October 15, 2009

Transcript:

Hi , it’s Haitham again and this is another one of our.. how-to videos around the Power calculator on Supercharger Performance .com

What I’m going to talk about today…
is timing advance and tuning your ignition curve.

First we’re going to talk about some of the underlying concepts of ignition timing and timing advance

Then we’re going to look at some simulated timing curves

Then we’re going to summarize with some insights on how you can use this information to increase your horsepower by advancing or retarding timing, in the right places to gain more power.

First of all, let’s look at a 4 stroke engine animation …

This is top dead center… when the piston is all the way up the bore
This is bottom dead center … when the piston is all way down the bore.

Usually when when we talk about timing advance, we talk about ‘B.. T.. D.. C.. ‘ or before top dead center.

Usually the spark plug ignites the mixture before the piston reaches top dead center in the compression stroke.

Now the reason why we fire the plug ‘early’…
is that the mixture of air and fuel (and possibly water, methanol, or nitrous) takes some time burn…
and so the flame front takes some time to travel outwards and consume all of the air inside this top volume (comprised of the cylinder head volume, and the piston surface volume if the piston is dished)…

once the flame has consumed a large portion of the air fuel mixture… this flame, trapped between the piston and the cylinder head, creates ‘peak cylinder pressure’ and it is this cylinder pressure and expanding flame that pushes down on the piston making it rotate.

Now the trick here is that you have to synchronize the piston rotation, with the flame front burn rate so that you can hit the piston with peak cylinder pressure just after it cross top dead center and thus deliver ALL the force of the combustion into the rotation of the engine.

If you advance timing too much, you catch the piston on it’s way up and slow down it’s movement losing power
If you retard the timing too much, the piston outruns the flame front and very little power is transfered from the combustion into rotation

So there is a ‘perfect’ timing setting based on these two things:

piston speed: which is affected by rpm, and by Rod/Stroke ratio
flame front travel speed: which is affected by factors like mixture density, fuel to alcohol ratio, compression ratio…etc

Now let’s talk a little bit about the two main factors affecting timing advance:

The first is RPM. As RPM increases, the piston speed increases.

The thing is that the piston speed increases linearly with RPM … but the flame front travel speed only increases slightly with RPM due to more turbulance in the cylinder and a better mixture of air and fuel which allows the ‘fractal’ moving flame front to travel faster.

So going from 700 rpms to 7000 rpms, the piston increases it’s speed by a factor of 10, but the flame front only increases it’s speed by about a factor of 3.

So in a sense the piston is OUTrunning the flame front, and to re-synchronize the mixture so that we can catch the piston at top dead center we need to further advance the ignition timing.

To make up for this effect, the timing advance at 7000 rpms needs to be about 3 times the timing advance at idle and that’s how ‘mechanical timing’ came to be starting with a base timing of something like 10 degrees BTDC and growing out to 32 BTDC near redline if we’re talking for example about a large bore engine).

The second factor is mixture density. Which is typically measured by the car’s ECU with RPM, Flow and temperature sensors… But for this example we’re going to measure it in terms of volumetric efficiency since we can calculate that figure.

As the mixture increases in density, then so does the number of air and fuel molecules available for the flame front to expand outwards. This denser mixture allows the flame front to travel faster, as is typically the case with forced induction engines such as turbocharged and supercharged vehicles that cram more air and fuel into the cylinder.

If we leave this mixture alone at stock timing, then the air fuel mixture will out-accelerate the piston and catch it BEFORE top dead center which slows the engine rotation and exerts power rather than making it.

So as mixture density increases we retard timing to catch the piston once again just as it crosses top dead center. As the mixture density decreases, we advance timing to prevent the piston from outrunning the now slower moving… & less-dense mixture.

Of course there are other things that affect flame front travel speed besides mixture density…

such as:
octane rating (higher octane fuels burn slower)
the presence of burn accelerators such as nitrous oxide and oxygenated alcoholic fuels like ethanol and methanol,
or the presence of flame retardants such as water injection and high humidity.

Now that we have a basic understanding of timing curves, let me show you some simulated timing curves based on the 5.7 liter LS1 engine.

Now let’s talk about the advantages of knowing this information…

Dyno time
Different VE curves for modified cars
Not leaving any power on the table (nitrous or superchargers)

____________________________________________

The Apexi S-ITC is an older generation piggy back controller. The S-ITC stands for Super Ignition Timing Controller, and had a +/-15 degree advance/retard setting adjustable at 5 RPM points between idle and 7000 rpms.

At the time when this box was released, most timing controllers were either basic boost based retards such as the MSD BTM (boost timing master) or a fixed single externally triggered retard box for nitrous activation…etc

So, at the time, this product was much more advanced than the cruder forms of ignition tuning available by competitors. However, this product sold very poorly and was quickly discontinued.

The primary reason for the product’s failure was not the product itself, but rather lack of information in the general community about proper ignition tuning and the power potential that was being left on the table with un-tuned cars.

I hope that this how-to video here gives you a better idea about ignition tuning (since very few people actually discuss this aspect of supercharger performance and engine tuning in general). Even though the tools have changed over the years (and now we have full 16 X 16 timing maps that are 100% tunable to your desired timing), the basic theory remains the same, and the thirst in this community for this information is still there.