[Bill Hanlon]

This is a theoretical question, not specific to any particular vehicle, but would apply to anything with a gasoline engine and a distributor.

I think that centrifugal advance in a distributor is used to make spark plugs fire sooner as engine speed increases to make up for the fixed time it takes for the gasoline/air mixture to burn. The net result is the peak cylinder pressure aligned with the piston’s position. If I’m wrong, tell me why and this thread can be closed out.

Using specs for my ’52 GMC centrifugal advance actually starts coming in just above idle (400 RPM), is 22 degrees at 1700 RPM and is “all in” at 32 degrees at 2600 rpm. So, if increasing the timing advance between the advance up to 2600 RPM is a good thing why isn’t continuing to advance timing above 2600 RPM not a good thing?

My question is even more obvious when you consider a more high performance engine such as a 1969 Camaro Z28 302 V8. Specs for that engine shows an “all in” of 36 degrees at 3000 RPM. Factory red line is 6000 RPM, so no additional advance for the upper half of the engine’s RPM range.

These numbers are similar on almost any automobile engine with a distributor.

Just like to know.

[Kenth]

Make sure you read the specifications in degrees and rpm at the crankshaft OR at the distributor.
If you read them at the distributor, they must be doubled at the crankshaft. Much of the confusion about timing curves seems to arise when you miss this.

HTH

[steve25]

Everything mainly hinges on.

Cylinder pressure and its rise to peak cylinder pressure, and how the Cam in part controls that.

This is further controlled and subdivided by.

1) the motor’s compression ratio.
1b) how fast or slow the chambers burn .
1c) mixer turbulence in the chamber.
1d) intake air temperature.
2) the motors stoke .

3) the rod ratio.
4) intake port velocity.
4b) the condition of the fuel when it’s released into the chamber, as in its state of atomization.

All of the above is in no particular order in terms of the percentage of effect each factor has on timing needs PER rpm.

Also the fuel itself.

If where talking gasoline for example all fuels of the same octane can burn at a different rate from one another depending on there blend and how it reacts to temperature.

Some blends produce more power with a given air temperature than others, so if you where to test them on the same day this would yet make for another effect of a change in cylinder pressure.

What most folks miss, and always needs to be kept in the forefront of your mind throughout this whole timing adjustment process is that once a given motor reaches its peak level of torque then without fail the cylinder heads in use have reached the starting point of being taped out.

So for example if your street / strip motor makes peak torque at 4500 rpm, then at best you will have another 1200 rpm before the level of hp that the motor produces starts to greatly nose over.

The absolute only reason at that point that the level of hp keeps rising is due to the cylinders firing more times per minute and the sonic limiting level of the heads in use has not been reached yet, and the level of hp friction the motor produces has not exceeded the level of hp that it’s making,

In terms of best power here’s how David Vizard who I respect highly states what he has found overall.

He states.

Optimal ignition curves are never simple.

The advance should rise with increasing rpm, but once the motor comes on the Cam ( the rpm where the motor is in its peak torque band and however wide that rpm band may be )
The timing needs to retard some and then advance to a fixed point.

As the motors rpm rises pasted peak torque
the advance (which may have already been at
Fixed point) needs to advance far more slowly because the motors volumetric efficiency is falling off rapidly.
This is because peak cylinder pressure has been reached.

Once a motor is pasted the rpm of peak torque then the heads in use have been taped out, so the VE is falling fast and so in turn is the cylinder pressure.
End of quote.

I don’t know if I may have helped answer your question with this or made more questions?

I hope this has helped some.

[mgarblik]

Originally Posted by steve25

Everything mainly hinges on.

Cylinder pressure and its rise to peak cylinder pressure, and how the Cam in part controls that.

This is further controlled and subdivided by.

1) the motor’s compression ratio.
1b) how fast or slow the chambers burn .
1c) mixer turbulence in the chamber.
1d) intake air temperature.
2) the motors stoke .

3) the rod ratio.
4) intake port velocity.
4b) the condition of the fuel when it’s released into the chamber, as in its state of atomization.

All of the above is in no particular order in terms of the percentage of effect each factor has on timing needs PER rpm.

Also the fuel itself.

If where talking gasoline for example all fuels of the same octane can burn at a different rate from one another depending on there blend and how it reacts to temperature.

Some blends produce more power with a given air temperature than others, so if you where to test them on the same day this would yet make for another effect of a change in cylinder pressure.

What most folks miss, and always needs to be kept in the forefront of your mind throughout this whole timing adjustment process is that once a given motor reaches its peak level of torque then without fail the cylinder heads in use have reached the starting point of being taped out.

So for example if your street / strip motor makes peak torque at 4500 rpm, then at best you will have another 1200 rpm before the level of hp that the motor produces starts to greatly nose over.

The absolute only reason at that point that the level of hp keeps rising is due to the cylinders firing more times per minute and the sonic limiting level of the heads in use has not been reached yet, and the level of hp friction the motor produces has not exceeded the level of hp that it’s making,

In terms of best power here’s how David Vizard who I respect highly states what he has found overall.

He states.

Optimal ignition curves are never simple.

The advance should rise with increasing rpm, but once the motor comes on the Cam ( the rpm where the motor is in its peak torque band and however wide that rpm band may be )
The timing needs to retard some and then advance to a fixed point.

As the motors rpm rises pasted peak torque
the advance (which may have already been at
Fixed point) needs to advance far more slowly because the motors volumetric efficiency is falling off rapidly.
This is because peak cylinder pressure has been reached.

Once a motor is pasted the rpm of peak torque then the heads in use have been taped out, so the VE is falling fast and so in turn is the cylinder pressure.
End of quote.

I don’t know if I may have helped answer your question with this or made more questions?

I hope this has helped some.

The explanation from David Vizard matches what we see on the dyno. The shape of the combustion chamber and the piston head also have a big influence on optimal ignition timing. Just recently finished a Chevy 409 engine and ran it on the dyno. A horrible, inefficient chamber as the chamber is in the piston and constantly moving. Even removing allot of the piston dome to lower the compression for modern fuel, it wanted 42 degrees of total timing. About 32-33 by 3000 RPM. About 36 at peak torque and then 42 at 5000 RPM. Max HP@ 6300. Large bore, short stroke, which also effects optimal timing.

[zippy]

To simplify some of what Steve said above. When you get higher into RPM, and 'on the cam', the engine fills more efficiently. Meaning a greater charge at 3400 than at 2200RPM The act of compression, causes heat. More charge volume, creates more pressure and more heat. Faster compressing, (more RPM) creates more heat in the charge. Hotter charge burns faster, and can self ignite (Detonation) So as we get higher into RPM, and closer to our best VE (Volumetric Efficiency), the charge gets hotter and thus easier to ignite meaning more advance is not needed at this point. But after VE drops again. Heads maxed out, one could start adding more timing slowly, but this is insainly hard to do with a distributor.

One fun fact/weird point. In my other hobbies, which involve 2 stroke engines. It is quite common to RETARD the timing at high RPM. Advance up to almost max RPM, then slightly retard it. On a 2 stroke, which has a tuned pipe which works on exhaust gas pulse timing to assist pulling fuel into engine. Retarding the timing increases the pipe temperature, which makes the pulse bounce back faster (hotter gasses move faster) This can actually increase the engines RPM even more than the standard VE would normally allow in some cases.

[steve25]

Yes, if your scrapping the bottom of the barrel for the last spec of hp then electronic timing control is needed, but you had better know how to tune your motor and stay away from needing a cart to pick up your engines expelled pieces off the road or track.
For street or street/ strip use keep tune your motor to be 12 hp away from the ultimate HP it can make.

[Sun Tuned]

Bill,
The boys here above, as usual have already nailed it down.

I would like to add youre correct, you have to advance the curve ahead and somewhat faster as the RPM goes up to “get ahead” somewhat of what’s happening. There is a fixed amount of time to complete one combustion cycle, as the rpm changes, theres less time to get that done as the speed goes higher.

However theres a law of diminishing returns with this. After a certain point advancing anymore just doesn’t produce the gains you get early on. In fact up top it’s advantageous to pull a couple degrees back out.

To illustrate a point. If you hook a gas analyzer up on say a 360 Hp 389 with an 068 cam…. And set the timing dead stock @ 6 btdc, and set the carb and get a hydrocarbon and CO reading,then you watch it and as you bring initial timing up towards 12 , youll actually see the HC go down and clean up a bit. Now obviously as you do that the rpm will come up and you’ll need to readjust carb just a bit and reset idle speed but it will clean that up from 6 degrees.

But as I said there’s a diminishing returns thing at play, if you go past 12, on towards 14,16 etc.. you’ll see the HC start climbing back up again…

So there’s a sweet spot for a particular setup, and you can get greedy.

Hope that helped.

[chiphead]

When adding timing past peak torque, how much and how fast should it come in?

Lets just use my engine for example, since I know it and its a common combination. Stock pistons, 2802 cam, 62 heads, RPM intake, 93 octane.

My combination likes 29 degrees at 3000, 32 degrees at about 3300 RPM at WOT. Does it make sense to add more timing later, like 34* at 3500 RPM? Should I add another 1 or 2 degrees at 4200 RPM? This is possible with the PI distributor.

What is the sign you've added too much timing beyond peak torque? Is it reduced power, surging, pepper on the plugs or what?

[Shiny]

Originally Posted by Sun Tuned

Bill,
The boys here above, as usual have already nailed it down.

I would like to add youre correct, you have to advance the curve ahead and somewhat faster as the RPM goes up to “get ahead” somewhat of what’s happening. There is a fixed amount of time to complete one combustion cycle, as the rpm changes, theres less time to get that done as the speed goes higher.

However theres a law of diminishing returns with this. After a certain point advancing anymore just doesn’t produce the gains you get early on. In fact up top it’s advantageous to pull a couple degrees back out.

To illustrate a point. If you hook a gas analyzer up on say a 360 Hp 389 with an 068 cam…. And set the timing dead stock @ 6 btdc, and set the carb and get a hydrocarbon and CO reading,then you watch it and as you bring initial timing up towards 12 , youll actually see the HC go down and clean up a bit. Now obviously as you do that the rpm will come up and you’ll need to readjust carb just a bit and reset idle speed but it will clean that up from 6 degrees.

But as I said there’s a diminishing returns thing at play, if you go past 12, on towards 14,16 etc.. you’ll see the HC start climbing back up again…

So there’s a sweet spot for a particular setup, and you can get greedy.

Hope that helped.

Just curious tuning question..

Do aftermarket digital ignition and/or FI systems optimize timing curves this way across the entire rpm range? I would expect so... but not sure it could happen in real time. In other words, can a fuel ratio sensor in an exhaust stream allow "real time" optimization as an engine is revving or is there a lag before the AFR stabilizes?

If there is a lag, which I expect, do these timing computers provide "learning" or "tuning" procedures where the optimum timing is measured by dwelling at rpm intervals to build a full "base curve" that is retained in memory and modified by load-sensitive changes (ie vacuum, temperatures, etc.)?

It would be interesting to see a graph of that type of "optimized" advance curve vs a stock mechanical curve for different levels of performance on a single engine platform.

[steve25]

Peak cylinder pressure will always take place at the point / rpm that peak torque is made.

From that point on in terms of increasing rpm ( theoretically at least ) cylinder pressure will not go up , increased rpm means that once again in theory peak cylinder pressure is just being reached more times per minute.

Of course in real life depending on how inefficient the burn rate becomes with added levels of rpm then more timing may need to be added to counter act that degrading burn rate, but that rate of added timing needed at that point from what I have seen / experienced is that 1 to 2 added degrees of timing per every 500 rpm gain is about the limit.

But as you know there are a ton of factors that affect the burn rate, and there are more contributing factors to burn rate as the rpm climbs.

[chiphead]

Originally Posted by Shiny

Just curious tuning question..

Do aftermarket digital ignition and/or FI systems optimize timing curves this way across the entire rpm range? I would expect so... but not sure it could happen in real time. In other words, can a fuel ratio sensor in an exhaust stream allow "real time" optimization as an engine is revving or is there a lag before the AFR stabilizes?

If there is a lag, which I expect, do these timing computers provide "learning" or "tuning" procedures where the optimum timing is measured by dwelling at rpm intervals to build a full "base curve" that is retained in memory and modified by load-sensitive changes (ie vacuum, temperatures, etc.)?

It would be interesting to see a graph of that type of "optimized" advance curve vs a stock mechanical curve for different levels of performance on a single engine platform.

The self-learning ECUs can fine-tune the fueling but not the ignition timing. The user is expected to enter a baseline timing map and some other open-loop parameters. Then the ECU takes that and develops a fuel map to keep the AFR within the desired setpoints across the speed/load range.

Since there are no knock sensors on retrofit applications, you have to use the old school methods of fine tuning the spark map. Seat of the pants, listening for detonation, reading spark plugs, peak MPH at a dragstrip or with a Dragy data acq system.

I've seen that when my 400 goes into audible detonation, the AFR goes very lean. I assume detonation consumes the fuel mixture, and that drives the reported AFR very lean. Adding fuel and pulling 2* of timing when the data log shows abnormal lean spikes is a good idea, especially when you hear the popcorn.

[Shiny]

Originally Posted by chiphead

The self-learning ECUs can fine-tune the fueling but not the ignition timing. The user is expected to enter a baseline timing map and some other open-loop parameters. Then the ECU takes that and develops a fuel map to keep the AFR within the desired setpoints across the speed/load range.

Since there are no knock sensors on retrofit applications, you have to use the old school methods of fine tuning the spark map. Seat of the pants, listening for detonation, reading spark plugs, peak MPH at a dragstrip or with a Dragy data acq system.

I've seen that when my 400 goes into audible detonation, the AFR goes very lean. I assume detonation consumes the fuel mixture, and that drives the reported AFR very lean. Adding fuel and pulling 2* of timing when the data log shows abnormal lean spikes is a good idea, especially when you hear the popcorn.

Thanks... I'm kind of surprised they don't have "tools" or tuning procedures to generate these spark maps.

It's not clear to me that optimum timing is max before ping. In other words, will max torque always occur at max timing without detonation? If so, then a knock sensor is probably a good tool for generating a spark map.

If not, it sounds complicated to optimize.

Do they make "in-car" dynomometers? Something like a torque sensor on the drive shaft that transmits to a receiver (like tire pressure monitors only using a strain gauge)? If not, sounds like a good opportunity for someone.

[chiphead]

There is the concept of "minimum timing for best torque" MBT, which is the minimum amount of timing that produces the highest TQ and HP output.

If you keep adding timing beyond MBT, there is no gain or the power rolls off. When you go beyond the point of no returns, you get into detonation. So the idea is to back off timing in small steps until performance falls off and then add 1-2* of timing. Ideally you make peak power before the combo gets into detonation.

That's the issue with these old engines and modern pump fuel, the MBT overlaps with some detonation conditions. So you have to reduce total timing to keep it out of detonation, but then the engine can't make the peak power its capable of. This is where the PI distributor shines. If you can figure out exactly where the loaded danger zone is, you can pull timing right in that RPM range but add it everywhere else. Can't do that with springs.

I assume most of the aftermarket ECUs and distributors have suggested timing tables and/or software routines that will build a timing table. However, its still up to the tuner to read the tea leaves.

The factory stuff with the knock sensors runs it right up to onset of detonation and then pulls it back. There are settings for how aggressively it pulls timing when knock is detected and how fast its added back once the knock goes away. But that's only possible with the feedback from the knock sensors.