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Discussion Starter #1 (Edited)
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.
 

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I agree with pretty much everything omniman stated. However, the shape of the piston's dome is also very important. Not all domes were created equal. The really pointed domes are especially bad as they "divide" the combustion and actually push the a/f mix away from the spark plug. A well designed system will have plenty of quench area to keep the mix in the center of the cylinder and a broad/flat dome vs. a narrow pointed dome.

It's true that adding a bigger dome to a piston is not the best way to add compression, but it is the easiest way to add a lot of compression. If you pick a good piston, you can still net some nice gains.

The best way to add compression w/o adding piston dome is to weld the head. However, that is prohibitively expensive for most people in order to have it done right.
 

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..... 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.
Just wanted to clear things up so that no one gets the wrong idea.... Pistons for a 2.0L cannot work in a 2.4L block.

Everything else seems like good info.. :up:
 

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Sorry, I was busted tired the night before.
Another point that was hit on by Steve was reducing chamber volume that comes into contact with the flame front. By reducing surface area (Piston dome) you reduce the amount of material that comes in contact with the event, presenting less chamber heat loss. Keep the power in the cylinder.
However, it is important to keep small quench areas as well. Designing a chamber is a rough task of walking fine lines.
 

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From what i have seen hondas dont really care too much about filling the chamber with piston. there are some losses due to all this, but not nearly as much as some other engines.

a comp4 car that was kicking ass back in the day (running consistent 11.1's and 11.0's before there were many k's racing) had so much dome that they had to mill the center of the piston for spark plug clearance. I believe the builder told me something close to "fill the chamber with piston and it will go fast" may not be exact words, but it was at least 4 years ago.
 

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Discussion Starter #11 (Edited)
Well for a given group of parameters all things being equal and you change the bore size bigger with no other changes (piston volume etc.) the compression will increase. Try it on a compression calculator and see for your self. The reason it increases is a mathmatical fact. Compression ratio is simply the big volume (piston at bottom dead center) divided by the little volume (piston at top dead center). If you change the bore and nothing else the big volume will be naturally bigger and the small volume will remain virtually unchanged. The head gasket may see an increase making the small volume a tiny bit bigger but its no where near enough to offset the 99mm of stroke by 1mm of bore from the big volume increase.
 

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Just wanted to clear things up so that no one gets the wrong idea.... Pistons for a 2.0L cannot work in a 2.4L block.
YOu mean stock pistons will not work..

But AFTERMARKET PISTONS FOR THE K20A will work if they are the right bore size...

For example, a 12.5 CP piston for a k20a motor, 88mm bore... results in 14.1 compression in a k24a.. :)
 

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YOu mean stock pistons will not work..

But AFTERMARKET PISTONS FOR THE K20A will work if they are the right bore size...

For example, a 12.5 CP piston for a k20a motor, 88mm bore... results in 14.1 compression in a k24a.. :)
Correct.. It's all in the mm bore ... Just didnt someone who didnt fully understand whats going on to get the wrong idea about what can or cannot fit...
 

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i thought this was the other way around :confused: shouldnt it be....

less bore = more comp.

more bore = less comp.

?
No, because when you increase the bore, you now have more volume in the cylinder, but you are still cramming it all into the same tiny space in the combustion chamber, so you increase CR with bore.
 

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I agree with pretty much everything omniman stated. However, the shape of the piston's dome is also very important. Not all domes were created equal. The really pointed domes are especially bad as they "divide" the combustion and actually push the a/f mix away from the spark plug. A well designed system will have plenty of quench area to keep the mix in the center of the cylinder and a broad/flat dome vs. a narrow pointed dome.

It's true that adding a bigger dome to a piston is not the best way to add compression, but it is the easiest way to add a lot of compression. If you pick a good piston, you can still net some nice gains.

The best way to add compression w/o adding piston dome is to weld the head. However, that is prohibitively expensive for most people in order to have it done right.
I think that with these motors, unless you're trying to run 93 octane, you want the most dome you can possibly get if you're going all out. Someone surely isn't going to try and tell me that 14-15:1 CR pistons don't make power. I don't think anyone has hit 300whp with 12.5:1. I'm sure someone will try and say that there is a point where the dome size and volume negates the gains, but I just don't see it happening. Especially if you are free to run on race gas or e85/higher.

I've also yet to see any proof that a prepped piston can make more power than it's off-the-shelf counterpart.

These comments were directed at your remarks Chunky, just speaking in general.
 

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I think that with these motors, unless you're trying to run 93 octane, you want the most dome you can possibly get if you're going all out. Someone surely isn't going to try and tell me that 14-15:1 CR pistons don't make power. I don't think anyone has hit 300whp with 12.5:1. I'm sure someone will try and say that there is a point where the dome size and volume negates the gains, but I just don't see it happening. Especially if you are free to run on race gas or e85/higher.

I've also yet to see any proof that a prepped piston can make more power than it's off-the-shelf counterpart.

These comments were directed at your remarks Chunky, just speaking in general.
But like the man said there are better ways to gain compression than putting a huge domed psiton in your motor with the stock head... you get a custom 12.5:1 piston and put that in an Endyn welded and reformed head and your going to net more than 12.5:1 compression, and you will have a much more efficient burn and make more power on the same compression... It will create a combustion chamber with the same volume but have a more even area to spread the air/fuel in and create a more optimal burn. pair that with some nice port work and a good valve job and I gaurentee you will make more power on a propperly prepped head with a smaller domed piston than you will on your stock chambered head with a big domed 14:1 piston...

Same theory with a prepped piston, you will get rid of the hot spots and be more effcient and create less heat and avoid detonation... getting the plug to ignite all of your air/fuel and direct the power directly down instead of sideways one way or the other on your huge dome will be more efficient and create better more reliable power... getting the intake and exhaust charges out in a uniform manner is a different subject but equally important when talking about head work...

Great thread by the way...
 

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YOu mean stock pistons will not work..

But AFTERMARKET PISTONS FOR THE K20A will work if they are the right bore size...

For example, a 12.5 CP piston for a k20a motor, 88mm bore... results in 14.1 compression in a k24a.. :)
you can use stock pistons if a k24 was sleeved and kept at 86mm :p
 

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I wonder what some work like this would net compression wise?




thanks to GBR tuning for the pics
 
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