Another porting questions.... sorry :(

GoldenMotor.com

Pedrito0906

New Member
Jun 12, 2013
23
0
0
Puerto Rico
Hello Riders:

I want to ask you some questions about porting, I have read a bunch of pages really a bunch, some say that you should port match your intake/exhaust at 26.5 mm wide, some say you could high the ports a little, some say only round the edges, seriously I don't know what to believe. In those pics are my ported exhaust port (my intake port is the same as my exhaust port 26.5mm wide), my tuned pipe and my crankshaft. I don't know if I ported the exhaust port correctly because I'm a noob XD I want to learn!!! My friend took his brand new motor to a mechanic, he ported it and now he runs 40 mph, with stock muffler, stock head, stock carb and with a 41T sprocket, I only run 35 mph with a tuned pipe :-||, I could give it to him, but I dont want to pay $80, because I want to learn!!. I have a MM billet head, DaxRT carb with #70 jet, 39T sprocket, Bosch super plus heat range 8; the only thing I have not done yet is crank case porting and crank shaft balance, because I don't know what that is. I'm planning to study MotorBike Mechanic and I want to learn, so what better engine to start than this. Thank you for taking your time and answering our noob questions.

PS: I did those mods to the piston and really helped me a lot, also I now its backwards, because I'm a noob :'(
 

Attachments

mew905

New Member
Sep 24, 2012
647
9
0
Moose Jaw
higher ports mean more power at high end, but allows too much to blow through at low speeds (so you lose low end power). Raising the ports will also increase overall power, because RPM vs Torque gives you high end gains, your power curve will be shallower, but longer. You dont necessarily have to port match, it simply allows the motor to breathe easier (because the stock intake is actually a bit too small for the motor's size, it should increase RPM's too. If you choose to go with Reeds, if you run a 66cc, use Arrow reeds, they lower compression, but allow much more air flow)

in general, the bigger the exhaust port (in any direction, width or height) it will increase power. So to keep your current power band (say, lots of low end torque), simply widening the exhaust will boost your power. To reduce four stroking, you'll need to find a way to correct the transfer port angles. I used JB weld, I can elabourate on this if needed but it increases power and RPMs through better scavenging efficiency.

As for rounding the ports. Always round them. always always always, thats why your ports arent perfect squares, they're slightly rounded, being small, they dont need to be terribly oval. The bigger you make them, the bigger you need the curves, it's not for any power gains, its to prevent excess ring wear and/or ring snagging, breaking your rings, losing compression and damaging the cylinder, the roundness simply helps the rings get pushed back in gently at high speeds.

According to Jennings, 62% of your cylinder bore can be ported to your exhaust. For example, our 66cc motors are 47mm bore, 62% is just over 30mm, so port the exhaust 30mm wide and round AND bevel the edges to re-seat the rings easier. some high end motors use 70% bore width, but can cause short-circuiting (blowing your fresh intake charge straight out the exhaust). If you make it super wide, you gotta round like crazy, AND correcting the transfer angles will help prevent short circuiting.