What's the whole idea about compression and compression ratios? What exactly do they mean. I know it deals with the pistons and such, but how is that measured and what's the point of it? Is a 9.9:1 a better compression than 11:1? How does a supercharger effect your compression? Just some questions I've always wondered and figured some of you gearheads could help. I might decide to buy a stroker kit and need help on choosing which pistons to get the right compression.

 

The higher the compression the more powerful the explosion. For instance 11.0:1 is a higher compression than 10.0:1. The numbers 10.0:1 means the charge is compressed 10 to 1 at top dead center . When you put 10 pounds of boost in a cylinder you are actually compressing the charge by 10lbs when the piston is all the way down so on the up stroke there will be a higher compression because there is more in the cylinder to start. Also the higher the compression the higher the octane gas has to be used. Thats why in the Cobras and Mach 1's you have to use premium gas. The compression is higher. The higher the octane of the gas - the harder it is to ignite. i.e. 93 octane is harder to ignite that 89 octane.
On engines that are built for higher boost they usually make the compression less but I'm not totally sure why.

 

on built forced induction engines, they lower the compression ratio so that it is less prone to detonation (=fuel igniting before the spark plug sparks off the air/feul mixutre). you dont want a high compression with forced induction b/c the FI will raise the ratio naturally and cause detonation. so a lower ratio + forced induction = tolerable ratio and probably no induction. high ratio + forced induction = too high of a ratio and detonation occurs.

hope you could follow that

 

The compression Ratio (as stated above) is based on the volume of air from BDC to TDC (static Compression Ratio). A 10:1 CR will produce more power than a 9:1 CR, but the lower C/R will be easier on the engine and make it more drivable. Adding a supercharger adds more air to the engine at BDC that will be compressed at TDC. To compare apples to apples, "effective compression ratio" is used. The formula is ((boost pressure/14.7)+1)*Static Compression Ratio. So a car with a 10:1 static compression ratio with 10 psi boost will have an effective CR of about 16.8:1. If you made a car with a static compression ratio of 16.8:1, theoretically they would produce the same power. The difference is that the N/A engine with 16.8:1 would ALWAYS have that CR and would produce a tremendous amount of torque (think diesel engine). It would be very hard on the engine though, and it would probably require special gas. The supercharged one would only get the 16.8:1 CR when it was needed (WOT), otherwise, it would act like there was no S/C. What you want to do is build an engine that has a high enough compression ratio to give you good torque and power, then add the S/C to push the engine beyond. It used to be that you had to go with low compression for power adders, but with modern technology and controls, you can really push the envelope.

 

Sorry for a long and maybe comfusing post but, I think many do not totally understand the theory of Compression and compression ratio.

Compression as it is typically referred to, is the pressure created in the cylinder on the compression stroke. You can put a guage in the spark plug hole and measure the actual compression (pressure).

The higher the compression (pressure) in the cylinder (assuming fuel and air is the same) will create more power than lower compression. If the only change you make is raising Compression ratio, the compression (pressure) will increase.
You have to control the compression (pressure) to make sure it is not too high for your application, (i.e. detonation, blown head gasket, rotating assembly failure). Higher compression requires higher octane fuel, better head gasket sealing, stroker internals.

Compression ratio is the ratio; 1)volume of area above compression rings and cylinder head at BDC compared to 2)volume of area above compression rings and cylinder head at TDC. This is why you see different CR when heads have different combustion chamber sizes. You will also see the CR change for the same heads and pistons but you change the stroke length. Some other ways to change CR is dished to flat top pistons, deck block, head gasket thickness, mill heads. The trick is to find the compression ratio that gives you the ideal compression for your application.

The most power is made by getting the most fuel and air possible crammed in there without exceeding your applications target compression. One way to do this is to force more air in (fuel mixes based on mass of air) with forced induction. Since you are starting with more air on the compression stroke it maybe neccessary to have a lower compression ratio to make sure you do not exceed the target compression.

 

the reason you would need lower compression pistons on a forced induction engine is because you want to keep the compression low so that when you start to cram air into the engine(supercharged,turbo,or nos),the compression doesnt rise so much causing engine internals failure...once you use forced induction you are also raising the compression....

 

That will all depend on the tuning after the FI setup is installed.

You can boost with 10:1+ CR, and you'll make a lot more power than boosting on a 9:1... as you stated though it's more prone to knocking/pinging and causing damage but the part you left out is that will happen soon, unless you TUNE it.

You can have the best of both worlds, there are plenty of 11-11.5:1 CR Civic engines running 6-7 psi and making massive power after the ECU is chipped and a dyno tune is made. The reason people state lower your CR then boost is because it's easier and more safe to boost on a lower CR, but if you have an experienced tuner you can run high CR and boost with no problems.

 

I could talk/write on this subject for hours. I still see some do not understand the concept of compression, compression ratio and power.
You can not compare Compression ratio and boost of one engine to a totally different engine that has a different bore and stroke.
Power comes from Fuel, air, compression, given spark at the right time. Compression ratio is math to help you determine the compression (pressure) you will have at TDC. To make the most power you are trying to stuff the most air/fuel you can in there at the highest compression your assembly will handle without detonation.
I must go shopping with my wife now, but, I will come back to this one later today.

 

I understand that - my point is lowering the CR will reduce the amount of power you can make, persay.

I never compared two different engines with different bores/strokes - I simply gave an example of what's been done.

Now if you lower the CR and build the internals to handle the power, then yes you can boost more and make up for the lost compression.

It's easier to tune boost on a lower CR, but you're going to make more power on the higher CR. You're going to need to tune the setup anyway, so why not go with a slightly higher CR to start with?

 

We are on the same page just saying things in a different way. Matching the compression, boost, cam, heads, intake, exhaust, timing, quench area, surface temps, air charge temp, and fuel octane are all important. Each engine will have a different "Dynamic Compression Ratio" influenced by mechanical factors variables are adjusted in tuning to prevent detonation. Yes, you have more options when you start with a lower static compression ratio.