Inlet pressure

[arc]

I've been pondering..

I know basically how an engine works.. but one thing confuses me. Why is there a vacuum in the inlet manifold. My first thought was its because the engine is trying to draw more air than the induction is bringing in. But then i thought.. well, surely thats a bad thing..

What got me thinking was the boost gauge in my mates car, pootling along with low rpm and the gauge was showing near -1bar. If the engine is doing very little, therefore using less fuel and 'sucking' less air.. why is the pressure so low

am i making sense?


Cinq Sporting - and yes, its broom yellow!

 

[HP]

Yes, you're making sense.
The car is 'throttled' to make it go slower than flat out all the time. The butterfly in the throtle body is nearly closed during cruise, and the engine is sucking hard to try to get more air in. If you give it more air, you need to give it more fuel otherwise it'll run weak - and then it'll go faster. Fine on the open road, but not behind old Mrs Sidebottom in her micra doing 20mph when you need to keep a distance.

Pete.
1242cc cinq,turbo cinq,Cinqs&Seis Yahoo group,Clubcento

 

[MrDerv]

Arc.. When you connect your Vacuum gauge (if it ever turns up) it should read around the 20in.hg mark at idle. Just so you know when it's time to fit it

 

[arc]

ahh, that makes sense. cheers pete

brickfoot - the cheque has cleared for the gauges..so they should be en-route.. just need the pillers now. where did you run the pipe thru for the gauge btw?


Cinq Sporting - and yes, its broom yellow!

 

[arc]

follow up pondering..

ive always assumed the throttle controls the amount of fuel going into the engine, but does it in fact control the butterfly and then the fueling is just done as result of the amount of air going on?


Cinq Sporting - and yes, its broom yellow!

 

[MrDerv]

i ran the pipe thru the grommet on the bulkhead for the speedo cable, hope that helps

 

[mad_colm]

Originally posted by arc
follow up pondering..

ive always assumed the throttle controls the amount of fuel going into the engine, but does it in fact control the butterfly and then the fueling is just done as result of the amount of air going on?


Cinq Sporting - and yes, its broom yellow!

that's exactly it! that's what the airflow sensor is for


Met Cognac 99 Brava 100 HSX,

 

[arc]

what about on cars with carbs.. they run without ECU's dont they.. how do they control the fueling?


Cinq Sporting - and yes, its broom yellow!

 

[mad_colm]

wouldn't know! before my time! but it think the carb has a little inlet called a venturi which sucks the petrol in and is adjustable. not certain though

Met Cognac 99 Brava 100 HSX,

 

[HP]

Mad Colm, yep. It's a complicated mechanical thing which allows the right amount of fuel to be drawn in for the given conditions.

Check out http://auto.howstuffworks.com/engine16.htm

Pete.
1242cc cinq,turbo cinq,Cinqs&Seis Yahoo group,Clubcento

 

[arc]

ah cool, a carb works roughly how i would've guessed - cheers again pete


Cinq Sporting - and yes, its broom yellow!