- Local time
- 7:13 AM
- Apr 16, 2014
- Omaha, Nebraska
Wow that blowdown is horrendous, and is definitely the cause for the lack of higher RPM power. You could try to lower the jug to reduce the transfer duration and increase the timing ATDC to try and help improve the blowdown some. That would buy you a little room to raise the exhaust port. The downside is the port floor being a turbulence zone due to the piston blocking part of it would get worse. Plus doing so would only buy you a couple of degrees at best, and as you mentioned the jug is already decked so that will cause other problems.OK, tore it down again to get port timing.
Exhaust: 159.5 ( we can call it 160 I guess)
I am no porting pro....but what I have read it seems there is a discrepancy here, and these durations would give basically what its doing. Good low end, OK low-mid range, and mid-high falls flat fairly quickly.
Seems I want the BD to be significantly higher to get the power band up a bit more to the top, but I cannot do so without raising the exhaust port. I have already decked the jug for squish. Decking it more to raise the jug would get me into the top cooling fin and significantly reduce trapped volume. Would that be the way to go to spread the power band out a bit more toward the top?
Is my thinking correct here?
You could raise the exhaust port a little (A couple degrees) without likely causing many issues. 160 degrees of exhaust duration isn't too terrible. Another thing you can attempt to do is buy yourself as much evacuation as possible in that limited blowdown by widening the exhaust port at the top. The risk here is if you shape it wrong you will snag a ring.
Another option still would be to ramp the piston on the exhaust port side to get it to open sooner relative to the transfers. You will sacrifice a little compression and part of the squish band as a trade off but it may be worth doing in this case.
Obviously any combination of the three could also be made to work.