Boost porting the 68cc china engine

europorsche914 · #1 11-24-2008, 02:51 PM

Saw an interesting thread over on the 2 stroke pocket bike forum and I am plannin to perform this mod this week on my motor. It will be 1 port opposite the exhaust port, about 1/4" wide and 2 mm deep or so to the same height of the top of the transfer ports. I am going to utilize round port edges and angle the top to point at the spark plug. Ill let you guys know the results they claim atleast an extra 1000 rpm gain and more torque across the band. Heres the thread: Added a boost port tonight *pics* - Pocket Bike Forum - Mini Bikes

europorsche914 · #2 11-24-2008, 07:22 PM

Still thinkin this through since i havent torn the motor apart yet to modify it. I just realize that the intake port is currently directly across from the exhaust port so I may do two ports, one on each side of the intake to help scavenge the exhaust charge out of the cylinder. I havent decided what the measurements will be since I will be using two ports instead of one. A boost port acts as if another transfer port was fitted on the engine and also helps force the exhaust out of the cylinder (similar to having a second exhaust port).

Technocyclist · #3 11-25-2008, 12:08 AM

that's great... keep us posted... and give us some pictures to drool about...

europorsche914 · #4 12-02-2008, 01:09 PM

Here is the mod that i did to my motor. I attempted to balance the crank while it was apart and stupid me didnt look up any info on it... So it now vibrates like all **** at 25mph. I didnt half the weight of the reciprocating assy. But not all was lost... Before the crankshaft and boost port mod i could at most do 34mph according to my speedometer. Now the same speedometer read 43mph! Now im not going to tell everyone that it is dead accurate since i do not have a gps to test it but i know im going approx 40 mph since i was keeping up with traffic... The crankshaft can only be holding the engine back and the sound of the motor at that rpm is very smooth. Current mods to my engine include porting intake and exhaust, expansion chamber, homemade glass pack and steel pack muffler, 44 tooth sprocket, NGK b5hs, polish the head and piston top, and 2 boost ports. The two ports were about 1/4" wide and are as long as the ceiling of the transfer ports. They are about 3/64" deep and are centered between the intake and transfer ports. The end of each port is angled towards the intake port and utilizes a steep angle to aim at the sparkplug and help scavenge the exhaust charge out of the cylinder. Im satisfied with this mod and would do it again but will be tearing the motor down again to install a new balanced crankshaft... Please excuse the poor grinding on the one port, they ran out of grinding stones at the store and i will clean it up on the next tear down

jasonh · #5 12-02-2008, 01:24 PM

Very cool, glad it helped performance. Too bad about the crank though. I think velardejose balanced his crank. Should see if you can find his thread on it.

Home-made J · #6 12-06-2008, 09:57 PM

how do those boost ports not effect compression? Aren't you compromising the integrity of the rings' seal? I know you explained it a bit as being an extra exhaust port, but I would be into knowing some more before I do it...and I WILL do it if you're getting 43mph. I allready matched and opened the ports and machined the head, and I'm running a 34T Andy sprocket. If I do these boost ports, balance the crank, and throw a tuned pipe on her, then I will have taken Chinese motors to the wall (unless somebody starts fabricating bolt-on reed cages!). Some dude was making a reed setup, but never followed up on if the thing worked or not.....someday.

jasonh · #7 12-06-2008, 10:08 PM

SCSW does make reed cages for these motors. Contact Easy Rider for info.

I think it would only affect the ring seal when the piston is at it's lowest point, as they would get closed when the intake port is closed....I think....

europorsche914 · #8 12-07-2008, 03:59 PM

since the boost ports only go as high as the transfers, the compression is unaffected. Dont quote me on the 43mph but i know i keep up with traffic in 40mph speed zones. I may have been the one that you are thinking of, I had a thread about my homemade reed valve but I had a intake leak or possibly too small of a carb bc it wouldnt tune properly, i will hopefully have it mounted within the next 3 weeks. But be warned the boost port with out a reed cage will decrease low end torque (mid and high end torque seems unaffected) but increases rpms. I also noticed that at flat out i do not have much torque perhaps it is out of its power range at this point or it is because of my bad crank. My new crank will be installed within the next two weeks. Those considering this mod be aware that this allows your bike to turn some high rpms and therefore speed that could kill you if something happened...-Justin

i-paint · #9 12-09-2008, 09:25 PM

Let us know how the new crank performs. I would like to have one of mine balanced. It is a "vibrator" at all rpms, more at lower rpms though. It is a replacement engine from BGF and is the strongest running I have. I've had it on three bicycles same results. Would be worth a balanced crank.
Thanks. i-paint

europorsche914 · #10 12-09-2008, 10:37 PM

Heres some great info: it states to account for all rotating mass + 50% of reciprocating mass: http://www.neme-s.org/Model_Engineer...%20Engines.pdf. Here's another thread that states using 58% of the reciprocating weight: 4 Cycle Karting Forums - single cylinder engines can only be balanced at a set RPM
Reciprocating Mass is the weight of:
Piston
rings,
wrist pin,
pin keepers,
rod end that connects to piston.

Rotating Mass:
rod end that connects to crank

Stupid me didnt do any background checking on how to balance a single cyl and I balanced 100% of the reciprocating mass... Bad choice as I now know and wasted a lot of time grindin of hardened steel... Im plannin on using 57.5% of the reciprocating weight (60.375 grams) as the karting guys are doin. From what i take from his thread and pictures, Velardejose uses 52.3% of the reciprocating weight (55 grams) and says his works great but still has light vibration in the pedal area. (it will always vibrate a little since you can only balance a single cylinder to a certain rpm.) I will let everyone know the results and if I need to remove more I will. This way we will know which way to head (closer to 60% or closer to 50%). FWIW i have a one piece crank unlike velardejose's which is a three piece (larger weights attached to a small crank (older style)). Here's velardejose's thread: http://motorbicycling.com/f13/homema...cing-4498.html.
I also just ordered a shifter kit I cant weight to see how it hauls with these mods

maybe i will cruise at 70mph!!

-Justin