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hey guys, you might think this is a silly question, but i just dont have the time to read all the past posts about this subject.

why would most people change to an 18 to 1 from the stock 22 to 1? or am i reading this all wrong?
 

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Lowering the compression ratio lowers the cylinder head pressure even at higher boost pressures. This means you should be able to run 15 psi boost on your 6.5 with 18:1 and an IC without cracking pistons and heads.
 

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I'm still not following....

Empirically... what is the difference between 25psi at 18:1 and 15psi at 22:1?
Would head pressures not be comparable? With comparable head pressures, I would think that keeping the boost lower would be better due to less intake heating and less backpressure from the turbo.

Wrong???? :think::think:
 

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The simplest way of explaining it is, with lower compression you can fit more air into a confined space. Easy comparison you could take 2 cylinders of different sizes and pressurize them, the smaller tank will be more likely to blow up first. So if you lower the compression ratio you can squeeze the same amount of air in with lower pressures. The lower pressures means you can squeeze in more air before you reach the same pressure. And as we all know more air means you can add more fuel which equals more power.
 

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Some interesting discussion happened here about too much boost. Some numbers came up too about the difference between 18 and stock compression.
 

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The goal is to get as many little oxygen molecules into your cylinder as possible before the valve closes. Once the valve closes, you are stuck with that much oxygen, no matter what your compression ratio is.

Most engines are oxygen limited. Fuel will only burn properly in a strict ratio with oxygen. Too much fuel in the ratio, you will not have enough O2 to combust cleanly. As all piston engines (gas or diesel) are basically air pumps, they are air limited in how much power they make. In simple terms, getting air, and thus into (and out of ) the engine is the key to power. Turbos, tuned intake runners, port and polish, high lift cams, its all about getting as much O2 into the cyl as possible.

Lets look at a thoretical 1 cyl 1 liter normally aspirated diesel engine. This model assumes 100% efficiency at sea level.

When the piston completes its intake stroke, the cyl is filled with 1 liter of air at 14.7 psia. Oxygen content of air is ~20% so you have 200 cc of oxygen available for combustion. If the cyl compression ratio is 22:1 then that cylinders pressure would be 323 psi BEFORE factoring in pressure gains due to heating of the air. In the real world, the actual cyl pressure would be closer to 450psi. But for this instance, we will ignore heating pressure for the sake of simplicity.

So with only 200 cc of oxygen, we are limited to how much fuel we can inject, and thus, how much power we can make. We can inject WAY more fuel than this amount, but injecting too much will just produce smoke, and increase EGT's without making any additional power. There just isn't enough oxygen to combust this amount of fuel.

So, its obivous that we need more oxygen in the cylinder to allow more fuel to be burned. So we bolt on a turbocharger, and set it up to run at 14.7 psi of boost.

Now when the cylinder completes it intake stroke, It contians 1 liter of air at twice atomospheric pressure (29.4 psi) so you actually have DOUBLE the volume of air in the cylinder. So now we have 400 cc of oxygen available for combustion. we can inject TWICE as much fuel and make twice as much power. Life is good!

Except... Lets say our engine cannot take more than 550psi before bad things happen.

So with 14.7 lbs boost and a 22:1 com compression pressure at TDC is 646 PSI. and this would blow the head gaskets out of engine. So lets reduce the compression ratio to 18:1, this lowers the compression pressure to a more manageable 529 psi.

True, if you could build the engine to survive at 14.7 boost with 22:1 compression, it would out perform a comperable 18:1 engine, but a high boost 18:1 will make much more power than a low boost 22:1

So in summary, when we are limited to how much compression pressure the engine will tolerate, its better to pack extra air into the cylinder with higher boost, and compress it less, than to pack less air into the cylinder and compress it more.

That, in a nutshell is the benefits of an 18:1 compression engine. It allows more boost safely. The pistons in and of themselves will do nothing to enhance power. As a matter of fact, installing 18:1 pistons without increasing boost will reduce performance. They are the FOUNDATION to building power.

Tim

PS Don't flame me about intake air temps and exhaust backpressures and volumetric efficiencies and all that rot. They all factor into the "real world" but I kept them out of this model just to show the basic thought behind why 18:1 pistons.
 

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As Mr. Burns would say......

EXCELLENT......
 

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Basically the same as my explanation, just mine was the for dummies version(the way I learned it:lol: ).
 

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Hmm.... dunno about that Ferm'... I was still a bit lost until just now. It just wasn't clicking, but it does make perfect sense, and explains why even on gassers you'll see 8.5:1 with a turbo or blower rather than 12:1 or higher.

So as far as stress on the engine, (pre-ignition) cylinder pressure CAN be the same under high or low boost conditions given an inversely proportional compression ratio, but the key factor is the volume of the intake charge measured ATA, rather than under boost conditions.
 

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Great technical discussion! You just earned a link in the FAQ's.
 

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Discussion Starter #14
it makes alot more sence now!

thanks for the updates fellas....Grand Pa always said "you will learn something new everyday"
 
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