what is boost/how does it work

[MHanrahan]

what is boost/how foes it work

what exactly is meant by boost? And how does it work? All I really know is a supercharger produces boost. From my minimal knowledge of how a supercharger works, I'd say the super charger forces the air into the engine changing the compression ratio or something like that. I'm just curious and would like to know more about it.

 

[TarHeel085]

i think you got it right! but dont take my word for it. a lil off topic but how do you like your pirellis and howd you fit them?

 

[Stephen S]

Boost is the increase of intake pressure that the supercharger or turbocharger produces. A normally aspirated engine pulls in its own air/fuel mixture with the downward motion of the piston on the intake stroke. This creates a slight vacuum in the intake manifold and cylinder, so the cylinder is really not as full as it can be with the air and fuel mixture. A forced induction engine (supercharged or turbocharged) increases the intake pressure which increases the amount of air fuel mixture that flows into the cylinder on the intake stroke. The increased pressure literally forces the air/fuel mix into the cylinder at the boost pressure. This means that more air/fuel molecules are in the cylinder to be burned. More air/fuel makes more power.

 

[Alec]

Great description Stephen.

Depending on the motor, each pound of boost can equal anywhere from 7-15 hp. Boost depends on the design of the blower unit (turbo or s/c), engine speed, and the tolerances of the blow-off system.

 

[MHanrahan]

Thanks for the info, I also found an article about it on how stuff works, but yours was a bit more detailed Stephen. Me and my friends were talking about preformance and cars, etc. and boost came up, so I figured this was the place to answer my question.

As far as the pirellis I like them, they preform just as well as the firestones, well probably better. As far as the fit, well the only problem was when the steering wheel was at full lock, they rubbed and made a loud noise. Basically it was the tire hitting the inner corner of the fender flare, It took about a minute to cut off, and you can't even see it. Personally I wanted to go with BFG AT KO's, but no one would sell them to me, and they were a bit more expensive, I really like the tread pattern on the BFG's. But overall I like the Pirelli's alot and would reccomend them.

 

[MHanrahan]

one more question

On the howstuff works article it said that normal atmospheric pressure is like 14.7psi, so if you had 8psi of boost would the atmospherice pressure in the cylinder be 22.7psi? And so the increased pressure also sucks more fuel in just because there is more of a vacume type thing? I guess that was two questions.

 

[zensius]

There's not more vacuum. There's more pushing force. The superchager pushes more air into the combustion chamber. The end result is the same as more vacuum. More air = more fuel= biger bang= you feel more power.

 

[Alec]

On a forced induction application, the fuel system is changed so more fuel is provided under boost. That's why I have an FMU (fuel management unit) in addition to the stock FPR. The FMU can deal with boost and add the correct amount of fuel. Much more fuel is injected under boost than under vacuum. My fuel system has about twice the fuel pressure at WOT than the stock system does.

 

[Stephen S]

Re: one more question

Originally posted by MHanrahan
On the howstuff works article it said that normal atmospheric pressure is like 14.7psi, so if you had 8psi of boost would the atmospherice pressure in the cylinder be 22.7psi? And so the increased pressure also sucks more fuel in just because there is more of a vacume type thing? I guess that was two questions.

Technically, the air pressure inside of the intake on a normally aspirated engine is below atmospheric pressure. There is a vacuum condition inside the engine. The vacuum occurs because of restrictions on the air trying to enter the engine, like the air filter and intake tube. The engine can't get all of the air it would like into the cylinders because of this vacuum. This is why performance intakes and filters work to increase power. By decreasing the resistance of the air flowing into the engine (or by forcing it in with a turbo or supercharger) the engine will make more power.

 

[MHanrahan]

I see now, about the whole more fuel thing. I'm probably beating this into the ground but baiscally because of the increased pressure in the cylinder there are more air molecules per certain amount of space to combust with the fuel? So it's kinda like having a larger engine with a smaller one, or is that way off. You guys have deffinately been really informative.

 

[Alec]

MH,
You're absolutely correct that boost pressure (forced induction) makes up for displacement. More air = more allowable fuel = more combustion = more power.