Port Velocity

[Danger Dan]

When considering performance mods which is more important, port velocity or flow? In other words, is there an optimum intake and exhaust port velocity that should be maintained or should one just widen the port to 65% of bore or what the piston skirt will allow? :smoking:

 

[husky hustler]

I would have to say to build your engine to the extent of its physical limits. trying to achieve a said velocity versus maximum (allowable) port size seems like a simple answer to me.

 

[Al Smith]

Good question,I don't have the answer.

It would make sense though that by increasing the exhaust to a reasonable size that the more rapidly evacuated exhaust gasses would in effect draw the cylinder to a more negative pressure than the incoming charge accross the transfers.

As far as figuring out what the actual velocity is in relationship to size,I aint the one.TW or one of the other math wizzards could do it I suppose.

What little I do know is that if you get the transfers too large you stand a chance of over running the carb.It gets too complicated for my little pea mind.

 

[Danger Dan]

This might be one way to figure it out. :smoking:

http://www.wallaceracing.com/lpv.php

 

[buck futter]

Neat link,

it would work great if the port instantly opened and closed.

 

[Danger Dan]

Are you suggesting that port velocity is not relevant for two strokes or the calculator is only good for four strokes? :smoking:

 

[Al Smith]

You have to figure it out using time/area.MacDizzy and Jennings cover the subject.

 

[joesawer]

Imo, you are almost entering a world of black art, or maybe just asking an over simplified question.
The engine calculators only work if all the varibels are entered acurratly. Then they often are tuned to get better results.
For instance a change in the size of the outlet of a box muffler on a two stroke would change the optimum intake velocity. The whole thing must be considered as a system and every thing is affected and related.
But to answer your question about percentages. I no longer open my work saws up to the maximum. My saws were too peaky for my liking. I have worked on broader power bands with close to the same peak power. Having no way to measure this accuratly makes it very difficult. How ever, many freinds and and fallers who have been using modded saws for years, say my saws run very strong.

 

[Jacob J.]

Like Joe said, the whole thing is a single system. Change one part of it, then other parts will need changed in order to keep the system in balance. Balance is the key, a balance between flow and velocity.

 

[drmiller100]

a high velocity for sure is not a goal. a high velocity indicates a restriction, somewhere.

a large volume isn't really a goal.

what is a goal is a LOT of fresh charge injected into the combustion chamber such that it evacuates the exhaust efficiently.

direction of intake charge is very important.
mass of intake charge is very important.

 

[timberwolf]

On the exhaust, I don't think the width of the port changes the maximum velocity a great deal, gas velocity on an exhaust port (on muffler) is mainly affected by pressure. The pressure when the exhaust port opens is mostly related to the height of the exhaust port and how much fuel and air are being burnt. But how quickly the velocity rises and falls can be changed.

However as there is a fixed volume of unexpanded gas in the cylinder when the port opens how quickly the pressure drops will be directly related to how wide the port is, so a wide port will see the velocity drop much quicker. The key to the wider exhaust port is it drains the cylinder quickly so fuel can start filling the jug sooner and ultimatly the engine produce torque and maximum output at higher RPM. Port shape can make a big difference too, if the port is very round, it starts with a narrow opening and slowly widens out over 20-30 deg, the pressure pulse will be much longer, but less intense(slow rising curve dropping off slower), if the port was square the pressure pulse will be much shorter but more intense( more of a spike rising to full flow quickly and dropping off sooner. Problem though is with a square port the port width might be limited to only 50% of the bore or less without destroying the rings, but with a rounded port rings have done fine with ports up to about 70% of the bore in width.

Ironically a wider or more squared port may acctually raise the exhaust gas velocity as the engine is pushed to higher performance levels and is pumping more fuel and air to burn which increases cylinder pressures and (BMEP).

As was mentioned above exhaust port width is limited by physical constraints, how wide the skirt is and what the rings will take in realation to width of port vs bore diamiter.

 

[Danger Dan]

I cherry picked these statements off the web...think they're applicable or BS?: :smoking:

How Airflow Influences Engine Performance
- Volumetric efficiency (VE) is the measure of how well the cylinder is being filled with air, as a percentage of what it would be if it were filled to the same pressure as the atmosphere outside the engine.
- Kinetic energy packing air into the cylinders increases with the square of its velocity.
- The velocity of the exhaust pulse in the port and header creates a vacuum behind it.

 

[timberwolf]

I would say they are all valid statements. But when they say kinetic energy packing fuel in that only refers to the air its self, as energy canot be created or destroyed if the energy in it's velocity is increased, then some other energy must go down (temperature and/or pressure).

The third statement does not realy apply to a muffler saw, but a saw with a tuned pipe and header. A more square port opening will raise the pressure faster and the trailing edge of the pressure pulse will fall off faster, this will create a stronger following negative pressure pulse than a smaller square port or a rounded port of the same width

 

[Danger Dan]

Like Joe said, the whole thing is a single system. Change one part of it, then other parts will need changed in order to keep the system in balance. Balance is the key, a balance between flow and velocity.

I tend to agree. There have been writings that indicate there are optimum values for port velocities, (280-380 exh. / 240-355 in., ft/sec), although these figures may not specifically apply to 2 strokes, as the are based on 4 stroke engines, I do feel that there is an optimum velocity for 2 strokes as well. This may explain why haphazardly enlarged ports may result in decreased performance. :smoking:

 

[drmiller100]

first and foremost, a port that points the wrong way really screws things up.

second, anytime you change velocity of the charge, you are losing horsepower. so, if you are narrow, get wider, then narrow, you lost power.
likewise, a change in direction is a change in velocity, but there isn't a lot you can do about it.

third, all else being equal, the larger the cross section, the more flow you get for a given pressure differential.

fourth, turbulence is a REALLY bad thing in a transfer or boost port.

 

[glenn31792]

 

[Danger Dan]

Very interesting!
Your post provides a different perspective on transfer flow. The Yamaha engineers reduced incomming charge losses by reducing xfer port exit velocity. I would like to know more about their overall design criteria and how xfer port velocity relates to intake and exhaust port velocity. :smoking: