I have an old Metra 65 that's pretty worn, but still runs. I decided to start with a new one for a rebuild, so the old one is just sitting around. I'll take a chance porting the old cylinder, since I don't need it.
The new set-up is a ZA50, 20mm PHBG, 20mm intake, the Metra 65 (port matched), and a Homoet 6P.
I'm wondering if the Metra 65 can be ported to work better with the Homoet 6P, since the 6P seems to have a pretty high powerband and the Metra 65 doesn't seem to. I want to try to port the cylinder to match the pipe.
So blowdown refers to the crank angle between transfer port opening, and exhaust port opening? Is this correct?
What do you guys think about opening the exhaust port about 5 degrees earliar. Should I go more? Less? Is this completely the wrong idea.
Here's what I found on the Wiki...
The cylinders page says that the Metra 65 has an exhaust duration of 153 degrees, and blowdown is 19.5 degrees.
The other two 65cc cylinders are 164 and 165 degrees duration, and have 25 degrees for blowdown.
For an automatic, usable timings are:
150-160: Low exhaust timing. Torque, not high revs.
160-170: Very usable timing. Good torque and some power in the mid to high range.
170-180: High exhaust timing. Very limited use. Almost no torque with a narrow power band at high revs.
For the intake timing, calculate the blowdown.
Blowdown of 25: Low blowdown. Wide power band with little peak. Rides well for everyday use.
Blowdown of 30: Pretty narrow power band. Low power at low revs, higher power at high revs. You need a well tuned clutch to be able to ride with a blowdown as high as this, or it will stall out.
Blowdown of 35+: Way too high for an automatic. This is only ridable on a well tuned variomatic or shifter engine.
As a rule of thumb, a blowdown higher then 28 isn't for everyday use on an automatic. Don't go higher than 28 if you don't know what you're doing ;)