Optimizing Combustion Space by Omnipower

[Vteckidd]

Some of you may remember the thread i posted a long time ago that had drawgings of flame fronts and the pros of a flat top piston. Saw this over on K20a.org, very similar to the thread i had.

ALL MOTOR GUYS READ THIS

Optimizing combustion chamber space is often one of the most over looked aspects of an engine build. Although its a simple concept it has many rewards, including increased power, lower octane requirements, reduced fuel consumption and better overall throttle response. This is really important when your build has a compression increase as one of its modifications. Raising the compression ratio is equally important as the quality of the combustion space.



If your combustion space isn't well defined you wont see the full potential of the compression increase. Also your fuel octane requirement will be higher with a poorly defined combustion space.

The pent roof Honda 4 valve combustion chamber is one of the most efficient combustion chamber shapes, basically a triangle, with a flat piston. As the dome of the piston is increased the space becomes oddly shaped and not only takes longer to burn the air fuel mixture but also requires more ignition timing (starting the burn sooner) to do so. The reason it requires more timing is because the area becomes more like a winding passage vs an open triangle. This in turn promotes detonation and increases the octane requirement. As you can see in the diagrams below the large dome will cause the combusting air fuel mixture to hit the piston at 45 degree angles in some areas diluting and diverting the combustion energy. In the other diagram you can see the engine with the flat piston has a clearly defined area and combustion gasses will push directly on the piston downwards providing a more efficient piston push. The better defined combustion area requires less timing because of its superior burn efficiency.

Many things affect compression ratios. Here is a list of things that change compression with all other things being equal.

-Stroke length will change compression ratios, longer strokes increase compression while shorter strokes decrease compression.

-Bore size will change compression ratios. Bigger bores increase compression ratios while smaller bores decrease compression ratios.

-Head gasket thickness will also change compression ratios. Thicker head gaskets decrease the compression ratio and thinner head gaskets increase the compression ratio.

-Piston dome volume changes compression ratios. larger domes increase smaller domes decrease compression.

-Piston compression height (the distance from the center line of the wrist pin to the flat of the piston) {measured as if the piston was a flat top}. This can determine how much the piston is in the hole or out of the hole or flush with the block. the more the piston is in the hole (under the block deck) the less compression, the higher it is the more compression.

-Chamber volume changes compression ratios. The larger the volume the more it decreases the compression. The smaller the volume the more it increases the compression ratio.

-Spark plug depth also has a small effect on compression ratios. By taking off the sealing ring or washer you can add a small amount of compression to any engine.

In short anything that changes or can change the volume of the cylinder or combustion space will change the compression ratio. Even a valve job can sink the valves into the head slightly and increase chamber volume thus decreasing compression slightly.

When planning a built engine always try to increase the compression without adding to the piston dome. Here are some examples of ways to increase the compression without adding to the piston dome (keeping the flattest possible piston).

With so many areas affecting compression ratios its almost never needed to use dome volume to gain compression, although it is the easiest way. Certainly not the most efficient.

-flat faced valves can add .2-.4 on many engines
-milling the head can add upto a full point (be sure to account for piston to valve clearance changes).
-over boring can add .1+
-stroking can add quite a bit depending on how much you add to the stroke.
-decking the block to change the piston from negative deck height to flush or slightly/less negative.

When you buy pistons the advertised compression ratio is for one engine combination. For example if you buy 12:1 pistons for a 2.0 liter rsx engine and plan to put them in a 2.4 liter tsx enigne you will have a lot more than 12:1 compression.

[Vteckidd]

No one?

[allmotoronly]

makes sense. Less dome=less piston top surface area=less combustion resistance=more power.. so bascally a flat top or slightly curved dome piston top would be more efficient than a piston with a massive sharp angled dome?

Very interesting thread!!

[all-mota]

high dome pistons stop flame travel which kills flow and power

[TheChosenOne]

Nice! It's one thing to know the various parts and components that make up and engine. But its another thing to know the optimun function of each aspect!

Good thread.

[Thighs]

how does bigger bore increase compression? doesnt that make the chamber larger, decreasing compression?

[TheChosenOne]

I think he's assuming that when you bore the cylinder walls, you would also be mating them with oversized pistons. Atleast that is, if you want your motor to run.

Really it's all in the numbers. The piston surface area is increasing, while the actual combustion space is staying the same: that is what I cam up with in my own logic. I could be wrong, so by all means, someone confirm this or throw it out.

[Thighs]

I think he's assuming that when you bore the cylinder walls, you would also be mating them with oversized pistons. Atleast that is, if you want your motor to run.

Really it's all in the numbers. The piston surface area is increasing, while the actual combustion space is staying the same: that is what I cam up with in my own logic. I could be wrong, so by all means, someone confirm this or throw it out.

i see what your saying, but unless you increase the fuel/air intake volume along with your bore, you will be putting the same fuel volume into a larger space, lowering the compression. adding a high-compression piston to a bored cylinder would just bring your displacement back to normal by reducing the already-enlarged chamber. i may be wrong but its making sense in my head.

[TheChosenOne]

i see what your saying, but unless you increase the fuel/air intake volume along with your bore, you will be putting the same fuel volume into a larger space, lowering the compression. adding a high-compression piston to a bored cylinder would just bring your displacement back to normal by reducing the already-enlarged chamber. i may be wrong but its making sense in my head.

I see what your saying, but the stroke isn't being changed, so the space is only bigger when the air and fuel are taken into the cylinder. When it is compressed, I have a feeling the cobustion chamber isn't larger at all. But like I said... this is just my own logical theory.

[all-mota]

I see what your saying, but the stroke isn't being changed, so the space is only bigger when the air and fuel are taken into the cylinder. When it is compressed, I have a feeling the cobustion chamber isn't larger at all. But like I said... this is just my own logical theory.

you are correct the combustion chamber stays the same the whole time only the bore size and stroke changes the compression unless it's opened up on the head to match the bore size.