3rd trasfer port,exhaust timing,piston porting questions

mrfubs

mrfubs

Member
so i bought the reed valve kit from arrow, im not 100% aware of what advantages the 3rd TP adds? also does it bypass the intake port? so as soon as the piston starts going twords TDC from BDC creating a burst of suction when the intake port is opened; is that burst of suction gone due to the 3rd TP? does the port minimize intake charge?

My exhaust timing question, would i get a better breathing engine if i widened the exhaust port? or should i change the timing by a few if bringing the port up a little higher with out achieving too high of rpm?

My question on a piston port, would drilling a medium/small size hole in direct correlation with the wrist pin bearing upon intake stroke would that help lube the wrist pin bearing? and also would or would that not work if i added a 3rd TP?


Thanks everyone!
 
Jenning's book says you can widen the port safely up to 62% of bore width (if you run a 66cc, thats a 47mm bore, 62% of 47mm is 29.14mm, stock width I believe is ~24mm). Generally speaking, as long as you dont change the timing (the height), then it'll only add power, changing timing will move where that power is (its tuned for mid-low out of the box). Just remember if you do go this route, ROUND SIDES and bevel the edges so the rings dont snag as much. if you do neither, you could snag a ring and break it on the first stroke.

In terms of reeds, the third transfer adds extra directional flow. When you do it, try to make it dead center, and a shallow angle at the top so when the transfer happens it aims up and straight out. This will guide the other two transfer flow up and out to clean out the combustion chamber more efficiently. This is only possible with reeds because it comes straight off the intake port and would otherwise blow straight back out your carb. You piston port because otherwise the only way the transfer happens through the extra transfer is with the resonance effect of an expansion pipe that sucks a little extra into the chamber, and even then it wont add much power. The piston port will allow the downward motion of the piston to ram the air up into the chamber better through that extra transfer. The disadvantage to this is you lose transfer pressures because of the extra one. To fix this, stuff the crank case a bit (there are tutorials). You can do a boost port all the way to the bottom of the jug and leave the piston alone, but the advantages of piston porting are much better (lubrication and cooling flow through the wrist pin bearing will prolong its life, less weight on the piston reduces vibrations and prolongs all bearing life, and ride comfort improves). And a piston port without a booster would have the same effect as a boost port without the piston port, exhaust resonance sucks extra charge in, but it has to take a longer path through the transfers. Basically, it's do both, or dont bother.
 
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Awesome! i really appreciate the help. so at the end of your answer; when i get the 3rd transfer port/boost done and the ported piston basically upon the compression stroke the crank case will be sucking mixture through the port in the piston dumping it all over the wrist pin bearing but not upon the combustion stroke because the reeds will stop it if im not miss understanding?
 
mrfubs said:
Awesome! i really appreciate the help. so at the end of your answer; when i get the 3rd transfer port/boost done and the ported piston basically upon the compression stroke the crank case will be sucking mixture through the port in the piston dumping it all over the wrist pin bearing but not upon the combustion stroke because the reeds will stop it if im not miss understanding?
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nono, it'll go by the wrist pin and lubricate/cool it AS WELL as going into the combustion chamber to be fired. The lowered case pressures could be an issue though, I've never used arrow reeds but their huge size lowers case compression (but allows tons more flow than the RSE reeds, so it'll be non-restrictive for high speeds). Then again... I doubt arrow would sell you something that would harm the engine so you'll be alright.
 
gotchaaaa!! too bad pirate cycles is out of stock on there 12$ cylinder or id buy a few for testers.
 
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CarpsCustoms said:
mew905 do you recommend Jake's cut piston?? its only 20$.. or do you think it'd be worth it to personally cut your skirt?
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Having no idea of what it even is (cut-on??) I cannot really recommend it. However it depends if you're willing to shell out cash to save yourself any mistakes, however I like experimenting, but it also depends on what you're aiming for. Cutting the skirt was an experiment I did to try and reduce weight as well as open up the intake port 100% at TDC, but the ports in these motors are all wrong as it is. Inefficiency at its finest. The motor is going to try and suck 66cc (well, actually less, I cant be bothered to measure but basically the amount of air equal to the time the intake port opens, and depends on your exhaust and transfer efficiency, it's quite difficult to explain, but if you want REAL power, grab yourself some arrow reeds, otherwise toying with the intake timing (cutting any part of the skirt) will only change where your power is, and with how little these things make as is, you'll want your powerband to stay right where it is (the longer the time your intake is open, the higher your peak power will be, but of course the actual intake port in the cylinder is partially blocked, and rather large, reducing flow speed and causing turbulence where the skirt covers the intake. If you can reduce the restriction and/or turbulence without changing your intake timing, you should net yourself a bunch more power.

So in short all I can say is if you think it'll help, look up some reviews, or take the plunge. I prefer doing my own work as I can change the powerband to my liking, rather than someone else saying "increases power" or "increases RPM"
 
I have done many on kart racing engines.But I do them with my vertical milling
machine.If you do cut booster port by hand, just don't raise it higher then transfer ports.

Fly
 
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