Understanding Compression Ratio????

[Guest butlersZ]

Can someone help me understand the difference between dynamic and static compression ratio? In simple terms that is...

 

Also, Does the intake valve size have any effect on compression ratio? I'm curious because it would seem that a larger intake valve would fill the cylinder up more than a smaller valve given the engines are using the same cam which would result in a higher compression ratio right? More air/fuel occupying the same space right?

 

And wouldn't a slightly smaller valve, (1.94) have more intake velocity, be more efficient, and fill up the cylinder more than a larger (2.02) valve? Also the more velocity would increase the swirl of the fuel/air and promote detonation resistance right? I am still relatively new to building engines and am getting ready to do some head work on my Z. Engine already has cam, carb, intake, forged bottom end and pistons, etc. I'm just trying to understand compression ratio and pick the best valve size for my heads.

 

One more question, is there a way to figure out compression ratio by measuring the cranking compression of a motor with a compression gauge, like 200-210 psi. being equal to 11-1 comp. ratio, or there abouts. My guess is probably not because the cranking compression would be determined by how well the engine was built, the type of rings, the mileage, and such correct? Did I just answer my own question?

[blueovalz]

Grumpy (haven't seen him in a while) would have a plethora of great links for you to read, as this is a simple set of questions with no simple answers. My simple understanding of the difference between static and dynamic CR is this:

 

Static is the resultant ratio between the volumes of the space above the piston at BDC/same space at TDC (simple math). Dynamic compression ratio is the pressure change in the gas volume at RPM between the same two locations prior to ignition.

 

If the real world engine was always operating at 100% volumetric efficiency (VE), then the two would be identical (is that correct?), but most engines operate at less than this. Valve timing at low rpms allow some of the air pushed into the cylinder (on the intake stroke) to be pushed back out of the intake on the compression stroke, which results in a lower dynamic compression ratio (almost like decreasing the stroke of an engine, but not reducing the combustion chamber volume in the head). On the flip side, a properly tuned intake system may increase the intake charge to the point of pushing more than the normal amount of air into the cylinder resulting in a higher dynamic CR. Valve timing, I believe, will have a substantial effect on this.

 

Valve size could affect the CR. But a concave valve head, or deep valve seats would change that as well. My experience has been that valve size and shape are rather insignificant in regards to their effect in measured static CR being there is so little volume that is affected by this. Then you can get into valve shrouding, etc, and open a real can of worms.

 

Lastly, the compression test in itself will not tell you what the compression ratio is. This is really a dynamic measurement in that it can change with different cams even though nothing else is changed.

 

I hope I've not misled you on this topic and I'm sure better educated responses will follow.

[A. G. Olphart]

DCR is generally looked at in simple terms... true dymaic compression ratio involves enough variables that accurate measurement is unlikely.

 

Simply- static CR includes the entire stroke, as Terry mentioned. Generally DCR is figured from the point at which the intake valve closes, and actual compression can begin.

 

Good explanation and great free calculator here: http://members.uia.net/pkelley2/DynamicCR.html

 

I don't beleive valve size would enter into this unless they (and not port size/volume) are a significant restriction to flow. Logically this could only occur at higher RPM, so a true DCR might vary with engine speed....

 

Not nearly as thorough as Grumpyvette, but hth.

[zolorin]

I have not posted on the hybridz forum for the longest time, so lets get back to the question.

 

Static compression is if you assume that you would fill the cylinder fully with air and then close all of the valves and then bring the piston from BDC to TDC in a very slow manner (adiabatically). Now assuming no heat transfer (and this is what trully adiabatically means) from the compressed air to the outside the pressure will follow ideal gas law such that p1*V1=p2*V2 which means that the pressure will rise proportionally with the volume decrease

So if volume is reduced 10 times then pressure is up 10 times hence Static CR of 10:1.

 

This is cute but worthless in the sence that the engine is not compressing in the slow manner but does it very fast some times (7000 rpms is fast) So there is a velocity contribution to the pressure build up, so it can actually increase from being 10:1 to 11:1 or even 12:1 if you compress really fast.

 

Now to make this more complicated you have cam overlap and timing where the valves open and close which allow extra air to get in some cases or bleed off the dynamic pressure at others, that is why you run a lumpy cam if you have insane amount of CR.

[Dan Baldwin]

Here are my musings, but I'm no expert!

 

Compression ratio is a REFERENCE number. There is no need to bust your balls trying to find out what the ACTUAL compression is, just go by the geometrical ratio, as that's what just about all the useful comparison data out there will be based on. Dynamic or effective compression ratio is next to meaningless, it seems to me. A cam that gives good volumetric efficiency at lower rpm will require a lower compression ratio than a cam that gives good volumetric compression ratio at higher rpm, because of there's more TIME for detonation to occur at lower rpm, not because of less "effective" or "dynamic" compression ratio. If you can get close to 100% volumetric efficiency with a big cam (and you can, even MORE than 100% is possible over a very limited rpm range), then effective/dynamic compression ratio as I usually see it presented (based solely on when the intake valve closes) is sort of an invalid concept if you ask me, though it can lead to a correct conclusion (more cam => higher CR possible).

 

For the same reason, cylinder pressure during cranking doesn't really tell you anything conclusive as it's dependent on too many issues. A mild cam with high CR will yield greater cranking pressure of course, a big cam requires much higher rpm to fill the cylinder.

 

Just consider the geometric compression ratio, and know that with a bigger cam you can go much higher with it.

 

Like fastener bearing stress, it's not an ACTUAL stress, it's just the load divided by diameter times thickness. Test data is in terms of this REFERENCE stress, not actual which would be MUCH harder to measure. So the design, stress, and test engineers all have a geometrical reference value they all agree on. Same with CR. Absolutely no need to worry about the ACTUAL compressible fluid thermodynamics going on when all the reference data is going to be in terms of what geometric CR is possible given a fuel octane rating and cam.

[Guest butlersZ]

So static compression is the common type that most people talk about when matching parts to build a motor, right? So than with a bigger cam you can run more compression, is there a formula to figure out what or how much compression one can run given the cams duration,overlap, etc.?

 

One reason I ask about compression is because I see a lot of heads that for instance have a 58 cc chamber, that can be used on a 350SBC and see that the 305 SBC heads are 58 cc, but they can't be used on a 350 without having 12-1 or higher compression which is not good for street and pump gas. But those other 58 cc heads,some which have a better designed combustion chamber are being used on the 350 and with pump gas. I have a set of 305 heads that have been decently ported with larger valves, unshrouding, and such, I was wanting to run them on my Z but figured the compression would be way to high so I put a set of 70cc 327 heads on instead.

 

So without 100%VE, static compression will be lower right? And with a larger cam the compression is lower at low rpm and raises as does the rpm, correct?

[blueovalz]

Again, I've no numbers to illustrate this statement, but the quench area and design (of it) in the heads has a lot to do with compression ratio tolerance as well. A good, tight quench area (oh, that sounded good!) will allow one to run a higher compression ratio because it staves off detonation, verses a relatively small (or thick) quench area. If memory serves me correctly, once the quench area thickness (or height) gets deeper than about .040", it is just about useless, so this needs to be as tight as possible and I believe cover as much piston area as possible as well. But Dan's philosophy is right in that you can do so much research and thinking about this subject until you eventually may give up (unless you want that very last 10 ft/lbs) and just simply look at the cam manufacturers specs (or talk to them on the phone) and see what they recommend on CR for a specific cam and it's application, and then go with that.

[Kevin Shasteen]

Here's my attempt. FWIW, it is much easier to understand the DCR & SCR issues w/pictures.

 

To understand CR you must first mentally grasp the distinctions between a Compression Ratio -vs- that of Compression.

 

Compression is a pressure measured w/a guage of some kind whereas a Compression Ratio is a measurement between two different Volumes.

 

Pressure is not the same as a Compression Ratio therefore the size of the valve or the size of the port will not affect the CR, but will affect the Pressure in the Cylinder at different RPM's as well as the Airflow Velocity at different RPM's.

 

The two volumes being measured is the space immediately above the piston when said piston is at TDC -vs- the space above said piston when piston is at BDC.

 

SCR measures the space above the piston when said piston is at BDC and when the piston is at TDC yet wont take into account how much space remains above the piston when the Intake Valve Closes (IVC).

 

DCR measures the space above the piston with the piston position that corresponds w/the IVC, as well as the space above the piston when said piston is at TDC.

 

When approaching an engine build the DCR dictate the optimum SCR...not the other way around, and yes you can do the math but requires a Crank Angle Chart and this requires Trigonometry.

 

Kevin,

(Yea,Still an Inliner)