Chris White

Since I have mentioned this in a couple of other posts I though it was about time to toss this out for other folks to ponder on.

One of my firm beliefs on working with modifications is that you need to apply a ‘systems’ approach. This means thinking about the whole system and how it will react as opposed to comparing item A and item B by some non real world test (such as flow benches). If you want to take a ‘systems’ approach the first step is identify the end result – and that is what I will cover here.

In effect, this is a form of reverse engineering. Instead of looking at what we are trying to add to the system (MAF, turbo, blow off valves and such) lets look at what we want to do in the end. When it comes down to it, we want to have the most efficient/powerful combustion possible. Sounds kind of simplistic but that’s it.

Back when the four cycle engine was first explained to all of us it was usually mentioned that the mixture was ignited when the piston reached Top Dead Center (TDC) and the resulting ‘explosion’ started pushing the piston down creating ‘power’.

A couple of facts to ponder –

• Gasoline burns slowly (Really - I mean slowly!)
• Cylinder pressure is what pushes down on the piston to create torque (good thing)
• At 900 RPM (idle) the crank rotates one degree in .18519 milliseconds
• At 6000 RPM (rounded out redline) the crank rotates one degree in .02779 milliseconds

Uncompressed atomized gasoline burns at about .5 meters per second. Yep, that’s about 1 ½ feet per second. As you compress it and heat it the rate of combustion (velocity of the flame front) increases. Adding swirl and/or turbulence in the combustion chamber also increases the rate of flame propagation. Different studies have given different answers - It comes out to somewhere around 15 m/s +/- 50% for a NA engine (remember that this is after it has been mechanically compressed in the cylinder).

What is our goal with all this? We want to figure out when the spark plug should fire in order to get the most power out of the intake charge that we have spent big $ to stuff into the cylinder.

So – if we look at time of combustion in degrees we can use the following calcs –

• Combustion chamber is 100mm in diameter – max distance for flame travel is 50mm (work with me – I am rounding things out here)
• At 15 m/s that 50mm takes 1.333ms for the flame front to transverse the entire combustion chamber and create peak cylinder pressure (this is a very simple model that does not take into account swirl, quench induced turbulence and the tooth fairy).
• In a normally aspirated engine peak cylinder pressure should take place from 12 to 15 degrees after TDC – a little later for turbo cars (Exact timing is dependent on boost).

Armed with this info we can figure out exactly where the timing should be set (yeah right). The math says that an NA engine running at 6000 rpm will need approximately 36 degrees of advance in order to reach peak cylinder pressure at 12 degrees after TDC. Oddly enough that is approximately correct. The turbo, on the other hand, is set at about 16 degrees before TDC at 6000. This is approximately correct if we assume that the further compression caused by boost has increased the flame front speed to 45 m/s.

This is a complete and utter assumption sine I can’t find any sources that can agree on real flame propagation speeds under pressure and temp variations. As it turns out the flame propagation speed will change dramatically during the combustion because the pressure and temps are going up, way up, during the combustion. In fact, the speed can get near mach 1 at the very end of combustion. Very weird things happen near mach 1….

Why do I worry about crap like this? Well – that’s because you can alter the flame propagation speeds with modifications. Flame propagations is affected by the following things –
• Turbulence in the chamber (this is mostly caused by the interaction of the quench area – very important in the 8v heads)
• Swirling in the chamber (this is the big difference between on the 16v heads – they are swirl designs – hence no quench area and a central spark plug)
• Here is a table about other things that can change the combustion process (requiring changes to ignition timing) -


Bore/stroke ratio - You wouldn't have thought so, but this has an effect on rate of compression and piston dwell time at TDC - A long stroke/small bore requires as much as 10° more advance than does an equal displacement engine with short stroke/big bore
Camshaft with more duration Improved at higher speed, worse at low speed - - Less advance at high speed, more at low speed
Combustion chamber shape - - Compact designs take less time to burn to farthest reaches Minimized for compact designs, maximized for in-piston chambers
Fuel atomization - Liquid fuel does not burn Fuel globules have to be atomized by combustion in other areas before they will burn Minimized when atomization is complete - atomization also improves with speed
Improved exhaust efficiency or lowered backpressure - The presence of residual exhaust gas in the cylinder retards the flame front - Less advance required when exhaust extraction effect is working, ie, higher up in the rev range
Improved induction efficiency The engine has an easier time getting a 'lung' full Faster with improved Volumetric Efficiency - Less advance as Volumetric Efficiency improves
Increased bore size May increase valve shrouding and lower Volumetric Efficiency - More distance from plug to far side of cylinder More advance required
Increased compression ratio - A higher Compression Ratio results in faster burning - Less advance required for increased Compression Ratio
Mixture swirl in combustion chamber - - A well mixed cylinder charge will burn uniformly Minimized with good swirl. Westlake type heads have heart shaped chambers to improve swirl
Piston shape - - Pistons which force the charge into a confined space limit the distance the flame front has to travel Minimized for squish pistons, maximum for dished pistons
Spark plug position in head - - Varies with distance from plug to farthest point in the cylinder, ideally centered in cylinder Minimum when centered, more advance as it moves to the farthest corner of the cylinder
Air/fuel ratio - Anything weaker or richer than the ideal 14.7:1 will burn slower - Maximized at stoicheometric
Coolant temperature - Increased engine temperature affects final charge temperature - Less advance required as engine temperature increases
Fuel octane rating - Octane slows burn rate - More advance with increased octane, or, more importantly, less risk of over advancing without changing advance
Heat transfer rate - The higher rate at which the cylinder head can get rid of the combustion heat, the lower the final charge temperature will be - Less advance needed for poor cooling iron heads, slightly more for efficient turbo heads, even more for aluminum
Induction air temperature Colder charge means denser air and higher Volumetric Efficiency, warmer charge, lower Volumetric Efficiency Higher temperature charges burn faster - Less timing with increased temperature
Engine speed Volumetric Efficiency increases with speed - - As Volumetric Efficiency increases, advance decreases, but because flame speed is finite, it must be initiated much earlier with increasing speed
Ambient moisture conditions - Moisture in the cylinder charge will slow the flame speed - More advance required under humid conditions

(If anybody wants to tell me how to make tables show up I will make this a little easier to read - it was put together in Word as a table but does not copy over to Rennlist well.)

So, in a rather long winded way to get to the point – porting and polishing (and other mods) if done incorrectly can have an adverse effect on the turbulence in the combustion chamber and therefore screw with the ignition requirements of the ‘system’ and reduce efficiency.

If by some quirk of fate you dramatically increase the mixing and turbulence in the combustion chamber with the untested porting you could increase the flame propagation speed and get into detonation (bye bye engine) or if you reduced the turbulence designed in by Porsche you will create incomplete combustion – a drop in power.

More is not (necessarily) better. Better is better.

Chris White

PS – did anybody get anything out of this? (In other words – should I bother with more someday)

Sam Lin

Good post Chris, nothing new, but hopefully makes it clearer/easier to grasp for some.

Easiest way to do your table would be screenshot it and crop, post as a pic.

Sam

Russ Murphy

Heck yeah!

JimV

Looks good to me except the explosion part. I would say it's more of a controlled burn. I make good power using my flow bench as a tool. (Some guy's don't know how to use a wrench properly).

How does increasing bore size shroud the valves?

sweanders

Great stuff!

Chris White

You will get used to my slightly sarcastic style after a while. Of course you are right, that was part of the point - thinking about the whole system - many folks don't think about combustion.

You got me there, my mistake. That was supposed to be "valve size" not "Bore size"

fast951

Chris, a question.. Do I read this correctly? With a longer stroke you need more advanced timing?

You forgot one essential ingredient: MAGIC DUST

Duke

Great post and all but what happened afterwards, did the tooth fairy ever get married!?

No seriously, the most informative post in a long time. Thanks for contributing Chris.

Chris White

Since Jim mentioned flow benches I should add a comment – I am not anti-flow benches - flow benches can be used to make real improvements but comparing two items and basing the decision solely on one item providing 30 more CFM than the other is not necessarily going to result in an improvement in power output.

Chris White

schnellfahrer

So if I manage to stuff more air into the cylinders, the right a/f will not alone keep the engine from blowing up; I'd have to alter timing as well?

Chris White

I thought “Magic Dust” was more of an 80’s kind of thing.

“Do I read this correctly? With a longer stroke you need more advanced timing?”
I picked that up from a source that I trust…but it doesn’t make any empirical sense. I don’t think there is a real apples to apples comparison there. Too many variables. Feel free to ignore that one – I haven’t had the opportunity to test it.

Chris White

J Chen

Chris one more thing to add.
Connecting rod lenght / crank stroke ratio

Bengt Sweden

Thank you Chris for being so On Topic, contrary to many other posts lately.

Could you please explain this to me since I had a different understanding:

TIA
Bengt

Chris White

To get the most out of the system – yes you would have to change the timing.
But don’t get too worried – the stock timing is pretty conservative. I usually run at least 4 degrees more. You will not blow up due to the timing – if you ever notice pinging then up the quality of fuel (octane) as that will help reduce the flame front speed and therefore help with the timing.

Chris White

fast951

Could we add water injection to the variables? This is fun stuff, thanks Chris.