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Discussion Starter · #1 · (Edited)
engine 302
specs .
bore....................4.00
stroke..................3.00
rod lenthg.............5.00 ...(know they are longer)
compression ratio.. 8 to 1 static
int valve closing.....35* abdc
effective DCR stroke 2.80

ADD boost at 14psi

Static comp ratio then becomes equal to 15.42 to 1

DCR comp. ratio then becomes equal to ..14.31 to 1

not much of a LESSER DCR when boosted

Edit , the engine would have to have the higher c/r to equal the same psi at tdc. note there is no n/a DCR number . if you must know the n/a DCR , the info is in the specs to obtain it
 

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I mostly agree, with a little kink in the "Static comp ratio then becomes equal to 15.42 to 1", but I think it's just relative terms that I think are different. So, I'm not opposing it, just looking at it using different terms and factors. Let me take a shot:

Static compression ratio (SCR) is just that - static. It is a comparison of the swept volume from BDC to the volume at TDC in a closed cylinder. It's just a volume ratio. Once the engine is built, it never changes. Ever.

The dynamic compression ratio (DCR) is a fixed ratio factor that never changes and is a calculation based on the intake closing point, as opposed to the SCR compression stroke from BDC. It is always lower than static compression ratio in our engines. The ratio never changes. Ever.

Dynamic compression (DC - not ratio) is a variable and ever-changing (dynamic) level of cylinder pressure at TDC (compression). The DC can be well above both the calculated SCR and the DCR (which is always lower in our typical engines than SCR). This is where the 'magic' happens, as an engine with a 'hotter' cam and later intake valve closing (smaller DCR) can make more power than one with less intake valve timing (larger DCR).

Volumetric efficiency (VE) is the engine's ability to fill the cylinders under various conditions, and becomes the primary factor in your DC (not DCR). Due to various effects (harmonic tuning, charge mass, inertial flow, etc.), the VE can exceed 100% - meaning more air in the cylinder than would fill it by by both DCR and SCR calculation in a normally aspirated engine. This is the other factor that pumps the magic in the DC result.

So, while I generally agree, due to dynamic factors, there is no actual "static compression ratio equivalent" for boost. Apples and oranges. That's comparing a dynamic effect with a calculated ratio. Yes, twice the manifold pressure will obviously give somewhere in the very general neighborhood of twice the DC (not DCR). But, due to the different (dynamic) flows caused by the greater pressure differential, the "equivalent to SCR" is not a simple and precise single calculation. That's my only beef here. On the other hand, it does give a simplistic view of how the dynamic cylinder pressure will obviously increase with boost. So, although not very accurate, it is useful in a very general sense for point of demonstration.

This is highlighted by a common phenomenon, where doubling the manifold pressure of an engine by using boost generally results in more than double the dyno horsepower. How can that be? Dynamic effects and parasitic losses.

David
 

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Discussion Starter · #3 · (Edited)
Dynamis Compression should only be called cylinder pressure .

can call it anyting even percentage of fill , long as everone knows what the meaning is
actual boost at tdc to n/a given static comp ratio is just equalizing the higher compression ratio needed to equal the same psi .

a pound is a pound

the VE has nothing to do with compression ratios , total different subject
 

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If you're really looking for a good comparison between turbo, supercharged, N/A, even 2-strokes, then BMEP (Brake Mean Effective Pressure) is a good yardstick to calculate the comparative "effectivness" of the engine at making power.

BMEP (psi) = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci) for a four stroke,

BMEP (psi) = 75.4 x TORQUE (lb-ft) / DISPLACEMENT (ci) for a two stroke.


This will tell you how much "use" you're getting out of your displacement. Don't confuse it with cylinder pressure either, it's just an abstract number to represent the relative production of torque per cubic inch.

Brake Mean Effective Pressure (BMEP): The Performance Yardstick for more info.
 

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Discussion Starter · #5 · (Edited)
If you're really looking for a good comparison between turbo, supercharged, N/A, even 2-strokes, then BMEP (Brake Mean Effective Pressure) is a good yardstick to calculate the comparative "effectivness" of the engine at making power.

BMEP (psi) = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci) for a four stroke,

BMEP (psi) = 75.4 x TORQUE (lb-ft) / DISPLACEMENT (ci) for a two stroke.


This will tell you how much "use" you're getting out of your displacement. Don't confuse it with cylinder pressure either, it's just an abstract number to represent the relative production of torque per cubic



Brake Mean Effective Pressure (BMEP): The Performance Yardstick for more info.
Thats not the subject . its about compression ratio's only

not power output
 

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Right. If it's only about compression ratios, then there are two ratios and both of them are fixed. Period.

If you want to expand the scenario and add boost, then that is dynamic, and a bunch of other nerd stuff comes into play.

BTW - yes, DC is just cylinder pressure, but the term is often used to differentiate it from post-ignition cylinder pressure.

David
 

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Discussion Starter · #7 ·
Right. If it's only about compression ratios, then there are two ratios and both of them are fixed. Period.

If you want to expand the scenario and add boost, then that is dynamic, and a bunch of other nerd stuff comes into play.

BTW - yes, DC is just cylinder pressure, but the term is often used to differentiate it from post-ignition cylinder pressure.

David
the reason for the post is ....what else , another forum member stated the DRC can be higher than static ratio .

just numbers from Wallace that I was refer too

love the "nerd stuff" comment
 
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