Portal 2
Some weeks ago I started porting the cylinder on the DT1. The last time I ported a motorcycle cylinder (head) I was maybe 16 and the bike was a Honda Z50. (it was an overhead cam engine, so the intake and exhaust ports were on the actual cylinder head. On two strokes, the ports are built in to the cylinder itself. The 2 stroke cylinder head is nothing more than an aluminum chunk with a combustion chamber, nothing else) I also had plans to supercharge that bike with a Coleman battery powered air pump and PVC pipe adapters.
Porting is the enlarging, smoothing, and streamlining of intake and exhaust ports. Sometimes it’s called porting and polishing (polishing is what it sounds like) the idea is that smoother, hand shaped ports allow air flow to move in greater volumes with less restrictions. The result- more power. Or, if you’re from East Texas, more “pahhrR.”
I’ve learned most of everything mechanical through reading, studying, looking at pictures and most of all by actually doing it and messing up. But my philosophy on life has changed, and that’s transferred over in my approach to this porting project. My philosophy now is take baby steps forward instead of a huge leap. And as much as possible…don’t overdo things.
Some grinding done to the xfer ports before I got there
So I started with the transfer ports. Big chance to help smooth air flow, small(er) chance to really ruin things. When I got the cylinder, I could have sworn something had been done to the head. I had high hopes that it was a rare performance part from Yamaha, but having no way to substantiate that, I’m 98% sure it’s a factory, run of the mill cylinder. Most likely the 14 year old kid who had the bike before me, or the owner before that, did some work to the transfer ports. It's cool to think that some old guy back in the 70s did this to hop up his bike. You can see the color difference between the regular cast material and the material that was ground down years ago. Given the fact that the inside of the engine was so clean and the bike in general looks like it was never used, (wheel bearings perfect, hubs still showing clean grease, transmission gears showing almost 0 wear) I think this is old work, not done by the kid I bought it from.
2 strokes don’t use valves and camshafts to control air/fuel and exhaust. They use “ports” which are really just holes in the cylinder that allow gasses to flow through at certain times.
The 2 stroke relies on the crankcase to hold the air/fuel/oil mixture, relies on the transfer ports to get this mixture in to the cylinder, and relies on the exhaust port to get the spent gasses out of the cylinder.
Stock transfer ports on a GYT Cylinder (stolen from Vallyrider and his rebuild thread, http://advrider.com/forums/showthread.php?t=557469
In the photo to the right you can see a thin dark "stained" area for lack of a better work on the flat cylinder surface. To contextualize, the cylinder is upside down in this pic. When installed on the motor, this surface is in contact with the crankcase. This dark stained area over hangs in to the crank case, disrupting and obstructing a portion of the flow. This part is easily ground away to port match the cylinder to the cases.
Why port/case matching helps. The person before me filed down the center divider to smooth air flow.
If you look closely in the center of the picture where the case (grey silver color) comes in contact with the port (the two black/brown open holes) you can see the difference in surfaces.
Close up of the surface and material removed with a carbide bit to match the transfer port to the case. This will be smoothed with sand paper and then be finished. You can also see the scoring in the cylinder wall, which is why it needs to be bored.
The engine manages to do all this with nothing but a piston, the changes the piston creates in air pressure, and it’s path of travel up and down the cylinder. Piston Port induction. The earliest, simplest, (most inefficient) form of 2 stroke awesomeness. There’s some good info online about how they work, but here’s a good animation from Hooniverse.com
So porting…why not make the port as big as possible, polish it mirror smooth, and profit?
That’s pretty much the plan on 4 stroke engines (pretty much everything today) with the caveat being that if you port too much, the port walls are too thin. Behind the port walls of almost every 4 stroke engine- water jackets that transport coolant. If the ported wall is too thin you can get a leak, and that’s all sorts of bad news.
For 2 strokes…since the efficiency of the engine relies solely on the volume of air the piston is able to move in 1 stroke, if you make the ports too large, the gasses will not move with adequate velocity. The whole deal with two stroke porting is balancing velocity and volume. The other mess that comes in to play is port “timing,” or when ports open in relation to one another and for how many degrees of the rotation of the crankshaft. There are some good DT1 port maps out there based on the DT1MX, which came with a higher performance cylinder. Doing that level of porting requires a lot of time, skill, and faith. All of which I have…but I have it in baby steps.
How it looked when it was first opened...
I was going to buy a new cylinder for this engine anyway. The kid before me ran this thing without oil (he tried…he spliced the oil line in 3 places with 3 different kinds of tubing, and he did put 10w30 in the oil tank (not the right oil, kid…)) and the piston did some damage to the cylinder wall. I broke out the dremel to begin smoothing out the transfer ports from the work that was done before. I feel like I did well. I need to do hand sanding, but I didn’t take much off and smoothed transitions where possible.
After looking at prices of cylinders on ebay (and their terrible condition) I decided to seek out a machine shop around Houston that can do the work. Harold’s machine came up as the place from a couple of recommendations, and as of today that is where the cylinder sits.
I’ll need to get a piston before he can finish the work, so there is that. But once done, I can finish the ports by hand, powder coat, and put it all together.
It has dawned on me that I have nowhere to start this thing around here, living in a small apartment. It may have to be transported to San Antonio in pieces and assembled there.
