DirtRider . Net MX, SX, Arena Cross, Off-Road Community
Dirt Rider . Net Text Version Home
Dirt Bike Dirt Bike Dirt Bike Dirt Bike

This is the text version of DirtRider.Net
Click Here for the Full Version


Pages: 1

four stroke porting for a newbie

(Click here to view the original thread with full colors/images)

Posted by: Jamesfromwv

I have an '06 KX250F that I am currently rebuilding(piston, valves, etc.) and I can't seem to ignore my inquisitive nature and need to improve things. I would like to increase my machines power through some means of my own doing. I have rebuilt many 2 and 4 stroke engines with good success but I would like to be able to do more than just restore a machines original performance. So my question is, are there any basic "rules" of four stroke porting? (I am a junior in mechanical engineering and my Fluid Mechanics text is on the ready) Also, does anyone have experience with modifying my particular engine?

Thanks in advance,
James


Posted by: john stu

"basic rules" if you dont know how to correctly port you engine then do not do it if because most likley you will hurt power or worst the engine........send the head to eric


Posted by: Jamesfromwv

I appreciate your concern and I understand the possibilty of hurting my bikes performance. But my main goal is not to simply have a more powerful bike, it actually has plenty of power for me. I want to learn the concepts behind porting because it is something that interests me.


Posted by: Rich Rohrich

Theory and Practice of Cylinder Head Modification by David Vizard is an excellent introductory text on the subject.


Posted by: RACER557

where can i pic up this book? i have done many 2 strokes in the past. polishing, cutting new ports in, and so on. would like to learn process for doing 4 stroke engines. thank you.


Posted by: Chili

Quote:
Originally Posted by RACER557
where can i pic up this book?


In this day and age of extensive search engines I'm curious if most people are that inept at using them? By simply typing in the title of the book Rich mentioned into google I found multiple online sources to purchase it within seconds.


Posted by: oldguy

Quote:
Originally Posted by Chili
In this day and age of extensive search engines I'm curious if most people are that inept at using them? By simply typing in the title of the book Rich mentioned into google I found multiple online sources to purchase it within seconds.
And I would guess one is located along side a large river in the Brazlian jungle


Posted by: cujet

I truly hate all those disclaimers. There is information out there and if someone knows it, I feel they should post it.

A basic rule is to remove any casting imperfections. Pay particular attention to the valve seat area. The manufacturers often leave a ridge here.

Those with flow bench experience will note that a straight shot port works well with a slightly necked down area just upstream of the valve seat. 90 degree intake ports are a bit more tricky.

The valve guide area is also a place to look. Often there is more material around the valve guide than necessary. Careful removal can result in better high lift flow numbers.

However, I must add that modern ports are often good to great. The camshaft is most often the flow control.

Chris


Posted by: Ol'89r

James.

In my experience, there are two basic types of ports. The straight port and the deep pocket port, with variations in between.

What is most important is, what is the engine going to be used for? You can modify the way an engine performs by modifying the port. A deep pocket port will produce torque but, it will stop flowing at a relatively low RPM. A straight port will flow at a higher RPM and produce better HP at a higher RPM but will not have as much bottom end torque. So, if you were building an engine that has to perform at a low RPM with gobs of torque right off the corner, a deep pocket would be best. If you were building a high RPM engine like a road racer, a straight pocket port would be best.

The best flowing port would be a straight open pipe with no obstructions. But, since you have valve guides and valve stems and different angles messing up your straight pipe, you have to figure a way to make those obstructions less of a factor. This can be done by blending the rough spots left by the factory and just cleaning up the port.

Older engines usually have a port that starts out level with the carburetor and makes a 90 degree turn just after the valve guide. These types of ports make excellent torque. The large port gives a place for the intake gasses to build until the valve opens. These types of engines usually use large diameter valves to complement the large ports. But, there again, this type of port will not flow at a high RPM due to the 90 degree turn in the port. Modern engines are getting away from this design and as a result are becoming more of a high revving type of engine.

A straighter port must have more of a down angle from the carb to the head. This lessens the angle of the port at the pocket and allows the port to flow better at a higher RPM. The degree of angle in the intake manifold is limited to the carb angle. Since a carb uses a float it has to be mounted in a level position this limits how much the intake tract can be angled down.

I can see some major improvements coming to dirtbikes in the very near future due to fuel injection being used. With FI the injector can be mounted at an angle and this will compliment the steeply angled intake tract design. If you are into street bikes you will notice that the modern crotch rockets use this design. The injectors are mounted high above the engine and the intake tracts are steeply angled down into the head. This allows a straight intake tract and straighter angle at the port itself. Therefore, a port that flows better at a higher RPM.

Like cujet said, the modern engine ports are very good. Very little clean-up is required to make them excellent and extensive modification of the ports usually results in a loss of power. Also, these ports are very thin. There is not a lot of material around the ports to make them larger. Instead of using a large valve they use two or three small diameter valves so, they are also limited by the valve diameter. This type of port is better served by a different cam and cam timing.

There is a lot more to it than this. Cams, cam timing, intake length, pipe design and length and many other factors are involved in getting an engine to perform the way your particular style of racing requires. Proper machining of the valve seat and valve angles are very important also. There are many books on the subject. As Rich suggested, that would be a good place to start.

Just my $ .02
Ol'89r



This is the text version of DirtRider.Net
Click Here for the Full Version

Text Version Home





vBulletin Copyright ©2000 - 2008, Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2008 - Created by Stefan "Xenon" Kaeser