Squish you squish me

[180bfj20det]

Reading through another thread I am confirming my limited knowledge of the 4 stroke engine.

Please someone post a pic of where the squish area is on a head. I think it would help some of us less educated people understand more easily.

Does compression ratio change squish?

Headgasket thickness?

If you modify the combustion chamber?

Anything else thatll effect it?

Importance bt turbo and NA engines?

Cheers

[proengines]

In the photo that's hopefully below, the area with the red squigles on it is the quench/squish are. Basically it is the area where the flat surface of the head face sits over the flat areas on the piston. "quench" is because these areas run cooler than the rest of the chamber as very little combustion actually happens in these areas. "squish" is because these areas squish the air/fuel mixture in towards the spark plug.

In order with your questions,

1. There is no direct relation between quench area and compression, although moving the piston closer to the head will increase compression. The closer the piston runs to the head, the more compression you can run without detonation or at the same compression you can run more timing.

2. Head gasket thickness does affect quench, the closer the piston sits to the head without hitting it, the more complete combustion you will have as a shock wave is formed between the flat areas, pushing the air/fuel mixture in towards the spark plug. A thicker head gasket will move the faces further apart and result in less efficient combustion.

3. If you modify the combustion chamber on the flat areas it will affect it, Jun machines most of the quench pads away and their engines make great power, I'm not sure of the exact reason why they do it though. It still leaves a ring around the outside of the chamber that sits over the piston though.

4. Machining the top of the block will move the pistons closer to the head, machining the head will not make any difference to piston to head clearance, it will increse compression and decrease piston to valve clearance though.

5. I'd say it's equally important in both turbo and n/a engines.

There are a lot of different thoughts on quench area. Endyne in the US do a lot of hondas and say that the actual compression doesnt matter in a turbo engine as long as the piston top has the right profile. They also shape their pistons to push the air/fuel mixture towards the spark plug and the exhaust side of the chamber. It's an interesting idea and they have the numbers on the board to show it works. If you're interested, have a look at www.theoldone.com theres quite a few days reading there.

[Dash]

Awesome post,thanks for sharing.

Do nissan engines usually have a large quench area compared to other motors or is it more of a ratio?

I'm assuming 2 valve heads would have a lot more room do they run more quench given there not limited by the the valves?

Thanks

[StockyMcStock]

here's a copy of a post i made on another forum regarding squish/quench on an RB25/30, it's dumbed down a fair bit but only to make it easier to understand. fluid mechanics can be a bitch to get your head around if i use the proper terminology:

when the piston gets to top dead centre, and the head surface is 40thou away from the piston edge, the air/fuel mixture which is there encounters a bit of a problem. it's trying to be compressed (locally) very forcefully by huge local pressures and hence wants to get the f*ck out of there. it therefore gets pushed at huge velocity the only way it can go: inwards. this creates a lot of turbulence in the combustion chamber which mixes the fuel/air mixture even more and also causes it to swirl at huge velocity. it has the added effect of forcing the mixture up to the middle of the combustion chamber (at such huge piston velocities the local pressure changes actually stay as changes rather than homogenising for a fraction of a second which is all it needs) where the spark plug is.

so when it sparks, you get a very well mixed, turbulent mixture igniting. it reduces local "hotspots" and improves the efficiency (completeness) of the burn.

the squish band is illustrated here: the flat edges of the piston which come very very close to hitting the squish areas on the head, shown in the 2nd picture. the universal standard for squish is about 40 thou on a high-revving engine to allow for rod/crank/piston stretch at high RPM. you can go as low as 30thou safely though.

so if you think about it, if the flat areas on the edge of the piston are exactly level with the top of the block (as in an RB30) at top dead centre, and the head shown here (RB25) has flat squish areas exactly level with the head face, then the only thing stopping the piston hitting the head on the edges is the head gasket thickness. luckily for us, it's 47 thou which crushes to about 38-45 thou at full torque.

[180bfj20det]

Thanks guys. Really really helpful!

[Sydneykid]

Great post Craig, not easy to explain in plain English. My 20 cents worth on this bit;

3. If you modify the combustion chamber on the flat areas it will affect it, Jun machines most of the quench pads away and their engines make great power, I'm not sure of the exact reason why they do it though. It still leaves a ring around the outside of the chamber that sits over the piston though.

I think Jun are caught between a rock and hard place, They need to lower the compression ratio to run the huge boost, but they know they will have detonation issues if they stick a thick head gasket in there. Using a bowled piston would have the same effect. They can't machine out the combustion chamber as there is not enough metal to the water jacket. So they remove the squish ramps. Certainly not ideal, but the best of a bad bunch of choices.

[DoughBoy]

.. and a side effect of unshrouding the valves?

[Cubes]

As a general rule if you can't get squish under 50-60thou you should completely remove the squish as leaving it there will only increase detonation.

This supports SK's thoughts.

[WetGTR]

In real world terms what would this acheive, eg: better efficiency & how better, more overall response, more HP, smoother revving, etc

If some people have followed the Japanese trend and have installed 1.6mm head gaskets which have cost a small fortune - is it worth removing and obtaining squish because they would of obviously lost this.

[Cubes]

Better 'average' power, response and fuel economy.

A motor will also make its best power with the pistons ever so slightly kissing the head.

Not something you really want to do though.

[Craved]

Nice thread guys ... this info would be something that will come in handy one day!