Finding Port Durations With A Degree Wheel (2 Methods) Feb 14, 2010 16:27:31 GMT -5
Post by 90GTVert on Feb 14, 2010 16:27:31 GMT -5
Finding Port Durations With A Degree Wheel
Making a degree wheel is easy. You can find printable degree wheels online. They can be printed out and stuck to a CD, cardboard, or whatever you like. I find it easiest to print on sticker paper, available at lots of office supply stores pretty cheap.
Here's a free degree wheel that I made up to make life easier when checking out 2Ts.
I started by putting a piston stop on my cylinder. If you don't have one, they're very easy to make. I just cut mine out of some flat bar I had around and drilled 2 holes for the cylinder studs to pass through. If your piston doesn't reach the top of the cylinder you'll need to drill a hole in the center so you can put a bolt through with a nut or by threading your piston stop. Otherwise the piston won't hit the stop and the stop does you no good. You also need to secure the cylinder so it won't move while you're working.
I removed the side cover, fan, and flywheel earlier so I just used the flywheel nut and a large washer to attach the wheel to the crankshaft. I had to use some washers behind the wheel to get enough clearance from the engine case. You'll want to keep the wheel just a little loose for now.
As you can see here, I used a paper clip bent and bolted to a muffler mount location as a timing pointer. You can use whatever works for you in whatever location you desire.
Then I rotated the crankshaft clockwise until the piston touched the piston stop. You only want to use gentle pressure, don't try to force the piston farther than just touching the stop. I made a note of the reading on the degree wheel. I then turned the crankshaft counter-clockwise until the piston was once again at the stop and made a note of the reading on the degree wheel. TDC is exactly in the middle of the 2 readings. Adjust the degree wheel accordingly. If properly adjusted, the piston should stop at the same reading on each side of 0/360 degrees. In my case, it was 345 degrees ATDC and 345 degrees BTDC. Once you get it dialed in, tighten the degree wheel so it doesn't move. Go through the steps again and make sure you haven't thrown anything out of adjustment when tightening the wheel. If it is only slightly off you may be able to bend/reposition your timing pointer instead of the wheel itself.
Once everything is secure, the piston stop can be removed so the engine can be rotated to 0/360 degrees (TDC).
I rotated the crankshaft clockwise until the exhaust port was just about to open. I made a note of the reading on the degree wheel. You can shine a flashlight through the opposite end of the exhaust port to make this easier.
I then rotated the crankshaft clockwise until the piston had just covered the exhaust port again and made a note of that reading. If you setup the degree wheel properly, both opening and closing numbers should be the same. Say you get 87 degrees BTDC and ATDC when you measure the exhaust port open and close times. That would show that your degree wheel is setup properly, provided you are recording the numbers at the same piston location. If you end up with 87 and 89 degrees, you may want to try again to see if you still come up with the same numbers. If you come up with different numbers again,you could go back and setup the degree wheel again to be accurate on 0 degrees at TDC, or you can average your 2 readings.
Once you have an exhaust event timing figure, you can come up with a port duration. I'll use 87 degrees as the exhaust port opening/closing timing for an example. Subtract 87 from 180, and you get 93. Now multiply 93 by 2 and you will get your port duration of 186 degrees. Here's the formula :
(180 - Port Opening/Closing Degree) x 2 = Port Duration
Here's another way to get the same result. Use whichever you prefer.
360 - (Port Opening/Closing Degree x 2) = Port Duration
You can get your transfer durations the same way.
Finding TDC :