Upshur's opposed twin engine

[Steamchick]

Happy Thanksgiving. Enjoy family time.
😊

 

[Brian Rupnow]

I just received this email from R Klopp, who successfully built and ran six (not eight) of these Upshur opposed engines.
Hi Brian

I based my Upshur valve timing on the Wyvern design and came up with these numbers:

Wyvern timing (based on crank angle) is reportedly as follows:
Inlet: 230º duration, open 5º BTDC, close 225º ATDC, lobe centered at 110º
Exhaust: 255º duration, open 235º BTDC, close 20º ATDC, lobe centered at 107.5º BTDC
Offset between the lobes based on cam angle is therefore 108.75 º
Both valves are equally open 7.5º ATDC.

I machined the Upshur cams to these numbers, and I think the reality came pretty close to the numbers. I suppose the main thing for you is to set the angle between cam lobe peaks to 108.75° or thereabouts. You can then shift the cam-to-crank gear engagement tooth by tooth until you get the intake opening as close to 5° before TDC as you can.

This post has a decent explanation: https://www.model-engineer.co.uk/forums/topic/centaur-valve-timing/page/2/

Some old info here: https://www.homemodelenginemachinist.com/threads/upshur-twin-valve-timing.32825/
and here: https://www.modelenginemaker.com/index.php/topic,3793.msg68882.html#msg68882

Please consider copying this info to the main thread so the rest of the community can benefit.

Good luck!

 

[rklopp]

Brian is way too generous. I didn't build 8 . I wish my teachers gave me that much extra credit back in the day.

 

[Nerd1000]

That is when the exhaust pressure in the cylinder is still quite large and it will affect the intake stroke.


When you build an engine with a compression ratio of 4-1, 5-1 or 6-1 the entire cylinder volume is calculated and when you give it 45 degrees after BDC, what will be the remaining compression ratio of the engine !?


I usually tune the engine this way, maybe a little more but never 45 degrees after BDC

These numbers are not unusual for a faster running engine. The reason is that the air in the intake takes some time to accelerate and slow down, so by having an intake pipe of the right length one can arrange for air to keep flowing into the cylinder even after the piston has passed BDC. Even 60 degrees after BDC would be possible on a racing engine, however it would run poorly at lower speeds.

A similar idea applies with opening the intake early. If the exhaust pipe is the right length one can arrange for the escaping pulse of exhaust to produce a temporary vacuum at the exhaust valve at the end of the exhaust stroke. By opening the intake early, that vacuum can pull some mixture into the exhaust and thus flush exhaust gases from the cylinder. Of course this is bad for fuel consumption and pollution, but can allow a high performance engine to make more power.