quick question that anybody can answer.

[Guest ON3GO]

does boost add compression to your motor?

my friend is telling me that when u have 9 to 1 compression, then when u get into boost its higher.

true or false?

 

mike

[Guest greimann]

Static compressio ratio is a relationship between the total volume of the cylinder and head at BDC and the total volume of the combustion space at TDC. This cannot change unless you physically change a dimension of either the cylinder, head, piston or gasket.

 

Dynamic compression ratio takes into account cam timing, and other factors that affect the actual total volume of air/ fuel that is compressed. In the strict definition, dynamic compression ratio does not account for forced induction.

 

Boost does add to the overall cylinder pressure generated at combustion because of the greater volume of air/fuel.

 

So no, boost does not add compression.

[deMideon]

Compression is the volume of the cylinder at the bottom of the stroke versus the volume left at the top of the stroke. Adding a turbo increases the amount of air/fuel that comes into that volume. So strictly speaking it is not the same thing. The volume of the cylinder will not change with a turbo. The amount of air pushed into the cylinder by the turbo is variable according to how fast the turbo is spinning. That said, they end up doing similar things. Increasing the compression increases the density of the air / fuel mixture at the top of the stroke, since there is less volume available. A turbo increases the amount of air / fuel mixture in the cylinder, increasing density at the top of the stroke. There are alot of other factors involved (as usual) but hopefully this is a valid basic idea of the difference.

[Kevin Shasteen]

Compression and Pressure are often misunderstood; simply because the differences are usually assumed to be understood and therefore the how-to mags dont teach it.

 

Take the Compression Test, you are not testing the Compression Ratio: what you are actually testing is the Pressure in the engine as it compresses. Yet we dont call it a Pressure Test we call it a Compression Test; therefore the "Compression Ratio" is mentally associated w/the Compression Test; because that is how our minds work. Confused yet?

 

The distinction between the Compression Test and the Compression Ratio is the word "Ratio". The ratio, Static or Dynamic, as said earlier is a mechanical function of mathematical relationships between the Piston design, Stroke, Connecting Rods, Block Deck Height and for Dynamic Compression Ratio's you would include the Cam's Intake Valve Closing.

 

Basically we are always interested in the amount of air above the Piston-relative to the volume of the cylinder.

 

The Pressure in the cylinder at any moment depends on the Volumetric Efficiency of how your engine was constructed; how well it draws air in AKA: Intake System, and in a Turbo/Supercharged engine the pressure will go up: also how well your engine combusts and exhausted the airflow.

 

Compression "Ratio's" and Pressure are not one in the same but the two will work hand in hand positively...if you have chosen your part components correctly. If you have chosen poorly then they will work against you.

 

As a hot rodder that is all we are trying to do: effect the airflow velocity in such a manner that the internal air pressures are max'd at an rpm range where we need it most.

 

To answer your question about what your friend said: A turbo doesnt change the "Compression Ratio" instead what it changes is the Velocity of the Air which effects the Density of that air and fuel as it enters the engine.

 

When building any engine you must account for the relationships between three basic categories, which all fall under the main category of Pressure, and they are:

 

I) Pressure

 

1) Volume

2) Velocity

3) Density

 

this is the juggling act every rodder struggles with.

 

Until it understood it can be confusing at times.

 

Hope this helped.

 

Kevin,

(Yea,Still an Inliner)

[pparaska]

Not sure I follow that, Kevin.

 

Here's the way I look at it. This is the way I understand it, but I'm probably wrong somewehre:

 

A turbo does nothing but add pressure to the

intake. Well, it heats that air and you might see some increase in the velocity of the air past the intake valve, but that's a secondary effet.

 

When you do a compression test you are measuring the dynamic compression at low rpm. You read a pressure, that's a function of the stroke, bore, rod length, and intake closing point. And the atmospheric pressure where you are.

 

BTW, I have a spreadsheet on my site that does static and dynamic compression ratio calcs. You input bore, stroke, rod length, combustion space dimensions, intake valve closing point (ABDC), and the ambient pressure in the intake tract. If it's turbo'd add the amount of boost to the ambient pressure before the turbo (14.7 psi or darned near that). SOme examples of builds I'm planning for my 406 are there, but you can change the numbers to suit what your engine parameters are. Inputs are in the yellow boxes.

 

http://mywebpages.comcast.net/pparaska/tools/VvsP.xls

[blueovalz]

Does boost add compression? It adds pressure as (has already be stated) to the intake system ahead of the valves. When the valve opens, a NA motor will have (depending on the motor's configuration and rpm), about 15psi forcing air into a lower pressure cylinder (a difference of pressure across the valve?). The turbo increases this differential so that now more air is forced past the open valve than would be for a NA motor. Thus, VE increases, resulting in an increase in cylinder pressure. Compression ratio will remain the same, but overall pressure in the cylinder will increase. A 9:1 compression ratio using say 12psi at BDC = 108psi at TDC (NA). Start off with a little more pressure to begin with at say 20psi (turbo), then the same 9:1 CR will produce 180psi at TDC. This is a very simplistic way to look at what is really happening, and leaving a lot of variables out, but it will give you an idea of what is happening. If nothing else, it will cause you to take pause and think about the terminalogy of compression, compression ratio, and pressure.

[Kevin Shasteen]

Originally posted by blueovalz:

.., If nothing else, it will cause you to take pause and think about the terminalogy of compression, compression ratio, and pressure.

That is basically the point I was trying to make. To over simplify: Compression means to squeeze something into a smaller state of existance. Keeping this in mind a Compression Test is not measuring the Compression Ratio-it is measuring the Cylinder Pressure in the Cylinder.

 

Compression Ratio is a mathematical relationship that doesnt change after the Engine has been built, doesnt matter if it is a Static or Dynamic Compression Ratio-once the engine is built those two Ratio's never change. Unless of course you can engineer a "Variable Bore & Stroke" as well as a "Variable Camshaft Intake Valve Closing"-then you can alter the Compression Ratios: till then-the Ratios will always remain the same.

 

It is the pressure of the chamber that is altered when a "Compressor" such as a Turbo/Supercharger is added: or a Chemical Compressor such as Nitrous is added.

 

Pressure deals in density, as in Moisture Content of the Air Molecule, while the Compression Ratio deals in the Mathematical Relationships of the Cylinder Displacement (Volume) relative to the Total Combutsion Chamber (Volume).

 

Both the Cyl.Displacement & Total Combustion Chamber Volumes can not be altered once you have put your engine together.

 

It is only the Intake Pressure (PSI) of the Intake System, that is altered relative to ambient pressures (Density of the Air Molecule) that a Compressor will effect.

 

The Density of the PSI is also what we are effecting when we add an Intercooler to a Turbo; we are amplifying the Turbo's ability to Pressurize the intake system.

 

That is how I understand the issue of Pressure, Compression Ratio and Volumes.

 

Kevin,

(Yea,Still an Inliner)