Cam and compression ratio compatibility
 Matching cam to compression ratio
Sometimes it's difficult trying to explain to fellows who are new to hot rodding that you have to match the characteristics of the camshaft to the static compression ratio of the motor along with the operating range where the cam makes power. The camshaft is not a stand-alone piece. It must be coordinated with other motor and drivetrain parts in order to arrive at a combination of parts that will all work together toward a common goal.
This chart is nowhere near scientific or definitive in its scope. Larger or smaller cubic inches, different lobe separation angles and other variables will alter these figures, but at least it's a starting point in the explanation of using a certain range of cam with a certain range of static compression ratio. The figures shown in degrees are camshaft degrees of duration measured at 0.050" tappet lift.
The chart will show why it is foolish to install a 230 degree cam in an 8.00:1 motor in an attempt to get a lope out of the motor. It might lope, but it will lack the power to pull the hat off your head. There seems to be an obsession with lope these days. Lope is simply the sound of the motor being inefficient at low rpm's because the ascending piston is pushing fuel/air mixture back up the intake tract through the still open intake valve and disrupting the metering abilities of the carburetor.
On the other hand, it's equally as foolish to install a short cam into a motor with a fairly high static compression ratio. You get into an area of such high cylinder pressure that no fuel short of alcohol or racing gasoline will prevent detonation.
You can move 1/2 point of SCR either way and be in the ballpark. In other words, if you have a 9.00:1 motor and you want a little more cam, you can move up to a cam that would be used in a 9.5:1 motor and be OK. On the other hand, if you wanted to go with a little less cam, you might use 1/2 point less compression, down to an 8.50:1 cam.
Why is it necessary to know the Compression Ratio of an engine in order to choose the correct cam?
The compression ratio of the engine is one of three key factors in determining the engine's cylinder pressure. The other two are the duration of the camshaft (at .050" lifter rise) and the position of the cam in the engine (advanced or retarded). The result of how these three factors interact with one another is the amount of cylinder pressure the engine will generate. (This is usually expressed as the "cranking pressure" that can be measured with a gauge installed in the spark plug hole.)
It is important to be sure that the engine's compression ratio matches the recommended ratio for the cam you are selecting. Too little compression ratio (or too much duration) will cause the cylinder pressure to drop. This will lower the power output of the engine.
With too much compression ratio (or too little duration) the cylinder pressure will be too high, causing pre-ignition and detonation. This condition could severely damage engine components.
|Static Compression Ratio
|Intake Valve Duration
(degrees @ .050" lift)
 Additional reading/engine theory
- top dead center (TDC) is when the piston is at the very top of the stroke
- bottom dead center (BDC) is when the piston is at the very bottom of the stroke
Modern engines are designed so that the intake valve doesn't close until the piston travels after bottom dead center (ABDC). This sounds counter-intuitive, but this aspect of camshaft design was a breakthrough when introduced, allowing modern engines to make more power as well as run at higher RPM.
The reason closing the intake valve ABDC works to make more power is because air is a gas, and as such is compressible (unlike liquids or solids). When air enters the engine it does so at a relatively high velocity. When the piston reaches bottom dead center, the air stream will still continue moving into the cylinder due to the momentum of the mass of the charge. Even though the piston has passed BDC and is now moving ABDC (back towards TDC, in other words), the air is compressing upon itself, and is able to move into the cylinder because the intake valve is still open. So up to a point, the longer the intake valve stays open, the more air can be packed into the cylinder.
 Intake valve closing point
The length of time the valves stay open is measured in degrees. The intake valve closing point (IVC for short) is one of, if not the most important determining factors in how the camshaft impacts engine performance. Manufacturers give the IVC point in degrees after bottom dead center (ABDC). As an example, the Comp Cams XE274H cam's intake valve closes 64º ABDC. The other opening and closing points as well as lift, lobe separation angle (LSA), valve lash (where applicable), etc. are all called out on the cam card (see example below) that's supplied with the camshaft or in the manufacturers information.
 Static compression ratio vs. Dynamic compression ratio
- The static compression ratio (SCR) is computed using the full swept volume of the cylinder from BDC to TDC compared to the volume of the combustion chamber, valve reliefs (or dome), and all the other volumes not occupied by the piston stroke.
- The dynamic compression ratio (DCR) is computed from the position of the piston at the point of IVC to TDC compared to the volume of the combustion chamber, etc. (instead of using BDC to determine the swept volume of the cylinder). The relationship between SCR and DCR when different IVC points are used can be seen here.
Most factory production cams are considered to be short duration cams. This means that the intake valve closes much sooner ABDC than a high performance cam might, and thus less of the intake charge is pushed (reversed, thus the term reversion) past the intake valve back into the intake tract. Since less of the intake charge is lost to reversion, more of it stays in the cylinder, so the compression stroke can yield a satisfactory DCR with a relatively low SCR.
An engine having a lower SCR and an early IVC figure will produce power in the lower RPM range. Since there is little reversion, these engines idle very smoothly (no lope) and will have high idle vacuum around 20 in/Hg. If you were to put a compression tester on these engines you would see something in the 150-190 PSI range.
Performance cams with longer durations begin to exhibit the opposite traits of a short duration cam. At low RPM more of the intake charge is lost to reversion because the intake valve stays open longer ABDC. Hence there is less cylinder pressure built up on the compression stroke. To compensate for this, a higher SCR is used. As the RPM climbs, so does the intake charge velocity. Now the later closing intake valve will allow more air to cram into the cylinder. At higher RPM this type of cam is able to more completely fill the cylinder, so will make more power.
An engine having a high performance cam (later IVC point) needs a higher SCR in order to keep the DCR within an optimum range. Because the late IVC point can cause reversion (along with an increase in overlap and/or a tighter LSA), this type of tune can cause the engine to idle rough (have a lot of lope). Idle vacuum will be lower and if you were to put a compression tester on these engines you would see something in the 125-150 PSI range.
 Estimating intake closing point
If the intake closing (IC) point isn't known, it can be calculated:
- Divide the intake duration by 2
- Add the results to the lobe separation angle (LSA)
- Subtract any ground-in advance
- Subtract 180
 Dish volume
Most dished pistons have published volumes. Those that don't or with pistons that are unknown as to brand can be estimated by using the following formula:
- 3.14 x [radius of the dish]² x depth of the dish x 16.4 (converts cubic inches to cc) = Volume in cc
- Add 2 or 3cc for the valve reliefs if they extend beyond and/or below the dish.
 Compression calculators
Note: Different calculators use different ways of expressing piston dome or dish volumes. Some calculators assign a negative number for a dish or dome, others will use a positive number. The same thing goes for advertised figures published by different manufacturers when they describe piston dish or dome volumes. So be sure of the sign (negative or positive) that is needed to get the correct results from the calculators.
 Static compression ratio calculator
 Dynamic compression ratio calculators
 Cam phasing
You can fine tune a particular cam by advancing or retarding it. This procedure does nothing to alter the lift, duration, lobe separation, etc. of the cam. What does happen is the phase of the camshaft is advanced or retarded in relation to the crankshaft position. Advancing the cam closes the intake valve sooner. This will build more pressure in the cylinder and shift the power band lower. Retarding the cam will cause the intake valve to close later, this will build less pressure in the cylinder and shifts the power band higher.
Modern engines with variable valve timing are able to phase the cam while the engine is running. This represents the best of both worlds because the same cam is able to optimize valve events based on engine RPM. This means the VVT engine is capable of:
- advancing the cam (early IVC) at lower RPM to produce more torque, less intake reversion, and less lope at idle
- retarding the cam (late IVC) at higher RPM to produce more horsepower
Of course the SCR remains constant despite cam phasing. The DCR will vary based on RPM since this effects intake charge velocity.
 Compression ratio calculators
 Dynamic compression ratio
 Static compression ratio
- Dynamic compression ratio
- How to choose a camshaft
- Camshaft/Compression Ratio relationships
- Comp Cams CamQuest dyno simulation program
General Application Chart * Advertised Camshaft Duration Rear Gear Ratio Engine RPM Range & Compression Ratio Converter RPM Stall Speeds ** Stock to 260° Stock to 3.23 1000-4600 8 to 9.5 1500 to 1700 Stock to 265° Stock to 3.23 1400-5000 8 to 9.5 1600 to 1800 265° to 280° 3.00 to 3.73 1600-6000 9.5 to 10.5 2200 to 2400 280° to 300° 3.55 to 4.56 2000-6500 9.5 to 11 3000 to 3400 280° to 310° 3.73 to 4.88 2800-7000 10 to 11.5 3500 to 3800 290°+ 3.73+ 3400-8000 10.5 to 12 3500+