Compression ratio is usually only thought about when selecting new pistons for an engine build. Most use the piston manufacturer’s reference table to determine what compression ratio they’ll end up with based on piston type and head chamber volume. A Hughes Engines Tech Center article is careful to explain that compression ratio has much more to do with the sum of an engine’s parts and not just the pistons.
Compression ratio has a great effect on engine performance, but cylinder pressure has an even greater influence on power output. Cylinder pressure is what you get when you install a pressure gauge in the spark plug holes and spin the crankshaft with the starter. This cylinder cranking pressure number is the result of several important engine components.
- Compression ratio: a higher ratio = higher pressure, and a lower ratio lowers the cylinder pressure.
- Altitude: As the altitude gets higher, the air pressure drops. That is why airplanes pressurize the cabins (to keep your pressure up). The altitude effect on an engine is a gradual condition. It can have a small effect around 1,000’ – 1,400’. From 2,500’ and up you must find some way to offset the pressure loss. The easiest way is to raise the compression ratio. We have customers who run 14:1 with 91 octane gas at higher altitudes.
- Cylinder heads: Cylinder heads made of aluminum dissipate more heat than cast iron heads. Less heat means less pressure in the combustion chamber. The effect on power is to reduce it approximately the same as lowering the compression ratio about 1 ½ points. Some “experts” will tell you that you “can” raise your compression ratio to make up for the heat/pressure loss when using aluminum heads. I’m saying you “better” raise it or you are giving HP away.
- Camshaft: Actually, camshaft size is based on duration (when the valves open and close), not lift. The compression stroke begins when the intake valve closes. The bigger the cam, the later past bottom dead center the valve closes and the compression stroke starts. Later closing means a shorter effective compression stroke. The shorter the compression stroke, the less volume is compressed and the lower the cylinder pressure will be. This is why big cams need high compression ratios to make cylinder pressure and power.
A discussion of cylinder pressure is not complete without addressing octane ratings. The subject of compression ratio and fuel octane comes up more when dealing with customers who want to run pump gas rather than with customers running race gas.
The octane number tells you how much cylinder pressure the gas can handle without detonating. That detonation is not a bad thing; it is a very bad thing. Can you say, “The motor blew up”? You not only can say it, you can do it with detonation. Cast iron heads can tolerate somewhere around 165 psi of cylinder pressure with 93 octane gas. Switch to aluminum heads and the cylinder pressure can go to 195 psi with 93 octane (both at sea level). There are ways to build your engine to tolerate more cylinder pressure with the same 93 octane gas, such as very tight quench heights, but generally the above numbers are safe.
Now, based on the preceding information, it looks like it is a juggling act when putting an engine together to get all the power you are paying for, right? Exactamondo, grasshopper!
An 11 ½:1 compression ratio pump gas street engine at sea level is not uncommon, and that is with a warm street cam and aluminum heads. With a 250º @ .050 cam and aluminum heads you may be getting to 12 ½:1 compression ratio area.
When we design engines, short blocks, or engine kits for our customer, we take all these factors into account to get the most “bang for the buck” for each customer, regardless of how much they complain. This is because they read every engine on pump gas should be 9 ½:1 compression ratio. But that’s not for our customers.