for a turbo.
Tune the cams (without turbo) as you would for a truck: low RPM torque peak. This reduces "apparent" turbo lag and helps initial spooling.
With cam timing changes on n/a (not electronicly controllable) you can have one of three mutuallly exclusive goals:

1. Max Power
(move torque peak up and maximize it at higher rpms)

2. Max torque at low end (low rpm torque peak for towing)

3. Economy
(leave intake open partially into compression stroke, or open earlier during exhaust stroke. Makes compression stroke essentially shorter than expansion and rises efficiency, but costs power and torque).

Of course your intake and exh. system and valve/port sizes determine what's achievable. A truck intake/exhaust won't make a truck engine into a high-rev race engine with only cam changes.

Cam tuning procedure:
Set a rich AFR of say 11 or 10.5 across the WOT row and keep low ign. advance.
With a speed density or alpha-n system you log AFR, RPM and TPS. Make WOT acceleration runs in high gear (at least 10 secs for a sweep, using brake as load if needed to stay at legal speeds). Select the WOT part of the run only and look in LogWorks at an x-y plot of AFR over RPM of the selected data.
Changes in cam timing that result in better breathing (or VE) will make AFR leaner because more air enters. Your goal is to make AFR as lean as possible in as many RPM ranges as possible, but especially in the area you are interrested in, depending on your goal.
With a MAF based system you log MAF voltage or inj. duty cycle instead of AFR.
Once you are done with that, don't touch cam timing again for further tuning.
After that you get your AFR into a more reasonable range.
Best for detecting knock are det-cans. Do a search.

Regards,
Klaus