Can some one please explain Dynamic compression

[dadnova]

Can 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.

I know I can't install the cam Intake closing @ 21 ATDC degrees to raise these numbers to:
Effective stroke 3.9
Dynamic compression 10.75
Crank psi pressure 232.7

Help me understand

[stroker1]

Maybe this will help. It's a good read.

http://www.empirenet.com/pkelley2/DynamicCR.html

[sunsation540]

that makes a good read, however in the shortest terms the static compression is figured off the stroke bore and chamber volume as well as the dome or lack of it to the piston... Dynamic comp has to do with the valve timing of the cam and captured cylinder pressure... as a test a 468 with 312/322 722 lift cam had 110. lbs of cranking compression just changing the cam nothing else 296/ 304 714 the compression, dynamic is now 150 and the car runs like 8 tenths faster...these things are juggled around for many reasons

[dadnova]

That is very good reading, I also found this one as well.
The benifit of knowing is so I know what I can and can't do.


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.