123 replies to this topic

[Dave6179]

When I was porting my head I thought the same way. Without the valve I could insert a straight piece of wire and 'see' the apex on the short turn. This is what I removed... to a degree. Not knowing how much metal was there I erred on the side of not f*#king it up!

[EunUCh]

Something else i just found out ? ,when looking at an 8 port head, (a real engine 12 port may be similar?) ,when we look into the port,on the "port divider" side we see that the side that is on the shrouded area of the valve is shaped a bit like a wing, along to some degree with the floor with it's short turn radius , seems like it might tend to stream to "one corner" for some reason then "hit" the back of the valve and do some fancy foot work while going in to the pot.

Edited by EunUCh, 24 March 2015 - 04:42 PM.

[N/A-PWR]

[EunUCh]

     

[EunUCh]

Air speed in sloooow mo.

 

would be nice to able to "see" what is going in there with gear like this ?

you can also google sonic boom of jet planes close to water,incredible stuff?

 

dare i say it ...an engine should "sing" when sort of "tuned" or not ?

 

just as an example...one of the local stock car boys was running a blue 202 , now, this bloke just put them together as cheap as possible

with no major modifications other than a bit of a solid cam/3 Delorto's ,extractors and who knows what in the port department if any?,he is well known for being a "tester" of engines..i.e ,stand on it hard and make the right boot do isometric exercises on the floor pan to get more and see what happens.

This thing would "sing" a tune up high,he was giving the $10k 351 a run and probably would have taken the night but the second hand 

head gasket let go.

 

[EunUCh]

Will spend a bit more time looking at the CFD stuff,found a bit, but must be more out there.

 

So if we are looking at the attributes of the way the port/valve/chamber play a role,even though the "wing" and short turn radius do

some good,what would we expect to see at the higher rpm end of things when the air can no longer stay attached to the radius of the short turn, but on the wall with the wing shape things stay a bit cleaner,there must be some transition of where things change as the volume/speed increases and change the way the the air flows not only in the port but through the throat and over the back of the valve? 

 

[N/A-PWR]

Thought this was a good read:-

 

Manifold Design Criteria

 

As mixture flows, the mixture picks up heat that evaporates the liquid fuel droplets, gradually changing it into a gaseous air-fuel mixture.

 

The manifold size must be large enough to allow adequate flow for maximum power and small enough to maintain sufficient velocities to keep the fuel droplets in suspension.

 

The charge flow through the intake manifold is dependent upon the number of abrupt runner bends, the smoothness of the interior walls, and the cross-section of the runners.

 

Sharp bends tend to increase fuel separation as air is lighter than fuel droplets.

 

Rough interior runner surfaces add drag and turbulence to the charge velocity disturbing charge distribution.

 

The corners cause turbulence that helps evaporate liquid fuel on the surface.

 

Passenger car engines are primarily designed for economy at light load, and part throttle operation.

 

Their manifolds, therefore, have a much smaller cross sectional area to maintain adequate mixture velocities throughout their normal operating range.

 

On the other hand, the engines designed especially for racing have large manifolds for attaining maximum power, but their size allows fuel separation at low speeds.

 

The problem of fuel separation can be avoided on the engines that have fuel injection into the manifold near the inlet valve.

 

These engines can operate satisfactorily at low speeds, even with large manifold cross-sections.

Small ports for Low rpm, and big ports for High rpm.

[EunUCh]

Interesting,some say that evaporating fuel in a warm intake runner will tend to take up more space but result in a more "gaseous" mix

resulting in a "cleaner" burn which is good for emissions,on the other hand , a cool dense smooth mix in the runner with minimal "fuel drop out"  that then "sees" a hot valve/chamber will result in the expansion of the fuel into a vapor in the cylinder thus aiding in a mix that takes up more space aiding in the 'volume" of the mix expanding in the chamber which in turn "boosts" compression/power and result in just as good emissions but with more power?

 

Heard a story that said fuel when it is evaporating in a hot environment will take up nearly 10 times the amount of space of that of which 

it was a cool mix...don't quote me on that ...was just pub talk.

 

[TerrA LX]

I think manifold heat is far from any concerning factor. Unless we are talking extremes not experienced in normal conditions.

The intake, in a N/A engine, operates under vacuum, this Vacuum better allows the fuel vaporize, the lack of vacuum, in a big runner, big carbed engine, down low, out of its power band, is unable to hold the fuel in suspension where it can condense on the port and runner walls.

 

You only have to study a vacuum gauge in relation to throttle openings vrs RPM/engine load to understand how important vacuum is in a N/A engine.