Cylinder Head Exhaust Ports

[Zak33]

OK....a brief guide. This is not my engine, but its similar.

The cylinder head in the ALLOY SILVER bit, lower part of piccie, runs left to right...it is the "lid" on top of the block, where the pistons are...it has spark plug holes (black and shadowed) and ports for the gas to escape from...they are all silvery.
Each exhaust port has 2 bolt holes, one above, one below....to have an EXHAUST MANIFOLD bolted on...

if the exhaust gas came straight out it would be belching flames onto the radiator AND sound so loud you might die of shock

So this is the front of the Vauxhall engine....the BACK (can see) has 4 similar ports (to let air in).

INSIDE, hidden from view are VALVES that open and close to allow air in the back and then exhaust gas out the front.

[Zak33]

right....THIS is an exhaust manifold...(not mine, but a good picture)


it BOLTS to those ports....it takes the red hot exhaust gas down.

This is a 4-2-1manifold....it combines 4 exhaust ports (from a 4 cylinder engine) into 2 pipes...then those 2 become one.

On my car, the 4 become 2 and stay seperate right under the car, in 2 pipes and THEN join.

The further they stay seperate, the more room the gas has to move quickly without getting all jumbled...the faster we go

[Zak33]

now...imagine THAT Manifold (cast iron, rough as a badgers arse ) has been bolted onto my head for 3 rallys...

its done 600 miles on the road as well....
exhaust gas is full of soot...back soot...red hot, sticky, black soot...

when you take the manifold OFF.....in THEORY, the exhaust gas and soot should JUST be IN the pipes on the manifold....

[Zak33]

here's another...



and another...



ignore the sticky out things....these cast iron things take gas away from the engine..cheap to make....cheap to design...and sadly they dont always line up with the cylinder head ports in the way you'd want

[Zak33]

now the hard bit to explain.

When I took mine off, the EDGES of all the ports/ducts....the dam hole where the gas goes, were SURROUNDED by a black sooty edge...

in some cases, their was a black border, like a picture frame, around the entire manifold hole...

that MEANS the cylinder head is throwing exhaust gas (an soot) out of a big hold, straight into a smaller hole.

Its like having a door that is one side, but the hole in the wall, is too small...and as you walk through, you catch your head on the top, and trip over the bottom...

All I needed to do was use a drill and a grinder (like a spinning cotton reel sized thing, made of rough stone to grind away at cast iron)

and MAKE the ports the SAME SIZE as the sooty marks....

I WISH I HAD PHOTOS>..but I dont.

In effect...the cylinder head port holes, are NOW pushing the exhaust gas into holes of SIMILAR SIZE AND SHAPE and therefore there is no blockage or disruption.

Same on the "down pipe" .....having done the top of the manifold, I looked at the bottom of it where the gas comes out in two pipe....

the MANIFOLD in this case was perfect....2 HUGE holes, but the pipes that bolted to it (DOWN PIPES) were too small...

hence my other piccies.....I had to grind the inner edges wider to match

[schrickvr6]

this was a nightmare to do on mine, as the vr6 has a crossflow head to get maximum power the front cylinders exhaust ports needed to be larger than the rear bank and vice-versa with the inlet ports

[blockers]

So..have i got this right basically all this is to get the exhuast gases to move out of the engine quicker?

How does this give you more power?

[schrickvr6]

So..have i got this right basically all this is to get the exhuast gases to move out of the engine quicker?

yep thats right or get gasses in quicker if your 'match-porting' the inlet manifold....same principle

[Zak33]

yup..to get exhaust gas out faster.

I'll give you an example.....imagine trying to suck air into a piston when its already full of old air and exhaust gas.

Right....now...imagine what happens at 6000 revs per minute.

That means every 10 seconds the engine turns 1000 times
That is 100 times per second...

imagine ALL THE AIR BEING SUCKED IN....

now, the REASON a car goes at all, is cos the air sucked in, and the fuel mixed with it, go BANG and EXPAND>...BIG TIME.

So...if this engine has 4 cylinders, and is turning 100 times PER SECOND, and the air being puled in is being expanded by blowing up petrol inside it....

imagine HOW MUCH air you have to get OUT again....

For reasons we wont go into, a 4 sylinder engine, when it's turned ONCE has had two explosions...

If the engine is 1600 cc, each cylinder is 400cc volume. Lets imagine the air expands to double that...(its a lot more, but I dont know HOW much more)

every 100 revolutions, 200 explosions have occured.....each is creating 800cc of exhasut gas....

200x800 = 160,000 cc...that 160 litres of exhaust gas....

per second....

thats filling 80 plasitc two litres coke bottles PER SECOND....

whaere's it gonna go?

and if it DOESNT go.....its gonna stay in the cylinder, and not let the next induction of air/fuel in..and it wont support combustion anyway, cos the Oyxgen is gone and its all CO2 and other stuff....

no bang.

Wanna know what happens if you stick a load of banana's up an exhaust pipe?

Watch Beverly Hills Cop....the car just stops

[blockers]

So it isnt giving more power as such, it is just making the engine more effecient.....?

[Zak33]

one creates the other Efficiency in the EXHAUST system will lead to the pistons drawing in MORE AIR/FUEL for better power...

but you must remember...there are 4 pistons...2 go up, 2 go down, simultaneously.....

SO if ONE is finding it HARD to push the exhasut gas out, because its restricted, its like having a BRAKE on the other 3.....

so yu SLOW the engine, and LOSE power because each cylinder is suferring...while ONE cylinder is turning the crankshaft , another is slowing it down by having a bad exhasut system...it ,must ALL work in tandem

[Blub2k]

200x800 = 160,000 cc...that 160 litres of exhaust gas....

per second....

thats filling 80 plasitc two litres coke bottles PER SECOND....

Zak if these are real life figures then this is the best argument I have ever seen for alternative energy sources ever. This has scared me, am glad I dont have a car, and now I KNOW why.
Sorry to go a bit OT.
[/Hijack]

[Zak33]

Blub...thats the worst hijack I have ever seen...

but to be fair...my figures are on the LOW side mate....very low.

[hotpurple]

the thing you don't realise is that most of the air going into and coming out of the engine is untouched as it is nitrogen (about 80% of air is nitrogen). The level of actual emissions is very low in reality. You are talking around 100g of CO2 per km for a fairly small engine.

Chris

[Zak33]

/\ good point...well made

[sdp]

As a footnote to a top explanation, if the step is the other way, i.e. the port is a wee bit smaller than the exhaust manifold, there are some cases where it will actually improve gas flow slightly.

But unless you got a flow bench and half dozen heads to play with it's not something you can really evaluate.