Real life consequences re: squish gap

[weefek]

A few light swipes across the 120 will give you a quick indication on where the high spots are, once the high spots are level, then it's a matter of sanding a bit and measuring, sand a bit, measure, sand a bit, measure, you get the picture. Probably a full six pack job.

That's basically exactly what I did as you can see from the photo. There were two other measurements taken past that photo to get me to 1.2mm

*EDIT: It's kind of inconsistent because I was measure from head deck to the casting fin. Castings are not perfect at all. Overall it gave me a number I was comfortable with, if that make sense.

 

[ImpulseRocket]

The bigger nerdy science behind the squish gap also has to do with thermal efficiency, swirl velocity, and combustion efficiency.

.8 to .9mm is considered "ideal" because it strikes a balance between how tight the gap is and potential clearance issues at high rpm. Ideal squish gap in theory is zero, but due to physics, that isn't possible. For many "race" builds people will run gaps of .5 to .6mm.

For swirl efficiency and velocity it goes back to one of Bernoulli's principles. That tight gap forces the entire volume of charge in a smaller volume of cylinder. This causes the charge swirl velocity to greatly increase. Faster swirl = better fuel atomization and mixing and faster flame front propagation = better bang for your buck.

Combustion efficiency is related to the swirl and the gap. Since the tighter gap forces more charge to the center, there is less charge swirling on the outer edges. The further the charge swirl is away from the center the slower it spins. The slower it spins the less energy it has, and that lack of energy creates a temperature differential in the charge that causes it to burn more slowly. Often resulting in as much as a 25% loss of fuel charge going unburned. The tighter the gap, the lower the loss of efficiency.

As for the torque increase, you feel it more at lower rpm because the increase in cylinder pressure means more torque. As rpm increases the lower the effect is felt because the combustion process is happening so much faster.

 

[weefek]

The bigger nerdy science behind the squish gap also has to do with thermal efficiency, swirl velocity, and combustion efficiency.

.8 to .9mm is considered "ideal" because it strikes a balance between how tight the gap is and potential clearance issues at high rpm. Ideal squish gap in theory is zero, but due to physics, that isn't possible. For many "race" builds people will run gaps of .5 to .6mm.

For swirl efficiency and velocity it goes back to one of Bernoulli's principles. That tight gap forces the entire volume of charge in a smaller volume of cylinder. This causes the charge swirl velocity to greatly increase. Faster swirl = better fuel atomization and mixing and faster flame front propagation = better bang for your buck.

Combustion efficiency is related to the swirl and the gap. Since the tighter gap forces more charge to the center, there is less charge swirling on the outer edges. The further the charge swirl is away from the center the slower it spins. The slower it spins the less energy it has, and that lack of energy creates a temperature differential in the charge that auses.it to burn more slowly. Often resulting in as much as a 25% loss of fuel charge going unburned. The tighter the gap, the lower the loss of efficiency.

Correct me if I'm wrong but aren't there a lot of other factors that go into this specifically?

I'm not trying to be argumentitive I'm just trying to learn. I have great respect for your knowledge to be honest.

Let's not get into the 'zero' squish gap it reminds me of infinite inflow of power on start up which technically is true but never realized in real life.

What you're saying makes complete sense to my science brain. I'm thinking once I get my driver's license back I will look into a custom head rather than this stock aluminum cast garbage... I've always s**t on cast aluminum because lets face it, for the most part it sucks. I'm coming from an Industrial standpoint and from here cast aluminum is a nono, just like cast iron for the most part to be fair.

Thank you @ImpulseRocket89 your wisom never fails me.

 

[ImpulseRocket]

Correct me if I'm wrong but aren't there a lot of other factors that go into this specifically?

I'm not trying to be argumentitive I'm just trying to learn. I have great respect for your knowledge to be honest.

Let's not get into the 'zero' squish gap it reminds me of infinite inflow of power on start up which technically is true but never realized in real life.

What you're saying makes complete sense to my science brain. I'm thinking once I get my driver's license back I will look into a custom head rather than this stock aluminum cast garbage... I've always s**t on cast aluminum because lets face it, for the most part it sucks. I'm coming from an Industrial standpoint and from here cast aluminum is a nono, just like cast iron for the most part to be fair.

Thank you @ImpulseRocket89 your wisom never fails me.

Oh yeah, there is a lot involved lol. I gave the most simplified answer possible for a subject that is way more complex and filled with tons of variables.

 

[weefek]

Oh yeah, there is a lot involved lol. I gave the most simplified answer possible for a subject that is way more complex and filled with tons of variables.

Right up my alley to be honest... But not right now. I'll see it and deal with it tomrrow in the AM or maybe PM if you know what i mean. I've had a few pops.

 

[darwin]

OK, ignorance is bliss huh? What exactly is squish gap? I thought it was compression of the head gasket.

 

[Wrench]

Squish gap is the amount of dog sh^t in between you toes after you step on it barefooted

Or maybe squish gap it's the distance between the top of the piston and cyl head @TDC

 

[Cannonball3]

Doesnt it have to do with forcing a bit of the charge into the gap where it cools the head/piston as the charge us burned?

 

[darwin]

So compression of the head gasket does affect squish gap. Dang I'm good!

 

[Cannonball3]

From Gordon Jennings. The profile of the piston must closely match the head. An example of a poor combo is a CG head on a Minarelli piston. I suppose you could get a good squish reading at the outer area of the piston. However the squish profile is all wrong so it negates the effect. The same is true of swapping heads of one manufacturer to another with out checking profile. A bad squish profile is prone to detonation.