DYNAMIC COMPRESSION AND CAMSHAFT SELECTION
Dynamic compression is the effective compression ratio of an engine due to intake valve closing. Unless an intake valve closes immediately after BDC (bottom dead center, bottom of the stroke) compression is delayed, as the cylinder is not sealed. Exhaust intake opening does not effect compression as it always occurs after top dead center (ATDC). For this reason it is important to choose a cam that has an intake closing point ABDC that keeps the dynamic compression at a reasonable level.
Too high of a dynamic compression leads to detonation and loss of power. Too low of a dynamic compression robs power. In effect you want to run compression as high as possible without detonation.
With most modern import's this means you want a dynamic compression between 9.0:1 and 11.0:1 for a NA motor. For most domestic engines you want a dynamic compression between 7.5:1 and 9.5:1. The difference between import and domestic compression requirements has to due with head design. Most imports have more efficient head designs that allow higher compression without detonation.
Higher compressions are possible if a higher octant fuel is used (ie 110 octane race fuel). Octane rating measures a fuel's resistance to detonation, higher octane equated to greater resistance.
Figure 1: Piston, Rod, & Crank Diagram.
R = ½ stroke
To determine dynamic compression we determine the amount of compression that occurs after the intake valve closes. To do this we must first calculate the effective stroke. Once the effective stroke is found we can use this to determine the effective cylinder volume, volume of the cylinder remaining for compression. This effective cylinder volume allows us to calculate dynamic compression.
Effective Stroke (Se)
The effective stroke is the actual stroke travel during compression (after the intake valves have closed). To determine this we find the piston position (P) at intake closing (ATDC intake cam data) and then add ½ the stroke and the rod length to determine the remaining stroke.
For more information on how P is found please review the RS Ratio tech article.
Se = R + L + P
(1) Se = R + L + R cos(q) + (L2 - R2 sin (q)2) 1/2
Effective Cylinder Volume
To find the effective cylinder volume we use the displacement formula, but replace stroke with effective stroke. Note to determine cylinder volume we do not multiple by cylinder count.
(2) Ve = B2 (ES) .7854
Effective Compression Ratio (CRe) / Dynamic Compression Ratio
To determine dynamic compression we us the normal compression ratio equation, but replace cylinder volume with effective cylinder volume.
(3) CRe = (Ve + H) Ve
H is camber volume. Camber volume is the volume of the head per cylinder minus piston volume (above deck height). If H is not known the following equation can be used to determine H from static CR and Cylinder Volume.
H = V (CR-1)
See basic engine calculations for more information.
Measured Compression/Cranking Pressure
Compression can easily be readily measured. Compression is the cranking pressure of the cylinder and is directly related to compression ratio. The equation below gives you the expected compression for an engine with a specific compression ratio (CR).
(4) CP = CRe1.2 (A) - A
(Thanks to Panic, http://victorylibrary.com/mopar/mopar-tech.htm, for equation 4)
It is important to take into consideration compression with camshaft selection (and vice versa). For an import engine you want the dynamic compression to be in the range of 9-11:1. An engine with 11:1 dynamic compression in a daily driver fueled with pump gas (premium) is living on the edge. An engine with 9:1 compression will be slightly anemic and an engine with 11:1 compression will be living on the edge. If the engine has been bored-out (reducing sleeve width) for added displacement I suggest running lower compression that with stock sleeves.
In the chart below is the dynamic compression of a number of Honda B series motors.
As you can see the dynamic CR are in the suggested range of 9-11:1.
Some people suggest choosing a camshaft and piston combination that creates the highest cranking pressure and that ideally has a cranking pressure in a magic number range 15-20% higher than the stock. This methodology is flawed for several reasons.
1) Camshafts effect much more than compression and cranking pressure. Duration and lift of a cam greatly effect engine performance. A higher cranking pressure doesn't always mean greater performance.
2) Cranking pressure is a measure of compression ratio (see equation 4). There is nothing magic about 20% higher than stock cranking pressure. In general this MIGHT give you a dynamic CR near 11:1, but often this isn't the case. Choosing a CR higher than 11:1 based on cranking pressure increases is dangerous to you motor.
3) Increased CR (and cranking pressure) increases HP, but the gains are by no means dramatically different within the CR suggested range. Increasing CR gains HP in diminishing returns. Expect 3-4 hp/l for every CR point increase between 9-12 or until detonation occurs.
To help illustrate the point lets look at the numbers in the table above. The B16A cams in a JDM ITR motor give higher cranking pressures than any other stock (11.10:1 CR) configuration. Using the cranking pressure methodology would suggest that the B16A cams would make more power and is false. Both the ITR and the Skunk2 stage 2 cams make significantly more power in this set up than a B16A cam; the Skunk2 Stage 2 cams make approximately 20whp more. Further to reach the 20% increase in cranking pressure the dynamic CR would have to be raised to 11:5:1. This CR is too high to safely run on pump gas.
The proper method of selecting a camshaft is complicated and involves many factors including cam lift, duration, compression, and engine breathing. Compression to some extent is the smallest piece. As long as dynamic CR is between 9 and 11:1 the cam is appropriate for the application. For a race motor aim for 10-11:1.
I've used a B20VTEC (CRVTEC) hybrid motor to explain proper cam selection based on compression in the table above.
The top three rows show a B20 block with aftermarket pistons that raise the compression ratio. As you can see the dynamic CRs are all with in the suggested range. Since this is the case I'd look at other aspects of each cam and the breathing characteristics of the engine itself to determine which camshaft I'd use in this application.
I've marked the fourth row in yellow because the Dynamic CR of this combination is borderline. 11.06:1 dynamic CR is very high and this can be a big problem in a block like the B20. The B20 block is a special case in the Honda Blocks as it has relatively thin sleeves, a large bore, and a short RS ratio. The thin sleeves in combination with high side loads due to the short RS ratio, and the large bore make detonation FATAL in this motor. To run 11:1 dynamic CR in this motor would require very good conservative tuning. I've heard of several B20 engines cracking their sleeves at these CR's. As such I'd suggest not running higher than 10-10.5:1 dynamic CR as a limit for these motors unless you are an expert tuner.
The next two rows are stock B20Z blocks (98-01 CRV). The 5th row is running B16A cams, the 6th row is running Skunk2 Stage 2 cams. The 5th row shows that the B16A cams should work fine in this motor. The 6th row (in yellow) shows that the Skunk2 Stage 2 cams will show disappointing gains in this motor and that compression should be raised if you wish to use these cams. As a side note the Skunk2 Stage cams are also border line on this motor due to possible valve to piston contact.
Finally the last two examples (red) help to demonstrate why some mild B20 Hybrid motors make 150whp and others make 180whp. The B20B blocks (96-97 CRV) have a lower compression than the B20Z blocks. Using them with stock pistons in a hybrid motor means your dynamic CR is terribly low and the motor will be anemic. If you are going to build a B20 Hybrid with stock pistons use the B20Z block.