Eric: port time-area formula?

[jaguar]

Gordon Jennings authored a port timing article (http://2stroker.net/jennings/aspirin_takers.pdf) in Cycle magazine which
referenced a port time-area calculation method obtained from an SAE paper about hi speed racing engines. I want to know if the target range is the same for a low rpm engine as well. Jennings said these formula results are desired:
exhaust .00014 - .00015
transfers .00008 - .00010
piston port intake .00014 - .00016
The higher numbers are more for high revving engines, and the lower numbers are for low rpm power engines.
The formula goes like this: (seconds of port duration open)x(first half of port area in squared cm)/(engine displacement in cc)
The formula for finding the center rpm for the each port is (degrees x area)/(center value x 6 x engine cc). "Center value"
is the middle number between the two extremes listed. .000145 is center of .00014-.00015

The port duration would actually be the active half of its opening. If the exhaust opens from 90 to 180 degrees then the duration would be the time it is letting gasses go out during that 90 degrees. Figure like this: (rpm/60)=rps 1/rps=time of one cycle. Divide this by 360 to get time of one degree. Multiply by degrees port is active. So a 90 degree exhaust has .0015 seconds of port duration at 10,000 rpm.

The first half of port area is the area open from beginning of opening to the half way mark in degrees. So figure or measure the half way mark on the port and figure or measure the area in millimeters and then divide by 100 to get the area in squared centimeters.

The intake port timing is only critical for non-reed valved engines.

For my little 48cc engine the rpm necessary to get a formula output of .000145 is 3852rpm.
Please Eric or Rich, can you comment on this formula and its application for enduro engines?

 

[helio lucas]

http://www.amazon.com/Design-Simula...6850/ref=sr_1_1?ie=UTF8&qid=1294658240&sr=8-1

all your answers are here described in a way no one else can do...
buy the book, worth twice is wheight in gold. will pay for itself in less than a week.

 

[jaguar]

I wish I had $170 to splurge. That's the reason I am asking here, it's w/o cost!
What's weird is Jennings emphasis on the combo of port timing and area dates way back to 1973 (year of my first bike: a Penton 125). But no one ever references it. Why? But maybe the good engine porting software programs include this calculation. I am using this formula to report my 48cc engine. Previously I had just ported by timing derived from recommendations by Eric Gorr. Now I am including the port area.

 

[helio lucas]

But no one ever references it. Why? But maybe the good engine porting software programs include this calculation. I am using this formula to report my 48cc engine. Previously I had just ported by timing derived from recommendations by Eric Gorr. Now I am including the port area.

eric gorr recomendations are for mx bikes, not for a chainsaw engine. totally different beast...

time-area and angle-area have been here long ago. and yes, it´s widely referenced...
gordon jennings was a guru at his time, his writings where great, still they are. but, time has changed, new technologies evolved etc... his principles are still right but things are different now. even a industrial engine like yours will accept a very wide exhaust port, what in that time was called "race only" or "high wear spec". his formula to calculate time-area is crude but may still give you a good ballpark figure.

the concept of time area is very easy to understand. and the 170$ book is really the two stroke bibble, a compilation of loads of sae papers.

i will give you this link again:
http://www.dirtrider.net/forums3/showthread.php?t=180270
check rich´s posts. every one, they are super informative...

 

[jaguar]

Actually Eric gives specs for enduro type powerbands also, and I found some literature of his that says he does use the time-area calculations.
I wonder why Eric or Rich didn't respond to this post.
Anyway I did port using this "new" (to me) info and it helped a little but I just tried a "trick" for piston port engines of putting an extender between the carb and the engine to boost the middle of the rpm band of power and it worked really good (gave a 15% boost). Next I will try my homemade reed valve once I get the carbon fiber sheets in to make the reeds.
Where are you located Helios? I am in Ecuador.

 

[helio lucas]

portugal. the tail of europe.