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Dynamic CR Calculation Done Correctly ?

11K views 20 replies 7 participants last post by  Equil 
#1 ·
I used the web site that is linked from here in the advanced CR tech article. writtten by Michael Delaney, which I find all of his articles very informative and in-depth.

I know the mm to in conversion is not 100% but this is what I got

Camshaft, Rod Length, Boost and Altitude Correction to Compression

Your engine summary is as follows: Bore 3.15 inches,(81.55mm) stroke 3.45 inches,(87.2mm) rod c-c length 5.430 inches, with a static compression ratio of 11.5 :1. Your camshaft specifications call for an inlet valve closing of 46.0 degrees ABDC (after bottom dead center).

Your chamber volume is 41.96 cc's. With this camshaft your dynamic, or effective stroke is 3.07 inches. Your dynamic compression ratio is 10.28 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 219.22 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 0 PSI is 10.28 :1.

Knowledge is power.

I have had the same rod lenght and stroke I just got oversized pistons +.020. I don't know if this is efficent or not. Any input would be great. it seems like it would be decent to me at least with the stroke not being effective .38in of the stroke. Which is to be expected with a stage 3 cam. Not to mention this can all be changed to a certian amount with adj. cam. gears. Right????
 
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#2 ·
I. Conversion Factor -> 1 in. = 25.4 mm

To convert the mm dimension into inches divide by 25.4


II. Stock Intake Cam ABDC Spec (in degrees)

LS (b18a) 31

LS (b18b) 23.5

Civic Si with bseries (b16a all generations) 40

GSR (b17a & b18c1) 40

ITR (b18c5) 45

JDM ITR (b18c) 45



III. Stock Dynamic Cranking Pressures (assuming 600 ft altitude in psi) [dynamic CR]

Using the dynamic CR calculator ( http://www.rbracing-rsr.com/comprAdvHD.htm ):

LS (b18a)176 [8.67:1]

LS (b18b)181 [8.84:1]

GSR (b18c1) 188 [9.11:1]

ITR (b18c5)197 [9.46:1]

JDM ITR (b18c)207 [9.82:1]



-----------------------------------------------


you want your cranking pressure to be at least the same as stock and no lower. A 5% increase is a significant increase. Most people try to get from 15% to 22.5% increase in dynamic cranking presures from stock with big cams.

0.020 in. over is a 0.51mm increase in bore. stock bore is 81mm.

what is the stage 3 cam's intake ABDC spec?
 
#3 ·
when I went to Toda B's and had to increase the CR with 12:1 Endyn-Wiseco pistons, the dynamic cranking pressure went up 23% from stock. The dynamic CR went from 9.11:1 to 10.43:1. Ideally, those cams like 10.69:1 dynamic CR when you plug in the ideal static CR for those cams.

when I was running stock 10.0:1 CR with those Toda B's, the dynamic CR was 8.69:1 (LESS THAN STOCK GSR CAMS WITH STOCK GSR PISToNS) and the cranking pressure (WHAT TURNS THE CRANK) was 177 psi (LESS THAN STOCK GSR CAMS WITH STOCK GSR PISToNS). In fact, my cranking pressures were LOWER than those on an LS with stock LS cams and LS pistons!!

And it showed on the dyno. We made 168 whp with Toda B's and stock GSR CR after hours of fuel tuning. That was less than what I had made with my ITR cams in a GSR with stock CR (182 whp)!!

Imagine spending $1000 for a pair of cams and $400 for valvesprings (and then having a ***** of a time installing them with a broken timing belt and bent valves along the way & more money tuning them) to find that you lost power.

This isn't just true for Toda cams. It's true for ALL big cams for upgrading when you use the wrong static CR with them.

This is what you call OVERCAMMING THE ENGINE: choosing a cam that's too big for the engine. You lose cylinder pressure and that's what turns the crank to generate power.

Now with the right CR, the engine makes well over 200 whp.

So it isn't about getting bigger. Bigger is not always better.

It's about knowing what parts affect other parts and how they affect them.

It's about knowing which parts work well together not just about knowing the 1 part and it's specs by itself.

This is called knowing the engine package or engine combination.

It's true not only for cams. It's true for all parts that you upgrade. You have to look at the BIG PICTURE
(The forest from the trees) BUT still keep an eye on the details like the specs (Know the trees from the forest).

Nothing worthwhile is always simple.


P.S. here's more discussion on this topic and a practical application of the ideas presented in this thread.


here's another thread with a practical example of using the dynamic CR calculator to find the best static CR to go with your cams
 
#4 ·
Here are some aftermarket cams' intake ABDC closing specs:

GSR:40

ITR:45


Sk2 Stage 1: 43 (please check)

Skunk2 Stage 2 : 49 (please check)



Toda B: 50

Toda C: 53


Buddy Club Spec 3 Plus: 45 (vs. some web reports of 76 please check like on honda tech..bogus?)


Buddy Club Spec 4:52



CompCams/Zex Stage 1:35

CompCams/Zex Stage 2 :44


Rocket M22: 50


Jun 3:


Spoon:


Crower 63403 : 46

Crower 63404:


Piper BP285:


please add to this database if you have the cam spec sheets info after buying aftermarket cams.
 
#5 ·
Wouldnt the RPM of the engine play a role in the dynamic CR?

I am thinking this because the calculator uses altitude (for air density) and as the RPMs increase so does the volumetric efficiency (ie. more air stuffed into the cylinder at higher RPMs). The increased amount of air would be similar to going to a lower altitude. right?
 
#6 ·
you would have to firgure the "boost" pressure caused by a tuned intake system and at what point that occurs.

Then you factor that into the formula by adding it to the 14.7psi atmoshpheric pressure when solving for Volume/Pressure Index and cranking pressure increase.
 
#7 ·
how does one figure out this "boost"? As the RPMs increase so does the "boost". Also try to put in any amount of boost, it only changes the dynamic CR not the cranking pressure (calculator not right?).

Seems logical to me that the more air you stuff into the cylinder the higher the cranking pressure.
 
#8 ·
CR is the ratio of uncompressed charge to compressed charge.

If you have 14.7 psi of air pressure at BDC and a CR(dynamic) of 10:1 you will have a pressure of 232.98 psi (absolute)


If you have 16 psi of air pressure at BDC and a CR(dynamic) of 10:1 you will have a pressure of 253.58 psi (absolute)


CP Cranking Pressure (absolute) CP = (CRE ^1.2 × AP)
CRE Compression Ratio, Effective (dynamic)
AP Atmospheric Pressure 14.7 psi @ sea level (zero elevation); use the correct lower figure for higher elevations

The 16 psi value is assuming you had reached a resonant frequency and was getting a 1.3 lb "boost" from acoustic supercharging. the Boost is like the AEM hump, the sonic waves forec air in, but only at certain engine speeds. Below =no boost, above=no boost.

The secret tomb of knowledge
 
#9 ·
V/P Volume/Pressure Index V/P = CP × VE × N × .3% (.3% or .003 is a correction factor to return a useful 2 digit number roughly proportionate to torque)
 
#11 ·
The calculator shows you what cylinder pressure you lose out of the intake cam as it closes on the compression stroke (at a given altitude and boost which also affect cylinder pressure).

The IM or ITB or rpm isn't in the calculation. We are well past the ram effect or extra filling effect of the IM or ITB on the intake stroke.

We are in the compression stroke which is a prelude to the power stroke. If your peak cylinder pressure at the sweet spot of the power stroke (i.e. 10-15 crank degrees ATDC) is lower, the force that turns the crank is less.

So the effect of the intake cam's duration on the cylinder pressure bleed off during the compression stroke sets the starting point for the Cranking Pressure during the power stroke,...regardless of the rpm since it will be proportional at each rpm.
 
#14 ·
it's proportional within the compression stroke...at low vs. high rpm.

yes cylinder filling (flow capacity) increases as the rpm's rise. We know this because peak VE occurs at peak torque.


however, the amount of bleed off is proportional so that even though the amount of pumping into the cylinder increases as the rpms rise, the %age of loss to the total volume/pressure is the same...at least that's what this model presumes.

take a look at this cylinder pressure analysis (cylinder pressure vs. crankshaft angle) that was in the ignition article here.:




this is 5 consecutive combustion strokes in one engine where zero is TDC and the spark was ignited 30 degrees BTDC which is what you see on some Tegs these days in the upper rpms.

the solid lines are the pressure due to the expansion of gases during combustion after the mix has been ignited by a spark.

the dotted line is the cranking pressure (or "start up" pressure before combustion and reflects your pumping losses secondary to poor ring seal and blow by or loss from reversion or valve seal loss.


you can see that the total cylinder pressure dwarfs the cranking pressure in the relative scale. But the importance of cranking pressure is that it sets the "springboard" from which the combustion takes off. If it's too low, your peak pressure at the sweet spot of the combustion stroke will be proportionately lower at each power stroke (at any rpm). If you start off with lower cylinder pressure before combustion, you will have less "push" down onto the piston top to turn the crank.

And as little as being 5-20 psi off can make that much of a difference, especially as the frequency of "pushing down onto the piston top to turn the crank" increases with increasing rpms. The cumulative effect of that and the momentum of the crank turning is noticeable.

I have to tell you that when I ran the Todas with slightly above stock CR, it lagged around like a dog panting on a hot muggy Louisiana summer day. When I dropped in the 12:1 pistons, it was like someone flipped a switch and the engine woke up.


to show you just how sensitive this effect is, even a loss in cranking pressure of 5 psi below the stock cranking pressure (stock cams at stock CR) dropped my whp output by 8-12 whp with the Toda cams at stock CR compared to the stock cams (ITR cams) at stock CR.


with altitude, you are dealing with another factor though: namely atmospheric pressure and oxygen content.

The leaner mix at a lower "push" into the cylinder spells lean condition risk.

I would be much more conservative with the CR increase unless you know you can fuel it properly to control the burn rate properly .


I would consider the more conservative range of 15% (max. gain in static CR) rather than the usual 20% that we shoot for at lower altitudes. This is an educated guess on my part since I've never run an engine at altitude.

There's several altitude correction equations for cams and cranking pressure. Panic's website with the victory library articles (victory motorcycles) is an excellent read.
I think I hyperlinked to it in the cam-static CR article here.




PS you had asked me for some technical books on the topic: I would google engine cycle analysis and cylinder pressure. For books, check out the sae.org website's bookstore and papers online and visit your local university or college's mechanical engineering dept. and see if they have a formula sae group who can help point you to some material and hands on experience.
 
#15 ·
So here is a differing opinion quoted from the victorlibrary site linked to above about the dynamic compression ratio concept:


DCR Errors

“Dynamic Compression Ratio” (DCR) is offered (elsewhere) as a method of approximating how much power will be lost by extended intake closure (i.e., cylinder pressure is reduced).

DCR is very useful in predicting what octane is needed at lower speeds to help setting idle (initial) spark vs. advance curve length, stall speed, &c. However, this is only partially accurate due to several errors and misconceptions.


1. The actual power loss is only present below the points where both full capture (where intake reversion ABDC stops) and positive flow-through at overlap occur (although these may not be close together).

Above that point power is increased (over the milder cam), either by improved volumetric efficiency (and cylinder pressure, since a larger percentage is trapped in the cylinder), higher RPM reached @ the same efficiency level, or both.



2. The power loss is not solely due to reduced cylinder pressure (which is the output of the Kelley, RSR & DCR calculators), since the lower pressure is also acting through a shorter effective stroke (measured from intake valve closure ABDC) and therefore, suffers from two reductions.

My own method factors in the effective cylinder displacement as well, for a closer approximate at how much power is developed at low speed.


3. DCR is widely considered to be an accurate barometer of knock resistance (e.g., “up to 8-1 DCR can be used with XX octane”).

This is not true, and not safe, since (in a high compression motor with its DCR reduced by late intake closure) after the early low-pressure period expires (at the beginning of the torque curve) actual combustion pressure will be at least as high with the bigger cam than it was previously - even though the DCR is lower, and therefore suggests that lower octane is safe. However, in a motor with 14-1 static CR the gas pressure at its torque peak is not at all reduced by a very late intake closing point, although the (lower) DCR may indicate that 92 octane &c. is sufficient.


Since the knock will only occur at high speeds it may not be audible, and will reduce power (lower MPH) even if no damage can be detected. In my opinion, the power sometimes gained by retarding spark in high gear is actually an attempt to recover some of this loss - but would be better served by reducing the static ratio slightly.


DCR is a curve or slope of cylinder (not combustion) pressure, with Position 0 (the absolute low end) at cranking speed, then a small rise to idle speed, then another rise to the capture point &c. After this point (and especially near the torque peak) the static CR becomes more important, since almost the full stroke length is captured and compressed, regardless of the intake closing point.


Any cam will determine the “slope” (or rate of rise) of the DCR curve. A long-duration cam with its attendant late intake closing point will have a high degree of rise, a mild cam less, &c. A longer cam will also extend (stretch) the range of RPM that the slope covers, sometimes over several thousand more RPM.


The static ratio determines the height of the cylinder pressure line at Position 0 (cranking speed). With high static ratio the entire curve is higher, with the curve's upward intensity being governed by the intake closing point.


It's possible to design a DCR that looks promising, but will not provide any more power, by assuming that there is no limit to either static ratio or intake closure - and, of course, neither is true. Some motors cannot turn fast enough (due to stroke length, weak valve gear, high reciprocating weight) to reach their capture point if the intake closure is too late, and will produce more power with more conservative cam timing. A motor with limited static ratio (flathead) must conserve cylinder pressure by limiting intake closure for the same reasons.


Another error in use of DCR calculations for low-speed power prediction lies in the fact that a smaller volume of mixture being compressed to a higher ratio. Even though the pressure gauge reading taken during cranking or idling is higher, the total of cylinder pressure times the actual mixture volume captured may still be lower (compared to the original milder cam and moderate compression ratio).

To sum up:

DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics.



Of course, the opinions above come from experience with V8's, but they make for interesting reading. So where is the data that supports the fact that you need to keep the dynamic compression ratio higher by up to 25% than it was before you changed the cams to ones with longer duration and thus extended ABDC opening angle. Do we have dyno plots of cam changes that lowers the DCR without changing the Static CR and then of the same cam with an increase in DCR by increasing static CR. I like to see proof and I do not think I have seen any. I want to believe, but concrete data showing the effect will help me believe.

For example, say you change the cam and it lowers the DCR by 8%, can you not gain power everywhere due to other effets on the engine?
 
#17 ·
right now the dcr is 13% higher than stock on my engine(b18c),i'm planing on using ss2 cams, with those installed the dcr will be 9% above stock

are those acceptable numbers?or would it be overcamed?should i try to make the dcr 15% above stock?or does 9% sound good?
 
#18 ·
yes as long as you are above the stock CP you will be fine. however, the ideal range is closer to 15-20% from looking at the motor packages and reputable unmanipulated dynosheets (i.e. did not rig the dyno run or correction factors to artificially inflate the numbers) that I've seen over these past 8 years of experience.

edit: sorry to bring this back from the dead and for missing this back then.

I didn't see the last question until now and had to at least give an answer for future members researching this topic in the database.
 
#21 ·
Sorry to bust this out the grave btw... can anyone confirm my above statement and have a link for cranking pressure formulas?
Edit.
Re Edit. It works, you can deduce chamber volume with CR-1 dividied by CC volume. The calculator works
 
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