Which would make better power in a ported 346xp a pop-up or a flat top piston with 200 psi?
I have worked on and I have ran a few of the 346s. I have cut the chambers and I have done the pop ups. I still cant tell a difference performance. When I built my personal 346 I cut the piston and not the chamber. If I had thought that I would gain anything by cutting the chamber I would have done it.
Try to stay under 200 pound compression on the 346, anything over that and it will slow down.
Somehow your thread got changed from your original question on your 346 to flat vs domed piston in race saws and modern 2 stroke dirt bikes. On a woods ported 346 saw I dont think anyone can tell the difference.
Here is a quiote from Eric Gorr. Please google him if you dont know who he is. It will give you a good idea about domes vs flat pistons.
Q: I have a question relating to flat top and domed 2 stroke pistons, as
currently used in 125MX engines. What are the advantages and disadvantages to each type?
Flat pros
1) greater mechanical efficentcy because less surface area for combustion
pressure to push against
2) more available port-time-area because of the difference between the port
roof angles and the piston top angle (0).
3) weight savings potential
These factors make this design better for mx applications where quick
acceleration is more important than sustained wide-open throttle.
Flat cons
1) greater pressure per square inch exerted on the top of the piston
2) expansion tends to deform the top exhaust side resulting in deformation
of the top ring groove, very evident on over-heated piston where the ring
fails and breaks that edge of the piston.
3) Lower scavenging efficentcy at high rpm because the shape of the piston
top and combustion chamber have flats rather than curves, burnt gasses get
trapped in corners.
Domed pros
1) thick crown absorbs heat and grows up towards the center rather than
towards the edge near the ring.
2) thicker crown less likely to burn through under heavy thermal-load
(shifter karts, long track, end of long straight)
3) better scavenging efficiency because the curved crown and combustion
chamber enable the loop scavenging pattern to form properly and clean all
the burnt gasses from the edge of the combustion chamber, enabling more
room for a fresh charge. This is why all modern roadracing 2-strokes use
domed pistons.
Domed cons
1) lower mechanical efficentcy because of high surface area
2) inherently heavier design
3) less port-time-area because of compromise in port roof angles and crown
angle.
Q: Why would someone convert from a domed piston to a flat top?
A: Because they're looking for better acceleration performance, mainly for
supercross application. Also because its like gaining more port-time-area
without doing any porting.
Later
Dan
opcorn:
