How an Exhaust Manifold Works

today I'm going to show you what's

inside one of these exhaust manifolds

and how it works over here at the back

of the engine underneath this heat seal

is where the exhaust manifold is located

the exhaust shield is out of the way I

can work on removing the exhaust studs

to get it off the engine and here's the

exhaust manifolds removed from the

engine now here at the back of the

engine is where the exhaust manifold

mounts to right so you can see you've

got the entire exhaust laid out here

we've got the exhaust manifold up at the

front here the catalytic converter in

the middle and the resonator I'm going

to tell a pipe that leads out to the

muffler so here's the exhaust manifold

removed from the vehicle this one here

from the 4-cylinder engine is a four to

one manifold you can see this is what

the backside looks like and this is what

the exhaust gasket looks like now over

here we've got the heat shield and we've

got the collector over here followed by

the exhaust flange and then we've got

the air to fuel sensor before it goes

out to the catalytic converter now I'm

going to cut this open to see what it

looks like inside and that's what that

looks like

oh there's double walls so inside the

down pipe here I was actually surprised

because it is a double wall pipe that's

why it was so difficult to cut through

you can also see the air to fuel sensor

inside of there which tells the computer

if it's burning to lean or to rich now

you can see the double wall on the other

end here

it runs from about here all the way up

to the top here and over here with the

exhaust flanges it's also double walled

alright let's do one more cut down over

here alright let's open that up there

and that is pretty cool so if we take a

closer look at the collector here we've

got the four individual pipes that join

on each corner with this cross in the

middle if you look from the back here

you can see that cross and here we've

got the exhaust manifold you can see

that the outside cylinders have a much

longer pipe compared to the inside

cylinder so um has specifically tuned it

so it has optimal performance and sound

for this engine and the function of the

exhaust manifold is to collect all of

the exhaust from each of the cylinders

in this case have drawn a four to two to

one exhaust manifold now each cylinder

releases the exhaust gases at different

timings and those exhausts come out in

pressure pulses with the front having a

high pressure and the back having a low

pressure so now the pressure pulse

travels along here it creates a

low-pressure zone over here which helps

suck out exhaust gases from the cylinder

now this effect is known as scavenging

when the

low pressure from the exhaust pulse

helps to evacuate remaining exhaust

gases from the cylinder and it also

helps to bring in fresh air from the

intake at the moment when the intake

valve and exhaust valves are open at the

same time in the cycle that helps to

fill the cylinder with fresh air for the

next cycle and the manufacturer has

chosen the diameter and the length of

each pipe to optimize the scavenging

effect that this manifold has on the

cylinder now aftermarket exhaust headers

will usually go from the skinnier om

design to a much larger pipe to make it

a little bit more free-flowing and the

piston easier to push out the air now a

free-flowing pipe allows for a much

easier escape of the exhaust gases

compared to a narrow pipe where pressure

is building up however the trade-off

with that is on the narrower pipe you've

got a much higher velocity of your

pressure wave which increases the

scavenging effect allowing fresher air

to come in to your exhaust on the other

hand with a larger pipe your velocity

decreased significantly and so does the

scavenging effect now when designing the

exhaust header we need to strike a

balance between these two in order to

have a sufficient scavenging effect

while at the same time not including too

much force on the piston due to back

pressure and who would have thought

there was so much engineering that goes

behind a simple exhaust manifold