static vs dynamic compression ratios

[HeinzE]

When discussing compression ratios the topic of static ratios vs corrected (dynamic) ratios invariably gets mentioned and there seem
to be some differences of opinion on which is the more important and whether either one can tell much about the actual pressures
within the cylinder due to the changing intake port pressure as an engine revs up and down and the throttles is opened and closed.
A complicated process to be sure. However, given that we have to start somewhere, and agreeing that the intake valve closing point
will change the above piston volume available for compressing and that the engine is being run at full throttle and within it's intended
powerband, and that the ignition timing and fuel are good, what is the maximum corrected compression ratio most common today. Also, how much difference between lquid cooled and air cooled engines should there be, if any? I have heard as much as 12 to 1 corrected for liquid cooled but not much more than 9 to 1 for air cooled. Also I imagine that these ratios will be different for an engine run for only seconds vs one running
for hours, but in any case what are the ranges you guys have seen?

[panic]

Dynamic compression value as a comparator drops off sharply when the engine starts its torque band.
"Engines with low static CR can't use late intake valve closing points": Harley-Davidson spent $$$ proving this wrong from 1940-69.

[CamKing]

You're just wasting your time.
To show how meaningless DCR is, take these 2 cams.
Cam#1: 300/304 @ Seat, ,500"/.500" Valve Lift, 114 LSA, 112 ICL
Cam#2: 320/336 @ Seat, .750"/.720" Valve Lift, 106 LSA, 102 ICL

According to every DCR calculator, these would have the same "corrected" compression ratio.

[David Redszus]

You're just wasting your time.
To show how meaningless DCR is, take these 2 cams.
Cam#1: 300/304 @ Seat, ,500"/.500" Valve Lift, 114 LSA, 112 ICL
Cam#2: 320/336 @ Seat, .750"/.720" Valve Lift, 106 LSA, 102 ICL

According to every DCR calculator, these would have the same "corrected" compression ratio.

DCR is one of the most important parameters that can be used to determine engine performance.
Remember that both SCR and DCR are merely ratios, not absolute values.

Consider two engines with the same SCR but with different DCR numbers, under the same inlet
conditions.

SCR 9.50..14.5 psi... 100F deg (at inlet)
DCR 7.02..225psi......752F deg (at TDC)
DCR 8.02..271psi......817F deg (at TDC)

Even though both have the same SCR, the absolute cylinder conditions at TDC are quite different.
SCR has no real value, except as a rough comparison of two engines that are identical in all other areas.

SCR cannot be used to compute either compression pressure nor temperature.
DCR can be used to determine a compression pressure ratio which allows the computation of thermal efficiency.
SCR does nothing

Remember that DCR is still only a ratio; inlet conditions must be considered in order to determine actual
compression pressure and temperature. And those inlet conditions will change with throttle, resonant
induction tuning, boost and temperature.

Does camshaft IVC timing affect DCR? Yes, indeed it does. DCR is shown in red.

IVC btc. ..12.....13.....14.....SCR
90..........7.3....7.9.....8.4
100.........8.2....8.9.....9.5
110.........9.0....9.8....10.6
Changes in camshaft IVC will have a substantial effect on DCR.

[HeinzE]

Cam King, I think this could only be correct if the cams were installed in engines with identical bore and stroke and chamber volumes and
the cams had identical intake valve closing points. Change any of these parameters in either one of the engines and I believe the "dynamic" cr would measure differently. Obviously it is clearly possible for cams with different intake closing points to have the same measured dynamic ratio, but one of the engine parameters would have to be different between them. As you well know, like static cr the dynamic ratio is just a measurement of volume ratios and says nothing about cylinder pressure or fill density, but then niether does static cr. As I said in my original post, the intake process is
a complicated affair and no one number or ratio is going to explain a whole lot about it. Certainly intake port pressure throughout the intake
cycle is, in my opinion,way more important than most other factors as far as achieving good VE is concerned, but neither static nor dynamic cr will tell you much about that. In my own engine builds I certainly will measure the static as well as corrected cr if for no other reason than to know what
they are, and have found in some engines that by either changing the static cr or intake closing point specifically to change the dynamic cr have
been able to eliminate pinging or improve performance. In the end all these ratios and pressures are just numbers, and some combination
of those numbers will work better in some engines than others. Some tuners put more store in static cr than others and some obviously pay attention
to dynamic as well. I have no issue with either. In Don's book -The Horsepower Chain- he spends several pages addressing dynamic (he calls it
"effective" ratio) and mentions it again in his "additional items" list in the closing to his chapter on compression, and I take that to mean
that there is at least some value in knowing it and understanding how it fits into the overall picture of the compression process. My interest
in it is based on my own experience in working with it and manipulating it to see what changes in engine performance might result. If others
have done the same then I would like to hear more about their conclusions. And, if as you say, I'm wasting my time, well it won't be the
first time. But I'm retired now, and just shy of 75, so I spend my time mostly on what I like and try not to worry too much about it.

[HeinzE]

Guys,
I did not see the post by David Redszus prior to my last posting. If I had, I probably wouldn't have bothered since David did a much better
job at tackling the topic than I could ever have. Anyway, still very interested in hearing anything any of you have to say on the subject.

[CamKing]

You're just wasting your time.
To show how meaningless DCR is, take these 2 cams.
Cam#1: 300/304 @ Seat, ,500"/.500" Valve Lift, 114 LSA, 112 ICL
Cam#2: 320/336 @ Seat, .750"/.720" Valve Lift, 106 LSA, 102 ICL

According to every DCR calculator, these would have the same "corrected" compression ratio.

DCR is one of the most important parameters that can be used to determine engine performance.
Remember that both SCR and DCR are merely ratios, not absolute values.

Consider two engines with the same SCR but with different DCR numbers, under the same inlet
conditions.

SCR 9.50..14.5 psi... 100F deg (at inlet)
DCR 7.02..225psi......752F deg (at TDC)
DCR 8.02..271psi......817F deg (at TDC)

Even though both have the same SCR, the absolute cylinder conditions at TDC are quite different.
SCR has no real value, except as a rough comparison of two engines that are identical in all other areas.

SCR cannot be used to compute either compression pressure nor temperature.
DCR can be used to determine a compression pressure ratio which allows the computation of thermal efficiency.
SCR does nothing

Remember that DCR is still only a ratio; inlet conditions must be considered in order to determine actual
compression pressure and temperature. And those inlet conditions will change with throttle, resonant
induction tuning, boost and temperature.

Does camshaft IVC timing affect DCR? Yes, indeed it does. DCR is shown in red.

IVC btc. ..12.....13.....14.....SCR
90..........7.3....7.9.....8.4
100.........8.2....8.9.....9.5
110.........9.0....9.8....10.6
Changes in camshaft IVC will have a substantial effect on DCR.

You're fooling yourself.
When the intake valve closes is when compression starts, and when the piston is at BDC is not when positive flow stops.
Positive flow stops somewhere between BDC and IVC, and that point changes throughout the RPM range, and many factors effect where that point is.
Compression starts somewhere between BDC and IVC, and that point changes throughout the RPM range, and many factors effect where that point is.

[CamKing]

Cam King, I think this could only be correct if the cams were installed in engines with identical bore and stroke and chamber volumes and
the cams had identical intake valve closing points.

That's my point. If you put the two cams I listen in the same engine, according to every DCR calculator, they would have the same DCR, but in reality, they would be "Dynamically" completely different.

If you can look at those 2 cam examples, and think they would have the same DCR or "corrected" compression ratio, you don't have a very good understanding of how engines work.

In the same engine, they would have the same IVC, but the mass in the cylinder would be completely different. The port velocity would be completely different. The pressure differential between the port and cylinder would be different. Combustion temp would be different. Inlet temp would be different. Exhaust temp would be different. Amount of left over exhaust gasses in the cylinder would be different. At any given RPM, the point where positive flow stops, would be different. The difference in pressure between the top and bottom of the cylinder will be different.

All DCR is good for, is calculating cranking compression, it has no connection, to what's going on, in a running engine.

[David Redszus]

In the same engine, they would have the same IVC, but the mass in the cylinder would be completely different.

Whether they have the same mass or not is irrelevant to the topic, which is compression RATIO, a value based only on comparative volumes.

The port velocity would be completely different. The pressure differential between the port and cylinder would be different. Combustion temp would be different. Inlet temp would be different. Exhaust temp would be different. Amount of left over exhaust gasses in the cylinder would be different. At any given RPM, the point where positive flow stops, would be different. The difference in pressure between the top and bottom of the cylinder will be different.

It is not adequate to assert that additional parameters may exist, if they do. It is necessary to explain how each asserted variable would affect the compression RATIO. Combustion temperature is not valid; neither are exhaust gases, and exhaust temperature in the computation of volumes.

Delta pressure would be useful to know in order to determine the effective inlet closing angle. Actual measured cylinder compression pressure, at each crank angle in a running engine, would provide the necessary confirmation of
calculations. The task of measuring compression pressure in a running engine is not difficult to do and should be done.

All DCR is good for, is calculating cranking compression, it has no connection, to what's going on, in a running engine.

If you think DCR has limited value, then what can be said about SCR, and its totally useless value?

Which DCR calculator do you use?
How does it predict compression pressure?
How does it compare to actual measured pressures?

[rebelrouser]

OK I may be all wet, but over the years many times I get engines that just don't run as well as they were expected to. Many times I get theses engines and the owners have no idea what camshaft was used or what the static compression ratio is. So I do a cylinder leak down test to verify rings and valves will hold air, and then a cranking compression test to get an Idea of what kind of camshaft and dynamic compression ratio is. So while a lot of posts say dynamic compression ratio does not matter to a running engine, I can tell you from a diagnostic point of view the cranking compression test tells me a lot about what kind of miss matched parts may be in the engine. So lets say I get an engine that likes to foul plugs, idles poorly, low intake vacuum, I do a leak down test, it is average 10% cranking compression test is 110 lbs, engine is 383 CID with a 750 carb, and holds 10 inches of vacuum at idle. The ten inches tells me that it is not a huge cam, but the 110 cranking compression tells me it has maybe like 8.5 static compression. So to fix this engine, " biggest bang for the buck" do you raise the static compression or buy a camshaft with less duration and a wider centerline?

[David Redszus]

OK I may be all wet, but over the years many times I get engines that just don't run as well as they were expected to. Many times I get theses engines and the owners have no idea what camshaft was used or what the static compression ratio is. So I do a cylinder leak down test to verify rings and valves will hold air, and then a cranking compression test to get an Idea of what kind of camshaft and dynamic compression ratio is. So while a lot of posts say dynamic compression ratio does not matter to a running engine, I can tell you from a diagnostic point of view the cranking compression test tells me a lot about what kind of miss matched parts may be in the engine. So lets say I get an engine that likes to foul plugs, idles poorly, low intake vacuum, I do a leak down test, it is average 10% cranking compression test is 110 lbs, engine is 383 CID with a 750 carb, and holds 10 inches of vacuum at idle. The ten inches tells me that it is not a huge cam, but the 110 cranking compression tells me it has maybe like 8.5 static compression. So to fix this engine, " biggest bang for the buck" do you raise the static compression or buy a camshaft with less duration and a wider centerline?

An engine with a SCR of 8.5 would typically have a DCR of about 6.5. A this should produce a cranking
pressure of about 186 psia...if there is no leakage. I might look for a valve and/or ring leak. What was the leakage?

[Nut124]

An engine with a SCR of 8.5 would typically have a DCR of about 6.5. A this should produce a cranking
pressure of about 186 psia...if there is no leakage. I might look for a valve and/or ring leak. What was the leakage?

Is there a mine field warning here I missed? Should there be? Popcorn?

How does a 8.5 SCR engine produce 186PSIG cranking pressure?

Mine has a SCR of 10.5 and cranking pressure is 180psig.

[EXCELLENCE AUTO]

An engine with a SCR of 8.5 would typically have a DCR of about 6.5. A this should produce a cranking
pressure of about 186 psia...if there is no leakage. I might look for a valve and/or ring leak. What was the leakage?

Is there a mine field warning here I missed? Should there be? Popcorn?

How does a 8.5 SCR engine produce 186PSIG cranking pressure?

Mine has a SCR of 10.5 and cranking pressure is 180psig.

Umm, If you have a 24 Voly battery this can be achieved... LOL!!!
Yes, minefield. Yes, popcorn. I am here to learn from a man with more real world experience, than most around. When Mikey speaks, me listens...

[hoffman900]

Looking at how compression ratios have changed over the years in the OEM front, where static12-13:1 is doable on pump gas, from the factory, with large bores, it all seems meaningless to compare across different platforms, and have rule of thumb.

Using a 1D sim and beyond, I think makes you realize they're all (static / dynamic compression ratios) are meaningless other than just having a starting measuring point. The topography of the chamber / piston, homogeneity of the air/fuel ratio, cooling of the cylinder, mixture turbulence, etc. all have a way bigger role.

It’s like asking what tire pressure is best. The answer is “it depends”.

[BradH]

I've seen this debate come up on many occasions and have almost universally sided with the "DCR doesn't tell you what it think it does" crowd, primarily because of people who use the associated cranking pressure as a guideline for octane sensitivity / requirement. Those folks aren't taking into account that "Dynamic ÇR" is really "Static CR adjusted for intake valve closing point", and it doesn't reflect the changes going on w/in the engine across the RPM range, which means to me that there's nothing "dynamic" about it.

A race-fuel engineer in an interview that I read many years ago took issue w/ people Band-Aiding high CRs on low(er) octane fuel by throwing in "big" cams w/ late IVC timing. He boiled it down to that being OK up until a certain point in the RPM range, after which the cylinder pressure being created was still going to exceed the anti-detonation capabilities of the low(er) octane fuel. When I've tried to present it in those terms, those same folks still don't seem to recognize the flaws in that approach. "You can't see what you can't see", I guess.

What David Redszus is bringing up makes sense when you can factor in all of the variables that require a far more sophisticated tool set for analysis purposes. I get that there's simply more to the picture than what Joe (Above?) Average does -- or can? -- take into account. From someone whose day job is software engineering, although in a completely unrelated field, that's some pretty cool $hit he must have access to.

Is there a middle ground for those of us not at either extreme of either a) having a massive database of successful R&D and actual builds to leverage, or b) having the ability to model things using highly sophisticated programs requiring extensive inputs?