head porting for 350x

[350xrider911]

how much will i gain with this, i have a stock head on a 425x will i lose the wheelie power or gain in it. who will port it for and how much?

[honda_atc200es]

i am also curious what gains are to be had with a good port and polish

[SWIGIN]

a stock head on a 425 is like you running a mile while breathing through a straw.

there's all different kinds of port jobs for how the motors built and what its used for.

a good port job will be a night and day difference if done right

[350xrider911]

i want one thats good for drags and trail riding

[Dirtcrasher]

It's kinda impossible to port for both of those conditions....

One is wide open top end power and the other is looking for low end torque and grunt in the lower revs.

[350xrider911]

well is ther a such thang as a mild port great for mid and top end

[Gag_Halfront]

Only sort of. For the mid range you don't want to change the port cross-sectional area, but rather just smooth things out a bit. Clean up any casting flaws and stuff like that. For the top end, you actually want to enlarge the port (and obviously the cam, carb and manifold to match), but as soon as you do that you start to lose the bottom end because of the reduced velocity. With a slower mass of air you get crappy fuel metering and crappy mixing and the cylinder has a harder time filling properly at the low RPM. That's what's going to kill you on the trail.

In the automotive world you have things like mid-rise split-plane manifolds, trick headers that are optimized for scavenging and 4bbl carbs to help mitigate the trade-off. On a single cylinder engine there's really only so much you can do to create balance. There's no such thing as scavenging with only one cylinder. With only one tube, there's no such thing as a manifold plenum to play with. There are multiple carb setups which could help, but might just be more trouble than they're worth for a trail bike. Really, once you open up the ports, intake, and exhaust to get that top-end power there's very little you can do to keep the low-RPM power from dropping off.

IMHO, if you want to be strong in both trails and the strip, you probably want to build gobs of bottom end torque using dead simple technology and then use gears and tires to determine how you put that power to use. I don't think you're going to need a whole lot of porting to make that happen. Probably a mild cam and a very minor head clean-up with maybe just a slight bump in compression.

I think that's probably going to be about the best compromise.

[300rman]

read Motoman's write up on Velocity porting.
Read here http://mototuneusa.com/thanx.htm

making the ports smaller seems to give greater power advantages

[350xrider911]

i have and 11;1 compresion and mid and top end cam to me it wont hurt to lose bottom end i want on to be good on power and still be reliable. i have the 400ex carb and it pull like there anit not tommoro.