Intake velocity vs turbulance

[JDelaney]

Eric, when working on the intake side of a newish fuel injected bike what do you think is more important: velocity or turbulance? Should the intake track be polished or should it have more of a rough finish?

[Dirtbag]

Not Ed, but I'll try to condense.

It is always beneficial to have the highest velocity that you can through the port until the point at which flow is restricted and cannot meet the demands of the engine for a given RPM. If the port is too large and adequate velocity cannot be maintained, the the flow will become turbulent. This causes a restriction and the flow numbers will decrease even further.

A finish that is not smooth helps to create a surface turbulence which tends to keep the boundary layer (slower moving surface flow layer) from growing thicker and reducing airflow. A polished Intake port will usually show a small drop in CFM from a slightly rough surface. The longer the port length the more significant the affect that polishing may have.

[Tosh Togo]

Eric, when working on the intake side of a newish fuel injected bike what do you think is more important: velocity or turbulance? Should the intake track be polished or should it have more of a rough finish?

Never give the inlet ports a mirror-finish. Although it'll impress the customer (maybe), those too-smooth port walls will make the boundary layer deeper, as your first reply stated. THAT will make the intake port behave as though it's slightly smaller than it really is.

Mirror finsih is OK on the exhaust side, as it will slightly impede the heat transfer from the outgoing gas plug back into the head.

Port shape counts for far more than does size/finish/length, and bigger's rarely better.

btw- turbulence can be a very good thing when it happens in the combustion pocket. :)

[Ed Who?]

sorry i missed this one...but the above two should give ya some good ideas. Also a guy Tim Radley in UK does great work and is on the same line as me/dirtbag/tosh.

http://www.racedevelopments.co.uk/


there is never a "right way for every engine" to do a head. A LOT depends on HOW guys is riding (drag race? Top speed/bonneville? Roadracing tight or long track?) and what kind of compression, cc, other supporting mods, race gas, etc, etc..

[JDelaney]

It's the whole dimples on a golf ball thing. I've found that port work is a little bit of science and a whole lot of art. You must have an understanding of the science before you can perfect the art. Thanks guys!

[JDelaney]

Next question: Why do velocity stacks have smooth bores? By roughing up the bore of the velocity stack, wouldn't the boundary layer be minimized and increase intake velocity?

[Dirtbag]

Many velocity stacks used to be spun aluminum and had a coarse finish inside. Many nowadays are cast or molded that generally accounts for the finish.

The inlet stack is at the beginning of the tract so it is much like the front of a vehicle in the wind. Air begins to compress and accelerate at the inlet or front. The boundary air layer begins to develop back from the leading edge due to pressure and airspeed. At the unions between Stack, TB and Head there is a stripping effect due to the joints. Not to say that it is irrelevant but even the flow loss in the Intake due to the surface finish is minute. But since the port is the last section in the inlet tract and there is shear involved in the turn at the bowl it is more a point of focus. If you know that the port works better with a rough finish and it is within your control, that's what you do.

Carbon fiber Velocity stacks would look horrible sanded anyway! :D

[Tosh Togo]

Next question: Why do velocity stacks have smooth bores? By roughing up the bore of the velocity stack, wouldn't the boundary layer be minimized and increase intake velocity?

Velocity stacks are a just tuning aid for changing the length (and resonant frequency) of the inlet tract.

What hooman beans call "smooth" is far from that, when you ask the airflow what it really wants. Fingers can't feel what the incoming fuel/air does.

The machined or as-cast inner bore's finish is one of those things that, IF you could discover and somehow install a better one, would have improvements measurable only in your imagination.

[Tosh Togo]

.... But since the intake valve(s) seat(s) are the last section in the inlet tract and ....

Fixed that one for you. :)

[mpp12]

Ed.... I have a Tim Ridley head in my gsxr1000 K57 (K7 bottom and K5 head) and the bike simply is impresive.... No mirror ports here.

He did the intake epoxy ports and did weld the exhaust ones, send me a ignition map to reflash the ECU....he is to say the least a genious.

I think that the layer effect affects only when you are dealing with gasoline/air, if the surface is smoth small condensations of gasoline will develop like the front glass of a car in a cold condition or a very small rain.
When is air alone smoth does not matter....

I remember that motoman (Pat Mc Givern) talked about that when he started to talk in the early ages about epoxy porting, he did not recomend mirror finish ports.