Blocking off boost ports

[manbearpig]

looking for some information about the theory behind blocking off the boost ports i a 2 stroke motor

i have recently purchased an 81R that runs well. PO said that he took the motor to Scribbs and they ported it for top end while they were in there. said it made it unrideable for him. i rode it, its not that scary. pulled the intake off and looked inside the intake to find that the boost ports have been sealed off and the intake has been opened up and reshaped.

ive encountered this before in a ported cylinder i got in a trade, but the boost ports were not sealed up completely, but were restricted heavily. they were sealed up and then had a small tube inserted to create a restricted passage.

why are people doin this? im sure its performance oriented, but it contradicts all of the big wallowed out boost ports that im used to seeing

just incase my terminology is incorrect, im referring to the two ports in the intake on either side of the large opening in the center. ive heard people call these boyessen ports as well



^^this is the intake from the nicholson 250R's cylinder

[manbearpig]

heres one of the cylinders i referenced. just found the pictures. this cylinder now belongs to Hondaridr

[3 Wheel Drive]

Better gas mileage? Lol!

[yaegerb]

Here you go. Below is link to the patent number from Eyvind Boyesen himself which was submitted in 1975. Its a long read but tells you about his hypothesis for performing these lower transfer port modifications or how it came to be known as "boyesen ports". Also at the end you will find many more citations from this patent and then reference to other patents he created to help further the advantage of this particular type of port work.

http://www.google.com/patents/US4051820

[barnett468]

Hello manbearpig


Unfortunately I have no definitive answer for you so if that’s the only thing you are looking for please disregard the following.

Sorry for sounding stupid but are these ports stock on the air cooled 250 R’s. I f they are then they are most likely to improve bottom end and midrange performance while sacrificing some too end in the process since top end is easy to come by. I don't know if the kxt's had these either but i don't remember them.

You may get many opinions posed as facts ie my lap times were better after, my bike wheelies more now, joe said that pete said that steve said that he got another 100 hp out of his alt 50 doing this etc. The fact is that the only proof in determining exactly how they affect an engines performance is in a back to back before and after dyno test which i personally have never seen for this. There are many engine dynos around so why hasn’t some porting guru done this and posted the results? All I can say is that if someone installs them and is happy with them then that’s all that matters but it’s not scientific proof as to their effectiveness.

There is apparently a book written by a person named Gohr that makes some mention of these ports but I don’t know if there are any dyno sheets in the book either.

Arlen Lehman of LED racing has also done some work with these see links at bottom of page.

The Banshee owners apparently have the most experience with these ports from what I have read. If you go to their sites you will see many that installed them said they made more poser whatever that may mean, and there are just as many that installed them and said either that they reduced power or just didn’t like what they did to their power band etc and filled them back in, lol.

If these ports were that beneficial in some applications I’m sure the motorcycle mfg’s would have installed them on the production bikes where they benefited the target market. I personally have not seen them on production bikes but I haven’t seen many that are 1988 or newer.

Generally speaking every port change in an engine ads power somewhere but takes just as much away somewhere else.

Irregardless of what performance gains they may actually add, there ”should” be a certain maximum size calculated for these ports to achieve the optimum desired results. Increasing these ports beyond that size will actually reduce engine performance due to loss of air speed /pressure thru the ports and decreased crankcase volume. This might explain why one of your cylinders has large ports and one has small ones.

Anyway I think the best source of info on these is Arlen at the links below, just call him up and see if he has dyno tests and see what he says.

[barnett468]

Hello


Here you go, forgot to include them in my post above.


ARLEN LEHMAN PORTING A 350 CYL WITH BOYSEN PORTS

http://www.youtube.com/watch?v=X_WfxWfpp3c


DYNO CHARTS CALL TO SEE IF HE HAS BEFORE AND AFTER DYNO CHARTS FOR BOYSEN PORTS ONLY

http://www.ledperformanceengines.com/trx250.htm


ARLEN LEHMAN LED RACING HOME PAGE

http://www.ledperformanceengines.com/

[manbearpig]

heres the port work that sparked my inquiry

[christph]

I can't imagine how blocking off the ports could improve any part of the power band. I'll have to read the highly technical article by Boyesen (above).

[El Camexican]

I’ve been looking around for info about this since you posted this, mostly because I had to work on both the original boost port and additional one on my Yamaha engine based on nothing other than what the so called Wrench Report called for.

In the most dummied down terms (the only ones I understand) it seems boost ports are thought to add bottom end to engines that have had their exhaust ports raised beyond what would be considered normal for an application. If yours are sealed up someone either thought they were useless to start with, or was shooting for max top end. Did they achieve it? Only the dyno knows for sure.