360 to 180 degree crank

[LOUD MOUSE]

Your ideas seen OK and they bring to question why the 360 BRITISH engines pretty much didn't use the 180 crank timing and HONDA used both the 180 and 360 cranks with 2 carbs.. ................lm

The reason for the 180 vs 360 crankshaft is the head and the carb(s).

Although I'm new to these bikes, I think figured that one out. If the engine had a 180 crank and a single carburetor, the carb would see two pulses of air, one at 0 degrees and the next at 180 degrees, followed by a long pause at 360 degrees and 540 degrees. That ragged firing order would probably cause significant problems with metering air/fuel for a single carb feeding two cylinders.

When they went to a more open framed bike, they could fit two carburetors, one for each cylinder, and could balance the crankshaft. I don't think you can fit the two-carb head to the Dream because the carburetors will foul the bike's frame.

I also suspect that the Dream engine feels smoother at lower RPMs with the 360 firing order; bang-pause-bang-pause for the 360 vs. bang-bang-pause-pause for the 180. At higher RPMs the needs of mechanically balancing the engine becomes more important. The forces increase at the square of the speed, so the higher performance, high RPM bikes get a 180 crank and dual carbs.

[G-Man]

I think the British layout was all down to production cost and the good fortune that the 360 crank bikes made a nice smooth sound. The vibration was no worse than the singles they replaced and performance was much better. The basic cranks were one-piece forgings or castings with a central flywheel bolted on. A 180 crank was difficult to do. Getting oil to both big ends from one feed is easy on a 360 engine with plain big ends.

With the expensive 'pressed-up' design of those roller-bearing cranks and three-piece camshafts Honda had freedom for a number of permutations without actually changing the basic components.

The follow-on CB250/350 lost some of the versatility and cost. Pressed-up cranks but solid camshafts. No 360 motors in that range.

Honda then found a "better" solution with the later 250 and 400 Hawks (Super Dream in UK) of the late 70s and early 80s where they had solid 360 shell-bearing cranks, balance shafts and twin carbs, not to mention the one-piece cams and 3-valve heads.....

Engineering is all about compromise ......

G

Your ideas seen OK and they bring to question why the 360 BRITISH engines pretty much didn't use the 180 crank timing and HONDA used both the 180 and 360 cranks with 2 carbs.. ................lm

The reason for the 180 vs 360 crankshaft is the head and the carb(s).

Although I'm new to these bikes, I think figured that one out. If the engine had a 180 crank and a single carburetor, the carb would see two pulses of air, one at 0 degrees and the next at 180 degrees, followed by a long pause at 360 degrees and 540 degrees. That ragged firing order would probably cause significant problems with metering air/fuel for a single carb feeding two cylinders.

When they went to a more open framed bike, they could fit two carburetors, one for each cylinder, and could balance the crankshaft. I don't think you can fit the two-carb head to the Dream because the carburetors will foul the bike's frame.

I also suspect that the Dream engine feels smoother at lower RPMs with the 360 firing order; bang-pause-bang-pause for the 360 vs. bang-bang-pause-pause for the 180. At higher RPMs the needs of mechanically balancing the engine becomes more important. The forces increase at the square of the speed, so the higher performance, high RPM bikes get a 180 crank and dual carbs.

[DJM]

The reason for differences between otherwise identical engines but with 180 or 360 degree engines is due to pumping losses in the crankcases.

I read a paper on this a while ago but don't remember the exact details.

With a 360 degree crank, the crankcase volume increase / decrease by 250 or 305 cc every revolution, this air has to be 'pumped' in somehow, or at least produce a cyclic pressure variation in the crankcases.

In the 180 degree engine the crankcase volume and hence 'pressure' remains constant, but the air being displaced by the descending piston needs to move to fill the partial vacuum created by the opposite ascending piston. Hence air is being pumped backwards and forwards across the two halves of the crankcases resulting in 'pumping losses'

These losses aren't just proportional to engine speed, indeed, looking at the comparison graphs in the CL 72 Shop Manual, the 360 degree engine produces significantly MORE power at low and medium revs only loosing out by a couple of horsepower at the highest engine speed.

I don't claim to fully understand this but this is why so called 'big bang' engines produce more power although the BMEP acting on the pistons doesn't change.

[G-Man]

DJM

More interesting thoughts to consider. I remember a Yoshimura paper where he recommended curtting holes in the crankcase to ease the flow of that pumping air to reduce losses.

The speed relation is also fascinating and involves the area of the passageway, the volume and compressibility of the air. For the same reason, 'blow-by' can also be speed realated.

I used to drive an old David Brown tractor and the dipstick/oil-filler was a push fit, held in with o-rings. If you left it ticking over at idle for any length of time it would build up enough pressure to blow the filler out with an amazin pop!

PCV valves on cars and big diesels with crankcase blowers aim to maintain a small vacuum in the crankcase to reduce the tendensy to push oil our of the joints. Triumph engines had a timed breather attached to the camshaft which aimed to let pressure out but resisted the engine 'sucking' air in through the breather

Going back to Hondas, it intrigues me that they used the same breather mechanisms on both engine types. Cost-control again I suppose. Even on the 180 engines, blow-by will always be present and will slowly pressurise the case unless there is a breather.

G

The reason for differences between otherwise identical engines but with 180 or 360 degree engines is due to pumping losses in the crankcases.

I read a paper on this a while ago but don't remember the exact details.

With a 360 degree crank, the crankcase volume increase / decrease by 250 or 305 cc every revolution, this air has to be 'pumped' in somehow, or at least produce a cyclic pressure variation in the crankcases.

In the 180 degree engine the crankcase volume and hence 'pressure' remains constant, but the air being displaced by the descending piston needs to move to fill the partial vacuum created by the opposite ascending piston. Hence air is being pumped backwards and forwards across the two halves of the crankcases resulting in 'pumping losses'

These losses aren't just proportional to engine speed, indeed, looking at the comparison graphs in the CL 72 Shop Manual, the 360 degree engine produces significantly MORE power at low and medium revs only loosing out by a couple of horsepower at the highest engine speed.

I don't claim to fully understand this but this is why so called 'big bang' engines produce more power although the BMEP acting on the pistons doesn't change.

[pcmenten]

Very interesting information about the crankcase volume. So here's a stupid idea; use the pumped air of the crankcase to feed the carburetor. It would require a reed valve on the crankcase air inlet, but the result would be a supercharged engine.

[G-Man]

That's an interesting thought. You'd effectively have two pistons moving down feeding the one piston (in a 360 crank engine) but that would only work for one cycle........

You'd have to suck in more atmospheric air and the energy would not come for free. That air already in the cases is only available once. When you have pumped it out you have to pull it back in from somewhere as the pistons rise, like a two-stroke. Unlike a two-stroke, though the crankase volume is very large compared with the engine swept volume so you would not get much crankase compression to move the air around.

You've almost designed one of these.......

http://fjstuart.blogspot.co.uk/2012/12/ ... 500cc.html

In the case of the 360 crank Honda engine the breather is not big enough to let much of the air out and pull it back in on every stroke. In that case the air in the crankcase just acts as a spring. Compressing and expanding as the pistons rise and fall.

You might have invented a 4/2 stroke engine or a perpetual motion machine.... :-)

G

Very interesting information about the crankcase volume. So here's a stupid idea; use the pumped air of the crankcase to feed the carburetor. It would require a reed valve on the crankcase air inlet, but the result would be a supercharged engine.