Well the ol' Sahara 150 is without her engine at the moment,
doing a 155 big bore mod. While Im at it porting and polishing
the head is in order, my question is how large should I make
the exaust port and intake port? I know that the intake port
can be larger than the intake manifold opening but the exaust
port should not be larger than the muffler flange opening. Is
my thinking correct? _________________ Kinroad Sahara 150,chrome intake
1.5 uni,130 main jet, 9g rollers
gates belt,prodigy clutch/bell
mrp high performance exaust
g-force 5pt. harnesses,t-handle
hardtop and fabricated fenders
Keep the ports Matched and keep them as uniformed as possible, when the tube has a location in it that is larger than the rest of the tube you end up with a void, this void creates a vacumn on the intake side and if large enough it could cause a..... kinda like a flat spot on acceleration, although you may not notice it on the dirt, if it were a prostreet you'd notice it in your time at the end of the track, it also has an effect in the exhaust , but like I said you may not notice it , I got to typing and... well, were not tring to shave a couple of tenths off our time, but you understand it will affect perfomance, so keep it as uniformed as possible _________________ Turbo Turtle(for the little man)
XYKD260-1,
Stock; air in the tires and wind in my face!. Everything else has been Simacated!
Active member of MBRA and NEBA.
yes keep the intake port uni form and port matched to the intake manifold, but do not polish it to a mirror shine, in fact you want the intake port surface to be sligthly rough because it helps with keeping the fuel molecules atomized in the air. a miror shine smooth finish can cause the fuel to puddle in the runner and valve bowl. the exhaust side can be as smooth as you want it, but it is actually good to have a small step up from the exhaust port to a slightly larger exhaust manifold. this small change in diameter leaves a little 'edge' that creates a vacuum that prevents exhaust gas reversion. this lip doesnt affect flow measruably because the exhaust is still flowing from a smaller dia pipe to a larger dia pipe (the smaller port to the larger exhaust manifold).
hope this helps _________________ 2006 Roketa 800cc - K&N + precharger, rejetted, manual carb secondary, Desertkart spark arrest muffler, limiting straps, & lots of repairs...
Thanks for the advice, a steady hand and lots of patience is a key to doing this thing right. Hopefully the outcome will create
a better performing 150, oh I forgot, 155, Im buying the big-
bore really soon. _________________ Kinroad Sahara 150,chrome intake
1.5 uni,130 main jet, 9g rollers
gates belt,prodigy clutch/bell
mrp high performance exaust
g-force 5pt. harnesses,t-handle
hardtop and fabricated fenders
I had my engine rebuilt by Tom at SYC Power Sports. He will port and polish your valve head for you. www.sycpowersports.com.
He does excellent quality work. I highly recommend him. _________________ '07 Roketa 150cc
32mm MRP Carb/165 Jet
HP Manifold, Coil & CDI
62mm BIG Bore/Strkr Crank
Port/Polish Lg Valve Head
A12 Cam/39t Sprkt
Dr. P Hit Clutch Kit & Variator
K&N Air/E.S. Exhaust/Iridium Plg
F:22x7x10 Holeshot XCR
R:22x11x10 Holeshot XCT
...but do not polish it to a mirror shine, in fact you want the intake port surface to be sligthly rough because it helps with keeping the fuel molecules atomized in the air. a miror shine smooth finish can cause the fuel to puddle in the runner and valve bowl.
I would tend to disagree. The air is moving so fast in these chambers that a rough surface will slow the flow down. I am going to throw a gauge on soon but I would expect these engines to be around 16-18 -inHG. That is a signifigant amount of suction and will not allow the fuel to linger at all.
This type of thing is opinion but it has been my experience that the majority of internal combustion engines benifit from a polish on both sides, intake and exhaust. _________________ -Alex
www.kidnmekarts.com
-Service Tech Extraordinaire
Articles
