SWEDE
New member
Over the years I've noticed some 2 cycle engines have a hole on the side of the piston.What does this really accomplish? It seems like I've heard of this being added to some engines as well.....
Holes in the intake side of 2 stroke pistons are typically used on reed valve engines to extend the intake timing of the engine... or...in other words allow the piston to draw more fuel and air for a longer crankshaft rotation thru the intake port and carburetors to better fill the crankcases. That way more fuel/air mixture can travel up the transfers to the combustion chamber to be compressed and ignited. More fuel and air...more power!
A four stroke would accomplish this same outcome by changing camshaft profiles to keep the intake valve open longer.
On the old piston port engines we used to accomplish this by shortening the piston skirt on the intake side.
A four stroke would accomplish this same outcome by changing camshaft profiles to keep the intake valve open longer.
On the old piston port engines we used to accomplish this by shortening the piston skirt on the intake side.
Rustman
New member
How do you determine the size of the hole to make?
SWEDE
New member
Nice explanation there Bob, that makes sense, but I'm not ready to go and drill holes in my pistons just yet:-)sideshowBob said:
The location and size of the piston hole is specific to each engine amd intake port/boost port configuration and not applicable to all engine types. Adding or changing these is best left to the manufactures and engine performance gurus...its all about the Math.
YAMMIEGOD3:16
Active member
GREAT INFO MANsideshowBob said:
3:16 (yammie tony)
Rustman
New member
Thanks, I was asusming so but I thought I would ask. I;ve seen holes in stock 93 vmax750 pistons that are quite small, but then Ive seen modified engines with pistons that had massive holes in them. Just wondering.
sideshowBob said:
Bob gave a excellant description , heres some other things that come with doing the mod as well. The larger hole also provides better lubrication of the wristpin bearing, some engines like to turn the bearing and pins blue from heat. Another thing that occurs is you can better cool the underside of the piston crown because of the increased amount of fuel/air charge being trapped under it on the down stroke of the crank.
Its all pretty good stuff as long as its timed right. I add holes to alot of engines that dont come with them and theres power and benefits to be had but.... exactly like Bob said its all about the math and you need to have a degree wheel on a engine to know where and how big(duration) of a hole or slot to put in the piston. You cant just put a hole in the back its gotta be in the exact right spot.
, heres some other things that come with doing the mod as well. The larger hole also provides better lubrication of the wristpin bearing, some engines like to turn the bearing and pins blue from heat. Another thing that occurs is you can better cool the underside of the piston crown because of the increased amount of fuel/air charge being trapped under it on the down stroke of the crank. Its all pretty good stuff as long as its timed right. I add holes to alot of engines that dont come with them and theres power and benefits to be had but.... exactly like Bob said its all about the math and you need to have a degree wheel on a engine to know where and how big(duration) of a hole or slot to put in the piston. You cant just put a hole in the back its gotta be in the exact right spot.
SWEDE
New member
That is kinda what I figured and thanks for the reply Don!!!!mrviper700 said:Bob gave a excellant description
Its all pretty good stuff as long as its timed right. I add holes to alot of engines that dont come with them and theres power and benefits to be had but.... exactly like Bob said its all about the math and you need to have a degree wheel on a engine to know where and how big(duration) of a hole or slot to put in the piston. You cant just put a hole in the back its gotta be in the exact right spot.
Mac
Member
Got Boost - Match the ports
Extra boost can be obtained through port matching piston and cylinder. Look at your cylinders and notice the square port. The piston has a round port.
Extra boost can be obtained through port matching piston and cylinder. Look at your cylinders and notice the square port. The piston has a round port.
I opened these holes up to 1/2" on an srx 600 once when we did a rebuild and other than that I cleaned up the transfers on the bottoms of the jugs and that 600 ALWAYS gave my and a buddy's 700's a realy good run. Not sure if opening the holes helped but it sure didn't hurt.
Do you guys recommened opening them slighlty on an srx????
Do you guys recommened opening them slighlty on an srx????
SWEDE
New member
Good stuff guys,much appreciated!