Tank Slap - Motorcycle Forum

After a discussion with a colleague it quickly became apparent that there are different ideologies behind Cylinder Head Porting.

My colleague and friend believes that increasing the size of the ports increased the volume you can pump in (volumetric flowrate)

My belief is that you want to improve the velocity of flow and not just the volume. This means potentially adding material (welding if necessary) and making the smoothest (in terms of radii and not surface texture) port possible - essentially you're making the racing line larger so you can go faster. This is about all my knowledge on engines as I don't know what goes where, where it comes from or how it necessarily works but I would imagine you would need to increase fueling (or rejet for a carb'd bike) to make up for the increased air flow and keep the ever-important A/F Mixture correct.

I'd like to know your own thoughts on this as I don't believe there is just one way of doing this.

From my experience which is pretty dated now, it is both. While removing material to increase flow rate you try to smooth as much as possible to improve airflow, not just in velocity but also reducing turbulence.

I've done it with just a die grinder and sandpaper and checking volumes with the oil filling method. But the method I've seen in the past I'd look into is Abrasive Fluid Machining. Basically they force an abrasive fluid through your ports at a set volumetric rate and by nature of the fluid flow it ports in an ideal fluid path, polishes and keeps all intakes at the same volume. I've seen one head done on a rally car I helped spanner on, and it looked trick and the owner was impressed with the results.

Your friend is wrong, if you just increase the size of the ports the velocity will drop. It's more about shaping and polishing the port to make it more efficient and flow better. It's a chuffing art!

just turbo or supercharge it, you'll get more air in that way guaranteed!

Monty wrote:Your friend is wrong, if you just increase the size of the ports the velocity will drop.

No always true Monty. I agree with the basis that increased area equates to decrease velocity if the volume of fluid remains constant (due to displacement). But if the airflow is being restricted by a pressure build up at the orifice (Valve inlet/outlet) you get full bore flow. This full bore flow will be lower the smaller the orifice. Increase orifice size, increase pressure downstream = increase flow rate.

When it comes to air a smoother surface actually hinders flow - airplane wings aren't smooth, they're deliberately textured to allow the air to move more freely thus creating less drag.

I would love to make one from scratch using ANSYS (FEA) software to calculate a theoretical best result and then get it manfucatured. Inevitably there are numerous issues such as calculating how much flow you can actually achieve with the bike in motion which is something I do not have the equipment nor expertise to do.

/starts to read thread

Cav wrote:When it comes to air a smoother surface actually hinders flow - airplane wings aren't smooth, they're deliberately textured to allow the air to move more freely thus creating less drag.

I'm going to sound argumentative on this thread, but I'm not sure where you got this info Cav, but it's not right. I spent my first 16 years of working life as an Aircraft Maintenance Tech in the NZ Air Force and as a civi for the Australian Air Force.

The airflow over a wing is complex and it is hard to discuss one effect in isolation, but the ideal airflow over a wing is laminar. The boundary layer (airflow closes to the wing) has a great effect on the laminar airflow. Generally the smoother the surface the thinner the boundary layer is and the better your laminar flow is.

There are other conditions that you purposely disrupt the laminar airflow. Vortex generators are a good example. Towards the trailing edge of a wing you have problems such as laminar flow departure and transverse airflow (across wing rather than along it). So one of the ways they maintain airflow along the chord line is inducing a small series of vortex toward the rear of the wing or in front of flight control surfaces to direct the air in a straighter line.

actually you dont want polished intake ports ,exhaust yes but not intake

kiwikrasher wrote:

Monty wrote:Your friend is wrong, if you just increase the size of the ports the velocity will drop.

No always true Monty. I agree with the basis that increased area equates to decrease velocity if the volume of fluid remains constant (due to displacement). But if the airflow is being restricted by a pressure build up at the orifice (Valve inlet/outlet) you get full bore flow. This full bore flow will be lower the smaller the orifice. Increase orifice size, increase pressure downstream = increase flow rate.

Yes that's why I said "If you just increase the size" meaning only that which is what Cav's mate seem to imply and added "It's a Chuffing art"

Monty wrote:

kiwikrasher wrote:

Monty wrote:Your friend is wrong, if you just increase the size of the ports the velocity will drop.

No always true Monty. I agree with the basis that increased area equates to decrease velocity if the volume of fluid remains constant (due to displacement). But if the airflow is being restricted by a pressure build up at the orifice (Valve inlet/outlet) you get full bore flow. This full bore flow will be lower the smaller the orifice. Increase orifice size, increase pressure downstream = increase flow rate.

Yes that's why I said "If you just increase the size" meaning only that which is what Cav's mate seem to imply and added "It's a Chuffing art"

fair point

I think Cav is confusing Aeroplanes with Golf balls

Monty wrote:I think Cav is confusing Aeroplanes with Golf balls

^^This !!

Hahaa, I mean there is a texture to the material on the wings. Fair enough if my source is incorrect but the explanation with it made sense

Cav wrote:

Monty wrote:I think Cav is confusing Aeroplanes with Golf balls

^^This !!

Hahaa, I mean there is a texture to the material on the wings. Fair enough if my source is incorrect but the explanation with it made sense

Golf balls are dimpled to reduce profile drag because they are round as there is a large low pressure area behind a shpere in an airflow. The dimples help the air into this low pressure to lessen it. (google how if you need to, it's very long winded!) Airplanes are streamlined and by design don't suffer that problem to a point where where imparting skin drag would give a greater gain over profile drag.

In answer to the original question, my understanding of cyl head porting is something along the lines of:

(1) increase and/or rectify the swirl effect within the combustion chamber to achieve a fuller and more efficient burn - thus increasing power.
(2) to ensure the incoming fresh charge has properly atomised fuel distributed completely throughout it - thus ensuring adequate oxygen for said fuel and more power.
3, to obtain a surface finish that prevents laminar airflow - which prevents good distribution of fuel within moving air. As I recall laminar airflow will hold fuel laden air in "strata" preventing that even distribution as it enters the combustion chamber and begins to swirl.
4, if possible to increase maximum airflow whilst still maintaining 1/2/3, after all, more air is more oxygen, which allows more fuel, which should be more power.
5, exhaust ports should be a mirror finish in order to prevent carbon from adhering to the port walls, which would change the flow characteristic.
6, All ports should be exactly matched to their respective manifolds, with no discernable lip at any joint to disturb the carefully cultivated air/fuel flow.
7, valves and valve guides should be smoothed and shaped as required to ensure no disturbance of airflow and if possible to enhance it.
8, all inlet tracts to be the same volume, unless design demands different, same for exhaust tracts.

That's about as much as I know about the subject - most of it gleaned from articles read over the years and discussions with a couple of tuners I've met. Hth DG.

What do I think porting is??

Secret, magic, clever stuff what makes the engine thing work better.

[clarkson]POWER[/clarkson]

I like Portishead, does that count?

I hand ported the heads on my 1976 Porsche 911 when I rebuilt the engine ,it made 300 Bhp from a 3 litre so I just have done something right

This book will be worth a read if this sort of thing interests you.

http://www.stanstephens.com/" onclick="window.open(this.href);return false;