Breathing/Combustion Chambers

Squish Band

This is defined as the area in the combustion chamber where the piston (at TDC) comes extremely close to the head. It's usually around the perimeter of the piston, and no mixture is expected to burn there. Physical contact is the only factor determining the 'height' of the squish band, so practically no mixture will be expected to be there as the flame front moves from the spark plug outwards.

Here's how it works: the upcoming piston squishes out that mixture, forcing it to blend with the rest around the plug. This action creates extra turbulence and homogenises the resultant mixture, which means that it burns cleaner and quicker, requiring less ignition advance. As a bonus, the outmost edges of the combustion chamber are closer to the plug tip, further reducing the need for extravagant ignition advance. All this leads to efficiency, especially in off-boost situations, where the mixture is not well homogenised (there are rich and lean spots within it)

Under boost, the role of squish takes the backseat. The compressed air is already agitated enough for the squish turbulence to make any major difference. Therefore race engines designed to run mainly on-boost don't have to pay much attention to it.

Here's an interesting paper on mixture preparation strategies

 

Measuring the LET's squish

It comes up to roughly 650millimiters, or 0.65cc, around 2.35% of the total Chamber volume.
This means that the designer has sacrificed the burning of 2.35% of the mixture, in return of better burning of the rest. It's actually less than one would expect from such an efficient design. Probably the mixture was expected to already be quite turbulent - the air coming from the turbo would certainly help in that respect.

 

Thicker gaskets?

If someone fits a gasket thicker by 1mm (or steel plate to that effect) the new, deformed chamber will be sacrificing 4.16% of the volume with not much squish effect
These are static figures, would be accurate at revs while cranking the engine by hand.
In real-life rpm there's a lot of inertia involved, and the stock LET squish can easily be below 1% while the 'deformed' one over 5%. The static CR would be about 8.3:1 in this scenario.

Is that bad then?
Yes, because it takes a fixed number of milliseconds at those revs for the flame front to propagate from the spark plug to the edge of the charge. (It also depends on the strength of the mixture, effective scavenging, etc. Check out the ignition section)
The whole piston crown is NOT the bottom of the combustion chamber. The mixture is squeezed into a homogenous blob around the plug tip just before TDC

Fitting a thick gasket DEFORMS the combustion chamber shape, creating an extra 'donught' a couple of mms tall and around 8mms thick. Most of the mixture in this space will NOT be burnt - it's too far from the plug tip.
Some of it will indeed burn, but too late, after the power stroke (not good)
In the process you've lost quality of the rest of the charge that DID burn, because it wasn't stirred up as thoroughly as the designer intended it.

That's why running double (or more) the stock boost with stock CR is not how the manufacturer would have done it. There are complicated formulas to calculate *how* much the CR should be (it's not linear!) but whatever the figure, one has to make sure that the combustion chamber is not being deformed in the process. Combustion chamber efficiency is the engine's most powerful tool against detonation.

 

How do we reduce the Compression Ratio then?

The DIY methods of lowering the CR range from botches to acceptable. The majority is in the first category - thicker gaskets, double gaskets, steel plates, machined pistons.
The only acceptable one is to use a lower-compression piston that retains the squish band incorporated by the OEM design
Call the piston manufacturer and ask them - if they don't know what you're talking about, look elsewhere. People have been known to skim metal out of XE pistons to reduce compression. This is a serious botch, it weakens a piston that's not strong enough to begin with!

Of course there's always the cheap'n'cheerful way of altering the cam timing to achieve a similar result, as I describe in the 'cams' section.

 

...So is thicker gasket a no-no?

Lets not get paranoid here.
We're talking about a drop of efficiency of a few percentage points.
It could have been a lot worse, the squish area of the LET is already relatively small.
If you're hell-bent on running 2 bar on stock pistons, then you'll have to fit a 2-3mm gasket because you've got no other choice (intercooling isn't enough at this stage). In this scenario, retaining squish is a bit of a luxury. Off-boost efficiency will be sacrificed for on-boost reliability.

If, however, you're only doing up to 20psi overboost, and no more than one bar is sustained, then it might not be worth upsetting the delicate balance of a well-designed combustion chamber. For a reliable high-boost setup that is expected to survive prolonged full-throttle, a proper set of pistons will bring the static CR down to 8:1 or thereabouts.

In-cylinder cooling provided by such means as water injection or small shots of nitrous oxide can also work wonders in keeping the engine together in such conditions.

The soft head article, quite interesting: (courtesy of Animal)

 

Here are some thoughts on performance head gaskets (local copy here)

Combustion chamber design from street rods

Even more on combustion chambers (local copy here)

 

On to the Exhaust...

 

 

 

 

extract from Supra site

basically, a cylinder leak-down test consists of pressurising a cylinder with shop air and listening for leaks. fashion an adapter from an old sparkplug so you can hook up an air hose from your air compressor. rotate the engine until the cylinder to be tested is at tdc compression. be sure to get it exactly on tdc. next slowly turn up the air pressure regulator on your air compressor or slowly open the supply valve to pressurize the cylinder. go slow because the engine may try to spin - keep hands and tools clear of belts, etc. once the cylinder is fully pressurized listen for air leaks. do not confuse a "seashell" sound for a leak. air leaks will be very distinct sound and you may even feel a rush of air. open the throttle and put your ear next to the intake opening. a rush of air indicates a leaking intake valve (bent valve? misadjusted or sticking?). next put your ear to the tailpipe opening, air rushing out means a leaking exhaust valve (bent, misadjusted, sticking, or burnt?). listen at the oil filler cap. you will hear a slight hiss of air. this is normal 'blow-by' leakage. how much is normal? well, many clt tools have a flow meter to measure how much air is coming by the piston rings and out thru the oil filler cap. usually less than 15%. you probably don't have an air flow meter to hook in-line with your air hose, so instead try to remember what each cylinder sounded like and compare them to one another. engines with good compression and good rings will sound even, slightly louder than a 'seashell' and you will not feel any air rush. lastly, take the radiator cap off and look for bubbles. bubbles indicate a blown head gasket or maybe a cracked cylinder head. doing a clt along with a compression test will tell you a lot about an engine's condition.

leak down test by dennis haynes
a number of people have asked "What is a LEAK DOWN test?" and "Can I do it myself?". first, let me explain the concept. we already use a compression test to determine an engines condition. The problem with this test is that there are too many variables. it can only be used to check engine condition by comparing cylinders to each other or a past norm. variables such as cam profile, engine cranking speed etc, will affect the readings.

a leak down tester uses air from a compressor and measures the rate at which it leaks through the engine. this is done with the engine not running and the beauty of this is that toy can find the source of the leakage by listening for the escaped air. internal leakage is found by air bubbles in the cooling system. tools need are a leak down tester and an air compressor. the tester is available from Milton at about $60.00. compressor should be at least 2hp and deliver 90psi.

procedure: remove spark plugs. set engine to tdc #1. calibrate test gauge per the instructions. Lock engine so it can not turn. connect hose to spark plug hole. connect pressurized gauge to hose connection. read leakage. if looking for coolant leaks and nothing obvious shows up, bypass gauge and connect shop air direct to cylinder. open radiator cap before this. if coolant sprays out, you have a head problem. do same test on rest of cylinders. remember to set tdc of each piston for compression stroke. this test is also great for air cooled head leaks and valve problems. note that all engines will have some leakage past the rings. i always do full pressure test when i suspect a problem. make sure engine is secured with full pressure test. it will spin violently. with gauge connected, you can rock crankshaft to see if leakage changes. if so, this is a sign that the ring lands are wearing, new engines will also do this until the rings are seated.

i know these instructions are kind of flaky, but I hope this gives some insight as to this type of test. if having a mechanic work on your vehicle, he should be familiar with the leak down test. i would be concerned if he isn't. this is a basic troubleshooting tool that all fleets use. especially on diesels.

 

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