UkLegacyT

as topic. is it best to match the port size in the inlet manifold to the port size in the heads?
im not sure what to do, as the step created by the manifold can help fuel mixture etc due to turbulence...

any ideas?

thanks
ian

Pavlo

Well, I am going to stick my neck out here!

It has been recommended to make the header about 3mm larger (on the radius) than the port on the head. Also that this sould be increased a little to about 4mm on the edge closest to the direction in which the header points. On the scoob this would be forwards I think, whereas an upright inline 4 you would increase the step on the bottom edge, as the manifold turns down.

The idea being that the larger step creates tumble which helps the gas cling better to the inside radius of the header, rather than all collecting on the outside due to momentum. In reality nobody really knows what's going on, although you could simulate it, or measure it using laser doppler anemometry (don't ask).

What I do know for certain is that a friend of mine openened up the head port to match the header and it was worse. So he got the head welded to put the material back.

Paul

Adam M

I would go to a head expert and ask them, or get them to do it.

that step idea sounds liek it is creating too much turbulence.

Not only that but tumble and swirl are better for charge stratification and leaner burning engines.

For max power these things are not so important, and homogenious charge is ok.

UkLegacyT

thanks for the replies

Paul, are you on about the exhaust manifold there?

Pavlo

Adam, I am talking about tumble coming out of the head and into the header. So the port in the header would be bigger than the port in the head. Gas flow would from small to large hole, not the otherway round. I think the theory behinds making the header larger, is that the pressure waves in the exhaust have an easier time going out than into the head, so exhaust gases are encouraged not to get sucked into the cylinder.

Swirl and tumble are terms usually applied to gas flowing into cylinder, but not exclusively so.

Flow bench figures are not the whole story, as they can't really replicate the pulsing present in the exhaust. Problems can occer because of exhaust/inlet overlap, such that as one cylinder is exhausting, it can push gas into a neighboring cylinder that shares the same header, something you don't want.

Paul

Pavlo

yeah, for some reason my brain completely removed the "inlet" from the thread subject!

So yes, I was rambling about exhaust headers/manifold!

Matching is probably a better bet for the inlet, duh!

Paul

Adam M

most head experts I know of dont believe in flow benches, certainly the three I was considering don't.

the reason I suggested a head expert is that they ahve been doing these things soleley day in day out for a long time. I would presume they would know from past experience what works well and what doesn't, rather than specualting based on perceived impressions of fluid dynamics. These guys are not scientists, they work practically and so tend to know what is goign to work or not surely?

Adam M

Paul I aprpeciate what you are saying regarding pressure and can understand that swirl and tumble can apply to air leaving the cylinder, but I fail to see why one would actively generate tumble in air attempting to leave the cylinder, any steps whether up or down will generate turbulence due to discontinuity, this promotes eddie currents which reduce pipe diameter and hence increase back pressure, in this respect I would rather go for a smoother narrower pipe than a stepped wider pipe regardless of which direction the charge is flowing.


Incidentally, I cant see a problem with egr if not on boost as it improves efficiency, although it does come at the expense of heating up the incylinder temps. Surely when on boost the boost pressure of incoming charge will significantly aid in purging the cylinder of spent gas during the overlap period?

Pavlo

I don't know exactly why tumble is encouraged on the entry to the exhaust manifold. I agree that turbulence is seldom the path to great flow, but the exhaust manifolds are complicated affair as there's all sorts going on inside, so who knows whats best in terms of overall power. The step would also cause a negative pressure wave (sucking) to be reflected back up the exhaust port, maybe this helps I don't know.

I'm only relaying what I have been told/seen/read about/studied.

Paul

SMG

Ian unless you can match the port perfectly to the inlet you will see little gain, match the manifold to the gasket and make the port
about 1mm bigger, the early type manifold has the injector boss in the inlet track anyway, later manifolds dont. Unless you are going to run higher lift cams you will see very little increase on the inlet side of things the gains are .300 and above.
Adam
I never used to use a flow bench, but then i saw one in operation
and i would not be without it. Velocity tubes are great you can
do gas speed check and this will show what i call a dead spot
which is apart of the port which has little gas speed and this
will occour becouse a part of the port is sheltered by an other part of the port. If your running a cam with .450 lift and your port will only flow .400 then you can loose flow, so easy work can be made for cam selection. You can see how well your ex port copares to your inlet.
As said ex primary lenght and diameter can effect the pulse in the
primary, this pulse can be used to hold the incoming charge in the cylinder but can also be used to suck it into the cylinder.
On the inlet side of things the inlet track lenght can determin
what the rev range will be, there is a term called supercharge effect, if you hit a hammer on a hard surface it will bounce
well that is a pulse a similer pulse will acour when the valve
shuts, this pulse will travel up the port and when it reaches the end it will return back to the cyl, this is the super charge effect it has up to 4 differant strenghts and inlet track lenght is critical to what pulse is used if any. I done some workings out and subaru use this pulse for lower down power.
As fare as porting without a bench if you were to go to america and attend the Advanced Engine Technology confrence, and that is where you will find the top boys indi and NASCAR engine builders they would most certanly show you were and how to find power.
Lads this is just the tip of the ice burg.
Deck heights, quench zones, rod lenghts, surface finishes, coatings it goes on for ever.
Have fun Steve.

SMG

whoops thats a bit of a long bugger.

Pavlo

Can modern flow benches give super sonic airflow?

Problem with small kart engines is nobody knows what's going on at high revs due to suspected transonic airflow at the inlet port.

When you say velocity tubes, are you meaning a small probe you can go in and measure point velocities with?

BTW, I see that extrude hone are now doing commission work, they will do your head, inlet manifold, turbo compressor casing etc etc.

Paul