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Static Compression VS Dynamic compression

General engine tech -- Drag Racing to Circle Track

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RevTheory
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Re: Static Compression VS Dynamic compression

Post by RevTheory » Tue Jun 18, 2019 10:24 am

I almost hate to ask about a rule-of-thumb on here but I will anyways :D

I know this topic is complex but can't it be somewhat useful in the beginning phase? Say the initial combo shows around 7.5:1 DCR, you might think "cool, I should be safe on pump swill for my hot, street." Then mission creep sets in thinking you can get away with more static compression and it's now showing over 8.5:1 DCR.

Even as a crappy rule-of-thumb, couldn't that be used to tell you that you're going too far and need to reconsider?

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Re: Static Compression VS Dynamic compression

Post by David Redszus » Tue Jun 18, 2019 11:21 am

RevTheory wrote:
Tue Jun 18, 2019 10:24 am
I almost hate to ask about a rule-of-thumb on here but I will anyways :D

I know this topic is complex but can't it be somewhat useful in the beginning phase? Say the initial combo shows around 7.5:1 DCR, you might think "cool, I should be safe on pump swill for my hot, street." Then mission creep sets in thinking you can get away with more static compression and it's now showing over 8.5:1 DCR.

Even as a crappy rule-of-thumb, couldn't that be used to tell you that you're going too far and need to reconsider?
Compression ratio, either static or dynamic, are merely steps along the path to useful information.

Starting with static compression, we compare chamber volumes at TDC vs BDC. Those volumes are determined by bore, stroke and chamber volume. The displaced volume starts by assuming compression begins with IVC at BDC.

But, in an actual engine, the IVC point is not at BDC; it is at some later crank angle. Now we will consider that compression begins after IVC, which can be wherever we choose based on the camshaft. Obviously, the DCR will vary with IVC, if everything else remains constant.

But those are only ratios and have no effect on engine performance. What we need to know is chamber conditions during the compression stroke, such as compression temperature and pressure.

An excessively high compression pressure make it more difficult to fire the spark plug, requiring more firing voltage. A high compression temperature makes it easier to fire the spark plug. The closer the compression temperature is to fuel boiling, the easier to evaporate. The closer to fuel auto-ignition temperature, the easier it is to ignite the fuel.

But more factors are at work such as rpm, squish velocity, throttle position, mixture, and a host of others.

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Re: Static Compression VS Dynamic compression

Post by CamKing » Tue Jun 18, 2019 11:35 am

RevTheory wrote:
Tue Jun 18, 2019 10:24 am
I almost hate to ask about a rule-of-thumb on here but I will anyways :D

I know this topic is complex but can't it be somewhat useful in the beginning phase? Say the initial combo shows around 7.5:1 DCR, you might think "cool, I should be safe on pump swill for my hot, street." Then mission creep sets in thinking you can get away with more static compression and it's now showing over 8.5:1 DCR.

Even as a crappy rule-of-thumb, couldn't that be used to tell you that you're going too far and need to reconsider?
If I came up with a rule of thumb that said,"Starting with 9:1 static compression, for every X amount of seat duration above 260 degrees, you can add X amount of static compression for 87 octane". It would be pretty easy to come up with something like this, and would help some people out. It wouldn't be correct, and it wouldn't teach anyone anything, but some beginners would find it useful. n the cases it's not correct, it could cause engine damage, if the engine builder believed it as gospel.
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Re: Static Compression VS Dynamic compression

Post by Stan Weiss » Tue Jun 18, 2019 11:48 am

Using this same engine as my other posts. Our 290 psi cranking compression in a running engine would be about 49% VE. What this graph shows is what the cylinder pressure would be @ IVC for a give VE.

Stan

ab-cgp-ve.gif
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Re: Static Compression VS Dynamic compression

Post by Enigma » Wed Jun 19, 2019 3:48 am

CamKing wrote:
Tue Jun 18, 2019 10:13 am
Enigma wrote:
Mon Jun 17, 2019 5:07 pm
"dcr", which is a fixed, calculable ratio........
No, it's not.
It assumes the cylinder stops filling at BDC, and compression doesn't start increasing until the intake valve closes.
This is 100% false. That's not how a running engine works.
No, it does not. It assumes that the cylinder stops filling at the ivc point, which it does. Unless you know something I don't. That's the whole point of it, the cylender continues to fill after bdc, so dcr (that r stands for ratio, a simple formula) tends to include some of the effect of the air column ram. But beyond that, it has nothing to do with the PRESSURE, and is just a formula. Again, you guys are confusing dynamic cylinder PRESSURE (which is affected by physics, fluid mechanics and dynamics, and whatnot) with a simple RATIO. Saying it's worthless is no different than saying that calculating SCR is worthless because it doesn't take into account water grains and fluid mechanics and ram filling. Dcr is nothing more that a fixed, calculable ratio. Volume at IVC point, versus volume at TDC. It doesn't change (unless you want to get detailed enough to spin it and add in bounce. Sure, it doesn't take into account pressure and ram filling and fluid dynamics, but it's not supposed to. That's the job of DYNAMIC CYLINDER PRESSURE. Dcr is just a fixed, calculable ratio. Argue that all you want, but that's what it is.

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Re: Static Compression VS Dynamic compression

Post by digger » Wed Jun 19, 2019 5:28 am

Yep it's just another rule of thumb that can be totally useless when used to the extreme . It's a bit silly to call it dynamic compression though when it's essentially constant (excluding vct or valve motion discrepancies )...lol

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Re: Static Compression VS Dynamic compression

Post by Enigma » Wed Jun 19, 2019 8:20 am

digger wrote:
Wed Jun 19, 2019 5:28 am
Yep it's just another rule of thumb that can be totally useless when used to the extreme . It's a bit silly to call it dynamic compression though when it's essentially constant (excluding vct or valve motion discrepancies )...lol
Exactly. Just a general guideline, like lobe intensities (I know mike will love that😄). That's why it's called dynamic compression RATIO. Pressures and deltas and all that jazz are something entirely different, and quite difficult to calculate with any degree of accuracy. They also constantly change throughout the operational rpm, but the dynamic simply measures the ratio of volume available, and does not change. It's not meant to take into account what's ACTUALLY in the cylinder. It doesn't care if it's 110% full of air, or has three spoonfuls of pudding in it. Ratio.

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Re: Static Compression VS Dynamic compression

Post by Stan Weiss » Wed Jun 19, 2019 10:01 am

Static compression ratio is just that once the engine is built it is static. But while the engine is seating in the car I can change dynamic compression ratio by advancing or retarding the camshaft. If solid lifter by loosing or closing valve lash. All of these changes will show on a cranking compression test.

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Re: Static Compression VS Dynamic compression

Post by CamKing » Wed Jun 19, 2019 10:09 am

Stan Weiss wrote:
Wed Jun 19, 2019 10:01 am
Static compression ratio is just that once the engine is built it is static. But while the engine is seating in the car I can change dynamic compression ratio by advancing or retarding the camshaft. If solid lifter by loosing or closing valve lash. All of these changes will show on a cranking compression test.

Stan
What they show on a cranking compression test, may or may not relate to what they show at RPM, when the engine is running.
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Re: Static Compression VS Dynamic compression

Post by Enigma » Wed Jun 19, 2019 3:44 pm

Stan Weiss wrote:
Wed Jun 19, 2019 10:01 am
Static compression ratio is just that once the engine is built it is static. But while the engine is seating in the car I can change dynamic compression ratio by advancing or retarding the camshaft. If solid lifter by loosing or closing valve lash. All of these changes will show on a cranking compression test.

Stan
But once it's set, it's set. Adjusting it will show on a compression test, but so will adjusting the stroke in the short block. Same concept, just more work. Dynamic is what it is when it's set, just the same as static.

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Re: Static Compression VS Dynamic compression

Post by Stan Weiss » Wed Jun 19, 2019 4:01 pm

Not sure if this is going old school or just plain ancient. Look up vari-cam. Today how about VVT.

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Re: Static Compression VS Dynamic compression

Post by Enigma » Wed Jun 19, 2019 9:18 pm

Stan Weiss wrote:
Wed Jun 19, 2019 4:01 pm
Not sure if this is going old school or just plain ancient. Look up vari-cam. Today how about VVT.

Stan
I remember the split-lobe cam. And the one that adjusted itself much like a go kart type "adjustable gearing" (centrifugal force clutch), and you're correct. Just as with vvt. But it's still just a fixed ratio, once the adjustments are made it just happens to have two different ratios. Volume at IVC vs volume at TDC. Changing the IVC does result in a different dcr, but once the changes are made........same deal. Fixed ratio that cares nothing about pressure. You can't measure DCR with a pressure gauge, the same as not being able to measure SCR with a pressure gauge. It's a calculation. This versus that. And (in this case), you need fixed points of reference, not floating targets.

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Re: Static Compression VS Dynamic compression

Post by digger » Wed Jun 19, 2019 9:36 pm

It's not a fixed ratio if it changes drastically throughout engines operation because the cam position or event is variable by design. It's not a good way of wording it

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Re: Static Compression VS Dynamic compression

Post by exhaustgases » Wed Jun 19, 2019 10:08 pm

Here is an engine that has a dynamic compression ratio, https://www.youtube.com/watch?v=A6H66xfEZC4. :lol:
What ever happened to nitro? https://www.tfxengine.com/ This could help in this discussion.

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Re: Static Compression VS Dynamic compression

Post by cjperformance » Wed Jun 19, 2019 11:16 pm

Ok , lets say we have an engine with SCR at 11:1 and the given IVC says it has a DRC at 8:1
Think about the inputs that make you arrive at these figures.
These calculations both assume no change in air pressure be it -'ve or +'ve., they dont even take air or anything other than solid mechanical boundries into consideration.

The ONLY difference with DCR is that the IVC makes the cylinder LOOK mathematically smaller therefore reducing the Compression RATIO calculated from that point.

No variable cam timing etc, no fancy lifters, IVC effectively is not changing. We now have a mechanically fixed SCR and fixed DCR.
This is why DCR is a confusing term. Nothing that now happens mechanically within the engine is changing anything that alters SCR or DCR. (Im not getting into block thermal growth, valvetrain flex etc)

When you run the engine you do not change the dynamic or static compression RATIO , but you most certainly do change how much air you allow into that cylinder to be compressed. Does not matter wether you choke off the intake with a throttle plate and only allow that cylinder to fill to 1/8th capacity or run WOT on a good combo and allow 100% fill OR pump air/boost in and have that cylinder under pressure at BDC and IVC and have say 110% fill you do not change the RATIO at which you compress anything,, you only change the initial MASS, ie MASS in the cylinder prior to IVC, this altered MASS is then compressed at that same RATIO , BUT you achieve a higher or lower cylinder PRESSURE at TDC when you change the initial mass to be compressed.

From DCR you can calculate a basic cylinder pressure to give a WOT cranking speed cylinder pressure (compression test) reading. Once you throttle govern incoming air to that cylinder and have no spark/explosion present, the cylinder pressure reduces because you have allowed less air into the cylinder. Less air compressed at the same RATIO gives less TDC cylinder pressure as there was less mass present to compress.
If you had boost/positive pressure at the intake valve and did a WOT cranking compression test you will see a higher crankin cylinder pressure because you had a possitive pressure(extra mass) in the cylinder at IVC.

Dynamic or Effective Cylinder Pressure changes constantly in a running engine.
DCR is effectively fixed and its outcome depends entirely on the variable figure of mass in the cylinder at IVC.
SCR and DCR i think get confused a lot due to SCR being just a simple of one big space(BDC total volume) being compared to a small space(TDC volume) seeming so simple as compared to now adding an IVC into the thought process.
For basic DCR just forget that any portion of stroke/cylinder prior to IVC exists.

P.s. sorry for typos, i have a sore thumb
Craig.

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