Just surfing the web when I found a posting on your site about "Dynamic Compression Ratio" by one of your members back in April 2002.

I attempted to add to the thread but it would not allow me to add.

The question on that thread was asking for a formula for calculating a Dynamic Compression Ratio (DCR).

First let me say in registering with your site I like and appreciate your rules. I frequent another site http//www.hybridz.org who has the identical mindset: no put downs...only support: as hot rodders-this is all we should ever do, as in support those who need/request support.

Anyway, back to the this site's thread that caught my attention. The thread had posts which illuded to answer the initial question: which was asking for the formula for calculating DCR. Some of, most of, the answers were about Cylinder Pressure.

Cylinder [Pressure] is not the same thing as Dynamic Compression [Ratio]. A Ratio is not a Pressure: and therefore can not be calculated as a pressure.

The DCR is calculated just like one would calculate the Static Compression Ratio (SCR), such as:

Cylinder Volume + Total Chamber Volume / Total Chamber Volume = Comresson Ratio

The difference between the two ratios, DCR & SCR, is based upon how you determine your Cylinder Volume. The SCR's Cylinder Volume is calculated with the Piston's Position at Bottom Dead Center (BDC) and then Top Dead Center (TDC): then factor in the bore of the Cylinder and you have your Cylinder Displacement (Volume).

DCR differs from SCR in that the DCR doesnt begin its calculation of the Cylinder Volume w/the Piston at BDC: instead, it begins its measurement of the cylinder at the exact position of the Piston that corresponds with your Intake Valve Closing (IVC) event. The IVC is determined by your camshaft lobe's duration: which is calculated on a "Seat to Seat" measurement....not the "@ .050" measurement.

DCR and SCR as a "Ratio" is a mathematical calculation of the mathematical relationship between the Cylinder Volume & the Total Chamber Volume.

In order to calculate the DCR, you will need a Crank Angle Chart. To create a Crank Angle Chart you will have to know Trigonometry, or buy one of the various Engine Analyzer Program's: and DD2000 doesnt flip the bill.

The Crank Angle Chart is created from a process called "Slider Cank Mechanism". Engineers use this process to create their Crank Angle Charts. For the hot rodding crowd, the Slider is the Piston while the Crank is the Crankshaft and the two are connected via Connecting Rod. The center points of our Piston Wrist Pin, Connecting Rod's Upper and Lower Journal, and the Crank's Offset (1/2 the Stroke of the Crank) creates a mathematical relationship known as a Triangle. This is why one must use Trigonometry to create a Crank Angle Chart.

The purpose for the Crank Angle Chart is so that you will know the position of the piston for each degree of crankshaft rotation. Knowing the postion of the piston at any degree allows the engine builder to know the percentage of stroke remaining after the IVC. The chart will list 180*'s (* symbolizes the word Degree): which represents one full Swept Motion of the Piston From TDC to BDC. This Swept Motion factors in the Stroke of our Crank's Offset.

Once you know the %, percentage, of stroke remaining above the piston at the moment that corresponds to the IVC: you can then multiply that percentage by the SCR's Cylinder Volume. The sum to this equals your DCR's Cylinder Volume: calculated like so,

Cylinder Volume after IVC + Total Chamber Volume / Total Chamber Volume = DCR

The purpose for knowing your DCR allows the engine builder to choose the optimum SCR. DCR should be calculated first as it is your DCR that determines your SCR.

DCR takes your modifications into consideration which then determines the required SCR to correct whatever compromise your modifications have had on the airflow velocity.

The DCR & SCR are tools to building an engine: the first step is to know the intent of your engine and car: then choose your level of performance which requires a particular power need at a specific rpm range. Once you have chosen an engine displacement-then you choos a camshaft, it is the camshaft's profile: specifically the IVC that determines your DCR....then your SCR as said earlier.

Anyway that is about as simple of an explanation as I can give without making this post longer than it already is.

For an example of what a Crank Angle Chart looks like you can do a search on the net, or go to your nearest Barnes & Nobles Bookstore and find the Big Block Chevy Book titled:

Hot To Build Max Performance Chevy Rat Motors: Hot Rodding Big Block Chevys...by Ed Staffel: then go to page 108 for the Chart, and page 112-113 for the explanation on how to use the Chart.

Good luck & remember Performance Rules: and the American V8 really rules

Kevin,

PS: dont be bashful about visiting Hybridz.org as our site concentrates on swapping V8's into the Datsun's 240, 260, 280z, and 280zx's. Some have even done a Turbo Buick GN engines into their Z's. A few members have swapped the V8 into the 300z, but mostly our members are swapping the engine's into the earlier generations. I've seen a lot of similar interests here on your board: so I hope you will check us out.

 

I agree that DCR is definitely the most important factor in designing a motor buiild. However

"DCR should be calculated first as it is your DCR that determines your SCR."

Actually DCR is a measurement taken directly from the static compression figures. The cam is what changes SCR into DCR as you mentioned. In order to figure SCR you need the bore/stroke/rod length/piston dish,dome or valve relief volume/measured combustion chamber volume and measured deck height measurements. (I probably left something out:-)

Then with the calculated SCR you can calculate the DCR by using a formula that takes the chosen cam specs.

In short you don't want to build an 11.5.1 SCR motor and use a short duration narrow lobe cam...or it will detonate. Now take that same or higher SCR and add a large duration cam, then detonation or high octane gas isn't a concern, but then driveability is if using an automatic. You want to shoot for 7.5 to 8.0max DCR for a pump gas motor.

I have a pretty good DCR calculator if you want to try it.

 

Thanks for your DCR calculator offer-I have created my own on Window's Excel. If you dont learn to work the math w/out the use of a computer program then you will never see the cronolgical order in which that math must be worked: I believe this is where a lot of confusion lies.

Arent these discussions fun; especially if we all learn something new about cars.

I believe you are confusing Cylinder Volume with SCR; are you not(?). The engine doesnt see pressure until the IVC takes place: correct: As a result it doesnt care what your Total Chamber Volume is-therefore the DCR is not a function of SCR...it is the other way around.

The Cylinder Volume is not determined by whatever your Total Chamber Volume is: you dont need an SCR to pick a camshaft. Your power requirements, whatever they are, determine your performance characteristic you need...and this in turn determines what cam profile you will be running with, then whatever Cylinder Volume Remains after your IVC will determine whatever SCR is required.

The Total Chamber Volume is not determined by whatever your SCR is (that is backwards); your Total Chamber Volume is determined by whatever your power needs are based on the rpm range you wish your power to surface: after factoring in your Cylinder Displacement & Cam's Lobe Duration-this is where your DCR is then calculated. This requirement is based on whatever fuel your power need requires and that fuels ability to resist detonation: as in pump gas or race gas.

The link between the Total Chamber Volume and SCR is the DCR. A good street DCR is 7.5:1 to 8.5:1 while a good dedicated race DCR is 8.8:1 to 9.1:1. It will be these figures that determine your Total Chamber Volume.

The accepted norm of choosing an SCR and then trying to pick a cam is backwards. You need to build your engine around your camshaft & your car/engine's intended purpose. It begins with your car/engines intended purpose: which will require your power to manifest in a certain rpm range..., then shifts to the displacement of your cylinder..., then to the camshaft duration: from there all other issues are secondary.

Think about it: if compression in an engine doesnt begin until after the IVC-they why even concentrate on SCR at all? The purpose for SCR is to correct whatever compromise your modifications have made to the airflow velocity. You cant make a correction to your DCR simply because SCR doesnt determine what camshaft you need. Your camshaft choice is a function of your power needs while your SCR is a function of your airflow velocity needs. You can not know the characteristics of your airflow velocity until AFTER you have chosen your camshaft.

Camshaft requirement come prior to your SCR.

Kevin,

 

I agree that you need to pick the cam first entirely. Once you know the cam you need for the app then you will know what SCR to shoot for on the build.

Are you saying that any given cam will not lower the DCR from another cam? I hope not.

From what I am reading here, you say SCR has no meaning or relative propose in an engine build.

 

Moneypit,

NO, I am not saying that our DCR will remain the same regardless of what cam we choose. What I am saying is just the opposite: that the DCR will change with any cam upgrade whose profile differs from the profile it just replaced.

As far as the SCR not having any meaning: at least now we are addrssing the issue in the correct manner. SCR does have meaning providing it is approached in the correct manner.

As a brain teaser take a look at it this way: we can under cam an engine with a 9.5:1 SCR and it will rev to 5000 rpm's, we can build an engine with the correct cam with a 9.5:1 SCR and it to will rev to 5000rpm's, and we can also over cam an engine with the 9.5:1 SCR and it too will rev to 5000 rpm's. So of these three builds-which one is the correct build?

Kevin,

 

Bravo!!!

I love this conversation! Yes, it was me who proposed that you get your DCP from cylinder pressure, which is the real world way of measuring it. I applaud you for posting the information about IVC, it makes so much sense because that's when the compression really begins to build. However in a running engine, flow does not occur at valve lift less than .006", so the exact time of the IVC is not a critical as when flow becomes negligible and compression begins to build. The question was asked, "how I do find out my static compression ratio?" If you don't know your cams specs, you can measure that with a cylinder pressure gage. It is the peak pressure divided by the ambient atmospheric pressure. This may not be the empirical DCR, but it is the "actual" compression ratio. Obviously this does no good if you're planning an engine, but it is at least one answer to the question asked.

I am so glad that somebody else does their own formulas. I get a kick out of magazines when they post formulas with the numbers all in the wrong units. These formulas are oversimplified, and are not quite accurate. For what we're doing, they are more than adequate, but it helps build a much deeper understanding when you sit down with a pencil and calculator and figure it out for yourself.

This convo has inspired me to create my own crank angle chart. I'll post it when I'm done, but I know the piston's motion will resemble some transformation of a sine curve.

ROCK ON!!!!!!!!!! TK

 

You guys can try my calculator and compare to yours. Please let me know how far and where it is off so I can make changes. I didn't create this one but will create another or modify this one if you find it off. So far it has worked very well in the real world (vs theory). You can download it from the page I just created. The link is in my gallery.

 

TK,

You bring up some good points. First though you need to not confuse a [RATIO] with [PRESSURE]. Calculating Cylinder Pressure is the next step in planning an engine after you have calculated your DCR & SCR, still the two issues, Ratio and Pressure, are distinctly different issues.

A Ratio is a fraction...as in 50 units of 100 units written as 50/100; reduced to the least common denominator would be 1/2. A Ratio is not a Pressure. Yet once you understand engines-which you and Moneypit obviously do, then you obviously know that Cylinder Pressure at any particular rpm is what counts.

Therefore, after you have determined your level of performance: you then choose which DCR & SCR is correct for that level of performance. Then you can calculate Cylinder Pressure next. Your calculations are the foundation you build upon. After the engine is built you can then take actual readings from a Compression Gauge to see how close your cranking pressure is compared to your previous calculations. Pressure deals with VE% and a host of other things: yet a Ratio is simply that it is a Ratio and should never be confused with Pressure.

If you wish to calculate Cylinder Pressures (specifically Cranking Pressure) you can use the formula:

DCR^1.2 x Atmoshpheric Pressure = Theoretical Cyl.Pressure

When working this forumla be sure you use Atmospheric Pressure and not Barometric Pressure. Both are measuring the same effect-yet they are measured differently. Be sure you use Atmospheric Pressure.

Just remember that your cyl.pressures will increase once airflow builds: so when building your engine dont forget to give yourself 10psi or more as a tolerance.

Moneypit,

I bet there is nothing wrong with your DCR calculator. What TK & I have illuded to is the correct process for building an engine from the ground up. Most people usually decide to upgrade an engine they already have. This means they are already stuck w/cylinder heads and pistons: so their SCR is also set-unless they choose to replace those cyl.heads or pistons...or both.

This is why most magazines write their articles in the backwards manner. This backwards approach is: "Now that I have built my engine-how large of a cam can I get away with." When in reality you should not ask what you can get away with, rather you should be asking what is correct for your needs and you should ask this question prior to building your engine.

In the example, I illuded to where most individuals already have a complete engine and are only planning on a Cam, Intake Manifold, & Fuel System Upgrade w/out ever touching their pistons...this scenario means the builder has to make their alterations w/in parameters that are already set: as far as SCR is concerned. So, if no one ever comes along and tells you this approach is only correct if the scenario fits, then you will never know otherwise. There are many stages to an engine build and each builder has to understand what stage they are at, such as:

1) I have a short block-now what do I do,
2) I have a long block-now what do I need,
3) I have a complete engine & want to slightly upgrade it or revamp it completely,
4) I have no engine & would like to build one from scratch-what and how do I do this?

The real bottom line is you can work the math backwards or forwards: providing you understand the math. All that really matters is that your DCR & SCR compliment each other: as opposed to working against one another = detonation or mismatched engine components which = compromised power at the rpm range you need it most.

Kevin,

 

Yeah, I completely understand the difference between ratios and numbers. It is peak pressure alone however that dictates fuel octane requirements, etc. Another way to illustrate the difference is that a motor with a very high compression RATIO may detonate severely on pump gas near sea level in Florida, but in the Rocky mountains it may be drivable. It will be less powerful at high elevations, but the peak pressure is lower, because the ambient pressure is lower.

it's like this: let's say your DCP is 10:1. if the ambient pressure is 14.7 psi (atmospheric), then your peak cylinder pressure would be 10.0 x 14.7 = 147 psi. In the mountains, this pressure is lower than the 14.7 we use for sea level, so the peak pressure would be less. This is why NHRA dragsters run slower et's in Colorado, and break records all day at the Gator Nationals in Florida. TK

 

TK,

I agree about the magazine articles: I was just reading a Hot Rod article about this very issue-and they kept using the incorrect vernacular. It is obvious they are merely repeating what someone else told them.

I seriously believe that what we spoke of just is not being taught across the board and as a result too many folks have a serious black hole in connecting the process for building an engine with that of any real expected performance outcome.

About your Crank Angle Charts-when you are researching the Trig functions be sure you familiarize yourself with the "Ambiguous Angle". You will need a list of complete Trig Functions to draw from.

To understand the Ambiguous Angle you must first realise what is considered a Typical Angle. The typical angle will consist of two known sides and one known angle: this known angle will reside at the vortex of the two known sides...this type of triangle is very easy to solve for. The Ambiguous Angle on the other hand is in a triangle where the angle known is opposite the two sides that are known. The relationship of our Piston Wrist Pin, Connecting Arm Journals, and Crank Throw creates an Ambiguous Angle.

If you do not have the correct Trig Function; then your results will be wrong-also if you are not careful in how you approach the Ambiguous Angle you can come up with two different answers...hence the title "Ambiguous Angle".

Not all math books will speak about the Ambiguous Angle-this was a source of frustration for me when I was working out the problems.

BTW: for a complete set of Trig functions you can purchase the book, "Automotive Math Handbook" by Forbes Aird, published by MBI Publishing Company with a year 2000 Copywrite date. You will find his complete listing of all Trig functions on pages 32-33. This book is an orange book with a picture of a CITGO race car on the front: the book measures about 7" x 4".

Get that auto math book for the Trig functions and you will have to locate some actual math books to familiarize yourself with the Ambiguous Angle-maybe you will avoid the frustration I experienced.

Good luck.

Kevin,