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Discussion Starter · #1 ·
:smt100Can any one please explain dynamic compression to me, and where it should be for a race engine.

I have a 427cu SBC 4.125 pistons, 4" stroke, 6" rods @ 11-1 with the cam intake closing @ 71degrees ABDC.

Which would put my Dynamic Compression:
2.96 Effective stroke
8.4 dynamic compression
169 psi crank pressure

If my cam is set at Intake closing @ 61 degees my Dynamic would be
3.11 Effective stroke
8.95 dynamic compression
183.7 psi crank pressure

Where should I be, please explain to me, for I will admit I just don't understand. :smt100

I know I can't install the cam Intake closing @ 31 degrees to raise these numbers.
 

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Don't get too caught up in the dynamic compression concept. Put the cam where the engine wants it to be to achieve peak cylinder filling and never look back.

If dynamic compression was a critical measure, 9 to 1 compression ratio Craftsman Truck engines would've never made an ounce of power with their 64° closing points.
 

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Great question, for those who read this and would like to see how it is calculated go to http://www.wallaceracing.com/dynamic-cr.php I use that formula just so I have an idea what my cranking compression is. This way I can tell is everything is sealing up or something is way wrong.
To answer your question I don't think I have ever seen a cut and dry explaination as to where it should be for what application.
 

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Static compression ratio of 11:1.
Effective stroke is 3.67 inches.
Your dynamic compression ratio is 10.63:1 .
Your dynamic cranking pressure is 228.70 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 10.63 :1.
V/P (Volume to Pressure Index) is 261


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Discussion Starter · #5 ·
Spooled up, sorry but your way off, it depends on the cam timing. The numbers I gave are correct.

This is what I found on the web.


In our attempt to help our customers understand performance and what makes an engine produce power we are going to explain the concept of dynamic compression ratio (DCR). While seemingly esoteric, this is an essential concept in designing an engine for performance use.

The first thing to understand is that "compression ratio" (CR) as it is usually talked about is best termed "static compression ratio". This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 450cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 490/40, or 12.25:1. Conversely, if we hogged the chamber out to 60cc, the CR would now be 510/60, or 8.5:1.

Everyone knows that high performance engines typically have higher compression ratios. Simply put, higher compression makes more hp. Higher CR also improves fuel efficiency and throttle response. So why not bump up the CR even further? Once CR exceeds a certain point, detonation will occur. Detonation kills power and it kills engine. The amount of compression a given engine can handle is determined by many factors. These include combustion chamber design, head material, use of combustion chamber coatings, etc. Once these mechanical aspects of the engine have been fixed, the main variable is fuel octane. Higher octane = more resistance to detonation and the ability to tolerate more compression.

The above brings up the question that is often on the mind of performance enthusiasts and engine builders: how high should my CR be? Even if you know all about your engine and have decided what fuel you are going to use, the question cannot be answered as phrased. Why? Because without reference to the camshaft specs, talking about (static) CR is next to meaningless!

How is this so? Well, think about the Otto cycle and how a four stroke engine works. The power stroke has been completed and the piston is heading up in the bore. The intake valve is closed and the exhaust valve is open. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. The piston reaches TDC and starts back down. The exhaust valve closes and the intake valve opens. Fresh fuel and air are drawn into the cylinder. The piston reaches bottom dead enter (BDC) and starts back up. This is the critical point as far as understanding DCR. At BDC. the intake valve is still open. Consequently, even though the piston is rising up the bore, there is no compression actually occurring because of the open intake valve. Compression does not begin until the intake valve closes (IVC). Once IVC is reached, the air fuel mixture starts to compress. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. The DCR is what the air fuel mixture actually "sees" and is what "counts", not the static CR. Because DCR is dependent upon IVC, cam specs have as much effect on DCR as does the mechanical specifications of the motor.

DCR is much lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 10.0-12.0:1 range. Higher than this, there may be detonation problems with pump gas. Engines with "small" cams will need a lower static CR to avoid detonation. Engines with "big" cams have a later IVC point and can tolerate a higher static CR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Of course, race motors also have much larger camshafts which is another reason they can get away with such high static CR, often in the 13-15:1 range.

Note: there is some confusion about use of the term "Dynamic Compression Ratio". Some people use it to refer to the characteristics of an engine combo running at high speed. In that case, the engines volumetric efficiency will have a major effect on cylinder pressure. In this case, a larger cam will increase cylinder pressure when within its' rev range. Thus, more power and more cylinder pressure will be created. We prefer to think of this concept as "cylinder pressure" to avoid confusion.
 

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What you're ultimately trying to achieve is PEAK EXPANSION and that's got a LOT more to do with cylinder filling than squeezing. Don't get a mindset going that you must have 'x' cranking compression or whatever.

When 9 to 1 was the rule for CTS the engines made stupid power, given the cylinder head limitations for that era. It was also typical to see durations @ .050" up to 280° on the intake with these engines.....
 
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