Topic: Porting question...

[cdoggy81]

I've seen a lot of pretty port work done on some 750 & 550 heads & wanted to know by someone who does the work what all really goes into the port job? Do you just clean them up, change the design a bit, open them up or hog them out? The reason I ask is b/c I had some work done on my LS1 heads & got into a good debate about runner design, runner volume, flow, velocity, & how they all work together...

My feelings are with stock cubes & unmodified motors, you don't want to just start hogging out the intake and exhaust runners because the velocity will probably slow down even though you'll have great flow numbers. I think a lot can be gained on a stock motor by smoothing out the shape & transitions without opening them up a bunch. Typically with the bigger runners you lose some velocity which will affect your bottom end and part-throttle response but it can be said that sometimes it will be made up in the topend.

There seems to be a fine line of flow vs. velocity & the good porters know this. Runner shape and the cross section of the port can be more important (to a certain point) than just runner volume. A well designed smaller runner can flow better than a larger runner. Now all that being said, volume alone does not always dictate velocity. As long as the cross section is not too big, and has the proper shape, you won't lose velocity...
For example here are some head examples with made up numbers:

stock head 1 ~ 105cc runner & flows ~ 120cfm
ported head 2 ~ 115cc runner & flows ~ 140cfm
special head 3 ~ 105cc runner & flows ~ 180cfm

Head 2 has larger runners & flows more than head 1. OK, based on the above info you prob. think it gives
up a lot of velocity & low end b/c of the opened up runners. Not the case if ported by someone who knows what they are doing... I makes MORE power b/c it is a better "design" & still keeps the velocity up. How can head 3 have a smaller runners & flow more??? B/C they are a MUCH better design than the others (think aftermarket head design )

There is a lot more to it than all this but I wanted to mention a few key points & get other peoples thoughts & input. I am sure I missed a lot too & hope the pros can fill in the gaps. I really would like to hear from those who do the porting as to what all really goes into these port jobs? When do you know to stop? How far do you go for a stock motor, mild cam motor, hot cam & CR carbs motor? When have you gone too far & the port velocity suffers to increase flow?

[Ricky_Racer]

Maybe this will help... http://cb750k2.tripod.com/Technical/porting/Porting_the_Honda_Head.htm  RR

[turboguzzi]

"hogging out" ports is not something you do on an already pretty well designed motorcycle head, the term comes from hotrodding older cars with their contorted ports running  through pushrods and other obstacles.

its pretty common practice to actually fill the ports in strategic points to better manage the flow, mainly on the short side radius of the port(floor). a sort of explanation of why some seemingly smaller ports can flow better than hogged out ones without loosing speed.

that said, at least in the case of the CB500, the ports do need some enlargment, the area around the valve guide is simply a joke, got port silicon castings to prove that some polishing and cleaning will mostly make you feel good inside, the shape is the port itself is nothing to write home about and the main issue with the 500 at least is a basically small inlet that strangles flow above 10K. rule of thumb for a good sized valve is 50%+ of bore, with its 27mm valve the 500's is not even half the 56mm bore, hard to deal with that without some major re-angling of the inlets.

Join the speedtalk forum, you'll find there 10-page long or more posts on the subject

TG