- Location
- Spring Lake, MI
This past season I've been running a 771cc 85mm bore with aftermarket 44sbn's, bpipe, and zeel. Most of the year was 180psi, and a couple gallons at 200psi. I've been extremely pleased with the power output and have it tuned perfectly. No fouled plugs, no hesitation, no chugging. I'll be in possession of a Rad +5mm crank that I'd like to do some experimenting with. I have a good background in 2 stroke port timing, but more related to making sure the cylinder is spaced correctly and squish is set. I've scoured the forums and the rest of the internet to get an idea on what I want to do, and what the possible outcomes will be. I suppose this thread can document what I've learned and add additional findings that I couldn't dig up myself.
Stock 61x port timings are 39.6mm exhaust(178.7 degrees), 55.4mm transfers(118.26 degrees) and blowdown of 30.22 degrees.
measuring my engine using calipers for port heights, will rig up a degree wheel later on before teardown. 39.1mm exhaust(180.4 degrees), 55.1mm transfers(119.6 degrees), and 30.4 degrees of blowdown.
Using this port timing calculator: https://lambretta-images.com/tuningh/port-timing-calculators/
I was able to change the stroke to 73mm, and also adjust the piston height to be 2mm above the deck, considering it is currently 0.5mm below deck. Without changing any ports, just changing to the stroker crank, the port timings are now 182.65 degrees exhaust, 127.12 degrees transfer, and 27.77 degrees blowdown.
My understanding with going to a larger stroke is that it increases dwell time at the top and bottom dead centers. This shows how the transfer duration shot up and also cut down on blowdown. I can massage the port timings to be closer to original by lowering the cylinder by 0.5mm(limit of piston above deck) and raising the exhaust port about 1.5mm. This would give me: 185.76 degrees exhaust, 125.08 degrees transfer, and 30.34 degrees blowdown.
My concern is how important it is to get the blowdown back to original specs, and can a lower blowdown be compensated by having a larger transfer duration? I understand enough blowdown is needed to drop the cylinder pressure so the intake charge can be pushed in. If the blowdown is not quite enough, but the transfer duration is slightly longer, maybe it's no big deal. I know the current port timings work and the engine responds exactly how I want it, and I'd rather not turn it into a laggy topend machine. My thinking is to not modify anything except a set of domes to set proper squish, and see how it performs. Something else to think about is also a gain in port area in both transfers and exhaust when using the stroker crank. I measured the port bottoms of both and they hang roughly 2mm below the piston.
Any criticism or insight would be helpful. Ideally a lamey 5mm cylinder would show up but it would be nice to know if this could get me 80% there without spending another $2500 on a topend.
Stock 61x port timings are 39.6mm exhaust(178.7 degrees), 55.4mm transfers(118.26 degrees) and blowdown of 30.22 degrees.
measuring my engine using calipers for port heights, will rig up a degree wheel later on before teardown. 39.1mm exhaust(180.4 degrees), 55.1mm transfers(119.6 degrees), and 30.4 degrees of blowdown.
Using this port timing calculator: https://lambretta-images.com/tuningh/port-timing-calculators/
I was able to change the stroke to 73mm, and also adjust the piston height to be 2mm above the deck, considering it is currently 0.5mm below deck. Without changing any ports, just changing to the stroker crank, the port timings are now 182.65 degrees exhaust, 127.12 degrees transfer, and 27.77 degrees blowdown.
My understanding with going to a larger stroke is that it increases dwell time at the top and bottom dead centers. This shows how the transfer duration shot up and also cut down on blowdown. I can massage the port timings to be closer to original by lowering the cylinder by 0.5mm(limit of piston above deck) and raising the exhaust port about 1.5mm. This would give me: 185.76 degrees exhaust, 125.08 degrees transfer, and 30.34 degrees blowdown.
My concern is how important it is to get the blowdown back to original specs, and can a lower blowdown be compensated by having a larger transfer duration? I understand enough blowdown is needed to drop the cylinder pressure so the intake charge can be pushed in. If the blowdown is not quite enough, but the transfer duration is slightly longer, maybe it's no big deal. I know the current port timings work and the engine responds exactly how I want it, and I'd rather not turn it into a laggy topend machine. My thinking is to not modify anything except a set of domes to set proper squish, and see how it performs. Something else to think about is also a gain in port area in both transfers and exhaust when using the stroker crank. I measured the port bottoms of both and they hang roughly 2mm below the piston.
Any criticism or insight would be helpful. Ideally a lamey 5mm cylinder would show up but it would be nice to know if this could get me 80% there without spending another $2500 on a topend.
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