Diminishing returns - Compression vs Ignition timi

[G3-TEG]

Hey guys,
I been pondering over this for quite some time now and I thought I'd throw this out there and see what you think is the theory or general consensus behind it.

Basically, I'm wondering when you start to increase the static compression on pump gas (say 92) you must retard the timing X degrees to prevent detonation. However, you increase CR to increase power but at the same time, when you pull timing, you lose power.

So, my question is at what point does one start to have a diminishing return affect when the compression reaches a certain CR and the timing that is pulled starts to negate the fact that the CR is raised and you actually are losing power with the added static compression?

In simplest terms, would it be more benificial at a certain point to run LESS compression because you can then run more ignition timing (ie. closer to stock).

Prime example: We are running 13.4:1cr in our integra and we have to run 12deg BTDC ignition advance to avoid pinging. (92-octane) Would we see more gains if we lowered the compression so that we can run ~16-17deg timing?

My thoughts is that if the timing is (too) retarded for a certain CR, then at t higher RPMs the combustion event would occur too late in the power stroke whereas if one could run more advanced timing, you would see more gains up top.

But, there's the whole catch-22 thing again, would you actually be at the same place still? Since you lower the CR (less power) and increased the timing (more power) you'd still be in the same place that you were with more CR and less timing?

Ack! Any ideas?

[G3-TEG]

Another thought I had was, what if you stayed at the higher CR, but ran on the RICH side so that you could run closer to stock timing instead of lowering the CR at maintaining a 13.5:1 A/F or so.

Thoughts?

[blownLS]

Wow reading that hurts my head!!!

My opinion is all of this comes down to heat. Heat kills and engine. Heat is created by compression. An engine with 11:1 compression heats the air alot more than 10% compared to a 10:1 set up (its not proportional). Too lean of a A/F ratio also heats up the exhaust significantly which heats up the combustion chamber.

All octane is... Is a fire retardant. Everything on this earth has a flash point (temperature at which an object self ignites). If we had no octane in our fuel, we could not use the comp ratios and timing curves we have today. Diesel has such a low flash point that when you compress the A/F mixture until it heats up to the point were it self ignites (so no spark plugs are required on diesel engines). If you were to add octane to diesel it would not run, because you increased the flash point temp.

My point is that, the over abundance of HEAT is what causes detonation, not the other way around.

So if you can find a way to lower the heat build up, then you can run higher compressions at stock timing. Its always better to keep your stock timing and then increase your compression to the point that your current octane will allow you to go. If you can't go any higher on the octane then you can try lowering the temp different ways like more fuel, cold air intake, water injection, intercooler, lower coolant temps from bigger radiator or lower thermostat, alcohol injection, or just increase the octane with race gas.

Its a balancing act. The reason why the JR water injection kits work so well is water in itself is a natural octane. You can go back to your stock timing. Because you are running on stock compression with 92 octane gas. When the boost kicks in your compression increases by 40% (10:1 to 14:1) and then the water kicks in with the boost and is the equivalent of 98 octane fuel.

You ever try pissing on a fire???

Natural octane!!!!

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[UberLude]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by blownLS:
Wow reading that hurts my head!!!

My opinion is all of this comes down to heat. Heat kills and engine. Heat is created by compression. An engine with 11:1 compression heats the air alot more than 10% compared to a 10:1 set up (its not proportional). Too lean of a A/F ratio also heats up the exhaust significantly which heats up the combustion chamber.

All octane is... Is a fire retardant. Everything on this earth has a flash point (temperature at which an object self ignites). If we had no octane in our fuel, we could not use the comp ratios and timing curves we have today. Diesel has such a low flash point that when you compress the A/F mixture until it heats up to the point were it self ignites (so no spark plugs are required on diesel engines). If you were to add octane to diesel it would not run, because you increased the flash point temp.

My point is that, the over abundance of HEAT is what causes detonation, not the other way around.

So if you can find a way to lower the heat build up, then you can run higher compressions at stock timing. Its always better to keep your stock timing and then increase your compression to the point that your current octane will allow you to go. If you can't go any higher on the octane then you can try lowering the temp different ways like more fuel, cold air intake, water injection, intercooler, lower coolant temps from bigger radiator or lower thermostat, alcohol injection, or just increase the octane with race gas.

Its a balancing act. The reason why the JR water injection kits work so well is water in itself is a natural octane. You can go back to your stock timing. Because you are running on stock compression with 92 octane gas. When the boost kicks in your compression increases by 40% (10:1 to 14:1) and then the water kicks in with the boost and is the equivalent of 98 octane fuel.

You ever try pissing on a fire???

Natural octane!!!!

<HR></BLOCKQUOTE>


Err... hrmph.

But water injection doesn't lower the ignition point of the fuel, it simply cools the intake charge, so the chamber gets cooler air and the self-ignition point of the fuel isn't reached.

An analogy comparing octane ratings to urine. That's one for the books.

Cheers,
Ben

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99 Prelude SH
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The world isn't run by weapons anymore, or energy, or money. It's run by little ones and zeros, little bits of data---it's all just electrons.

[Jay Lee]

G3-TEG,

Just for starters, our internal combustion engines are HEAT engines. They would not function without heat, and it is through the movement of heat energy that we are able to extract mechanical energy; however, to clarify this, it is uncontrolled heat that will damage our engine.

In designing an engine, the static compression ratio is determined by the fuel that will be used. Since it's much more difficult to change the static compression ration ratio (Saab engineers have already designed a work-around to this), this spec is usually set from the beginning.

Ignition advance is used to compensate for the fact that the mixture takes time to ignite and then combust. There are a number of factors that can affect the rate of combustion but the ones we are most familiar are mixture, octane, and temperature.

The idea behind ignition timing is to minimize the occurrence of negative work. This occurs when the spark occurs while the piston is still moving up, this being in the BTDC region. The pressure in the cylinder will begin to increase, somewhat slowly at first as the combustion kernel forms near the spark, then very rapidly as the entire cylinder will be swept by the combustion wave. The ignition timing is an optimum value adjusted to maximize the positive work event, where the pressure rise pushes against the piston as it's going down in the ATDC region, and minimize the negative work event.

Your static compression ratio will determine the maximum energy you can extract from a given quantity of fuel, so along with airflow, fuel, and rpm (we can ignore frictional losses for simplicity), a maximum power figure can be calculated. You've probably already noticed that this is very theoretical, but there has to be an ideal configuration for us to base our approach in practice. You then adjust ignition timing to optimize the desired torque, whether it be broad or at high or low rpm, and with computer control these days, torque optimization is easy.

In answering your question, YES, it WOULD be more beneficial to run less static compression and more timing advance to get more power, but only if the fuel you're using is not optimal for higher the compression.

However, the general rule of thumb is [higher compression + less advance] is better than [lower compression + more advance], since there will be less negative energy production with less advance. The key is to approach the limits of detonation without encountering it, because this is when uncontrolled combustion and heat begin to do their damage.
I'm not even going to get into detonation due to space limitations...

Also, I'm not sure why you mentioned about less advance affecting power at higher rpms because changing the static compression ratio will alter the optimum ignition timing curve just about everywhere, and manipulating the distributor will shift the entire curve rather than only the sections where it might need change. Some sections might need to be retarded where other sections might be okay or even enjoy a little more advance. Again, computer control makes this relatively simple. From the looks of your post, you already seemed to know this so I found it a bit odd...

Obviously there are many factors but I've tried to cover some of the more or less basic concepts of static compression/timing optimization. Hope this helps...



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Am I the only FNG in here?