Confused...

[longlivetheZ]

I saw a thread kinda about this on here already, but not quite.

Why isn't lowering the compression ratio in order to run boost kinda "robbing peter to pay paul"? Both make power. I think I have some idea of how this works, but I just want to make sure and get a bit more clarity.

Thanks.

[Jet-Lee]

lowering the compression, then running higher boost doesn't require as highly advanced engine management as if you ran high compression and high boost.

I'd rather run stock compression and mild boost with just an fmu and make similar numbers as the guy who paid for forged low compression pistons and expensive engine management, and still be every bit as realiable.

There is a guy I talked to at one point, who had 9.5:1 compression, 10psi, nitrous, and alcohol injection on an Escort with a ~$300 MS system. It was his daily driver.

[-Josh-]

When you cram all that air in the cylinder and the piston wants to keep getting higher and higher but air pressure in the cylinder is pushing back, it can cause some problems. I tried to make that as simple sounding as possible.

[longlivetheZ]

Originally Posted by -Josh-

When you cram all that air in the cylinder and the piston wants to keep getting higher and higher but air pressure in the cylinder is pushing back, it can cause some problems. I tried to make that as simple sounding as possible.

I know that. Wasn't really what I was asking. I wanted to know why people would do something like this, as an example: lower the compression from 11:1 down to 8:1 or whatever in order to run 15 psi when they could, for instance, run the stock 11:1 compression and run, say...10 psi...wouldn't the outcome be about the same? All in all...why lower the compression in favor of higher boost pressures? I know the obvious "to prevent detonation" response...I'm looking for the mechanics of it, I guess you could say.

Here's the simplest way I can put it...why is it, or is it at ALL, beneficial to lower the compression ratio in order to run higher boost?

[nissanfanatic]

With high compression ratio pistons, you are filling the combustion chamber with metal to make better use of the existing fuel/air. With low compression ratio pistons, you are filling the combustion chamber with air/fuel then compressing it slighly less. So it makes for more air per cycle in a low compression ratio engine. Hence why you can make more horsepower on pump gas.

Low compression pistons are really good for street driven higher powered cars to allow them to make power on pump gas.

[beef_bourito]

if you think of it in terms of cylender pressure, on the one hand you can have high compression with low boost with X cylender pressure. on the other hand you can have lower compression with high boost with the same cylender pressure. both have equal chances of detonating (or preventing it) but one of them has more fuel end air in it and therefore more power (in case you didn't know wich one, it's the low comp high boost one). when you raise the compression you raise the cylender pressure when it's on the compression stroke, this makes a more violent, hotter, and quicker combustion and therefore more power. when you run a turbo or supercharger, you increase the cylender pressure creating a more violent, hotter, and quicker explosion but this time there's more to burn and more power. any time you want to make more power you need to think of 3 things: 1) making the engine and/or the combustion more efficient (lightening it, removing friction, removing rotating mass, higher compression) 2) putting more oxygen into the cylender (by removing restrictions in the intake, nitrous oxide (look it up if you don't know how it works), boost, getting colder air) this way is the easiest and the most common as a first step. and 3) by making the gases exit better (headers, getting a high flow cat or removing it entirely, cat back system, better flowing heads, etc). hope this helps.

[longlivetheZ]

The last couple posts helped a lot. It's basically what I thought anyway, just added a bit of clarity. Thanks, guys.