Piston with wieghts

How to Calculate Dynamic Compression Ratio

How to Calculate Dynamic Compression Ratio

Calculate Dynamic Compression Ratio
Calculate Dynamic Compression Ratio

 

Now i would like to say this before we get started. For a normal street engine the calculators you find online are accurate and work well if you follow there directions for the correct information in plugged into them they will give you the correct answer. Now if building a max effort racing engine you dont want to give up even one hp unless that trade off is for something valuable. Like longer life or less detonation with a given fuel type. So before you drive yourself nuts and look for batteries for that old calculator. You know the TI80 from school days. It may just be best to go right to the web and use the online calculators to get close enough. At the end of the article I will post a table of acceptable scr/dcr specs and cam timing.

Check the DCR dont follow it

Practiced engine builders have done these numbers so many times for different builds they know off the top of there head what cams will work and what cams wil be close but for racing they always run the numbers. If they are racing winning engine builders. Many now defer to computerized programs that calculate many factors at once. But not all of these programs are ideal the best ones all have one thing in common they are expensive. Some scratch paper and some time this cost can be wiped off the table for a one time build.

Before we can calculate the DCR of a given engine. We need an engine and some can spec for this I will use a Comp off the shelf cam and an engine with heads and rotating assembly that will produce power up to 8K rpm on paper. Since it is one paper and not in real life i will pick a cam with a little bit less rpm than the heads should support giving the engine power right up to the 8K mark and not running too far over and wasting rpm on turning the engine and not making power.

Example

Comp Cams  12-614-5 11:1 recommended

So for cams we grab an off the shelf comp cams  12-614-5   310B-4 drag race family cam. Recommends 11:1  and light cars good gears 350 cubes and up. I picked this out of rpm requirements solely and knowing it will make power where my heads and other parts can support with the correct setup.

 

Rpm range  5000 – 7700

Intake center line  104                                                                                                                 valve lash   intake  0.026               exhaust  0.028

Lobe separation        104                                                                                                              Duration    310                                     320                                            Duration   @.50       275                                              283

Valve lift                0.585                                   0.588

Lobe Lift              0.39                                             0.392

Valve Timing @ 0.02 lift

Exhaust   Closes 56 atdc,  opens  84  bbdc         Intake   opens   51  btdc,   closes  79   abdc

Not a small cam picking this cam with the wrong heads and other support equipment will result in now power being made and the cam never reaching its 7700 rpm potential. So it is important to find out if the engine can support the high compression and effective stroke needed to make this cam really work.

Intake Valve Closing degree

In order to find the Intake Valve Closing degree If its is not easy to find on your cam card or the cam card is incomplete. This is a simple math problem that can easily be solved.

Divide the intake duration by 2 so the 310 intake duration / 2 = 155

Add the results to the Lode Seperation Angle (LSA) 104   104 + 155 = 259

Now make sure to subtract any ground in advance 0 in this case

Subtract 180 from the total above  259 – 180 = 79

Where 79 is the correct intake valve closing degree for your Dynamic compression ratio calculation.

Standard Compression Ratio

Now we can go over the engine standard compression ratio. I added some deck height to make this more like a common street build. Since most race engines will be either zero or above deck I wanted to add this to the math so it can be used easily to build your street engine. Below Pie is represented as 3.14 using a calculator a larger more accurate number can be easily used by substituting 3.14 for the Pie button on the calculator.

 

Bore = 4.06 [ 3.14/4  X  4.060  X  4.060  X  3.48  =  45.0527 ]

Stroke = 3.48

Cylinder Volume  .7853  x  4.060  x  4.060  x  3.48  =  45.0471

Head cc = 64         [  64cc  x  0.061024  = 3.9055  ]

Piston cc dish  = 3    [    3cc  x  0.061024  =  0.1830  ]

Head Gasket = .039   [   (4.060 /2)x  3.14  x  .039  =  0.5049”    ]

Deck height = .025   [   (4.06 ÷ 2)2  x  3.14  x  .025”  =  0.3236  ]

 

Compressed volume    Head gasket  +  Deck Height  +  Piston Volume  +  Combustion Chamber  =

Compressed Volume:    0.5049  +  . 0.3236  + .1830  +  3.9055  =   4.917

Uncompressed Volume:  Compressed Volume   +   cylinder volume  =  49.964

Compression Ratio:   Compressed volume   /   Uncompressed Volume  =

Compression Ratio:   49.964  /  4.917  =  10.1614

Effective Stroke

now before we Start talking about dynamic compression ratio or effective stroke as its more accurately named. we know the cam will need more compression than the above engine numbers can support. 10.15:1 is not enough to build real power with this type of cam. Once you figure some higher than 1.5:1 ratio rockers and the need to adjust the cam specs to make the engine happy with the higher lift levels you will need even more than comp has in its spec’s. This is where degreeing the cam and making sure the specs are accuratly transferred to iron will be very important.

Would want to change the specs a bit to support the massive lift this cam and the heads that will support it can provide. Lets run the numbers again this time with 58cc heads and zero deck height. Since quench will be important in any racing engine a thinner gasket will also be in order. For racing applications copper shim gaskets are very strong and reusable for tear down and inspection.

 

Head cc’s = 58cc x 0.061024 = 3.5393

Head gasket cc’s 0.020 = 0.2589

Piston Volume is unchanged = 0.1830

New Compressed Volume = 3.9812

Cylinder Volume is unchanged = 45.0527

Total Volume is now = 49.0339

Compressed volume / uncompressed volume = cr

49.0339 / 3.9812 = 12.3163:1 CR

new compression ratio

Now our new compression ratio is 12.3:1 that is perfect for this large cam since it will run on pump gas and we have a few ways to change things as we go forward with gaskets. Since its all on paper if the CR does not work out to a proper DCR we can have the deck height changed along with some un-shrouding of the valves in the head to provide better flow and reach the numbers we want since it being done from a point of advantage rather than a lack of need cr. Its easy to remove metal a lot harder to put it back.

Now lets Calculate Dynamic Compression Ratio based on all the number crunching we did above.  More to come soon….

Dynamic compression general rule of thumb cam events. This chart has been published by several websites. KB, Crankshaftcolation.com and a few others. It is a general guess that works well for iron head v8’s. Four valve Pent roof heads can take a lot more compression because they have a more efficient combustion chamber. These heads can take dcr figures in the 10 or even 11:1 for regular 92 octane fuel. Wedge style two valve heads you need to be a lot more cautious.

  • Static CR….Intake closing point @ 0.050″….Dynamic CR.
  • 8.25……..10* ABDC……8.010………………
  • 8.50……..20…………8.012…………………….
  • 8.75……..27…………8.022……………………..
  • 9.00……..33…………8.018…………………….
  • 9.25……..37…………8.061……………………..
  • 9.50……..42…………8.029…………………….
  • 9.75……..46…………8.016……………………..
  • 10.00…….49…………8.038……………………
  • 10.25…….52…………8.043……………………..
  • 10.50…….55…………8.029……………………..
  • 10.75…….57…………8.069……………………..
  • 11.00…….60…………8.022…………………….
  • 11.25…….62…………8.038……………………..
  • 11.50…….64…………8.042…………………….
  • 11.75…….66…………8.035……………………..
  • 12.00…….68…………8.017…………………….